Chapter 12: Nervous Tissue --controls and integrates all body actvities within limits that maintain life --3 Basic Functions a. Sensing changes with sensory receptors (fullness of stomach or sun on face) b. interpreting and remembering those changes c. reacting to those changes with effetors (muscular contractions, glandular excretions) --Organization of the Nervous Sysme *CNS is brain and spinal cord *PNS is everything else a. consists of cranial and spinal nerves that contain both sensory and motor fibers b. connects CNS to muscles, glands, and all sensory receptors c. 3 subdivisions i. Somatic/voluntary nervous system (SNS) ~sensory neurons from cutaneous, skeletal muscle, and tendons, and special sensory receptors to the CNS ~motor neurons to skeletal muscle tissue ii. Autonomic/involuntary nervous system (ANS) ~sensory neurons from visceral organs to CNS ~motor neurons to smooth and cardiac muscle and glands --symapthetic division (speeds up heart rate) --parasympathetic division (slow down heart rate) iii. Enteric nervous system (ENS) ~involuntary sensory and motor neurons control GI tract ~neurons fuction independently of ANS and CNS --Neurons *functional unit of nervous system *have capacity to produce action potentials electrical excitability (muscles too) *Cell body a. single nucleus with prominent nucleolus b. rough ER (Nissl bodies) c. free ribosomes for protein synthesis d. neurofilaments give cell shape and support e. microtubules move material inside cell f. lipofuscin pigment clumps (harmless aging) *Dendrites a. conduct impulses towards the cell body b. Typically: numberous, short, highly branched, unmyelinated c. surfaces specialized for contact with other neurons (synapse) d. contain neurofibrils *Axons a. conduct impulses away form cell body b. single, long, thin cylindrical process of cell, may be myelinated c. arises at axon hillock d. impulses arise from initial segment (trigger zone) e. side branches (collaterals) f. end in fine processes called axon terminals g. swollen tips called synaptic end bulbs contain vesicles filled with NTs
--Axonal Transport *cell body is location for most protein synthesis NTs and repair proteins *Axonal transport system moves substances a. slow axonal flow: movement in one direction only away from cell body = anterograde, movement at 1-5mm per day b. fast axonal flow: moves organelles and materials along surface of microtubules at 200-400 mm/day --transports in either direction (anterograde and retrograde) --for use or for recycling in cell body *Fast axonal transport route by which toxins or pathogens reach neuron cell bodies a. tetanus (Clostridium tetani bacteria) b. disrupts motor neurons causing painful muscle spasms *Bacteria enter the body through a laceration or puncture injury a. more serious if wound is in head or neck because of shorter transit time --Summary of Neuronal Structure and Function Structure Dendrites Cell Body Junction of axon hillock and initial segment of axon Axon Axon terminals and synaptic end bulbs (or varicosities) Functions -Receive stimuli through activation of ligand-gated or mechanically gated channels -In sensory neurons, produce generator or receptor potentials -In motor neurons and interneurons, produce excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) -Receives stimuli and produces EPSPs and IPSPs throught activation of ligand-gated and mechanically gated ion channels -Trigger zone in many neurons; integrates EPSPs and IPSPs and if sum is a depolarization that reaches threshold, initiates AP Propagates nerve impulses from initial segment or from dendrites of sensory neurons to axon terminals in a self-reinforcing manner; impulse amplitude does not change as it propagates along the aon -have voltage gated Na + and K + channels -Inflow of Ca++ caused by depolarizing phase of nerve impulse triggers exocytosis of NT from synaptic vesicles -Voltage gated Ca++ channels --Functional Classification a. Sensory/afferent neurons: transport sensory information from skin, muscles, joints, sense organs and viscera to CNS (arriving) b. Motor/efferent neurons: send motor nerve impulses to muscles and glands (exiting) c. Interneurons/association neurons: connect sensory to motor neurons, 90% of neurons in the body (exclusively in CNS) most are multipolar --Structural Classification a. Multipolar: several dendrites and one axon *most common cell type b. Bipolar: one main dendrite and one axon *found in retina, inner ear, and olfactory c. Unipolar: one process only (develops from a bipolar) *are always sensory neurons
--Neuroglial Cells *half of the volume of the CNS *smaller cells than neurons *50x more numerous *cells can divide: rapid mitosis in tumor formation (gliomas) *4 cell types in CNS a. astrocytes ~star-shaped cells ~their processes extend into the basement membrane of capillaries and pia mater ~Functions: i. form blood brain barrier by covering blood capillaries ii. metabolize NTs iii. regulate K + balance iv. provide nutrients and structural support b. oligodendrocytes ~most common glial cell type ~analogous to Schwann cells of PNS provide electrical insulation ~each oligodendrocyte wraps around more than one axons in PNS c. microglia ~small cells found near blood vessels ~phagocytic role: clear away dead cells (brain macrophages) ~derived from cells that also gave rise to macrophages and monocytes d. ependymal ~form epithelial membrane lining cerebral cavities (ventricles) and central canal ~produce CSF (cerebrospinal fluid) *2 cell types in PNS a. Schwann cells ~cells encircling PNS axons ~each cell produces part of the myelin sheath surrounding an axon (only 1) in the PNS b. satellite cells ~flat cells surrounding neuronal cell bodies in peripheral ganglia ~support neurons in the PNS ganglia --Axon Coverings in the PNS *axons surrounded by a lipid and protein covering (myelin sheath) produced by Schwann cells *Neurolemma is cytoplasm and nucleus of Schwann cell: gaps called nodes of Ranvier *Myelinated fibers appear white a. jelly roll like wrappings made of lipoprotein = myelin b. acts as electrical insulator c. speeds conduction of nerve impulses *Unmyelinated fibers a. slow, small diameter fibers b. only surrounded by neurilemma but no myelin sheath wrapping *Myelination in PNS a. Schwann cells myelinate axons in the PNS during fetal development
b. Schwann cell cytoplasm and nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath c. Tube guides growing axons that are repairing themselves --Myelination in the CNS *oligodendrocytes myelinate axons in the CNS *broad, flat cell processes wrap around CNS axons, but the cell bodies d/n surround the axons *no myelin sheath is formed, no neurolemma *little regrowth after injury is possible due to the lack of a distinct tube or neurolemma --Gray and White Matter *white matter: myelinated processes (white in color) *gray matter: nerve cell bodies, dendrite, axon terminals, bundle o unmyelinated axons and neuroglia (gray color) a. in the spinal cord: gray matter forms an H shaped inner core surrounded by white matter (centrally located) b. In the brain: a thin outer shell of gray matter covers the surface and is found in clusters called nuclei inside the CNS (outer location) --Electrical Signals in Neurons *neurons are electrically excitable due to the voltage difference across their membrane *communicate with 2 types of electric signals a. action potentials that can travel long distances (always depolarizing) b. graded potentials that are local membrane changes only (proportionate to strength of stimulus, can be depolarizing and hyperpolarizing, found in dendrites and cell bodies) *in living cells, a flow of ions occurs through ion channels in the cell membrane --2 Types of Ion Channels a. Leakage (nongated) channels: always open *nerve cells have more K+ than Na+ leakage channels *as a result, membrane permeability to K+ is higher *explains resting membrane potential of -70mV in nerve tissue b. Gated channels: open and close in response to a stimulus and play role in neuron excitability *Voltage-gated: open in response to change in voltage *Ligand-gated: open and close in response to particular chemical stimuli (hormone, NT, ion) *Mechanically-gated: open with mechanical stimulation --Resting Membrane Potential *Negative ions along inside of cell membrane and positive ions along outside a. potential energy difference at rest is -70mV b. cell is polarized *Resting potential exists because a. concentration of ions is different inside and outside ~extracellular fluid rich in Na+ and Cl- ~cytosol full of K+, organic phosphate and amino acids b. membrane permeability differs for Na+ and K+ ~50-100 greater permeability for K+ than for Na+ ~inward flow of Na+ can t keep up with outward flow of K+ ~Na+/K+ pump removes Na+ as fast as it leaks
--Graded Potentials *Small deviations from resting potential of -70mV a. hyperpolarization: membrane has become more negative b. depolarization: membrane has become more positive c. occur most often in dendrites and cel body of a neuron *source of stimuli a. Mechanical stimulation of membranes with mechanical gated ion channels (pressure) b. Chemical stimulation of membranes wit ligand gated ion channels (NT) c. Voltage change: graded/postsynaptic/receptor or generator potential *Characterisitics of graded potentials: a. ions flow through ion channels and change membrane potential locally b. amount of change varies with strength of stimuli (not so for APs), aplitude expains strength c. Potential amplitude decreases over distance, dies out (no so for APs) --Action Potentials a. series of rapidly occurring events that change and then restore the membrane potential of a cell to its resting state b. Ion channels open, Na+ rushes in (depolarization), K+ rushes out (repolarization) c. All or none principal: with stimulation, either happens one specific way or not at all (lasts 1/1000 of a second) d. travels (spreads) over surface of cell without dying out --Phases of Action Potential a. Depolarizing Phase *Chemical or mechanical stimulus caused a graded potential to reach at least -55mV or threshold *Voltage-gated Na+ channels open and Na+ rushes into cell: ~in resting membrane, inactivation gate of sodium channel is open and activation gate is closed (Na+ c/n get in) ~when threshold (-55mV) is reached, both are open and Na+ enters ~inactivation gate closes in few ten-thousandths of a second ~only a total of 20,000 Na+ actually enter the cell, but they change the membrane potential considerably (up to +30mV) *positive/viscous feedback process b. Repolarizing Phase *when threshold potential of -55mV is reached, voltage-gated K+ channels open *K+ channel opening is much slower than Na+ channel opening *When K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops) *K+ outflow returns membrane potential to -70mB (repolarization) *If enough K+ leaves the cell, it will reach a -90mV membrane potential and enter the after-hyperpolarizing phase *K+ channels close and the mem potential returns to the resting potential of -70mV c. Refractory Period *Period of tiem during which neuron can not generate another AP *Absolute refractory period ~even very strong stimulus will not initiate another AP ~inactivated Na+ channels must return to the resting state before they can
be reopened ~large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible *Relative refractory period ~a suprathreshold stimulus will be able to start an AP ~K+ channels are still open, but Na+ channels have closed --Local Anesthetics *Prevent opening of voltage-gated Na+ *Nerve impulses c/n pass the anesthetize region *Ex: novocaine and lidocaine --Propagation of Action Potential *An action potential spreads/propagates over the surface of the axon membrane a. as Na+ flows into the cell during depolarization, the voltage of adjacent areas is affected and their voltage-gated Na+ channels open b. self-propagating along the membrane (unlike graded potential) c. AP travels at constant speed and its magnitude is constant (same size regardless of stimulus strength, unlike graded potential) *The traveling action potential is called a nerve impulse --Continuous versus Saltatory Conduction a. Continuous conduction: unmyelinated fibers *step-by-step depolarization of each portion of the length of the axolemma b. Saltatory conduction: jumping, myelinated fibers *depolarization only at nodes of Ranvier where there is a high density of voltage- gated ion channels *current carried by ions flows through extracellular fluid from node to node *faster speed of propagation --Comparison of Graded and Action Potentials Trait Graded Potentials Action Potentials Origin Arise on dendrites and cell bodies Arise only at trigger zone on axon hillock Types of Channels Produced by ligand or mechanicallygated Produced by voltage-gated ion channels channels Conduction Localized (not propagated, die out Conduct over the entire length of axon with distance) Amplitude Vary depending upon stimulus Constant, nondecremental Polarity of potentials Depolarizing or hyperpolarizing Only depolarizing Duration As long as the stimulus lasts, thus Have refractory period, no summation summation is possible Refractory period None Refractory period due to the nature of the noltage-gated channels --Encoding of Stimulus Intensity *How do we differentiate a light touch from a firmer touch? a. frequency of impulses: firm pressure generates impulses at a higher frequency b. number of sensory neurons activated: firm pressure stimulates more neurons than does a light touch --Speed of Impulse Propagation
to *The propagation speed of a nerve impulse (AP) is not related to stimulus strength ~larger, myelinated fibers conduct impulses faster due to sie and salutatory conduction (recall form physics that resistance is inversely proportional diameter) *Fiber types a. A fibers: largest and fastest (5-20 um and 130 m/sec), myelinated, somatic sensory and motor to skeletal muscle b. B fibers: medium (2-3 um and 15 m/sec), myelinated, visceral sensory and autonomic preganglionic c. C fibers smallest (.5-1.5 um and 2 m/sec), unmyelinated, sensory and autonomic postganglionic motor Fiber Type Speed (m/sec) Function A α (I) 80-120 Motor neurons, afferents form muscle spindles and tendon organs A β (II) 60 Tactile from skin A γ 40 Efferent to muscle spindles and from mechanoreceptors A δ (III) 20 Temperature, pain (in skin, fast) B 10 Preganglionic autonomic C (IV) 1 Postganglionic autonomic, visceral afferents (temp, pain, slow) --Action Potentials in Nerve and Muscle *Entire muscle cell membrane versus only the axon of the neuron is involved *Resting membrane potential a. nerve is -70mV b. skeletal and cardiac muscle is closer to -90mV (SA node is -60mV) c. smooth muscle varies but often around -60mV *Duration a. nerve impulse is ½ to 2 msec b. muscle action potential lasts 1-5 msec for skeletal and 10-300 msec for cardiac and smooth *Fastest nerve conduction velocity is 18x faster than velocity over skeletal muscle fiber --Signal Transmission at Synapses *2 types of synapses a. Electrical: ~ionic current spreads to next cell through gap junctions ~faster ~two way transmission and capable of synchronizing groups of neurons b. Chemical: ~one-way information transfer from a presynaptic neuron to a postsynaptic neuron ~slower ~neurotransmitter released: axodendritic, axosomatic, axoaxonic --Chemical Synapses *Action potential reaches end bulb and voltage-gated Ca++ channels open *Ca++ flows inward triggering release of NT *NT crosses synaptic cleft and binds to ligand-gated receptors (the more NT released the
greater the change in potential of the postsynaptic cell) *Synaptic delay is 0.5 msec *one way information transfer --Excitatory and Inhibitory Postsynaptic Potentials (EPSP, IPSP) *the effect of a NT can be either excitatory or inhibitory a. a depolarizing postsynaptic potential is called an EPSP ~it results from the opening of ligand-gated Na+ channels ~the postsynaptic cell is more likely to reach threshold b. an inhibitory postsynaptic potential is called an IPSP ~it results from the opening of ligand-gated Cl- or K+ channels ~it causes the postsynaptic cell to become more negative or hyperpolarized ~the postsynaptic cell is less likely to reach threshold --Removal of Neurotransmitter a. Diffusion: move down concentration gradient b. Enzymatic degradation: acetylcholinesterase c. Uptake by neurons or glia cells: NT transporters (prozac serotonin reuptake inhibitors) --Integration of information at single neuron level a. multiple inputs on a single neuron, both excitatory or inhibitory b. amount of neurotransmitter released and length of its presence c. timing of arriving stimuli d. summation of incoming information e. integration f. decision for outcome (to fire or not) --Spatial summation *summation of effects of neurotransmitters simultaneously released from several end bulbs onto one neuron (graded potential) --Temporal Summation *summation of effect of neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuron (looking at 1 synapse) *a single neuron receives numerous, excitatory and inhibitory, inputs. It integrates the information (sums it up) and decides whether to respond or not --Three Possible responses a. EPSP occurs: potential reaches -56mV only (below threshold)-no AP generated b. An impulse is generated: threshold was reached, membrane potential of at least -55mV c. IPSP occurs: membrane hyperpolarized, potential drops below -70mV --Strychnine Poisoning *In spinal cord, Renshaw cells normally release an inhibitory NT (glycine) onto motor neurons preventing excessive muscle contraction *Strychnine binds to and blocks glycine receptors in the spinal cord *Massive titanic contractions of all skeletal muscles are produced: when the diaphragm contracts and remains contracted, breathing c/n occur --Neurotransmitter Effects *NT effects can be modified: a. synthesis can be stimulated or inhibited b. release can be blocked or enhanced c. removal can be stimulated or blocked d. receptor site can be blocked or activated
*Agonist: anything that enhances a transmitters effects *Antagonish: anything that blocks the action of a NT Neurotransmitters --Neurotransmitters 2classes: 1. Small Molecules a. ACh b. Amino acids: GABA, gly, glu, asp c. Amines: (nor)epinephrine, dopamine, serotonin, histamine d. NO 2. Neuropeptides a. hypothalamic-releasing hormones b. pituitary peptides c. dual location/action peptides d. from other tissues --Characteristics of small molecule transmitters a. fast acting b. involved in transmission of acute signals/responses c. synthesized in cytosol of neuron terminal d. released within a ms or less e. vesicles are recycled containing all the machinery for synthesis/uptake/concentration of new NT f. interact with post-synaptic neuron within a msec g. most often activated an ion channel in the post-synaptic memebrane, occasionally activate a receptor h. most often cause excitation/inhibition, rarely may affect cellular metabolic pathways --Characteristics of neuropeptides a. larger molecules b. sites of synthesis and release different anatomical sites c. synthesized in cell body, processed (precursors split to smaller peptides and packaged into vesicles in Golgi) d. transported to terminal by axonal streaming = slow flow of cytoplasm (few cm/day) e. vesicles are autolyzed (not reused) f. lower amounts relased (potent chem. messenger) g. much more potent than small molecule h. activate a receptor on target cell (rarely open ion channels) i. cause prolonged action: change cell metabolism, gene expression; effects last from days to years --Small Molecule NTs a. Acetylcholine (ACh): released by many PNS and some CNS *PNS: 1. all motor neurons activating skeletal muscle 2. all preganglionic neurons of the ANS 3. postganglionic fibers of the parasympathetic ANS (and some postganglionic sympathetic neurons) *CNS:
1. at synapses involved in the acquisition of short-term memory. Drugs that enhance ACh levels are used in elderly patients with failing memory) 2. excitatory on NMJ but inhibitory at others 3. inactivated by acetylcholinesterase b. Amino Acids *Glutamate 1. released by nearly all EXCITATORY neurons in the BRAIN 2. inactivated by glutamate specific transporters 3. essential for loner term potentiation (long term memory pathways) 4. leads to depolarization by binding to: --ligand gated Na+ channels (FAST) --receptors that activate second messengers which open Na+ channels (SLOW) *GABA 1. Is INHIBITORY for 1/3 of all BRAIN synapses (Valium is a GABA agonist enhancing its inhibitory effect) 2. GABA hyperpolarizes the postsynaptic membrane by binding to: --GABAA receptors on the postsynaptic neuron opens up ligand- gated Cl- channels (FAST, about 1 ms) --GABAB receptors activates G protein and a second messenger that leads to the opening of K+ channels (SLOW, as long as 1 sec) *Glycine 1. is INHIBITORY 2. mainly in the SPINAL CORD (CNS) c. Biogenic Amines (modified amino acids) *Norepinephrine 1. regulates mood, dreaming, awakening from deep sleep 2. mostly EXCITATORY in BRAIN STEM and hypothalamus 3. found in postganglionic sympathetic neurons *Dopamine 1. in CNS 2. usually INHIBITORY 3. regulates skeletal muscle tone **Norepinephrine, epinephrine, and dopamine are derived from tyrosine and are collectively known as catecholamines *Serotonin 1. mostly INHIBITORY 2. precursor is tryptophan 3. control of mood, temperature regulation, and induction fo sleep 4. removed from synapse and recycled or destroyed by enzymes (monoamine oxidase MAO or catechol-o-methyltransferase COMT) d. ATP and other purines (ADP, AMP, and adenosine) 1. EXCITATORY in BOTH PNS AND CNS 2. released with other NTs (ACh and NE) e. Gases: Nitric Oxide (NO) 1. formed from the amino acid arginine 2. formed on demand and acts immediately: produced on demand, not sotored in
presynaptic vesicles, diffuses out of cell that produced it, diffuses into neighboring cells, changes metabolism (not mem. potential) 3. first recognized as vasodilator that helps lower blood pressure --Neuropeptides *3-40 amino acids linked by peptide bonds a. Substance P: enhances our perception of pain (originally found in GI) b. Pain Relief 1. enkephalins: pain-relieving effect by blocking the release of substance P 2. acupuncture, meditation may produce loss of pain sensation b/c of release of opioids-like substances such as endorphins or dynorphins c. Comparison of Neuropeptides Substance Substance P Enkephalins Endorphins Dynorphins Description Found in sensory neurons, spinal cord pathways, and parts of brain associated with pain; ENHANCES PERCEPTION OF PAIN INHIBIT PAIN IMPULSES BY SUPPRESSING RELEASE OF SUBSTANCE P, may have a role in memory and learning, control of body temp, sexual activity, and mental illness INHIBIT PAIN BY BLOCKING RELASE OF SUBSTANCE P, may have a role in memory and learning, sexual activity, control of body temp, and mental illness May be related to controlling pain and registering emotions Hypothalamic releasing and Produced by the hypothalamus; regulate the release of hormones by the inhibiting hormones anterior pituitary Angiotensin II STIMULATES THIRST; MAY REGUALTE BLOOD PRESSURE IN THE BRAIN as a hormone causes vasoconstriction and promotes release of aldosterone, which increases the rate of salt and water reabsorption by the kidneys CCK FOUND IN THE BRAIN AND SMALL INTESTINE, may regulate feeding as a stop eating signal, as a hormone regulates pancreatic enzyme secretion during digestion and contraction of smooth muscle in the GI tract --Neuron Circuits *neurons in the CNS are organized into neuronal networks *a neuronal network may contain thousands or even millions of neurons *neuronal circuits are involved in may important activities (breathing, short term memory, waking up) *Types: a. Diverging: single cell stimulates many others (motor units) ~Divergence into a single tract: amplication of signal output (corticospinal tract a single pyramidal cell can excite up to 10,000 muscle fibers) ~Divergence into multiple tracts: send info to several areas (dorsal columns diverge into cerebellum and to the thalamus) b. Converging: one cell stimulated by many others (detect weak stimuli) ~spacial summation from a single source or from multiple sources (where the inputs can be both excitatory and inhibitory interneurons in the spinal cord receive info form afferents, proprio, motor cortex)
~can be temporal summation too c. Reverberating: impulses from later cells repeatedly stimulate early cells in the circuit (short term memory) d. Parallel-after-discharge: single cell stimulates a group of cells that all stimulate a common postsynaptic cell (math problems) --Organization of Neuronal Pools (More than 1 neuron and their connections) * input and output * stimulatory field : neuronal area stimulated by each incoming nerve fiber *excitation: a postsynaptic neuron fires only if enough excitatory stimuli are present (temporally or spatially summated) = reach threshold *facilitation: signals that d/n add up to threshold but still depolarize the postsynaptic membrane are subthreshold a. dischard/excited/liminal zone: excite b. facilitated/subthreshold/subliminal zone: inhibitory c. threshold-excitation and subthreshold-facilitation *a single discharge from a presynaptic neuron almost never cause an AP in a postsynaptic neuron --Continuous Signal Output a. intrinsic neuronal excitability b. continuous signal emitted from a reverberating circuit c. rhythmical signal output (respiration, scratching reflex of a dog/cat hind leg; almost all are reverberating circuits; can change frequency and amplitude of signal output) --circuit s output lasts longer than the stimulus prolongation of a signal: 1. Synaptic after-discharge: as long as graded depolarization is at threshold (as with some long lasting NTs), the neuron will discharge APs 2. Reverberatory/oscillatory circuits a. one neuron only b. several neurons longer delay c. both, facilitatory and inhibitory feedback d. reverberatory circuit with many parallel fibers can have gradations b/w weak and strong reverberating signal depending on how many of the parallel fibers are involved e. reverberation stops suddenly when a component drops below a critical level --Most neurons have basal rate of discharge and thus, can signal both excitatory and inhibitory information a. excitatory input greatly increases signal output b. inhibitory input decreases output, thus decrease in output intensity is in itself a signal output --Inhibitory circuits as a mechanism for stabilizing nervous system function a. Types 1. Inhibitory feedback: from circuit termini to initial excitatory neurons 2. Synaptic fatigue *Short term: depletion of limited NT *Long term: up/down regulation of synaptic receptor --Epilepsy: second most common neurological disorder (affects 1% of population) *characterized by short, recurrent attacks initiated by electrical discharges in the brain a. lights, noise, or smells may be sensed triggers
b. skeletal muscles may contract involuntarily c. loss of consciousness *epilepsy has many causes including brain damage at birth, metabolic disturbances, infections, toxins, vascular disturbances, head injuries, and tumors --Regeneration and Repair *Plasticity maintained throughout life (sprouting of new dendrites, synthesis of new proteins, changes in synaptic contacts with other neurons) *Limited ability for regeneration (repair) a. PNS can repair damaged dendrites or axons b. CNS no repairs are possible --Neurogenesis in the CNS *formation of new neurons from stem cells was not thought to occur in humans a. 1992: a growth factor was found that stimulates adult mice brain cells to multiply b. 1998: new neurons found to form within adult human hippocampus *factors preventing neurogenesis in CNS a. inhibition by neuroglial cells, absence of growth stimulating factors, lack of neurolemmas, and rapid formation of scar tissue --Repair within the PNS *axons and dendrites may be repaired if: a. neuron cell body remains intact b. Schwann cells remain active and form a tube c. scar tissue does not form too rapidly *Chromatolysis: a. 24-48 hours after injury, Nissl bodies break up into fine granular masses b. 3-5 days: wallerian degeneration occurs 9breakdown of axon and myelin sheath distal to injury), retrograde degeneration occurs back one noe c. within several months, regeneration occurs: neurolemma on each side of injury repairs tube (Schwann cell mitosis), axonal buds grow down the tube to reconnect (1.5 mm/day) --Multiple Sclerosis (MS) *Autoimmune disorder causing destruction of myelin sheaths in CNS a. sheaths become scars or plaques b. ½ million people in US c. appear b/w ages of 20 and 40 d. females twice as often as males *symptoms include muscular weakness, abnormal sensations or double vision *remissions and relapses result in progressive, cumulative loss of function