Neuroscience: Nervous System Histology

• The text provides an introduction to neuroscience, focusing on the histology of the nervous system (SN).

Neuron Morphology Basics

• Neuronal morphology encompasses neuroanatomy, neuroscience, psychobiology, psychology, behavioral genetics, cognitive neuroscience, somatic intervention, biology, affective neuroscience, psychopharmacology, physiological psychology, psychophysiology, behavioral neuroscience, behavioral intervention and correlation studies.
• Neuroscience studies the nervous system which includes genetics of behavior and how it relates to affective and cognitive areas.
• The nervous system is complex and made up of numerous subsystems.
• A neuron is a cell (functional unit) of the central nervous system that communicates through synapses.
• Neurons do not undergo mitosis due to their specialization and loss of replication capability.
• Glial cells are located within the central and peripheral nervous systems, providing supportive functions.
• Average brain weight is approximately 1350 grams.
• Afference relates to the origin of a signal, such as the thalamus, while efference is signals coming from the thalamus.
• The human brain consists of approximately 10^11 neurons.
• There are 10 glial cells for every neuron.
• Neuron size ranges from 5-100 micrometers.
• Neurons lack centrioles.
• Communication between the cerebellum and brainstem occurs through axons.
• Cerebellar afferents send signals to the brainstem; afferents indicate signal reception.

Contributors

• Golgi shared the Nobel Prize with Ramon y Cajal for neuron staining techniques.
• Waldeyer lived from 1836-1921.
• Ramon y Cajal (1852) was a Spanish scientist, who also won the Nobel Prize for his neuron studies.
• Cajal illustrated neuron morphology, trophism, pathology, and function, discovered the synapse.
• Cajal is recognized as the father of neuroscience.

Neuron Function & Types

• Neurons function to transmit information.
• Typical information flow goes to the cell body of the neuron and then the axon.
• Neurotransmitters are part of neuronal physiology.
• A synapse is inhibitory when it occurs on the cell body of a neuron.

Neurons vs Other Cells

• Neurons cannot reproduce, and lack centrioles.
• Neurons need energy, synthesize carbohydrates, and possess a nucleus with DNA in the form of chromatin.
• As a unit, neurons must be delimited by a neuronal membrane, contain cytoplasm (or plasmolemma in the cell body), and corpuscles.
• Neurons transmit the same information they receive but they can receive information from multiple neurons simultaneously.

Neuron Components

• Cell membrane delimits the space and covers the entire cell, controls entry/exit of nutrients, oxygen, and neurotransmitters, changing its voltage.
• The cell membrane is a lipid bilayer with channels (proteins) which allow certain ions, anions, and cations to pass through.
• Cell body: Neurons receive substances from the environment to be used or eliminated via metabolism.
• The nucleus inside the cell body stores genetic information in DNA
• Golgi apparatus participates in distributing and processing substances inside the cell.
• Neurons always transmit the same electrical signal, activation requires a change in the axon hillock.

Signal Types

• Excitatory signals facilitate neuron activation.
• Inhibitory signals prevent neuron activation.
• Neurons transmit signals from the axon to the telodendrites if stimulation occurs.
• Telodendrites are the terminal part of the axon in the presynaptic region where it communicates with other neurons or cells.
• Not all signals in the nervous system are excitatory as some areas are inactive (inhibited).
• Movement results from excitatory and inhibitory signal interaction
• Neurons receive signals from various sources, process them in the axon hillock, and transmit an electrical impulse via the axon to communicate with other neurons if the voltage is sufficient.

Other Neuron Parts

• Nissl bodies are structures in the neuronal body that produce proteins essential for the neuron and form part of the axoplasmic flow.
• Dendrites are branched extensions coming off the cell body, receiving signals from other neurons via dendritic spines.
• Dendrites receive neurotransmitters and direct signals to the exterior.
• Axons are a single extension of the neuron that transmits electrical signals - connects to the axon hillock.
• Nodes of Ranvier are myelin-free gaps along the axon where the nerve impulse propagates faster.

Conduction Types

• Afferent conduction - signals travel into the neuron through dendrites.
• Efferent conduction - signals travel out of the neuron by way of the axon.

The Synapse

• The presynaptic neuron contains synaptic vesicles.
• The postsynaptic neuron contains receptors for neurotransmitters.
• Neurotransmitters are released into the synaptic cleft.

Axon Structures

• Neurofibrils are structures within the neuronal cytoplasm that run the length of the axon.
• Function: mechanical support - communication - transport of substances - neurological secretion.

Alzheimer's Disease

• Alzheimer's is a progressive neurodegenerative disease characterized by cognitive impairment.
• It affects memory, thinking, and behavior.
• Causes & effects: Beta-amyloid plaques and abnormal protein deposits accumulate between neurons, disrupting communication.
• Neurofibrillary tangles are formed by altered Tau protein which normally maintains neuron structure and facilitates substance transport.
• When Tau protein is altered, filaments become disorganized, disrupting transport and causing neuron death.

Neuron Types

• Unipolar neurons have one axon and one dendrite located at opposite poles, branching at the terminal ends.
• They receive sensory input from the environment to the central nervous system and information does not pass through the cell body.
• Bipolar neurons are found in the peripheral nervous system, designed to transmit sensory information via sensory cells to the central nervous system (visual, tactile, auditory).
• Multipolar neurons have one axon and multiple dendritic branches extending from the cell body allowing for association and interneuron connections.
• Short-axon multipolar neurons are association or interneurons.
• Long-axon multipolar neurons can extend several centimeters in length.

Brain Matter

• Gray matter consists of neuronal cell bodies and dendrites, found in the cerebral cortex and central nervous system.
• White matter comprises axons that transmit nerve impulses throughout the central nervous system.
• The spinal cord has gray matter in the center and white matter surrounding it.
• Spinal cord tracts have one direction of information flow.

Direction

• Anterior (ventral) part of the spinal cord transmits efferent (motor) information.
• Posterior (dorsal) part transmits afferent (sensory) information
• Spinal nerves emerging from the spinal cord are mixed, carrying both afferent and efferent information.

Nerve Fibers

• Nerve fibers’ arrangement: bundles of axons that are:
• Located in the central nervous system are called tracts or bundles: They have one direction that is either afferent or efferent.
• Located in the peripheral nervous system are called nerves and are usually for transmitting afferent/efferent information.

Myelin & Conduction Speeds

• Myelin acts as an insulation that increases the transmission speed of nerve impulses.
• Velocity depends on axon diameter.
• Myelinated fibers: velocity = axon diameter × 6.
• Unmyelinated fibers: velocity = axon diameter × 1.5.
• Myelin sheaths are not present from the beginning but are produced through development in the brain.

Glial Cells

• There are more glial cells than neurons in the nervous system which lack axons.
• Glial cells are not involved in the nervous impulse or affect the impulse by affecting conductivity.

Function

• Glial cells provide mechanical support (support) for neurons; provide physical and chemical protection.
• They produce myelin sheaths which insulate neurons that are electrically isolated.
• They deactivate neurotransmitters eliminated by neurons
• Glial cells clean up after injury, and help with tissue regeneration and controls the control composition of extracellular fluid.
• Glial cells transport substances to neurons, and waste substances away from neurons
• They destroy, or phagocytize dead neurons.
• Some glial cells (astrocytes) can act as stem cells, and can convert into new neurons in certain areas of the nervous system.
• The connected tissue fulfils is connective tissue.

Differences Between Glial Cells & Neurons

• No axon.
• Possess centrioles
• Able to reproduce by mitosis
• Indirectly involved in transmission of nerve impulses that affect conductivity.
• Glial cells play a connective tissue role within the SN (support).

Glial Classifications

• Macroglia includes: Astrocytes-oligodendrocytes & Radial glia.
• In the Central Nervous System (CNS) are Macroglia & Microglia & Ependymal cells.
• In the Peripheral Nervous System (PNS) are Satellite cells & Schwann cells.

Glial Types

• Astrocytes (macroglia that are formed by astrocytes (nucleus grande, glycogen granules and glialfilaments).
• Astrocyte Functions: Forms a barrier, sits in-between the capillaries and neurons.
• Fibrous atrocytes are located in white matter and associated with myelinated axons.
• Protoplasmic astrocytes have fewer filaments and are located in gray substance and associated with the neuron cell body and synapses,
• Oligodendroglia creates myelin sheaths & is found in both white and gray matter, emitting multiple arms and myelinating multiple axons.
• Ralial Glia appears during development & transforms after SN maturation.
• The primary function is the migration of the neurons.
• Microglia is responsible for phagocytosis of dead neurons with the ability to move destroy any neuron tissue & destroy harmful bacterial / foreign substances.
• Ependymal cells produce cerebrospinal fluid throughout the nervous system.
• Function: protection, movement & is found in the brain ventricles + the medulla spinal cord.
• Satellite cells are (astroglia packaging) that package the neuron cell bodies of the ganglia.
• It maintains the interchange of nutrients & metabolic waste.
• Nutritional relation, ensuring chemical equilibrium in the environment.
• Schwann cells are homologous which are cells related to oligodendroglia.
• These wrap the axon creating a myelin sheath, action + trophic.
• They nourish / protect & regenerate the neuron from the PNS while myelinizing a single axon!
• Connective tissue: glial cells create blood-brain barriers that use a filter to select molecules sent from the bloodstream, which regulates what enters and exits.

Summary

• Astrocytes -> Support and regulation in the CNC.
• Oligodendrocytes -> Myelinize the CNC.
• Células de Schwann -> Myelinize and help regenerate the SNP.
• Microglia -> Immune defense and cleaning in the CNC.
• Células ependimarias -> Produce and circulate the LCR.
• Células satélite -> Nourish and protect neurons in the SNP.