Neurobiology PDF

NEUROBIOLOGY The brain has changed a lot over time. Early nervous systems, like the nerve-net in jellyfish or the simple brain in flatworms, were very basic. But vertebrates developed more complex brains. Neurons and glial cells became more organized and complex, especially in mammals. While neuron...

NEUROBIOLOGY The brain has changed a lot over time. Early nervous systems, like the nerve-net in jellyfish or the simple brain in flatworms, were very basic. But vertebrates developed more complex brains. Neurons and glial cells became more organized and complex, especially in mammals. While neurons work in a similar way across species, they differ in number and organization. Invertebrates have simpler glial cells, while mammals have more. The brain is protected by the blood-brain barrier (BBB), which stops harmful substances from entering but allows necessary nutrients. This barrier is controlled by endothelial cells and helps protect brain function. Its structure differs slightly between species, with animals like drosophila having a simpler version. The brain has two types of tissue: gray matter (neuron cell bodies, dendrites, and unmyelinated axons) and white matter (myelinated axons). Over time, white matter increased, especially in vertebrates. The nervous system has two parts: 1. Central Nervous System (CNS) – the brain and spinal cord. 2. Peripheral Nervous System (PNS) – spinal and cranial nerves. The spinal cord sends and receives sensory and motor information. Sensory neurons are in the dorsal root ganglion, and motor neurons control muscle movements. Reflexes can be controlled by the brain, like when we quickly pull our hand away from heat. The brainstem controls basic survival functions. It includes the cerebellum (which coordinates movement and balance) and the pons (which controls breathing, movement, and sleep). The midbrain helps with movement, sleep, and arousal and is controlled by the thalamus and hypothalamus. The basal ganglia help control movement and are involved in diseases like Parkinson's and Huntington's. The limbic system controls emotions and includes the amygdala (fear and aggression), the hippocampus (memory), and the hypothalamus (hormone control). The brain has two halves (hemispheres), with each side controlling the opposite side of the body. The left hemisphere is linked to language and logic, while the right hemisphere handles spatial skills and creativity. The corpus callosum connects both hemispheres. Language is mostly in the left hemisphere. The Broca's area controls speech, while Wernicke's area is for understanding language. The brain's cortex is made of six layers, but the connections between neurons are what determine its functions. The frontal lobe controls movement and receives information from other brain areas. The sensory cortex processes sensory input. The parietal lobe handles touch and body sensations, the temporal lobe processes smell and sound, and the occipital lobe is responsible for vision. The motor cortex controls voluntary movement, and the sensory cortex processes information from the skin and sensory organs. The brain’s complex structure allows us to perform advanced tasks, with different regions working together to control body functions and thoughts. Summary of Techniques and Approaches to Study the Brain The study of the brain involves multiple levels of investigation, from individual neurons to complex brain networks, employing various methods that cater to different research questions and models. Below is an overview of the primary techniques and approaches used in neuroscience. Neurocentrism and Neural Network Models The Neuron Doctrine has been central to neuroscience, focusing on neurons as the basic functional units of the brain. Recent advances, however, reveal that neural ensembles and networks, rather than individual neurons, often give rise to brain function, cognition, and behavior. Neural network models are emerging as a new paradigm, integrating single-neuron data to explain complex brain phenomena like behavior and mental disorders. In Vitro Models of Brain Cells Primary neurons, glial cells, and neural stem cells (NSCs) are frequently cultured to study brain function. Neurons transmit signals, while glial cells (astrocytes, oligodendrocytes, and microglia) provide support and modulate brain activity. Induced pluripotent stem cells (iPSCs) allow for disease modeling by reprogramming adult cells into pluripotent cells, which can then differentiate into various brain cell types. 2D Pure Cultures are widely used for drug screening and high-content analysis (HCA), although they lack the complexity of in vivo models. Organoids and Co-cultures combine different cell types to create more physiologically relevant models for studying brain development, disease, and drug effects. Viral Manipulation of Brain Cells Viral vectors are commonly used to modify brain cell genomes or modulate gene expression. These vectors enable precise manipulation, such as overexpressing or knocking down genes to study their function in the brain. Chemogenetics and Optogenetics Chemogenetics allows the reversible control of cell signaling by using designer receptors activated by specific, inert drugs. Optogenetics employs light-sensitive proteins (opsins) to control neuron activity with high temporal precision, enabling researchers to study neural circuits and behavior in real-time. High-Content Approaches and Single-Cell Analysis Single-cell RNA sequencing allows unbiased examination of gene expression in specific brain cell types. This technique, alongside epigenetic studies, offers deep insights into how genes and cellular processes are regulated across conditions. Neurosurgery Methods and Lesion Studies Neurosurgery (historically used to study epilepsy) is now largely obsolete, with more advanced treatments available. However, animal lesion studies continue to be crucial for understanding brain function, with both permanent lesions (like aspiration and excitotoxic lesions) and reversible lesion models providing controlled environments to investigate neural circuits. Imaging Techniques Light microscopy (Golgi, Nissl stains) and fluorescence microscopy enable the visualization of neurons, synapses, and brain activity in both static and dynamic forms. Functional imaging techniques such as calcium imaging track neuron and glial cell activity. Electron microscopy and photon microscopy provide high-resolution images of cellular structures and synaptic activity. Super-resolution microscopy (e.g., STED, SIM) enables the observation of nanoscale processes, like synaptic changes and microglial behavior. In Vivo Imaging and Electrophysiology Advanced methods like two-photon microscopy and time-lapse imaging allow real-time, high- resolution observation of cellular activity in awake animals. These are often combined with electrophysiology to measure electrical activity within neural circuits. Electrophysiology remains essential for studying neuron firing patterns, especially in complex networks. The integration of advanced computational tools enables analysis of vast amounts of electrophysiological data. Genetic Animal Models and Fluorescent Labeling Genetically engineered animals (e.g., transgenic mice expressing fluorescent proteins) allow the study of cellular processes and brain activity in vivo with techniques like optogenetics and calcium imaging. Non-Invasive Human Imaging Structural imaging techniques like CT and MRI provide detailed anatomical data. Functional imaging methods like EEG, fMRI, and PET allow the study of brain activity and neurochemical processes in humans, offering insights into cognitive functions, diseases like Parkinson’s or Alzheimer’s, and neuroinflammation. Multimodal Approaches Combining various genetic, in vitro, animal, and human models with imaging, electrophysiology, and other techniques offers a more comprehensive understanding of the brain. This integrated approach is particularly effective in studying complex brain functions, diseases, and developmental processes. Conclusion The history and evolution of neuroscience methods reflect continuous advancements in technology and understanding. By integrating traditional techniques with modern technologies like optogenetics, high-content imaging, and single-cell analysis, researchers are gaining unprecedented insights into the brain’s structure, function, and diseases. The future of neuroscience will likely involve even more sophisticated combinations of these approaches, leading to breakthroughs in brain science and therapy. PRIMARY CULTURE Primary cells are: neurons, astrocytes oligodendrocytes microglial cell Secondary Culture →Cells taken from a primary culture and passed or divided in vitro. PRIMARY NEURON CELLS have a practical problem: they must be obtain from a tissue in which the cells are not already differentiated. Nowadays, we know exactly at which day of development each subtype of neuron starts to differentiate. The problem is that neurons mainly start to differentiate before the birth, during the embryonal period; this way we would need to kill the mother. THE CEREBELLAR GRANULE NEURONS continue to grow and proliferate even after the birth, so it’s possible to obtain primary cultures not from the embryos. SECONDARY CULTURES IPSCs, ASTROCYTES AND OLIGODENDROCYTES We start from a primary culture, which is a mix of glial cells from the new-born animal and then we detach the different types of glial cells. 2D CULTURE → this is the classical way to grow cells. Nowadays we know that this is not the perfect way to perform an experiment, because the cells in the tissue are not exactly represented. 3D CULTURE → representing in the perfect way the cells in the culture. Summit on Organotypic Cultures of Mammalian CNS → Organotypic cultures are a key tool for studying the central nervous system (CNS). The concept began in the 15th century with brain slices, such as cerebellum, placed in vitro but without supporting cell growth. In 1981, Gahwiler created the first organotypic culture system. However, a challenge was that only the surface of the slices received nutrients, causing cell death in deeper regions. In 1991, Stoppini solved this by introducing a semi-porous membrane, allowing nutrients to reach both sides of the slice and enabling long-term culture. This method supports studying brain regions like the cerebral cortex, hippocampus, cerebellum, and retina. Organotypic cultures are typically made from adult brain tissue, useful for studying brain physiology, while slices from embryos are used for development studies. These cultures can also be used in co-culture experiments to investigate conditions like neuroinflammation. This system helps maintain the brain’s architecture, making it ideal for studying processes like axonal growth. Summit on iPSCs and Their Role in Brain Modeling → Induced pluripotent stem cells (iPSCs), developed by Gordon and Yamanaka, have revolutionized brain research by offering an alternative to primary cultures derived from animals, addressing ethical concerns. iPSCs are created by reprogramming fibroblasts (or blood cells) through the overexpression of genes like Oct-3/4, SOX2, cMyc, and Klf4, which allows them to differentiate into various cell types, including neurons and microglial cells. Initially, viral vectors were used for this reprogramming, but modern methods using episomes are safer and more efficient. While iPSCs provide a way to model human brain cells, current cultures are still two- dimensional, limiting their complexity. Researchers are exploring three-dimensional (3D) cultures, such as using matrices to recreate models like "metabioplugs," which can be used to test drugs, such as those targeting amyloidosis. However, these models do not fully replicate the brain’s complexity. The next step is developing brain organoids, 3D structures that better mimic the architecture and function of the human brain. Summit on Brain Organoids → Brain organoids are in vitro-derived structures that self- organize to mimic certain aspects of brain development. While they are valuable for studying early brain development and neurodevelopmental disorders, they do not fully replicate the complexity of the brain. Organoids grow within a matrix, with cell organization resembling early neural tissue, including a central region and "rosette-like" structures where neurons differentiate. These organoids are particularly useful for studying neurodevelopment, genetic diseases, and for drug testing, offering a platform for screening potential treatments. However, they are not ideal for modeling neurodegenerative diseases, as their complexity and organization remain limited. As research advances, brain organoids hold great potential for expanding our understanding of the brain, though challenges remain in fully replicating its complexity. ANIMAL MODEL → Animal models are essential in neurobiology research. Key features for selecting model organisms include a rapid life cycle, large offspring for genetic studies, and small size to reduce costs and space. They should also propagate quickly on inexpensive food and have long-term storage options (e.g., freezing) for later studies. Commonly used animals have been chosen for practical reasons, like ease of care and space requirements. The cost of animals, such as rats and mice, is a critical factor in deciding which model to use for research. C. ELEGANS → C. elegans, a small, hermaphroditic flatworm, was chosen by S. Brenner in the 1960s to study nervous system development. It has a rapid life cycle (adults in a few weeks), is transparent, and can be frozen for long-term storage. It’s cost-effective, easy to maintain, and ideal for genetic and behavioral studies. With 959 cells, including 302 neurons, it provides a simple system for studying cell death, memory, and neurodegenerative diseases like ALS, Alzheimer's, and Parkinson's. Researchers have used C. elegans to identify genes involved in apoptosis, winning a Nobel Prize in 2002. Its simplicity, low cost, and lack of legal restrictions make it a popular choice for drug discovery and pre-clinical studies. DROSOPHILA MELANOGASTER → Drosophila, or fruit flies, are widely used in research due to their short life cycle, ease of maintenance, and ability to produce many offspring. They are ideal for studying genetics, behavior, and biological processes like neuronal development, memory, circadian rhythms, and neurodegenerative diseases. Drosophila has a simple blood- brain barrier, useful for drug permeability studies. Researchers induce mutations to study effects on behavior, such as memory and learning, through experiments like conditioned avoidance. The flies' large, observable chromosomes make them perfect for genetic mapping. Temperature also influences their development speed. Overall, Drosophila is key for genetic and drug research. ALYPSIA CALIFORNICA → Aplysia californica, a large marine snail, became an important model for studying learning and memory in the 1960s. Its simple nervous system, composed of 20,000 large neurons (up to 1 mm in diameter), made it ideal for research. The size of the neurons allowed scientists to observe them under a microscope and inject substances like drugs or genetic material for manipulation. Aplysia's accessible neural circuits made it perfect for studying the molecular mechanisms of learning and memory. The findings in Aplysia were later confirmed in mammals, showing that key processes are conserved across species, including humans. ZEBRAFISH → Zebrafish (Danio rerio) are small tropical fish commonly used in scientific research, particularly in toxicology and developmental biology. The first five days after fertilization are not regulated under European and Italian laws, making it easier to conduct studies during this period. Zebrafish are easy to breed, producing 100-200 eggs at a time, and their external fertilization allows for genetic studies. Researchers can create models with GFP (green fluorescent protein) to visualize the nervous system, making it easy to study brain development. Zebrafish are used in a variety of research areas, including: Behavioral studies (though challenging), Motor behavior, like studying ALS models, where affected fish stop swimming, Regeneration, as they can regenerate body parts, useful for studying healing, Developmental biology, offering insights into vertebrate CNS development, Neurodegenerative disease models for conditions like ALS, Drug discovery and toxicology, assessing environmental toxicity and chemical effects, Cancer research, studying solid tumors, Their transparency and fast development make zebrafish a powerful model for scientific research. MOUSE → Mus musculus (mice) are small mammals, weighing about 25 grams, and are one of the most widely used models for studying human biology due to their genetic and physiological similarities with humans. They are especially valuable for research in genetics, disease, and neuroscience. Mice are used to study age-related diseases, particularly neurodegenerative diseases, as they can live for about two years. Accelerated senescence mice (SAMP) are bred to age quickly, reaching old age by 6 months, making them ideal for studying aging and associated diseases. Humanized mice are genetically modified to have immune systems that are more similar to humans, making them useful for studying human infections. Mice are also used in various behavioral tests to study motor behavior, memory, and other functions. Additionally, transgenic mice with specific genetic mutations are widely used, with thousands of such models available for studying various diseases. These models allow researchers to study the effects of mutations in different genetic backgrounds, providing valuable insights into genetics and disease mechanisms. RAT → Rattus norvegicus (rats) are larger than mice, weighing around 250 grams, and are used in research for various purposes. They are more expensive and require more space than mice but are easier to handle due to their calmer and friendlier nature. Rats produce 8-12 pups per litter and have a lifespan of about two years. Unlike mice, rats are outbred, meaning they have more genetic diversity, which makes them more similar to humans but increases variability. This genetic diversity can complicate genetic studies, and creating transgenic rats is more challenging than with mice. Rats are commonly used for behavioral tests and biochemical studies, as they provide larger biological samples. They are also preferred for studying bioenergetics in vivo, especially for mitochondrial research, as they offer more tissue for analysis. Although there are fewer transgenic rat models compared to mice, rats are useful for gene silencing studies, where specific genes can be temporarily deactivated to study their function. Overall, rats complement mice in research, especially in studies of behavior, gene function, and bioenergetics, offering advantages like larger size and genetic diversity. NON-HUMAN PRIMATES → Non-human primates (NHPs) are the closest species to humans and are crucial for studying aging, neurodegenerative diseases, and mental disorders. In Europe, all drugs must be tested on NHPs before clinical trials, while this is not required in the USA. NHPs are valuable for studying complex biological processes, such as the interface between electronic devices and biological systems, especially in spinal cord injury and brain- machine interface research. Their aging process and immune systems closely resemble humans, making them important for pharmaceutical studies. While NHPs are essential for studying cognitive processes and psychiatric disorders, their use is limited by high costs, long research timelines (often decades), and ethical concerns. Genetic modification of NHPs is allowed in some regions but is not permitted in Europe due to ethical and practical considerations. Despite their importance, their complexity and the ethical issues involved make their use in research rare and highly regulated. PAPER PAPER PAPER !!!!!!! BRAIN DEVELOPMENT Fertilization begins with the union of male and female gametes (sperm and egg), leading to several key events: sperm penetration and membrane fusion, egg activation, and fusion of genetic material from both gametes. After fertilization, changes in the egg’s membrane prevent additional sperm from entering, ensuring only one sperm fertilizes the egg. Cleavage follows fertilization, where the zygote divides into many smaller cells called blastomeres. These divisions maintain the overall size of the embryo while increasing the number of cells. During cleavage, the embryo develops distinct regions: the animal pole and the vegetal pole. Yolk affects cleavage patterns, and in mammals, with little yolk, cleavage is more uniform, forming a blastocyst. The blastocyst has two main parts: the trophoblast, which forms the placenta, and the inner cell mass, which becomes the embryo. Gastrulation forms the primary germ layers: ectoderm, mesoderm, and endoderm. The ectoderm forms the nervous system (brain, spinal cord, nerves) and skin. The mesoderm gives rise to the skeleton, muscles, blood vessels, and connective tissues. The endoderm forms the digestive and respiratory tracts and internal organs like the liver and pancreas. Microglia, brain immune cells, originate from the mesoderm. Organogenesis, especially brain development, begins with the formation of the notochord, which guides the nervous system’s development. The process starts with the notochord, followed by the formation of the dorsal nerve cord. In neurulation, the notochord induces the ectoderm to fold and form the neural tube, which becomes the brain and spinal cord. The neural tube has a central cavity that resembles structures seen in induced pluripotent stem (iPS) cells, demonstrating how in vitro models mimic in vivo development. As the neural tube forms, the notochord remains beneath it, and neural crest cells, located on either side of the neural tube, give rise to structures like sensory neurons and components of the autonomic nervous system. The dorsal-ventral and anterior-posterior axes form, shaping the embryo’s structure and guiding nervous system and organ development. Organizers are groups of cells that release signaling molecules to drive morphogenesis and convey positional information. Finally, understanding this process involves recognizing how mRNA and proteins influence cell behavior during development. The nervous system is not only made up of the brain and spinal cord. In vertebrates, the brain develops along the rostral-caudal axis (head-to-tail). Early on, three primary brain regions form: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). These regions exist in all vertebrates and give rise to major brain structures. The brain later develops into five secondary vesicles: the telencephalon and diencephalon from the prosencephalon, and the metencephalon and myelencephalon from the rhombencephalon. Inside the neural tube, a central cavity remains, which becomes the ventricles in the brain. Cells around this cavity begin to proliferate and migrate outward, eventually differentiating into neurons and glial cells. There are two types of cell divisions in brain development: 1. Symmetric divisions: Both daughter cells stay as progenitor cells and continue to proliferate. 2. Asymmetric divisions: One cell keeps proliferating, while the other migrates to the outer layers to differentiate. Early on, most divisions are symmetric, but later, asymmetric divisions increase, leading to fewer proliferating cells and more cells differentiating into neurons and glial cells. This showed that neurogenesis (new neuron formation) continues into adulthood, contrary to the old belief that it only occurs during early development. Radial glial cells, which come from progenitor cells, guide neuron migration by providing a scaffolding. Neuroepithelial cells are the precursor cells of the brain and can become neurons, astrocytes, oligodendrocytes, or ependymal cells. Over time, these cells become neural stem cells, which can divide and give rise to either glial or neuronal cells. Glial progenitors produce glial cells, while neurogenic progenitors differentiate into neurons. The first glial cells are radial glia, which later transform into astrocytes, ependymal cells, or specialized glial cells. Astrocyte progenitors also continue to proliferate, producing astrocytes and oligodendrocyte precursor cells (OPCs). These OPCs later become oligodendrocytes, which are involved in myelination. On the neuronal side, neurogenic progenitors form various types of neurons. The number of neurogenic progenitors decreases as they differentiate into neurons. Microglial cells come from mesodermal progenitors in the yolk sac and migrate into the brain. They can proliferate and, in some cases, become macrophages if needed. The blood-brain barrier regulates, rather than completely isolates, the nervous system from the rest of the body. It controls which substances can enter the brain, but under certain conditions, cells from the body can cross the barrier and contribute to responses in the nervous system, especially during infections or diseases. This shows that the blood-brain barrier is not completely impenetrable, and its permeability can change based on various factors. DIFFERENTIATION OF NEURAL TUBE INTO SPINAL CORD → The differentiation of neurons in the spinal cord and brain involves a complex interplay of transcription factors, signaling molecules, and gradients. In the spinal cord, various neurons, such as motor and sensory neurons, are generated through specific transcription factors. Notably, motor neurons differentiate within the spinal cord, while sensory neurons originate from the neural crest. Coordination between the neural tube and neural crest is essential for developing the entire nervous system. The brain's development begins with three primary vesicles (prosencephalon, mesencephalon, rhombencephalon), which differentiate into five regions. Specific signaling molecules, particularly from the dorsal and anterior/caudal regions, guide the formation of different neuronal types through gradients. The olfactory bulb is an area of high neurogenesis, with olfactory neurons continuously replaced throughout life. Recent studies highlight the migration of olfactory precursor neurons to other brain regions and suggest viral infections can damage these cells, leading to cognitive decline. The subventricular zone, around the ventricles, is another key area for neurogenesis, with neural stem cells marked by "nestin" continuing to proliferate throughout life. However, neurogenesis declines with age as asymmetric proliferation increases. Transcription factors like Neurogenin 2 and NeuroD regulate the production of glutamatergic neurons in the hippocampus, essential for brain function. Epigenetic mechanisms, including histone acetylation, influence gene expression and neuronal development. During brain development, neurogenesis precedes gliogenesis, with radial glial cells facilitating neuronal differentiation and migration before the formation of glial cells like astrocytes and oligodendrocytes. ASTROCYTES IN NEURONAL DEVELOPMENT → Astrocytes play a vital role in supporting neuronal proliferation and differentiation, ensuring neurons develop into the correct types. They release various factors that promote neuronal growth, guide differentiation, and help maintain overall brain function. Astrocytes also produce neurotransmitters and signaling molecules essential for neuronal survival, protecting neurons from damage and degeneration, thus sustaining proper brain function. During both embryonic development and adulthood, astrocyte precursors, neural stem cells (NSCs), and oligodendrocyte progenitors (OPCs) follow similar developmental paths. Astrocytes can differentiate from precursors or radial glial cells, which also aid neuronal migration. This process is highly conserved across development and regeneration, with key genes identified that regulate astrocyte differentiation. Microglia, on the other hand, follow a separate developmental path. They originate from cells that enter the brain and become immune cells, regulating neuronal proliferation, migration, and differentiation. Microglia interact with astrocytes and neurons, playing a crucial role in brain development. The hypothalamus is an important brain region that links the nervous and endocrine systems. While it has limited stem cells, recent studies suggest stem cells may be present in areas like the hypothalamus, amygdala, and other regions involved in brain plasticity, supporting the connection between neurogenesis and brain adaptability. Neurogenesis is influenced by factors like environmental enrichment, exercise, and certain drugs, such as antidepressants. However, conditions like epilepsy and schizophrenia can disrupt neurogenesis. Ongoing research aims to understand how these factors impact not only neurogenesis but also the proliferation of neural stem cells and their differentiation into specific neuronal types needed for different brain regions. GLIAL CELL CENTRAL NERVOUS SYSTEM → Ependymal cells are found around the brain's ventricles and channels. They have cilia that help move cerebrospinal fluid (CSF) and form the choroid plexus, which produces CSF. CSF is constantly circulating and replaced daily to maintain proper brain function. Astrocytes support brain function by influencing synaptic activity, forming the blood-brain barrier, and releasing factors that help neurons grow and survive. Microglia act as the brain's immune cells. They play key roles in brain development, synaptic refinement, and provide support to neurons. Oligodendrocytes form the myelin sheath around axons. There are two types: those in gray matter that support neurons metabolically, and those in white matter that help with axonal activity. A single oligodendrocyte can wrap multiple axons, but it doesn't directly contact the axon. PERIPHERAL NERVOUS SYSTEM → Satellite cells and Schwann cells MIELINATION → Oligodendrocytes play a critical role in supporting neuron survival and function by releasing substances essential for neuronal activity. When oligodendrocytes are damaged, neurons begin to degenerate, which is particularly evident in demyelinating disorders where neurodegeneration occurs due to the loss of myelin. On the other hand, damage to neurons can also impair oligodendrocytes. The myelination process in the central nervous system is complex. Oligodendrocytes extend membrane structures to contact axons, initiating the wrapping of myelin around them. This interaction is vital for maintaining the health and proper functioning of neurons. The myelin sheath is not continuous along the axon, it is segmented, with myelinated sections interspersed by gaps known as RANVIER NODE. Here the action potential is regenerate. They contain a high density of sodium channels. To determine which axon to myelinate and where to position the nodes of Ranvier, oligodendrocytes recognize specific membrane proteins expressed on the surface of the axon. Oligodendrocyte membrane contains proteins that can recognize these axonal markers, facilitating the wrapping process. Once wrapped, the oligodendrocyte squeezes out the cytosol, leading to the formation of the myelin sheath. Astrocytes are crucial for manteining the organisation of myelinated fiber. In the PNS the Ranvier nodes are protected by the basal lamina of Schwann cells. In the CNS they are surrounded by perinodal astrocytes. Astrocytes interact with the axon through protein present in the Ranvier nodes. MYELIN FUNCTION → permits the propagation of action potentials, allow the metabolic coupling (astrocytes and oligodendrocytes help provide energy to neurons through the conversion of glucose from the blood into lactate and then lactate can be transported to the axon where became pyruvate to support mitochondrial activity and ATP production). Gap junctions and vesicular exchanges play an essential role in metabolic coupling and communication between cells in the brain. Gap junctions allow small molecules like sugars and amino acids to pass between cells, while exosomes and microvesicles facilitate the transfer of lipids, proteins, and RNA. These vesicles help glial cells, such as astrocytes and oligodendrocytes, communicate and support neurons Myelination in the peripheral and central nervous systems follows different models—jelly roll for Schwann cells and croissant-like for oligodendrocytes—resulting in the formation of myelin sheaths that support axonal function. DURING DEVELOPMENT → myelination is an intrinsic and spontaneous process. OPC proliferate, differentiate and wrap axons. IN ADULT → myelination becomes activity-dependent. Adult myelination involves OPC differentiation, axon contact, myelin production, and regulation by neuronal signals, ensuring proper myelination when needed. OLIGODENDROCYTES DIFFERENTIATION → the formation of myelin sheath start with OPS differentiate into pre-myelinating oligodendrocytes. Action filaments extend filopodia, which later develop into boarder lamellipodia. Inside the oligodendrocytes, various pathways regulate gene expression, leading to myelin gene transcription. Multiple transcription factors and miRNAs regulate OPC proliferation and differentiation. Astrocytes and microglia can release various factors and vesicles that influence oligodendrocyte precursor cells (OPCs) and oligodendrocytes. OPC proliferation and differentiation can be stimulated not only by chemical signals but also by enhancing central nervous system activity, which may, in turn, promote remyelination. MULTIPLE SCLEROSIS → MS is an autoimmune inflammatory condition in which the central nervous system cells are attacked, leading to cellular damage and even cell death. There are several theories as to why remyelination does not consistently occur in these diseases. Some propose that damaged axons fail to signal OPCs to proliferate and differentiate into oligodendrocytes. Other hypotheses suggest that axons might express proteins that actively inhibit remyelination, though these mechanisms remain uncertain. In the context of neurodegenerative disorders, one crucial aspect that could provide potential therapeutic targets is histone modification. Alterations in histone modifications and microRNA (miRNA) expression have been identified in various pathologies affecting oligodendrocyte precursor cells (OPCs). MiRNAs may serve as potential targets for genetic therapy to combat multiple sclerosis. Additionally, modulating immune cell activity may also play a role in addressing these conditions. ASTROCYTES – GLIAL CELL → one of the most abundant cell types in the brain. In pathological conditions, astrocytes respond to brain injury by transforming into “reactive astrocytes”, it increases the number and the size of the astrocytes playing an active role in disease processes. ASTROGLIOSIS → It’s an increase in the number and extent ofstrocytes especially in pathological conditions. MORPHOLOGY → 2 main types: PROTOPLASMIC in the grey matter (where neurons are partially myelinated) with multiple branching processes, with one side contacting blood vessels and the BBB, and the other side connecting with neurons. They are integral to the neurovascular unit. FIBROUS in the white matter and they provided support in the Ranvier node. They contribute to the regulation of action potential transmission and neurotransmission. MOLECULAR MARKER → GFAP → it’s the marker of astrocytes in research, but has limitation because its positivity increase under pathological conditions, making difficult to distinguish whether there is a true increase of astrocytes or if existing astrocytes are simply becoming more reactive. GLUTAMINE SYNTHETASE → only astrocytes involved in the glutamatergic system. And also, the protein S100B. PHYSIOLOGY → astrocytes creates a network through gap junctions with other astrocytes, and also with oligodendrocytes through chemical synapsis (calcium fluctuations). GAP JUNCTIONS → made of proteins called connexins (essential for cellular communications). Astrocytes express connexin 43 and 30. Oligodendrocytes express connexin 47 and 32. (connexin 43 pairs with 47, and 30 with 32). Astrocytes exhibit different behaviors when studied in isolation versus within a network. In isolation, signalling patterns like glutamate waves can be observed, but these behaviors change significantly in a networked environment. ROLE IN THE DEVELOPMENT → Astrocytes, once thought to be primarily involved in pathological conditions, are now recognized as essential for normal brain development. Radial glia, a subtype of astrocytes, guide neuronal migration during brain development, while astrocytes also contribute to synapse formation and synaptic pruning. They play a key role in myelination, supporting oligodendrocyte function. Astrocytes influence neuronal migration, dendritic development, synaptic refinement, and neuronal survival by releasing specific molecules. Their involvement in brain development is crucial, and deficits in astrocyte function are linked to neurodevelopmental disorders. Overall, astrocytes are vital for brain formation and function from early stages onward. Astrocytes play a crucial role in brain development by releasing various factors, including neurotrophins like Brain-Derived Neurotrophic Factor (BDNF), which support neuronal survival and differentiation. They also produce proteins essential for synaptic formation and release transforming growth factor beta (TGF-β), particularly isoform beta 1. Astrocytes surround and influence glutamatergic synapses, which are key to synaptic plasticity and memory. They release proteins like promispondin, ebin, and spart that affect synapse formation and receptor binding. Astrocytes also promote synapse maturation by releasing vinculin, which increases receptor numbers on synaptic structures. Additionally, they aid in synaptic pruning by releasing the C1q protein, signaling microglia to eliminate inactive synapses. These processes ensure the proper formation and maintenance of functional synapses during brain development. Astrocytes play a critical role in maintaining synaptic health and function in the healthy adult CNS. Initially recognized for their role in neurotransmitter uptake, such as glutamate and GABA, they prevent neurotoxicity and support synaptic communication. Astrocytes clear excess glutamate from synapses, convert it to glutamine, and recycle it to neurons, aiding in continuous synaptic signaling. They also regulate GABAergic signaling by cycling glutamine back to GABAergic neurons. Astrocytes are integral to the "tripartite synapse," where they interact with pre- and postsynaptic elements, not only clearing neurotransmitters but also releasing gliotransmitters like ATP, adenosine, and D-serine. These gliotransmitters modulate synaptic activity, with D-serine playing a key role in NMDA receptor activation. Astrocytes also help manage ion balance and provide metabolic support by releasing lactate, which neurons use for energy. Additionally, astrocytes engage in calcium signaling, influencing neuronal activity and cognitive functions. This highlights their essential role in maintaining brain homeostasis and synaptic health in the adult brain. Astrocytes play a crucial role in Ca²⁺-dependent gliotransmission, where an increase in intracellular calcium triggers the release of various molecules like D-serine, GABA, purines, and lactate. D-serine, in particular, acts as a co-agonist for NMDA receptors, enhancing glutamatergic synapse responses and contributing to synaptic plasticity. Astrocytes also support neuronal metabolism by converting glucose into lactate, which neurons use for energy, particularly for processes like memory consolidation. This lactate, along with other molecules, is transferred between astrocytes via gap junctions, facilitating intercellular communication and maintaining synaptic support. Astrocytes indirectly interact with neurons through signaling, releasing molecules into the extracellular space, and they are also connected through gap junctions, allowing them to coordinate and modulate neuronal activity. Furthermore, astrocytes interact with endothelial cells to maintain the blood-brain barrier and vascular function, highlighting their central role in both neural and vascular networks. Astrocytes play a key metabolic role in the healthy adult brain by absorbing, storing, and supplying energy to neurons. They store glycogen, which can be converted into glucose during high energy demands, and produce lactate to support neuronal metabolism. Astrocytes also help regulate blood flow, ensuring increased glucose delivery to areas with higher neuronal activity. They act as sensors for neuronal activity, adjusting glucose uptake and blood flow to meet the brain’s energy needs, particularly during tasks or cognitive engagement. This metabolic support is essential for sustaining brain function and cognitive health. Astrocytes play a critical role in maintaining the blood-brain barrier (BBB) by modulating the properties of endothelial cells through signaling pathways. While some debate exists about whether the BBB can function without astrocytes, it is clear that astrocytes are essential for its formation, regulation, and adaptability. They also regulate blood flow in the brain, adjusting vessel dilation to meet the brain's oxygen and glucose demands. In pathological conditions, astrocytes shift from a supportive role to a reactive one, potentially exacerbating neuronal damage. Their dual role underscores the importance of astrocytes in both maintaining brain health and contributing to disease progression. Astrocytes play a crucial role in myelin formation by supporting the proliferation and differentiation of oligodendrocyte precursor cells, which are essential for myelin production and remodeling. They release neurotrophic factors like BDNF and CNTF that promote oligodendrocyte maturation. Astrocytes also modulate their function through regulatory factors and the extracellular matrix (ECM), which provides structural support and biochemical signals for oligodendrocyte survival and myelin formation. In addition to supporting myelin production, astrocytes are key in maintaining and remodeling myelin sheaths, ensuring the overall integrity and function of the central nervous system. In pathological conditions, astrocytes lose their normal functions and become reactive. This leads to the release of toxic substances like ROS and excess glutamate, which worsen neuronal damage and impair the blood-brain barrier. Reactive astrocytes also contribute to chronic pain and neuroinflammation, playing a role in conditions like Alzheimer's disease. Astrocytes bridge communication gaps between neurons and other brain cells, including endothelial cells crucial for the blood-brain barrier. They form an interconnected system that plays a key role in regulating neuronal synchronization, supporting synaptic function, and providing metabolic support. The expression of connexins, like connexin 43 during development and connexin 30 in adults, varies depending on developmental stages and brain regions. Astrocytes influence cognitive processes such as memory, motor function, and sleep regulation, and are involved in psychiatric conditions, emphasizing their importance in both health and disease. Another important role played by astrocytes at the junctions is the modulation of potassium levels → Astrocytes play a critical role in regulating potassium levels, crucial for neuronal function, and modulate extracellular potassium in broader contexts, including interactions with blood vessels and oxygen regulation. They contribute to synchronization by conducting calcium waves that align with action potentials, enhancing neuronal communication. At the blood-brain barrier, connexins regulate ion and metabolite flow, ensuring proper permeability. These extensive electrical synapses among astrocytes and endothelial cells are vital for maintaining the brain's functional integrity and support efficient neuronal activity. ASTROCYTES IN NEURODEGENERATION → Astrocytes and microglia play crucial roles in brain health but become reactive in neurodegenerative diseases like Alzheimer's and Parkinson's. This reactivity is linked to aging and astrocyte senescence, where astrocytes halt replication and adopt a dysfunctional state. Senescent astrocytes exhibit changes like cell cycle arrest, increased GFAP expression, and release of pro-inflammatory factors. These alterations impair astrocyte functions, including protein degradation, contributing to neuroinflammation and the progression of neurodegenerative diseases. The altered interactions between senescent astrocytes and microglia further exacerbate these conditions. MICROGLIA – GLIAL CELL Microglia act as the CNS’s innate immune cells. They constantly survey the brain for inflammation, infection, and neuronal damage, monitoring CNS physiology across the brain and spinal cord. Identifying distinct microglial subpopulations is challenging due to the lack of clear morphological differentiation across brain areas. Unlike neurons, microglia do not have defined shapes, and they exhibit high motility, migrating between different regions of the brain. In a physiological state, microglia have highly branched, dynamic processes that enable them to effectively survey their environment. Many microglia are found near blood vessels, similar to astrocytes, particularly when they adopt a ramified morphology. ORIGIN AND ROLE → Microglia originate from myeloid cells, not from the brain’s original cell types. During brain development, they form a resident population but can also be replenished by circulating myeloid cells during injury or disease. Microglia are important for maintaining brain health, playing active roles in brain development, synaptic pruning, and regulation of neural circuits, not just responding to injury. MARKER → CX3CR1 Microglia are uniformly distributed across the brain but vary in morphology and density depending on the region. For example, they are denser in the hippocampus and adapt to the structure of the corpus callosum by becoming flatter. Culturing microglia for research is challenging because they are difficult to maintain in vitro. After isolation from rodent brain cultures, microglia quickly differentiate into macrophage-like cells and die within a few days. For many years it was thought microglia as “resting cells”, but it is an active participant in brain physiology. ROLE → During brain development the role of microglia is involved in phagocytosis, because during development, the brain generates more cells and synapses necessitating the removal of excess (SYNAPTIC PRUNING). SINAPTIC PRUNING → Microglia play a crucial role in synaptic pruning, eliminating unnecessary synapses during brain development. In healthy conditions, they refine synaptic connections, supporting brain function. However, in neuropsychiatric disorders like schizophrenia, microglia fail to prune excess synapses effectively, which may contribute to developmental issues. In contrast, in neurodegenerative diseases, microglia may excessively remove synapses, leading to synaptic loss and exacerbating brain dysfunction. The interactions between microglia, neurons, and astrocytes are vital for maintaining synaptic health, and disruptions in these interactions can contribute to both developmental and degenerative disorders. Microglia recognizes pathogens and responding to infection and also it modulates inflammation. Microglia contribute to repair processes in pathological conditions. Microglia also play a crucial role in supporting neurogenesis. The role of microglia in the synapsis → with the discovery of microglial involvement, we now recognize the existence of tetrapartite synapses, which include these four components: the presynaptic element, the postsynaptic element, astrocytes, and microglia. This framework applies not only during developmental stages but also in the adult brain. When a synapse is inactive, microglia can phagocytize and eliminate it, ensuring that only functional synapses remain. If a synapse is found to be non-functional, microglia can respond by eliminating it. The role of microglia in the brain's physiological conditions and their interaction with synapses has led to the hypothesis that microglia might influence synaptic plasticity. Research has indeed shown that microglia can modulate synaptic transmission by releasing signaling molecules such as TNF-α and PAMPAD. Microglia call continuously send signals regarding any pathological condition from one area of the brain to another Synaptic physiology and remodelling → microglia helps to maintain synaptic function. Microglia also is involved in myelin formation by supporting the proliferation into oligodendrocytes and it can quickly detect potentially dangerous situations. MICORGLIA ACTIVATION → activated microglia contribute to neuroinflammation, which leads to neuronal damage Microglia in neurodegeneration → In neurodegenerative diseases, the role of microglia is complex and multifaceted. Research suggests that these diseases may not only involve neuronal dysfunction but also a loss of physiological function in support cells like microglia and astrocytes. These glial cells can become senescent or dystrophic, especially in age- related conditions, which impairs their ability to support neuronal health. Initially, microglia actively survey the brain and protect neurons, but chronic stress or damage can cause them to become dysfunctional. This shift from a protective to a harmful role contributes to neuroinflammation, exacerbating the progression of neurodegenerative diseases. Microglia exhibit different phenotypes depending on the severity of activation. In healthy conditions, they act as vigilant sentinels. However, during pathological events, microglia can shift to an M2 phenotype, which is neuroprotective when the damage is limited or temporary. In contrast, sustained or severe damage can lead to an M1 phenotype, which is pro- inflammatory and contributes to neuronal damage. Traditional anti-inflammatory treatments fail because they inhibit both M1 and M2 microglial phenotypes. New strategies focus on promoting the neuroprotective M2 phenotype and preventing its transition to M1. Research into markers that distinguish M1 and M2 microglia is helping to develop immunomodulatory treatments aimed at shifting microglia toward the protective M2 state, offering promise for neurodegenerative disease therapies. Microglial phagocytosis is a vital function for maintaining neural health, both in physiological and pathological conditions. In a neuroprotective role, microglia actively perform phagocytosis, including the removal of synaptic debris and myelin. When microglia express the M2 protein, they are in an active phagocytic state, indicating their involvement in tissue maintenance. This process is essential not only for synaptic health but also for the turnover of myelin, as oligodendrocytes need to replace damaged myelin over time. Microglial phagocytosis is crucial for the long-term preservation of neural communities and proper myelination. Microglia play a crucial role in the clearance of myelin fragments and potentially harmful protein aggregates in both healthy and pathological conditions. In normal physiology, microglia facilitate myelination by clearing myelin debris, promoting remyelination. However, dysfunction in microglial activity, such as overactivation seen in diseases like multiple sclerosis, impairs this process, exacerbating inflammation. In neurodegenerative diseases, protein aggregates accumulate, and microglia struggle to clear them, leading to inflammation and contributing to neurodegeneration. In neurodevelopmental disorders like Rett syndrome, autism, and schizophrenia, microglia fail to support critical processes like cell proliferation, synaptic pruning, and neural differentiation. This dysfunction disrupts brain maturation and contributes to the pathology of these diseases, underscoring the importance of microglial function in both neurodevelopment and neurodegeneration. MicroRNAs (miRNAs) play a significant role in regulating the activation and function of brain microglia and macrophages. miRNAs can be released by cells as free molecules or within extracellular vesicles, enabling intercellular communication. This miRNA-mediated communication is important for microglial function and neuroimmune interactions. Emerging research suggests that miRNAs influence microglial activation, particularly the shift between pro-inflammatory (M1) and anti-inflammatory (M2) states. These miRNAs could serve as early biomarkers for neurodegenerative diseases, with potential applications in early diagnosis and treatment. Researchers are working to refine methods to detect cell-specific miRNAs, which could significantly improve patient outcomes in neurodegenerative conditions. NEURONS These cells are designed to transmit information from one part of the body to another. Morphology→ They are formed by a cell body (soma), one or more dendrites and a single axon, which may have multiple branches. The dendrites of the neuron are parts that receive and integrate information, while the terminal endings transmit information to other neurons or to non-neuronal cells. Based on their functions, we can classify neurons into SENSORY NEURONS which receive sensory information and MOTOR NEURON which send signals from the brain to the body. When considering the number and complexity of dendrites, we can categorize neurons into several types. Bipolar neurons are the simplest, featuring two processes. Pseudounipolar neurons have a single axon that branches off, allowing them to send information while receiving it through the cell body. Anaxonic neurons have only dendrites and are quite rare. Most neurons, however, are multipolar, characterized by many dendrites and a single axon. In a neuron, we can distinguish between the parts that receive information—primarily the dendrites and the cell body—and the part that transmits information, the axon. This process is unidirectional: information flows from the dendrites and cell body to the axon. The axon is a critical structure in neurons, responsible for transmitting information from the cell body to other neurons or target cells. When an action potential is generated in the axon, it propagates along its length, potentially branching out to multiple terminals. These branches allow the signal to be transmitted simultaneously across various pathways. The axon also plays a crucial role in converting electrical signals into chemical signals at its terminals, where neurotransmitters are released. This process is essential for communication between neurons, contributing to the overall functioning of the nervous system. MEMBRANE POTENTIAL – GRADED AND ACTION POTENTIAL The resting membrane potential of a neuron, typically around -70 mV, is crucial for generating both graded potentials and action potentials. Graded potentials are small, localized changes in membrane potential that depend on the strength of the stimulus, and they diminish over time and distance. These potentials can either be depolarizing, leading to excitation, or hyperpolarizing, leading to inhibition. However, graded potentials alone cannot trigger an action potential unless they reach a critical threshold. Once this threshold is reached, typically around -50 mV, it triggers an action potential, which is a rapid, all-or-nothing electrical signal that propagates down the axon. This process involves the opening of sodium channels, allowing sodium ions to enter the neuron, causing depolarization. At the peak of the action potential, sodium channels close, and potassium channels open, allowing potassium ions to exit the neuron, which leads to repolarization. The action potential follows the all-or- nothing principle, meaning it either occurs fully or not at all, depending on whether the threshold is reached. After the action potential, the neuron returns to its resting membrane potential as potassium continues to exit and the ion channels reset. This sequence of events allows neurons to transmit signals over long distances, enabling communication within the nervous system. REFRACTORY PERIOD → The refractory period plays a critical role in the propagation of action potentials, ensuring that signals move in one direction along the axon. It consists of two phases: the absolute refractory period and the relative refractory period. During the absolute refractory period, sodium channels are inactivated and cannot reopen, even if the membrane potential is depolarized. This prevents the action potential from traveling backward. In the relative refractory period, sodium channels can reopen, but only if the depolarization is stronger than usual. This ensures that the action potential propagates forward, from the cell body to the axon terminals, maintaining the efficiency of signal transmission in the nervous system. The propagation of action potentials along the axon is influenced by the refractory period, which ensures unidirectional flow. In unmyelinated axons, action potentials travel slowly from the cell body to the axon terminals. However, in myelinated axons, the presence of myelin and nodes of Ranvier greatly enhances conduction speed. At these nodes, high concentrations of voltage-gated sodium channels allow for rapid signal regeneration, significantly speeding up action potential propagation. The conduction speed is also influenced by factors like axon diameter and myelination. Larger axon diameters reduce electrical resistance, improving conduction, while myelination ensures faster transmission by insulating the axon and reducing signal loss. The frequency of action potentials, influenced by the strength of the stimulus, is crucial for neural communication. Stronger stimuli result in higher action potential frequencies, affecting neural responses. Neurons communicate via chemical and electrical synapses, with neurotransmitter release being vital for chemical synapses, while electrical synapses play an important role in certain nervous systems. This process is essential for efficient communication and proper functioning within the nervous system. ELECTRICAL SYNAPSES → can send information in both directions and allow the movement of small molecules with a molecular weight below one kilodalton, contributing to energy efficiency. ES are typically not isolated, it consist of connections formed by proteins that create various channel structures (trimeric, tetrameric and heterotypic) configurations. These channels are composed of proteins with four transmembrane alpha subunits, with N- termini located in the intracellular space. The proteins are synthesized through transcription in the nucleus, followed by translation in the rough endoplasmic reticulum, and are then transported to the cell membrane via intracellular vesicles. Once at the membrane, these channels align with similar structures on adjacent cells, forming functional electrical synapses. The lifespan of this channel is not permanent, they undergo turnover process regulated by lysosomes and proteosomes, ensuring that old channels are degraded and replaced. Astrocytes communicate through electrical synapses, electrical synapses between astrocytes allow the spread of calcium signals, which can be visualized through calcium imaging. PROTEIN → ES are formed by 2 families of proteins: CONNEXIN (cells that form the BBB and pericytes in the peripheral nervous system) AND PANNEXINS (in central nervous system cells). Astrocytes, through connexins and gap junctions, form a "functional syncytium" with other glial cells, pericytes, and endothelial cells. This network enables astrocytes to support synapses and interact with neurons, playing a key role in brain function and homeostasis. CHEMICAL SYNAPSIS OTTO LOEWI EXPERIMENT → Otto Loewi, a German researcher, demonstrated that synaptic transmission is chemical. He stimulated the vagus nerve of a frog's heart, slowing its rate. He then transferred the solution to a second heart, which reacted similarly, showing the chemical substance could mimic the nerve's effect. This experiment proved neurotransmission is chemical, and Loewi later won the Nobel Prize for identifying acetylcholine as the neurotransmitter involved. Neurons release both classical neurotransmitters and neuropeptides, but typically only one type at a time. Neuropeptides are stored in larger vesicles, visible under electron microscopy, and require high-frequency, prolonged stimulation for release, unlike classical neurotransmitters, which are released with low-frequency stimulation. Neuropeptides can be released from various parts of the axon, not just the synaptic terminals, allowing for broader signaling and interactions with neighboring cells like astrocytes, enhancing neural communication and protection. VESCICLE → are complex structure containing various proteins, these proteins are crucial because they facilitate the fusion of the vesicles with the membrane. NEUROTRANSMITTERS → Acetylcholine is one of the first neurotransmitters discovered, and it remains one of the most well-known. Other important neurotransmitters include glutamate, GABA, lysine, and catecholamines. Each of these neurotransmitters is synthesized by specific neurons through distinct biochemical pathways that involve various enzymes. NITRIC OXIDE → play a crucial role in synaptic plasticity, as a gas, it can diffuse freely across cell membranes. It’s a retrograde messenger that transmits signals from postsynaptic element back to the presynaptic neuron. Nitric oxide (NO) is synthesized by nitric oxide synthase (NOS), with three types found in the brain: neuronal (nNOS), inducible (iNOS), and endothelial (eNOS). nNOS is involved in glutamatergic signaling, where glutamate activation triggers nNOS, releasing NO. This NO acts as a retrograde signal, modulating synaptic activity from postsynaptic to presynaptic neurons. NO activates guanylate cyclase, increasing cyclic GMP (cGMP) levels, which activate protein kinase G (PKG) and drive synaptic plasticity. Inducible NOS, expressed by glial cells, marks microglial activation, while eNOS is involved in endothelial cells and the blood- brain barrier. RECEPTOR → all neurotransmitters can interact with two different categories of receptors: ionotropic and metabotropic. Most neurotransmitters can bind to both types, while some may only bind to metabotropic receptors. Ionotropic receptors are ligand-gated channels. When a neurotransmitter binds to these receptors, it causes the channel to open or close, altering the permeability of ions across the membrane. In contrast, metabotropic receptors, which are coupled to G-proteins, initiate a different mechanism. When a neurotransmitter binds to a metabotropic receptor, it induces a conformational change in the receptor, activating a G-protein. This G-protein can then activate or inhibit various effector proteins, leading to changes in the production of second messengers and subsequently affecting cellular signaling pathways. ACETYLCOLINE RECEPTOR (AChR): The first type of ionotropic effector that has been identified is the nicotinic acetylcholine receptor. This receptor is a classic example of a ligand-gated ion channel and is composed of five subunits, typically arranged with two alpha units. While the nicotinic acetylcholine receptor plays a significant role in the central nervous system, it is not as critical as the GABA receptor, which is essential for inhibitory neurotransmission. GABA RECEPTOR → The GABA receptor is highly important, particularly from a pharmacological perspective. Many substances, such as benzodiazepines, barbiturates, and steroids, can modulate the activity of this receptor. It's also worth noting that most neurotransmitters can interact with both ionotropic and metabotropic receptors. GLUTAMATE RECEPTOR → are crucial for synaptic plasticity and cognitive function, making them essential to our understanding of synapses and overall brain metabolism. Much of the information we have about synaptic mechanisms is derived from studies of glutamatergic synapses, which can sometimes be generalized to other types of synapses. There are three main types of ionotropic glutamate receptors: AMPA, NMDA, and kainate receptors. AMPA receptors are ligand-gated and primarily allow sodium, potassium, and sometimes calcium ions to pass through, influencing the postsynaptic membrane potential. NMDA receptors are unique because they are both ligand- and voltage-gated, playing a key role in synaptic plasticity and memory. NMDA receptors require both glutamate binding and membrane depolarization to activate, as magnesium ions block the channel at resting potentials. Additionally, NMDA activation needs a cofactor like D-serine, released by astrocytes, emphasizing their crucial role in glutamatergic synapses. METABOTROPIC RECEPTORS → Metabotropic receptors are G-protein-coupled receptors that, when activated, trigger intracellular signaling cascades, influencing second messengers like cAMP. This can either enhance or inhibit neurotransmitter activity, playing a crucial role in homeostasis. Unlike ionotropic receptors, which mediate fast synaptic responses, metabotropic receptors activate more slowly, leading to long-lasting effects and potential changes in gene expression. NMDA receptors, which allow calcium ions to flow, sit between ionotropic and metabotropic receptors, influencing both immediate cellular responses and longer-term gene expression. The diversity of receptors in a cell enables complex signaling and modulates the cell's overall response to stimuli. The tripartite synapse → Astrocytes play a crucial role in synaptic function by providing metabolic support and modulating neurotransmitter flow. They facilitate slow, prolonged modulation of neuronal activity through non-vesicular neurotransmitter release, essential for synaptic plasticity. Astrocytes release D-serine, crucial for NMDA receptor activation, and convert glutamate into glutamine for recycling. They also release purines like ATP and adenosine, which regulate both pre- and postsynaptic activity. Additionally, astrocytes help maintain the extracellular environment, influencing synaptic function and neuronal synchronization, with the extracellular matrix actively participating in signaling processes. The active milieu is the dynamic environment around neurons and synapses, composed of molecules and cells that influence neuronal function. The extracellular matrix (ECM) provides structural support and regulates neurotransmitter availability and synaptic plasticity. Astrocytes contribute by releasing gliotransmitters, modulating neuronal activity, maintaining ion homeostasis, and clearing excess neurotransmitters to prevent excitotoxicity. Together, these components form a complex network that modulates synaptic function, enabling precise regulation and adaptability in response to changing activity or environmental conditions. Astrocytes play a crucial role in synaptogenesis and neuronal development, influencing dendritic spine formation and receptor modulation. Glutamate activation triggers calcium waves, synchronizing cell differentiation. This underscores the active role of astrocytes in neural circuits. CYTOSKELETON → Maintain the complex morphology of neurons. Actin is present in all types of cells. In non-neuronal cells, actin is distributed throughout all components of the cell, in contrast in neurons actin is predominantly found in the cell body and along the axon, with particularly high concentrations at the axon terminals and in the nodes of Ranvier. In a brain cell, for example, actin and tubulin are localized differently. Tubulin is present in the cell body, along the axon, and in the dendrites. In contrast, actin is specifically localized in the dendritic spines and along the axon, in particular compartments of the neuron, but not in the cell body or along the entire dendrite. This distinct compartmentalization reflects the specialized roles of these cytoskeletal proteins in neuronal function. The neuronal cytoskeleton consists of three types of filaments: microtubules, microfilaments, and intermediate filaments. Microtubules, made of tubulin, are the largest and help maintain cell shape. Microfilaments, made of actin, are the smallest and contribute to cytoskeletal plasticity. Neurofilaments, a type of intermediate filament found in neurons, act as markers for mature, specialized neurons. These filaments together support neuron structure and function. FUNCTIONS → Cytoskeletal proteins have similar functions across cell types, providing a supportive and dynamic structure, particularly in proliferating cells. In both neuronal and glial cells, they are vital for cell division and communication within the cell. They also coordinate the transport and organization of macromolecules. In neurons, the cytoskeleton is essential for energy distribution and molecular movement. During development, it supports cell movement and migration. In neurons, it plays a key role in synaptic plasticity and allows cells to respond to neural activity and learning. THE MICROTUBULES → maintaining the shape of the neuron, even its complex and elongated structure. Microtubules are formed by 13 longitudinal rows of protofilaments. Each protofilament is made up of dimers of alpha- and beta-tubulin, which are organized in a helical arrangement. Microtubules have two distinct ends: a plus end and a minus end. The minus end is characterized by the presence of alpha-tubulin, while the plus end contains beta-tubulin. The beta-tubulin subunit is highly dynamic and plays a key role in the assembly and disassembly of microtubules, making it the more plastic component. In contrast, the alpha-tubulin subunit is more rigid and structural, providing stability to the microtubule. Microtubules in neurons play a crucial role in their dynamic properties, essential for processes like synaptic changes and neuronal growth. Both alpha- and beta-tubulin bind GTP and undergo post-translational modifications, such as acetylation and phosphorylation, which are key for microtubule function. Alterations in these modifications can impact microtubule stability and are linked to neurodegenerative disorders. Microtubules in neurons undergo continuous polymerization and depolymerization, regulated by GTP hydrolysis, enabling structural changes in terminal endings. Microtubules in axons are uniformly oriented with the plus end towards the axon terminal and the minus end towards the cell body. In dendrites, microtubules are oriented in both directions, with plus ends found near both the cell body and dendritic terminal. This difference in orientation helps explain the distinct protein movements and physiological properties of axons and dendrites. Microtubule-associated proteins (MAPs) are crucial for characterizing neurons at different stages of differentiation. In fully differentiated neurons, MAPs like MAP1A, MAP2A, and MAP4 are present, while Doublecortin (DCX) marks neural precursor cells committed to becoming neurons. These MAPs play vital roles in both the structure and function of microtubules and can serve as markers for tracking neuronal differentiation. Microtubules also interact with other proteins that attach them to various cellular components, facilitating the movement of organelles, proteins, vesicles, and RNAs. Motor proteins, such as dynein and kinesin, move along microtubules in opposite directions: dynein moves from the plus end to the minus end, while kinesin moves from the minus end to the plus end. This bidirectional transport is essential for maintaining cellular organization and function. Microtubule-associated proteins (MAPs) are vital for maintaining neuron shape and enabling the transport of proteins, organelles, and vesicles within the neuron. Neurons, especially motor neurons with long axons, require efficient transport of materials over long distances, such as from the cell body to axon terminals. Neurons are post-mitotic, meaning they do not divide, so proteins and organelles like mitochondria must be regularly transported to maintain cellular function. This includes the transport of neurotransmitter vesicles and the removal of waste products from the axon terminals back to the cell body. The transport is mediated by motor proteins: kinesin moves materials toward the axon terminal (plus end of microtubules), while dynein moves materials in the opposite direction (back to the cell body, minus end of microtubules). In axon terminals and dendritic spines, where microtubules are absent, actin filaments and the motor protein myosin are involved in transport. Thus, kinesin, dynein, and myosin work together to enable the bidirectional movement of macromolecules and organelles within neurons. Microfilaments, primarily composed of actin, facilitate movement along actin filaments and are key components of the neuron at the synapses. The combination of motor proteins interacting with both microtubules and microfilaments ensures efficient intracellular transport and the proper functioning of neuronal cells. Actin and myosin at the level of the CNS synapses → At synapses, myosin proteins move macromolecules along microfilaments, essential for cellular processes across tissues. Actin filaments, key for synaptic plasticity, provide a scaffold for molecular movement. In the presynaptic element, myosin moves neurotransmitter vesicles, while in the postsynaptic element, it transports receptor-containing vesicles like AMPA receptors. In glutamatergic synapses, the number of AMPA and NMDA receptors determines synaptic activity. Increased activity raises AMPA receptors at the synapse, enhancing the response. Actin-driven vesicle movement enables rapid changes in receptor numbers. This dynamic process, with continuous protein turnover, is vital for synaptic function and plasticity. THE INTERMEDIATE FILAMENTS → between microtubules and microfilaments. These filaments are composed of different proteins, which are encoded by various genes that are expressed depending on the developmental stage or regeneration state of the neuron. In neural stem cells, the primary intermediate filament protein is NESTIN that is co-expressed with other proteins, such as doublecortin (a protein associated with macrophages) in cells that are committed to becoming neurons. The intermediated filaments are typically made of neurofilament proteins in the CNS and of peripherin in the PNS, in the case of differentiated neurons. MARKER → Astrocytes can be identified using GFAP (Glial Fibrillary Acidic Protein), an intermediate filament specific to these glial cells. In cultured neurons, the differentiation level of cells is assessed by the intermediate filaments they express, such as nestin in neural stem cells. As neurons mature, different intermediate filaments are expressed, with proteins like tau, a microtubule-associated protein, being detected in more differentiated neurons. Neurofilaments in neurons can be stained with specific markers, such as those for alpha- internexin, which results in green labeling. These markers are useful for identifying neurons, as neurofilaments are expressed in the cytoplasm of all neurons, making them a prominent feature in stained brain tissue. Intermediate filaments, used as markers in research, are crucial for assessing cellular differentiation. These filaments are composed of coiled-coil dimers that assemble into tetramers, forming protofilaments which determine their tubular structure. The organization and condensation of these filaments are essential for their function. In neurons, neurofilament proteins, which vary in molecular weight, undergo extensive post- translational modifications, such as phosphorylation and glycosylation. These modifications affect neurofilament properties and neuronal plasticity, influencing the cell's ability to adapt. The polymerization and depolymerization of neurofilaments are regulated by proteins like kinesin and kinases, playing an important role in neuronal activity. Disruptions in the cytoskeleton, including neurofilament abnormalities, are linked to neurodegenerative diseases, highlighting their significance in neuronal health. Cytoskeletal disorders in neurodegeneration → one examples is Tau protein, a protein associated with microtubules, like in Alzheimer’s disease and other form of dementia, it become hyperphosphorylated that cause Tau aggregate and as a result microtubules being to disassemble. Cytoskeleton remodeling plays a crucial role in neuronal migration, particularly during development when neural stem cells proliferate, migrate, and differentiate to form neuronal networks. Microtubules and other cytoskeletal proteins are involved in this process, with migrating precursor cells interacting with radial glial cells that guide their movement. Cytoskeletal reorganization is essential for efficient migration, supported by energy from mitochondria. Pulsating waves of cytoskeletal activity drive continuous remodeling, allowing neurons to adapt and move. This process is vital not only in development but also in the adult brain, where neural precursor cells continue to migrate and integrate into existing networks. AXONAL TRANSPORT → Axonal transport plays a critical role in neuronal function and maintenance, facilitating the bidirectional movement of molecules and organelles along microtubules. In neurons, two types of transport are key: fast and slow. Fast transport moves smaller vesicles, proteins, lipids, and mitochondria, while slow transport is responsible for the movement of structural proteins like neurofilaments and microtubules. Motor proteins kinesin and dynein mediate transport, with kinesin driving anterograde transport (toward the axon terminal) and dynein facilitating retrograde transport (toward the cell body). ATP hydrolysis powers this process, ensuring efficient cargo movement. Axonal transport is essential for maintaining neuronal health. Mitochondria, for example, are constantly transported to regions with high energy demands, like synapses. Aged or damaged mitochondria are moved back to the cell body for recycling. Additionally, mRNA is transported from the cell body to the synaptic terminal, enabling local protein synthesis, which is vital for synaptic plasticity and function. In neurodegenerative diseases like Alzheimer's and ALS, axonal transport dysfunction is a hallmark, exacerbating disease progression. Neurodevelopmental disorders often stem from mutations in genes responsible for axonal transport, leading to disrupted processes such as neuronal migration and synapse formation. Thus, axonal transport is crucial not only for normal neuronal function but also for the proper development and maintenance of the nervous system. AXONAL TRANSPORT IN NEURODEGENERATIVE DISEASES → Alterations in axonal transport play a significant role in the progression of neurodegenerative diseases, particularly through the dysfunction of cytoskeletal proteins like tau. In Alzheimer's disease, tau becomes hyperphosphorylated, dissociating from microtubules, destabilizing them, and disrupting axonal transport. This leads to the formation of neurofibrillary tangles and impaired motor protein function, which exacerbates neuronal dysfunction. Tau-related disruptions in axonal transport are a hallmark of Alzheimer's and other neurodegenerative diseases, including Huntington's and Parkinson's diseases. In Huntington's disease, the deacetylation of microtubules impairs motor protein function, while in Parkinson's disease, mitochondrial dysfunction also affects axonal transport. These disruptions hinder the movement of vital components, such as mitochondria and neurotransmitter vesicles, leading to synaptic dysfunction and accumulation of misfolded proteins. The failure of efficient protein degradation further contributes to neuronal damage. These disruptions in axonal transport not only affect the normal functioning of neurons but also contribute to the accumulation of toxic proteins and energy deficits, accelerating disease progression. The understanding of axonal transport alterations highlights the critical role of the cytoskeleton in maintaining neuronal health and function, underscoring its importance in the pathophysiology of neurodegenerative diseases. Microtubules play a vital role in maintaining synaptic function and plasticity, particularly in the presynaptic terminal where they support vesicle movement and neurotransmitter release. During synapse formation, microtubules and other cytoskeletal proteins shape neuronal extensions and ensure proper synaptic signaling. Techniques like electron microscopy reveal the intricate roles of these proteins in maintaining synaptic integrity. Alterations in synaptic structure, such as changes in dendritic spine number and shape, are key features of neurodevelopmental disorders. Mutations in synaptic proteins, like Shank3 in autism spectrum disorder (ASD), disrupt synaptic function and spine morphology. Environmental stress can further exacerbate these changes, influencing neural structure. Disruptions in synapse formation and refinement contribute to disorders like ASD, schizophrenia, and Alzheimer's disease. Genetic mutations in synaptic proteins are central to synaptic dysfunction, highlighting the need to understand these changes for therapeutic advancements. SINAPTIC PLASTICITY The ability of the nervous system to adapt and change based on its activity, and are fundamental to the function and adaptation of neural circuits. There are various forms of synaptic plasticity, which can be categorized as either short-term or long-term. Plasticity involves changes in the activity of synapses, which can either enhance or decrease the synaptic transmission. Short-term plasticity involves transient changes, lasting from milliseconds to minutes, and generally occurs through alterations in the existing synaptic connections. These changes can either increase or decrease the synaptic activity or efficacy. Short-term plasticity refers to rapid, temporary modifications in synaptic activity, while long-term plasticity involves more persistent changes, which are often linked to cognitive functions such as learning and memory. Short-term plasticity primarily involves changes in the presynaptic neuron’s activity, which can influence the strength of synaptic transmission over brief time periods. These changes are critical for the immediate processing of information and the initial phases of learning. SHORT-TERM SYNAPTIC PLASTICITY → changes that occur at the presynaptic level (in the intracellular environment). This changes can alter the level of intracellular calcium. The focus on presynaptic calcium stems from its critical role in neurotransmitter release. When an action potential reaches the presynaptic terminal, it triggers the opening of calcium channels, causing calcium ions to enter the cell. This increase in calcium concentration is essential for the fusion of synaptic vesicles with the membrane, leading to neurotransmitter release. To enhance or sustain neurotransmitter release, elevated intracellular calcium concentrations are necessary. This mechanism is central to short-term synaptic plasticity, enabling synapses to adapt to varying levels of activity. Paired-pulse facilitation and depression are key forms of short-term synaptic plasticity, governed by the timing of stimuli. Paired-Pulse Facilitation (PPF) occurs when two stimuli are applied within a short interval (less than 20 ms). The first stimulus causes neurotransmitter release, and the second, arriving soon after, enhances this release. This is due to the elevated intracellular calcium concentration from the first action potential, which facilitates neurotransmitter release during the second. Paired-Pulse Depression (PPD) occurs with a similar short interval but results in a decrease in neurotransmitter release after the second stimulus. This happens because the first stimulus depletes the vesicular pool of neurotransmitters, and without enough time for replenishment, the second stimulus releases less. Additionally, a reduction in intracellular calcium levels inactivates voltage-gated calcium channels, further diminishing neurotransmitter release. Facilitation is observed with slightly longer intervals (20–500 ms), as calcium levels remain elevated between stimuli, allowing for enhanced neurotransmitter release from a fully stocked vesicular pool. OTHER FORM OF SYNAPTIC PLASTICITY → AUGMENTATION →is an increase in intracellular calcium and often an increase in neurotransmitter release, there is an elevation of calcium levels, enhancing synaptic transmission. POST-TETANIC DEPRESSION → prolonged stimulation, but the response is opposite with a decrease of neurotransmitter release and this depression may not only involve the presynaptic element but also affect the postsynaptic elements. there can be an inactivation of the postsynaptic receptors. This means that while neurotransmitters are released, the receptors on the postsynaptic neuron may become less responsive or unavailable for stimulation, impairing the synaptic response. In the case of depression, there is a reduction in intracellular calcium levels and a depletion of the vesicular pool, meaning there are fewer vesicles available for release. In contrast, during facilitation and augmentation, intracellular calcium levels remain elevated, and there is a larger pool of vesicles that are readily available for release. This increased vesicle availability, combined with higher calcium levels, enhances neurotransmitter release. Synaptic plasticity is influenced by changes in intracellular calcium. Three main hypotheses explain how calcium's effects are enhanced during repeated stimulation. The supercalcium hypothesis suggests that calcium levels remain elevated after the first stimulus, leading to more calcium influx with the second stimulus. The second hypothesis proposes that repeated stimuli increase the sensitivity of calcium channels, allowing more calcium to enter. The third hypothesis involves buffer saturation, where calcium-binding proteins become saturated, leaving more free calcium in the cytoplasm. These mechanisms together enhance neurotransmitter release during repeated stimulation. Synaptic plasticity is traditionally seen as occurring mainly at the presynaptic level, but this view is evolving. The introduction of nitric oxide, a gas neurotransmitter produced by the postsynaptic element, shows that plasticity can also involve retrograde signaling. Nitric oxide can travel back to the presynaptic terminal, influencing synaptic function. Additionally, presynaptic receptors for neurotransmitters, like those found postsynaptically, allow the synapse to regulate itself by modulating neurotransmitter release. Glial cells are also now recognized as active players in synaptic modulation, contributing further to plasticity. This broadens the understanding of synaptic plasticity as a dynamic, multi-level process. LONG-TERM SYNAPTIC PLASTICITY → involves changes in the number of synapses, we can observe the formation of new synapses and the elimination of existing synapses that are no longer functional. Learning and memory, therefore, seem to be mediated by structural changes in the brain, specifically the formation or elimination of synapses. HOMEOSTIC PLASTICITY → where the synapses adjust their activity in a coordinated way to maintain stability METAPLASTICITY → which refers to changes in the synaptic plasticity itself based on prior activity. One key aspect of plasticity involves receptors, especially in regions like the hippocampus, which play a critical role in both long-term potentiation (LTP) and long-term depression (LTD). These forms of plasticity are induced by different patterns of stimulation: high- frequency stimulation induces LTP, while low-frequency stimulation induces LTD. Both types require coincident activity—that is, the simultaneous activation of at least two neurons in both time and space. A critical part of this process involves glutamate receptors, specifically AMPA and NMDA receptors. AMPA receptors mediate fast synaptic transmission, and when glutamate is released, they increase membrane permeability, leading to depolarization. However, when the stimulus is insufficient or brief, the membrane potential changes, activating NMDA receptors, which are involved in more sustained changes and are crucial for inducing synaptic plasticity. RECEPTOR NMDA → important in the synaptic plasticity because of their duel property, they are both ligand-gates and voltage-gated. When the postsynaptic membrane is depolarized, it causes a conformational change in the NMDA receptor, displacing the magnesium ion that normally blocks the channel. Once the magnesium is removed, the binding of the neurotransmitter (glutamate) to the receptor opens the channel, allowing ions to flow through. One of the key features of NMDA receptors is that they are cationic channels, primarily allowing the influx of calcium ions. Calcium acts as a second messenger, triggering intracellular signaling cascades that lead to changes in synaptic strength, a fundamental process in synaptic plasticity. Most forms of synaptic plasticity, including long-term potentiation (LTP), occur at glutamatergic synapses, where glutamate is the primary neurotransmitter. These synapses, particularly those involving AMPA and NMDA receptors, are critical for plasticity processes. LTP (Long-Term Potentiation) has three main characteristics: Cooperativity: LTP can be induced when at least two synapses are activated simultaneously, strengthening the connection. Associativity: A weak input can be potentiated if paired with a stronger input, associating weak and strong signals. Input specificity: LTP is specific to the activated synapses, affecting only those connections. Initially observed in hippocampal synapses, LTP involves both AMPA and NMDA receptors. NMDA receptors are key, acting as coincidence detectors, requiring simultaneous presynaptic and postsynaptic activation for LTP induction. AMPA receptors primarily mediate sodium and potassium ion flow, causing depolarization. Calcium influx through NMDA receptors activates downstream signaling pathways, including PKA, CaMKII, and MAPK, leading to gene expression and long-term changes in synaptic strength, which are essential for learning and memory. Plasticity is the brain's ability to adapt, highest in early life but persisting into adulthood. It can be enhanced through intellectual, social, and physical activities. A key mechanism involves silent synapses, which have few surface receptors but contain AMPARs in vesicles. These receptors quickly move to the synaptic membrane, activating the synapse. This process is regulated by small GTP-binding proteins, promoting plasticity in both development and adulthood. The trafficking of AMPA receptors (AMPARs) during synaptic plasticity is driven by signaling proteins, particularly kinases that promote AMPAR insertion into the postsynaptic membrane, enhancing synaptic strength. Conversely, phosphatases like PP1 remove AMPARs from the synapse, weakening the connection. Synaptic stimulation requires precise timing to achieve cooperativity, which means simultaneous or closely timed activation of multiple neurons. This timing is essential for activating NMDA receptors (NMDARs), triggering downstream signaling and plasticity. Late-phase long-term potentiation (LTP) involves structural changes at the synapse and requires protein synthesis. These changes can occur through local translation of mRNA at the synapse or via gene transcription in the nucleus. Protein synthesis is essential for lasting synaptic changes; blocking it prevents late-phase LTP while allowing the early phase to proceed. Both local translation and nuclear gene transcription are critical for late-phase LTP. The translation machinery, including ribosomes, is recruited to the dendritic spines to synthesize proteins for synaptic strengthening. Newly transcribed mRNA from the nucleus is transported to synapses, where it is translated into proteins, contributing to long-term synaptic changes. Eric Kandel’s research on memory and LTP (long-term potentiation) in Aplysia and vertebrates has significantly advanced our understanding of the molecular mechanisms underlying memory. His work demonstrated that the mechanisms of LTP are similar across species, including in the hippocampus of rodents. In the early phase of LTP, glutamate release activates NMDA receptors, allowing calcium influx, which triggers signaling pathways like CaMKII activation, essential for synaptic strengthening. In the late phase of LTP, the activation of transcription factors such as CREB leads to gene expression for synaptic plasticity and structural changes, including new synapse formation. Retrograde signaling, particularly involving nitric oxide, plays a key role in modulating presynaptic neurotransmitter release, further strengthening synapses. While short-term plasticity strengthens existing synapses, long-term plasticity can induce the growth of new synapses and neurons, contributing to cognitive functions. Importantly, both LTP and LTD (long-term depression) can occur at the same synapses, with LTP triggered by high-frequency stimulation and LTD by low-frequency stimulation. This process underpins the dynamic nature of synaptic plasticity and learning. NMDAR – DEENDENT LTD → LTD at the same synapses means that the same molecular machinery within the synapses in involved, but it is activated in a different way to produce the opposite effect compared to LTP. The same synaptic element can undergo different forms of plasticity depending on the pattern of stimulation. The key factor is the level and timing of intracellular calcium concentration. The intensity and frequency of synaptic stimulation determine the intracellular calcium concentration, which in turn influences the type of plasticity that occurs. Low-frequency, low-intensity stimulation leads to moderate calcium influx (around 1 micromolar), activating protein phosphatases and inducing long-term depression (LTD). In contrast, high-frequency but low-intensity stimulation triggers a similar calcium influx, but not enough for long-term potentiation (LTP). For LTP, a higher calcium concentration (above 5 micromolar) is needed to activate protein kinases. Thus, the same molecular machinery (e.g., NMDA receptors, glutamate release) can produce opposite effects—LTP or LTD—depending on calcium levels, with metabotropic glutamate receptors (mGluRs) also playing a role in LTD through immune-related pathways. mGluR-Dependent LTD in cerebellum → in the cerebellum, there is a lack of NMDA receptors which rely instead on metabotropic glutamate receptors and when they are activated, initiate intracellular signalling cascades. This system triggers the release of calcium from intracellular stores into the synaptic compartment. This increase in calcium within the synaptic terminal can lead to long-term depression by triggering signaling pathways that ultimately internalize AMPA receptors from the postsynaptic membrane. Thus, the mechanism of depression in this case is fundamentally similar to that observed in other forms of LTD, even if the precise molecular signaling components involved differ. The key point is that, depending on the type of receptor activation and the calcium concentration reached, the synaptic response can shift between potentiation and depression. FUNCTIONAL ROLES OF LTP AND LTD → LTP is considered the physiological basis of declarative memory (in the hippocampus) so it’s the fundamental mechanism behind learning and memory formation. Additionally, LTD (long-term depression) and LTP are not only important in the hippocampus but also play vital roles in brain development, contributing to various forms of plasticity across different brain regions, including the sensory systems. The critical period is a crucial developmental time when the brain must receive appropriate sensory input to develop essential functions like vision, language, and motor skills. For example, if the visual cortex doesn’t receive input during this period, it can lead to amblyopia ("lazy eye"). Similarly, language acquisition requires exposure before age two, as delayed exposure can impair language learning. During these periods, mechanisms like NMDA receptors and transcription factors (e.g., CREB) are key for synaptic plasticity. Experiments on somatosensory cortex plasticity show how sensory input alters brain organization, such as when removing a finger leads to nearby brain regions taking over. The brain also reorganizes with experience, as seen in musicians or surgeons, where repeated practice leads to expanded brain areas. While plasticity is highest during early life, it can persist with lifelong learning and training, relying on similar mechanisms as those in long-term memory formation. LEARNING AND MEMORY APLYSIA → is an animal model that enables the observation of different forms of neural plasticity, both short and long term. Serotonin → it’s a neurotransmitter that plays a key role in modulating synaptic plasticity. Serotonin activates metabotropic receptors on the target neurons. These receptors are coupled to G-proteins, which activate adenylate cyclase, leading to an increase in cyclic AMP (cAMP) levels. Elevated cAMP activates protein kinase A (PKA), which in turn phosphorylates several proteins involved in synaptic transmission. By stimulating serotonin receptors, the animal's neurons undergo a cascade of molecular events that lead to changes in synaptic transmission. These events include the activation of transcription factors like CREB, which, as mentioned earlier, leads to histone acetylation and the transcription of plasticity-related genes. When serotonin is released from the sensory neuron, it stimulates the motor neuron, leading to the withdrawal reflex. The serotonin release also triggers molecular signaling pathways that enhance synaptic strength, making the motor neuron more responsive to subsequent stimuli. Short-term plasticity involves phosphorylation that increases calcium influx, strengthening synaptic transmission and enhancing responses to subsequent stimuli, typical of synaptic facilitation. In contrast, long-term plasticity is driven by more complex processes, such as activation of cAMP and PKA, leading to protein synthesis and the formation of new synapses. This mechanism is similar to long-term potentiation (LTP) observed in models like Aplysia, where sustained changes in synaptic strength are accompanied by the growth of new synaptic connections. LTP IN APLYSIA → is driven by the activation of transcription factors like CREB that is activated when