Week 6 - Physiology (Nervous Tissue) PDF

Week 6 - Physiology (Nervous Tissue) Objectives: - Classify the components and functions of the nervous system. - Describe the types of electrical signals generated within the neurons. - Identify the factors that generate electrical signals. - Recognize signal transmission and the effects of summat...

Week 6 - Physiology (Nervous Tissue) Objectives: - Classify the components and functions of the nervous system. - Describe the types of electrical signals generated within the neurons. - Identify the factors that generate electrical signals. - Recognize signal transmission and the effects of summation and neurotransmitters on synapses. - Describe the classes and functions of neurotransmitters. - Identify the events involved in regeneration and repair of nervous tissue. Outline: Organization of the nervous system Functions of the nervous system Various structures of nervous system: - Neurons & classification - Neuroglia - Myelination - Collections of nervous tissue Electric signals in neurons: - Ion channels - Resting membrane potential - Action potential & propagation of action potentials Signal transmission at Synapses: - Electrical synapses - Chemical synapses - Spatial & temporal summation of postsynaptic potentials Neurotransmitters: - Small-molecule neurotransmitters - Neuropeptides Regeneration & repair of nervous tissue: - Neurogenesis in the CNS - Damage & repair in the PNS Organization of the Nervous System: The nervous system is divided into two subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS). 1. Central nervous system (CNS) – consists of the brain & spinal cord located in the dorsal body cavity and is encased in bone for protection (skull & vertebral column). Specifically, the brain is in the cranial vault & the spinal cord is in the vertebral canal of the vertebral column. The brain & spinal cord are continuous at the foramen magnum (which cranial bone forms this structure?). 2. Peripheral nervous system (PNS) – consists of nerves & ganglia. Nerves are bundles of nerve fibers (bundles of axons). Cranial nerves & spinal nerves extend from the CNS to peripheral organs such as muscles & glands. Ganglia (singular – ganglion) are collections of nerve cell bodies outside the CNS. - Peripheral nervous system (PNS) – it is further subdivided into an afferent (sensory) division & an efferent (motor) division. The afferent or sensory division is responsible for the transmission of impulses from peripheral organs to the central nervous system (CNS). The efferent or motor division is responsible for conveying neural signals from the central nervous system (CNS) to the peripheral organs, thereby eliciting a physiological response or action. - The motor division is divided into the somatic nervous system (SNS) and the autonomic nervous system (ANS). - The somatic nervous system, also called the somatomotor or somatic efferent nervous system, supplies motor impulses to the skeletal muscles. Because these nerves permit conscious control of the skeletal muscles, it is sometimes called the voluntary nervous system. - The autonomic nervous system, also called the visceral efferent nervous system, supplies motor impulses to cardiac muscle, to smooth muscle, and to glandular epithelium. It is further subdivided into sympathetic and parasympathetic divisions. Because the autonomic nervous system regulates involuntary or automatic functions, it is called the involuntary nervous system. Functions of the Nervous System: Various activities such as smell, speech, movements, etc., are functions of the nervous system and can be grouped as: 1. Sensory function – Internal sensory stimuli such as blood pressure, partial oxygen levels, or external stimuli such as pain and touch are carried to the CNS. 2. Integrative function – The nervous system processes sensory information and produces appropriate responses - integration. 3. Motor function – If a sensory input needs an effective response, the nervous system elicits such a response by activating effectors such as muscles and glands. Neuron: - Neurons are the functional unit of the nervous system. It is the "conducting" cell that transmits impulses and the structural unit of the nervous system. The other types of cell are neuroglia, or glial, cells. The word "neuroglia" means "nerve glue." These cells are nonconductive and provide a support system for the neurons. They are a special type of "connective tissue" for the nervous system. - Neurons, or nerve cells, carry out the functions of the nervous system by conducting nerve impulses. They are highly specialized and (mostly) amitotic. This means that if a neuron is destroyed, it cannot be replaced because neurons do not go through mitosis. Parts of a neuron: 1. Cell body – or perikaryon or soma contains a nucleus surrounded by cytoplasm with cellular organelles such as lysosomes, mitochondria, & Golgi complex. The presence of a rough endoplasmic reticulum is called Nissl bodies. The cytoskeleton has neurofibrils (intermediate filaments that provide shape & support) and microtubules (which help to move materials from the cell body to the axon). 2. Plasma membrane (neurolemma) ranges from smooth to bumpy. These bumps are small projections called somatic spines, serving as receptor sites for binding to chemical messengers. - Nerve processes / Neurites – a nerve fiber is any neuronal process emerging from the cell body. There are 2 kinds – dendrites & axons. Generally, dendrites receive stimulus via binding of neurotransmitters to dendritic spines (similar to somatic spines). These are short, tapering, and highly branched. The number of dendrites on a neuron varies. They are called afferent processes because they transmit impulses to the cell body. - Axon – transmits signals from the cell body to another neuron or NMJ (neuromuscular junction) or effector organs. It is a long, thin, cylindrical projection attached to the cell body at a cone-shaped elevation called the axon hillock. The part of the axon closest to the axon hillock is called the initial segment. Nerve impulses arise at the junction of the axon hillock & initial segment, known as the trigger zone. Axons lack a rough endoplasmic reticulum, hence cannot synthesize proteins. The cytoplasm of the axon is called axoplasm, surrounded by a plasma membrane, known as axolemma. Along the length of the axon, side branches called axon collaterals are present at right angles to the axon. The axon & its collaterals end as axon terminals or synaptic bulbs. The site of communication between two neurons or a neuron & effector cell is called a synapse. - Transport systems – Some materials are transported from cell body to axon by either slow axonal transport system (1-5 mm per day) or fast axonal transport system (200-400 mm per day). Slow axonal transport systems are present in developing or regenerating axons & are transported in one direction (forward) only. Fast axonal transport system moves materials both ways. Anterograde (forward) direction moves materials organelles & synaptic vesicles from cell body to axon terminals. In retrograde (backward) direction, membrane vesicles and other cellular materials move from axon terminals to the cell body to be degraded or recycled. Classification of Neurons: Structurally, neurons are classified based on the number of processes extending from the cell body. 1. Unipolar neuron – Unipolar neurons have only one structure (axon) that extends away from the soma. The cell body lacks dendrites. 2. Pseudounipolar neuron – Most sensory neurons are of this kind. They have no dendrites, but the branched axon serves as dendrites as well. 3. Bipolar neuron – have one dendrite & an axon. Example – neurons in the retina of the eye. 4. Multipolar neuron – has several dendrites & one axon. Example – most neurons in the brain & spinal cord & motor neurons are of this type. Functionally neurons are classified based on the direction in which the nerve impulse is conveyed to the CNS. 1. Sensory neuron – or afferent neurons contain sensory receptors at their distal ends (dendrites). An adequate stimulus activates a sensory receptor, which produces a nerve impulse in its axon and sends it to the CNS via cranial or spinal nerves. The majority of sensory neurons are pseudounipolar/unipolar. 2. Motor neuron – or efferent (means away) conveys impulses away from the CNS to effectors (muscles & glands) in the periphery (PNS) through cranial or spinal nerves. Motor neurons are multipolar. 3. Interneuron – or association neurons are mainly located within the CNS between sensory & motor neurons. They integrate incoming sensory information from sensory neurons & elicit a response through motor neurons. They are multipolar in nature. Neuroglia: Neuroglia (glial cells or glia) are supporting cells of the nervous system. However, latest research indicates that they are involved in more functions. They are smaller than neurons and present in greater quantities. Brain tumors can be derived from glia, called gliomas. There are 6 types of neuroglia. Neuroglia of the CNS: 1. Astrocytes – star-shaped and have many processes; they are the most numerous of all neuroglia. Protoplasmic astrocytes have short branching processes and are found in gray matter in the CNS, while fibrous astrocytes have many long unbranched processes located in the white matter. They form borders between neurons & other body tissues (protection). 2. Oligodendrocytes – resemble astrocytes but are smaller & have fewer processes. They form & maintain the myelin sheath around CNS axons. 3. Microglial cells or microgliocytes – small cells with slender processes with numerous spine-like projections. They function as phagocytes, like tissue macrophages, removing cell debris and protecting against pathogens. 4. Ependymal cells – are cuboidal to columnar cells arranged in a single layer that possess microvilli & cilia. They are located in the ventricles within the brain & central canal of the spinal cord, which protects & nourishes by secreting cerebrospinal fluid (CSF). Neuroglia of the PNS: 5. Schwann cells – or neurolemmocyte. They encircle PNS axons. Like oligodendrocytes they form myelin sheath around axons. They participate in the axon generation. 6. Satellite cells – flat cells that surrounds the cell bodies of neuron in PNS ganglia providing structural support & regulates exchange of materials between cell bodies & interstitial fluid. Myelination: - Myelin sheath – multi-layered protective lipid & protein covering on the axons. The process of formation of the myelin sheath is called myelination. Neurons that have a myelin sheath are said to be myelinated, and others are unmyelinated. - Schwann cells (in the PNS) & oligodendrocytes (in the CNS) produce the myelin sheath. - Each Schwann cell wraps about 1 mm of a single axon. The Schwann cell’s cytoplasm and nucleus forms the neurilemma. When the axon is injured, the neurolemma aids in regeneration by forming a tube that guides & stimulates the regrowth of the axon. Gaps in the myelin sheath, called myelin sheath gaps (nodes of Ranvier), appear at regular intervals. - Oligodendrocytes myelinate parts of several axons. A neurilemma is NOT present as the cell body of the oligodendrocyte does not envelop the axon. Myelin sheath gaps are less numerous compared to Schwann cells. Since oligodendrocytes do not help in axon regrowth, the CNS lacks regenerative capacity. Collections of Nervous Tissue: Nerve tissue (neurons) are grouped in various ways in the nervous system. - Ganglion – (plural- ganglia). Refers to a cluster of neuronal cell bodies in the PNS. - Nucleus is the cluster of neuronal cell bodies in the CNS. - Nerve – bundle of axons in the PNS. Cranial nerves connect the brain to the periphery while spinal nerves connect the spinal cord to the periphery. - Tract – bundle of axons in the CNS. They connect the brain & spinal cord. - Gray & White matter – Gray matter contains unmyelinated axons and neuronal cell bodies. Nissl bodies in the cytoplasm give the grey color. White matter is myelinated axons. Ion channels: Nerve tissue (neurons) communicate with 2 types of electrical signals. 1. Graded potentials – used for short-distance communication. 2. Action potentials – occurs for long-distance communication. Movement of ions across the neuronal membrane results in an electrochemical change in the electric potential (nerve potential). Ion channels play a vital role in movement of ions. There are 4 types of ion channels. They are: 1. Leakage channels – gates of these channels randomly alternate between open & closed positions. Plasma membranes have more potassium ion (K+) leak channels than sodium ion (Na+) leak channels. Hence the membrane's permeability to K+ is much higher than Na+. 2. Ligand-gated channel – open & close in response to the binding of a ligand (chemical) stimulus such as neurotransmitters (ACh). ACh opens cation channels allowing Na2+ to diffuse into the cell & K+ to diffuse outward. Na2+ & K+ are cations (positively charged). 3. Mechanically gated channels – gates open & close in response to mechanical stimulation such as vibration, touch, pressure, stretching of tissue, etc. Example – eardrum (tympanic membrane) converting sound waves to mechanical waves and then finally to electrical waves. 4. Voltage-gated channel – opens in response to a change in membrane potential (voltage). Voltage-gated channels participate in the generation & conduction of nerve impulses in the axons of all types of neurons. Resting Membrane Potential: - The resting membrane potential of a cell is the electrical potential difference across the plasma membrane when the cell is in a non-excited state. This usually means that the inside of the plasma membrane (that is facing ICF - cytoplasm) is negative while the outside of the plasma membrane (facing ECF) is positive. - In neurons, the resting membrane potential ranges from -40 to -90 mV. Negative sign indicates that the membrane potential inside the cell (ICF) is negative compared to outside (ECF). Such neurons are said to be polarized because of the difference. - The resting membrane potential occurs due to 3 factors: 1. Unequal distribution of ions in the ECF & cytosol – ECF is rich in Na + and Chloride (CL-) ions. While ICF is rich in K +, protein & phosphate ions and these are high in negative charge. In addition, plasma membranes have more K+ ion channels than Na + leak channels. So K+ diffuses more to ECF than Na +. As more and more K + exit, the inside of the membrane becomes more negative and outside of the membrane becomes more positive. However, a less negative or more positive charge inside the membrane occurs in action potential generation. 2. Inability of most anions to leave the cell is because they are bound to non-diffusible molecules such as ATP and large proteins. 3. Electrogenic nature of Na+-K+ ATPases – during resting state, due to difference in concentration gradient, there is a tendency of K+ to leak out and Na+ to leak in. This offsets the resting membrane potential. To maintain normal state, Na+-K+ ATPases (sodium-potassium pumps) pumps out sodium and brings in potassium ions. Action Potential: Transmission of a signal within a neuron (from dendrite to axon terminal) is carried by a brief reversal of the resting membrane potential called an action potential. This is referred to as nerve impulse. It has 2 phases: 1. Depolarizing phase - the negative membrane potential becomes less negative, reaches zero, and then becomes positive. 2. Repolarizing phase - the membrane potential is restored to the resting state of −70 mV. Following the repolarizing phase there may be an after-hyperpolarizing phase, during which the membrane potential temporarily becomes more negative than the resting level. Action Potential - Definition: Action Potential – A nerve impulse occurs in the membrane of the axon of a neuron when depolarization reaches a certain level known as threshold (about -55 mV) - A subthreshold stimulus cannot generate a nerve impulse while a threshold stimulus can, which can depolarize the membrane. Suprathreshold stimulus is a stimulus that is strong enough to depolarize the membrane above threshold. Each of the nerve impulses caused by suprathreshold stimulus has the same amplitude and remains the same without depending on the stimulus intensity. Instead the greater the stimulus strength above threshold, the greater the frequency of the nerve impulse until a maximum frequency is reached. - A nerve impulse either occurs completely or does not occur at all. This is called all-or-none- principle. Signal Transmission: - Synapse – a region where communication occurs between two neurons or a neuron & an effector cell (muscle, glands, etc.) 1. Presynaptic neuron: Neuron that carries an impulse to another neuron or target cell. Postsynaptic cell is the cell that receives a signal. 2. Postsynaptic neuron: If the postsynaptic cell (from above) happens to be a neuron, it is called the postsynaptic neuron, which carries the impulse away from a synapse. If the postsynaptic cell responds to the stimulus at the synapse, it is called an effector cell. - Types of synapses – structurally, 3 types of synapses exist. 1. Axodendritic – between axon to dendrite. 2. Axosomatic – between axon to cell body. 3. Axoaxonal – axon to axon. Functionally, they can be classified as electrical and chemical synapses. Electrical Synapses: - Electrical Synapse – action potentials conduct directly between the plasma membrane of adjacent neurons through structures called gap junctions. - Each gap junction has hundreds of tubular connexons, connecting the cytosol of the two cells directly. - As ions pass across connexons, the action potential spreads between cells. Gap junctions are found in brain, cardiac, embryonic, and visceral smooth muscle. Two main advantages: 1. Direct communication: Faster than chemical synapse as it passes directly from presynaptic cell to postsynaptic cell. 2. Synchronization: They coordinate the activity of a group of neurons or muscle fibers. Chemical Synapses: - Chemical Synapse – pre & postsynaptic membranes do not touch each other unlike electrical synapses & are separated by a synaptic cleft, that is filled with interstitial fluid. Neurotransmitters from presynaptic neurons diffuse through this fluid to reach the postsynaptic neuron. - A postsynaptic potential is produced upon receiving the neurotransmitter, a type of graded potential (what is this?) occurs. - Presynaptic neuron converts an electrical signal to chemical signal (neurotransmitter), while postsynaptic neuron converts this chemical signal to electrical signal. - There is a 0.5 msec delay in transmission of signals. Differences between Synapses: Communication Mechanism: - Chemical Synapses: These synapses use neurotransmitters to transmit signals between neurons. When an action potential reaches the presynaptic terminal, neurotransmitters are released into the synaptic cleft, where they bind to receptors on the postsynaptic neuron, eliciting a response. - Electrical Synapses: In contrast, electrical synapses allow direct transmission of action potentials through gap junctions, which are formed by connexons that connect the cytoplasm of adjacent neurons. This enables ions to flow directly from one cell to another. Speed of Transmission: - Chemical Synapses: The process of releasing neurotransmitters and the subsequent binding to receptors introduces a delay, making chemical synapses slower compared to electrical synapses. - Electrical Synapses: These synapses facilitate faster communication because the action potential can propagate directly between cells without the need for neurotransmitter release. Directionality: - Chemical Synapses: Typically unidirectional, meaning the signal flows from the presynaptic neuron to the postsynaptic neuron. - Electrical Synapses: They can be bidirectional, allowing ions and signaling to flow freely in both directions between connected neurons. Modulation: - Chemical Synapses: These synapses can be modulated by various factors, including the type and amount of neurotransmitter released, receptor sensitivity, and the presence of modulatory substances. This allows for complex signaling and integration of information. - Electrical Synapses: Generally, they do not allow for such modulation; the signal strength is determined by the electrical properties of the connected cells and is less flexible than in chemical synapses. Examples: - Chemical Synapses: Found in most neuronal connections in the brain and peripheral nervous system, such as neuromuscular junctions and synapses in the central nervous system. - Electrical Synapses: Common in certain areas of the brain, cardiac muscle cells, and smooth muscle, where synchronized activity is crucial. Chemical Synapses - Steps: - Initiation of action potential – A nerve impulse in the form of action potential travels from the cell body down the axon to the synaptic end bulb. - Depolarization phase – The above step equates to a depolarizing phase, which opens voltage-gated Ca²⁺ channels, which are present in the membrane of synaptic end bulbs. Calcium ions flow from ECF (extracellular fluid) into ICF (intracellular fluid). - Release of neurotransmitters – Increase in calcium causes the synaptic vesicles to fuse to the plasma membrane of the synaptic end bulb and get exocytosed into the synaptic cleft. - Binding phase – Neurotransmitters in the synaptic cleft bind to neurotransmitter receptors in the postsynaptic neuron’s plasma membrane. This receptor is a ligand-gated channel, known as an ionotropic receptor. - Ionotropic receptors: A group of transmembrane ion channels that open or close in response to the binding of a chemical messenger (ligand) such as a neurotransmitter. - Metabotropic receptors: A subtype of membrane receptors that do not form an ion channel pore but use signal transduction mechanisms, often G proteins, to activate a series of intracellular events using second messenger chemicals. - Influx of ions – Binding of neurotransmitter on the postsynaptic neuron causes the ligand-gated channels to open and allow ions to flow in, causing a postsynaptic potential. Based on the type of ion that is let in, the electrical potential can result in either depolarization (excitation) or hyperpolarization (inhibition). - Conduction of nerve impulse – Once the depolarizing postsynaptic potential crosses the threshold, a nerve impulse is conducted through the postsynaptic neuron. Summation of Potentials: A neuron in the CNS receives input from 1000 to 10,000 synapses. A depolarizing postsynaptic potential is called excitatory postsynaptic potential (EPSP). More EPSP causes greater summation to reach the threshold to generate an action potential. There are 2 types of summation: 1. Spatial summation – is the summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time. Example – spatial summation occurs from buildup of neurotransmitters released simultaneously by several presynaptic end bulbs. 2. Temporal summation – is the summation of postsynaptic potentials in response to stimuli that occur at the same locations in the membrane of a postsynaptic cell at different times. Example – temporal summation results from buildup of neurotransmitters released by a single presynaptic end bulb two or more times in rapid succession. - A single postsynaptic neuron receives input from many presynaptic neurons, some of which release excitatory neurotransmitters and some of which release inhibitory neurotransmitters. The sum of all the excitatory and inhibitory effects at any given time determines the effect on the postsynaptic neuron, which may respond in the following ways: 1. EPSP - An EPSP without threshold occurs when the entire excitatory effects exceed the total inhibitory effects but are smaller than the stimulation threshold. Since the neuron is partially depolarized after an EPSP, additional inputs can sum to form a nerve impulse. 2. Nerve impulse(s) - If the entire excitatory effects exceed the total inhibitory effects and the threshold is reached, nerve impulses are triggered. As long as the EPSP is over threshold, impulses are created. 3. IPSP - Membrane hyperpolarization occurs when total inhibitory effects exceed excitatory effects. Postsynaptic neuron inhibition prevents nerve impulse generation. Neurotransmitters – Small-Molecule: - Neurotransmitters are endogenous chemicals that allow neurons to communicate throughout the body. - Small-molecule neurotransmitters include acetylcholine, amino acids, biogenic amines, ATP & other purines, nitric oxide, & carbon monoxide. - Acetylcholine – Released by PNS & some CNS neurons. It is often an excitatory neurotransmitter binding to ionotropic receptors opening cation channels. - It can also be an inhibitory neurotransmitter binding to metabotropic receptors coupled to G proteins that open K+ channels. - Example - ACh decreases heart rate at vagus (X) nerve parasympathetic neuron inhibitory synapses. ACh is inactivated by the enzyme acetylcholinesterase (AChE) by breaking it into acetate and choline fragments. - Amino acids – Several amino acids are neurotransmitters in the CNS. Glutamate (glutamic acid) & aspartate (aspartic acid) are excitatory neurotransmitters. - Glutamate synapses are prevalent in CNS opening cation channels (mainly Na+) producing an EPSP. - Gamma-aminobutyric acid (GABA) and glycine are inhibitory neurotransmitters. They open Cl- ions. GABA is found only in the CNS while half of inhibitory synapses in the spinal cord are by glycine. Anti-anxiety drugs such as diazepam (Valium) utilizes GABA. - Biogenic amines – Amino acids with carboxyl groups removed (decarboxylated) are referred to as biogenic amines. Examples – norepinephrine, epinephrine, dopamine, serotonin. - They bind to metabotropic receptors causing excitatory or inhibitory. Norepinephrine (NE) plays roles in arousal (awakening from deep sleep), dreaming, & regulating mood. Dopamine (DA) is active during emotional responses, addictive behaviors & pleasurable experiences. Also plays a role in muscle tone. Lack of dopamine neurons in Parkinson’s Disease causes muscle stiffness. - NE, DA & epinephrine are classified as catecholamines as they have an amino group (-NH2) & a catechol ring. - Catecholamines are synthesized from the amino acid, tyrosine. Reuptake of catecholamines are catalyzed by two enzymes catechol-O-methyltransferase (COMT) & monoamine oxidase (MAO). MAO inhibitors such as Isocarboxazid (Marplan) are anti-depression drugs. - Serotonin (5-HT; 5-hydroxytryptamine) is predominantly present in raphe nucleus and is involved in sensory perception, temperature regulation, control of mood, appetite, & sleep. - ATP & other purines – Various derivatives of adenosine such as triphosphate, diphosphate, & monophosphate (ATP, ADP & AMP) containing purine ring are excitatory neurotransmitters present both in CNS & PNS. In the PNS, ATP & NE are released from sympathetic neurons while parasympathetic neurons release ATP & ACh. - Nitric oxide – an excitatory neurotransmitter secreted in gaseous form in the brain, spinal cord, suprarenal glands, & nerves to the penis. Nitric oxide (NO) synthase (NOS) catalyzes the formation of NO from the amino acid arginine. NO is short-lived (10 sec) before being converted into oxygen & water to form inactive nitrates & nitrites. - Endothelial cells in blood vessel walls release NO, which diffuses into neighboring smooth muscle fibers & causes relaxation. This results in vasodilation, an increase in blood vessel diameter. This function is utilized in the drug (Sildenafil (Viagra)) - Carbon monoxide – an excitatory neurotransmitter produced in the brain in response to neuromuscular & neuroglandular functions. CO is related to dilation of blood vessels, memory, sense of smell, vision, thermoregulation, insulin release. Neurotransmitters – Neuropeptides: - Neurotransmitters consisting of 3 to 40 amino acids (large molecules) linked by peptide bonds are called neuropeptides. They bind to metabotropic receptors (G protein- coupled receptors) & have excitatory or inhibitory actions. - They are involved in a wide range of functions in the nervous system, including regulating pain perception, mood, stress response, appetite, sleep, and various physiological processes. - Neuropeptides are synthesized within neurons in the cell body and then transported to the nerve terminals. Unlike classical neurotransmitters, neuropeptides are often released slowly and can modulate neuronal function over more extended periods. - Several neuropeptides, such as substance P and enkephalins, are involved in the transmission and modulation of pain signals. - Neuropeptides like oxytocin and vasopressin play a role in regulating emotions, social bonding, and stress responses. - Neuropeptides such as ghrelin and leptin are involved in appetite regulation and metabolism. - Opioids target neuropeptide receptors to manage pain, but they also have the potential for abuse and addiction. Regeneration & Repair of Nervous Tissue: - Neural plasticity - also known as neuroplasticity, refers to the brain's ability to reorganize and adapt by forming new neural connections throughout a person's life. It involves changes in the strength and efficiency of existing neural pathways and the formation of new ones. Neural plasticity involves changes in synaptic connections between neurons, including synaptic strengthening (long-term potentiation) and weakening (long-term depression), as well as structural changes in neurons. - Regeneration is the process of regrowing or replacing damaged or lost tissues or body parts with new, healthy tissue. - Neurogenesis in the CNS - Neurogenesis in the Central Nervous System (CNS) refers to the process of generating new neurons within the brain and spinal cord. Neurogenesis primarily occurs in two regions of the CNS: the hippocampus (related to learning & memory) and the olfactory bulb (related to smell). - Neural stem cells (NSCs) are the precursor cells responsible for generating new neurons in the CNS. These cells utilize hormonal-like proteins, epidermal growth factor (EGF). - Lack of neurogenesis in other regions of brain & spinal cord are due to: a) Inhibitory influences from neuroglia, particularly from oligodendrocytes b) Absence of growth-stimulating cues that were present during fetal development. - That axons in the CNS are myelinated by oligodendrocytes rather than schwann cells, is the main factor in inhibition of neurogenesis. In addition, astrocytes (phagocytes in the CNS) rapidly proliferate forming a type of scar tissue that acts as a barrier to regeneration. - Neurogenesis in the peripheral nervous system (PNS) – axons & dendrites that are associated with a neurolemma may undergo repair if the cell body is intact, if the Schwann cells are functional, & if scar tissue formation does not occur too rapidly. Most PNS nerves have neurilemma-covered processes, and injury to axons in, for example, an upper limb, offers a good chance of recovery. - When axon damage occurs, changes affect the cell body, the axon distal to the injury site, and possibly the proximal axon. - Around 24 to 48 hours after peripheral neuron process injury, Nissl bodies break down into granular masses (chromatolysis). By the third to fifth day, the distal axon swells and fragments, and the myelin sheath deteriorates (Wallerian degeneration), while the neurilemma remains. - After chromatolysis, cell body recovery signs appear. Macrophages remove debris, and RNA and protein synthesis accelerates, aiding axon rebuilding. Schwann cells multiply and form a regeneration tube if the injury site allows, guiding new axon growth from the proximal area to the distal region. However, large gaps or collagen fiber filling can impede axon regrowth.