Ch.12 Nervous System

functional significance of segmented myelin sheath

**segmentation allows for nodes of Ranvier where new APs are generated to travel through internodes -w/out segmentation, an AP wouldn't make it through an entire axon** nerve fiber is too long for single Schwann cell or oligodendrocyte, so multiple cells contribute to myelin sheath internodes are myelinated sections and nodes of Ranvier are gaps between internodes axon hillock: location of generation of first action potential; works together with combined area called "trigger zone", including axon hillock and initial segment -initial segment is short section of nerve fiber b/w axon hillock and first glial cell; where APs are initiated; much higher density of voltage-gated ion channels here compared to rest of neural soma (unmyelinated section)

Action potential generation process

1) Local potential arrives at trigger zone and depolarizes membrane, causing a steadily rising LP. 2) If LP reaches threshold (-55mV), it opens the numerous voltage-gated ion channels at trigger zone. 3) All-or-none AP generated. Voltage-gated Na+ channels open quickly (initially only a few but more Na+ flowing in further depolarizes cell and causes the rest to open). Membrane voltage rises rapidly. -K+ channels are also beginning to open but very slowly so they don't have an effect currently (but will later in repolarization) 4) Once potential passes 0mV, Na+ channels begin closing, but by the time they all close, the voltage has reached about +35mV. 5) Slow K+ channels are now fully open, and K+ flows out of cell (bc of positive voltage). Causes repolarization back in negative direction. 6) Slow K+ channels stay open too long, so they actually cause hyperpolarization below RMP (by just a few mV). 7) Membrane gradually and slowly returns back to RMP by diffusion of Na+ into cell through conc. gradient. -CNS may aid in this be removing extracellular K+ by astrocytes) -Hyperpolarization stage is longest; otherwise very quick action Absolute refractory period: from generation of AP until voltage reaches RMP briefly, right before hyperpolarization. Relative Refractory period: during hyperpolarization, until steady RMP reestablished.

Tay-Sachs disease

A human genetic disease caused by a recessive allele that leads to the accumulation of certain lipids in the brain. Seizures, blindness, and degeneration of motor and mental performance usually manifest a few months after birth. hereditary disorder seen mainly in infants of Eastern European Jewish descent results from abnormal accumulation of glycolipid GM2 in myelin sheath -GM2 is normally broken down by lysosomal enzyme, but this enzyme is lacking in people who are homozygous recessive for Tay-Sachs allele as GM2 accumulates, it disrupts conduction of nerve signals and person suffers from blindness, loss of coordination, and dementia -inability to break down GM2 leads to build up of GM2 in lysosomes (bulging in pic), which causes cell death signs begin appearing in 1st year of life and victim dies by age 3 or 4

how signal is transmitted in a chemical synapse

AP arrives at axon terminal and causes calcium to flow into cell calcium causes synaptic vesicles to exit cell via exocytosis, releasing their neurotransmitter into the cleft

major classes of neurotransmitters

Acetylcholine (ACh): formed from acetic acid and choline; functions in NMJ and some CNS pathways, usually excitatory (skeletal muscle) but could also inhibit cardiac muscle -botulism can block release Amino acids: amino acids or slightly modified amino acids; includes glycine, glutamate, aspartate, and GABA; function in CNS pathways and can be excitatory (glutamate) or inhibitory (GABA) -GABA is most common inhibitory neurotransmitter in brain; functions in thalamus, hypothalamus, cerebellum, occipital lobe, and retina Monoamines: synthesized from amino acids through removal of -COOH group; includes epinephrine, norepinephrine, dopamine, histamine, and serotonin -Epinephrine, norepinephrine, and dopamine all part of single catecholamines group synthesized from tyrosine (makes dopamine and then dopamine can make other two) -Epinephrine and Norepinephrine: function in sympathetic nervous system and CNS pathways; involved in dreaming, waking, and mood; can excite cardiac muscle and excite/inhibit smooth muscle -Dopamine: functions in many CNS pathways in brain and is primarily excitatory; involved in elevation of mood and control of skeletal muscles -Serotonin: functions in many CNS pathways and can be inhibitory or modulatory; involved in sleepiness, alertness, thermoregulation, and mood -Histamine and serotonin in separate monoamine group and are synthesized from different amino acids -ritalin, antidepressants, cocaine, amphetamines, and ecstasy can increase release or block reuptake Neuropeptides: chains of 2-40 amino acids; includes CKK and endorphins; stored in secretory vesicles much larger than synaptic vesicles and can function as hormones or neuromodulators -endorphins suppress pain and can reduce perception of fatigue; "runner's high" Purines: includes adenosine and ATP Gases: include nitrous oxide (NO) and carbon monoxide (CO); synthesized as needed rather than stored in vesicles and simply diffuse out of axon terminal instead of being released through exocytosis; diffuse into target cells instead of binding to receptors -can have effects on large groups of cells

Overview of nervous system

CNS: contains brain and spinal cord PNS: contains sensory (afferent) and motor(efferent) division, each with visceral and somatic divisions based off what parts of body are involved -Sensory Divison: carries signals from various sensory receptors to CNS -Motor Division: carries signals from CNS to various sensory receptors; somatic subdivision carries signals to skeletal muscles; visceral subdivision is also called autonomic nervous system and carries signals to glands, cardiac and smooth muscles, usually unconsciously and involuntarily ANS: Sympathetic and parasympathetic divisions -Sympathetic: arouses body for action; fight or flight; inhibits digestion -parasympathetic: calms body and stimulates digestion

Electrical and chemical synapses

Electrical: ions flow directly through small synaptic gap through junction channels from presynaptic neuron to postsynaptic neuron -two neurons connected by small proteins and synaptic gap is small -quick transmission bc no delay for release and binding of neurotransmitter, but no time to integrate info and make decisions -uncommon; may be found among neuroglia, cardiac muscle, and single-unit smooth muscles -important for synchronizing activity of local suites of neurons in certain regions of brain Chemical: ions in presynaptic neuron cause neurotransmitter to be released and travel across gap to postsynaptic neuron -slower but allows for time to integrate info and make decisions -more common -synapse is much wider than electrical synapse -site of learning and memory, target of many prescription drugs, and site of action of addictive drugs

monoamine neurotransmitters

Epinephrine/Norepinephrine: function in sympathetic nervous system and CNS pathways; can excite cardiac muscle or excite/inhibit smooth muscle; involved in dreaming, waking, and mood Dopamine: functions in many CNS pathways in brain and is primarily excitatory; involved in elevation of mood and control of skeletal muscle Serotonin: functions in many CNS pathways and can be inhibitory or modulatory; involved in sleepiness, alertness, thermoregulation, and mood Epinephrine, norepinephrine, and dopamine all part of single catecholamines group synthesized from tyrosine (makes dopamine and then dopamine can make other two) Histamine and serotonin in separate monoamine group and are synthesized from different amino acids many act through secondary messengers like cAMP ritalin, antidepressants, cocaine, amphetamines, and ecstasy can increase release or block reuptake

4 characteristics distinguishing LPs from APs

Graded: LPs can vary in voltage depending on strength of stimulus. Intense and prolonged stimuli open more ion-gated channels and make them stay open longer, causing a greater depolarization. Decremental: LPs get weaker as they spread from origin. Happens bc of ion leakage (Na+ and K+ leaving) and also cytoplasmic resistance Reversible: if stimulus ceases, cell quickly returns to RMP Excitatory OR Inhibitory: Excitatory LPs depolarize a cell, making an AP more likely (i.e. ACh). Inhibitory LPs hyper polarize a cell, making it more negative and making an AP less likely.

comparison of APs and LPs

LPs are graded, decremental, reversible, and excitatory or inhibitory -also generated anywhere in dendrites or soma and are local APs are all-or-none, nondecremental, irreversible, and always start with depolarization (excitatory) -only generated at trigger zone and are self-propagating, having effects over long distances

Concentration gradients and Na-K pump in RMP

Na+ leaks into cell slightly and K+ leaks out of cell greatly -both are moving down their concentration gradients in doing so Na-K pump tries to compensate for this movement by pumping K+ back into cell and pumping Na+ back out -However, pump pumps more Na+ (3) out than K+ (2) it pumps in, leading to a net negative RMP of about -70mV pump consumes 1 ATP per cycle, accounting for 70% of energy use of nervous system -pump works continually to maintain equilibrium RMP

Cessation of Stimulation

Occurs through two mechanisms: cessation of the signals and clearing of remaining neurotransmitters Cessation of signals: cell stops releasing new neurotransmitters into cleft Clearing Neurotransmitter: 1)Neurotransmitter degradation: enzyme in synaptic cleft breaks down remaining neurotransmitters into fragments 2) Reuptake in synaptic knob: neurotransmitters or their fragments are reabsorbed by transport proteins in axon terminal, removing them from synapse and ending stimulatory effect. Fragments recycled. 3) Diffusion from synaptic cleft: neurotransmitters or their breakdown products simply diffuse away from synapse into ECF -Astrocytes in CNS and satellite cells in PNS can absorb stray neurotransmitters and return them to presynaptic neurons. Functional significance is that it allows for rapid successive actions by neurons. Avoids wasting energy and makes body more prepared to act.

Purpose of myelinated and unmyelinated fibers in PNS

PNS motor functions that require very quick conduction (like ANS sympathetic division) would be completely myelinated, but this takes up lots of room, so other not so important functions may be unmyelinated and just have single neurilemma wrapping from Schwann cell can't afford to myelinate all nerve fibers, so some just have single neurilemma from Schwann cell

Nissl bodies

Rough endoplasmic reticulum in neuron limited to soma, doesn't extend to axon

myelination in PNS

Schwann cells in PNS myelinate nerve fibers Schwann cells spiral repeatedly around a nerve fiber, laying down new layers over old layers; spiral outward (centrifugal myelination) outermost layer is a very thick coil called neurilemma (bc all of cytoplasm is pushed to end as myelin is being wrapped around nerve fiber, so almost all cytoplasm is in last layer called neurilemma) -contains nucleus of Schwann cell -external to neurilemma is basal lamina and endoneurium satellite cells cover soma of neuron

CNS vs PNS regeneration

Schwann cells, endoneurium, and neurilemma are required for regeneration. CNS lacks these and thus cannot regenerate. CNS is more protected than PNS and less susceptible to injury than PNS so this is ok. even for PNS, process of regeneration is not perfect. -Cell may die -Cell may connect to incorrect target cell. -Can take very long time (up to 2 years) -Usually functional deficit afterwards

Stronger stimuli in AP generation vs in Muscle Twitch generation

Stronger stimuli can produce stronger muscle contraction but, above threshold, they do not create "stronger" APs APs have all-or-none effect, so that they are created when the threshold is reached and stronger stimuli w/ stronger depolarization will still make the same action potential For muscle contraction, stronger stimuli can stimulate more nerve fibers in a motor unit, which thus leads to stronger muscle twitches -frequency and duration of stimulus also affect contraction

Signal conduction in myelinated vs unmyelinated fibers

Unmyelinated fibers use continuous conduction, where new APs are continuously being produced down the entire length of the axon. -Occurs bc unmyelinated fibers have voltage-gated channels along their entire length. -Chain reaction of new APs generated immediately distal to previous one. Continues uninterrupted down entire length of axon. Myelinated fibers use saltatory conduction, where AP travels through internode and weakens but arrives at next node with just enough voltage to trigger new AP. -Problem here is that myelinated areas (internodes) are scarce with voltage-gated channels, so new APs can't be generated in internodes -AP is generated at node, and magnetic fields of Na+ repel other Na+, causing flow of Na+ ions down node slowly but nondecrementally (due to ion leakage and cytoplasmic resistance). -AP moves much quicker in myelinated internode bc less ion leakage and no attraction between cations of ICF and anions of ECF. However it is decremental but arrives at next node w. just enough voltage to trigger new AP. Nerve signal in saltatory conduction is conducted by slow, nondecremental nodes and fast, decremental internodes, creating impression of "jumping" signal -most of axon is internode, so movement is mostly fast -saltatory much faster than continuous, so faster signals travel on myelinated fibers (continuous conduction is like using the space bar to move your cursor while saltatory conduction is like using the tab key) AP itself isn't traveling down entire axon, but nerve signal is through creation of multiple successive APs.

Herpes and axonal transport

acquired through close contact w/ infected person who is shedding virus from skin or genital secretions enters body through skin or mucosal surfaces, although doesn't require open cut on skin virus is surrounded in lipid-membrane and breaches epidermal cells, where they start replicating and damaging the skin (lesions) eventually, virus invade the dermis and then invades peripheral sensory neurons innervating the skin -takes advantage of fast retrograde axonal transport to travel to CNS virus is inactive in dorsal root ganglion of CNS before utilizing axonal transport to travel to other body areas, like genitals local trauma and systemic stimuli can trigger reactivation in dorsal root ganglion and cause virus to travel back to skin

Neuroglia

also known as supporting cells or glial cells -protect neurons and help them function Oligodendrocytes: octopus w/ belly and arms; each arm reaches out to nerve fiber and spirals around it, creating myelin sheath that insulates nerve and speeds up signal conduction -single oligodendrocyte can service many neurons Ependymal: resemble cuboidal epithelium lining internal cavities of brain and spinal cord; produce CSF and helps circulate it Microglia: small macrophages that develop from WBCs; wander through CNS and probe fro cellular debris or other problems; defend brain and function in immune function through phagocytosis -conc. in damaged areas and also aid in remodeling Astrocytes: most abundant glial cells in CNS; cover entire brain surface and most non synaptic regions of neurons in gray matter; very diverse functions; have long cytoplasmic processes associated w/ neurons and blood capillaries; functions in creation of blood-brain barrier and aids in structural integrity

trigger zone

area that includes axon hillock and initial segment initial segment is short section of nerve fiber b/w axon hillock and first glial cell; where APs are initiated; much higher density of voltage-gated ion channels here compared to rest of neural soma -(unmyelinated section)

Axodendritic, Axosomatic, and Axoaxonic synapses

axodendritic: between axon of presynaptic neuron and dendrite of postsynaptic neuron axosomatic: between axon and soma axoaxonic: between axon and axon allows for modifying of pathway behavior

kinesin vs dynein

both are motor proteins that hydrolyze ATP to "walk" down the microtubules, but, Kinesin works in anterograde transport as it transports cargo to the positive end of cell; usually the axon terminal Dynein works in retrograde transport as it transports cargo to the negative end of the cell; usually the cell body

Soma

cell body of a neuron contains organelles including rough ER (Nissl body surrounding nucleus), mitochondria, lysosomes, Golgi complex, and numerous inclusions inclusions include glycogen and lipid droplets (functioning as energy stores for neurons) Centrioles are lacking in mature neurons (after adolescence), meaning they can no longer undergo mitosis and so neurons that die are usually irreplaceable -in CNS, there are some unspecialized stem cells that may regenerate some nervous tissue to a certain extent -in PNS, nerves can sometimes repair themselves

effectors

cells and organs that respond to motor/efferent signals

centrifugal vs centripetal myelination

centrifugal is "away from center" and occurs in Schwann cells of PNS Centripetal is "towards center" and occurs in oligodendrocytes of CNS no neurilemma or endoneurium in CNS myelination bc of centripetal myelination

resting membrane potential (RMP)

charge difference across the plasma membrane in a resting, unstimulated cell -70mV result of K+ diffusing out, Na+ diffusing in (slightly) and the action of the Na-K pump -pump exchanges 3 Na+ out for 2 K+ brought in, which leads to the eventual negative charge Resting plasma membrane is more permeable to K+ than it is to Na+, so K+ has greatest influence on RMP at equilibrium, K+ is 40x more concentrated in cell than out of cell -Na is about 12 more concentrated out of cell than in cell -still diffusion in and out at equilibrium, but no *net* overall diffusion K+ is more concentrated in cell rather than Na+ bc the membrane is more permeable to K+ and the cell is attracting positive charges bc anions in cell can't diffuse out -cell also attracts Na+ but since membrane is much less permeable to Na+, K+ ends up being far more concentrated in cell and Na+ more concentrated out of cell Concentration gradients: Na+ leaks into cell slightly and K+ leaks out of cell greatly -both moving down their concentration gradients -Na-K pump tries to compensate by pumping K+ back in and pumping Na+ back out, but it pumps out more than it brings in so RMP is negative

Brain tumors

consist of a mass of rapidly dividing cells; neurons rarely form tumors bc not really mitotic Brain tumors can arise from meninges (protective membranes of CNS), metastasis of a tumor from other part of body, or (more commonly) from tumors in glial cells most tumors in adults are composed of glial cells, since these cells are mitotically active -called gliomas -usually grow rapidly and are highly malignant -most common is in astrocytes (bc they're most common glia?) and they are very aggressive and very likely to metastasize usually not treated w/ chemotherapy bc of blood-brain barrier; treated with radiation or surgery most common us glioblastoma

myelin sheath

covers the axon of some neurons and helps speed neural impulses consists of glial cells' plasma membrane and is about 80% lipids and 20% protein lipids in myelin causes whit color, thus white matter in the brain appears white bc of myelin (gray matter is unmyelinated) myelin sheath is formed by oligodendrocytes in CNS and Schwann cells in PNS -schwann cells spiral outward concentrically -oligodendrocytes spiral inward unmyelinated axons still enveloped in Schwann cells in PNS or astrocytes in CNS myelination beings early in fetal development and isn't completed until late adolescence (dietary fat important for development of early nervous system for myelin's high lipid content) segmented bc nerve fiber is much longer than single oligodendrocyte or Schwann cell, so multiple cells have to sheath nerve fiber in segmented fashion schwann cells have neurilemma and endoneurium, oligodendrocytes lack both

conduction speed of nerve fibers

depends on two things: diameter of fiber (thicker=faster) and myelination (faster) Fiber diameter: large fibers have *more surface area* and signal conduction occurs along surface, so this means thicker fibers can conduct signals more rapidly -large nerve fibers do require large somas and a lot of energy, so again, nervous system has to pick and choose important nerves to be thick Myelination: further increases conduction speed bc it insulates neuron to prevent ion leakage and prevent attraction of cations in axon to anions in ECF -again, can't afford to have all myelinated, so pick and choose important neurons to be myelinated

Excitatory Adrenergic Synapse

employs the norepinephrine neurotransmitter (NE) like other monoamines, NE acts through secondary messenger like cAMP; its receptor is not an ion channel but a transmembrane protein associated with a G protein on the inner face of the membrane Slower to respond than cholinergic synapse but has advantage of signal amplification -single NE molecule can induce formation of many cAMPs, each capable of having large effects of their own Unstimulated NE receptor is bound to G protein. NE binds to receptor, causing G protein to dissociate from receptor. G protein binds to adenylate cyclase and activates it, converting ATP to cAMP, which can induce several effects -One effect is that it can produce an internal chemical that bind to ion channel from inside cell, causing it to open and depolarize cell -Another effect is that it can activate cytoplasmic enzymes in cell that can lead to diverse metabolic effects -Another is cAMP can induce genetic transcription, causing cell to produce new enzymes for metabolic effects

universal properties of neurons

excitability: all cells are excitable and respond to stimuli conductivity: quickly conduct electrical signals to other neurons secretion: secrete neurotransmitters at synapse to stimulate next cell signaling generally alters electrical and chemical (electrical signal transmitted down neuron and chemical neurotransmitter b/w neurons) -allows for modification and refinement of signals as they are transmitted

axonal transport

fast anterograde: (kinesin) moves mitochondria, synaptic vesicles, other organelles, components of axolemma, calcium ions, enzymes, glucose, amino acids, nucleotides to the axon from the soma fast retrograde:(dynein) returns unused synaptic vesicles and other materials to soma and informs soma of conditions at axon terminals slow anterograde: (kinesin) anterograde process that works in a stop-and-go fashion, making frequent stops -moves enzymes and cytoskeletal components down axon, renews worn-out axoplasmic components in mature neurons, and supplies new axoplasm for developing or regenerating neurons -damaged neurons regenerate at speed determined by slow anterograde transport motor protein kinesin used for anterograde transport; moves from - to + motor protein dynein used for retrograde transport; moves from + to - microtubules made of tubulin are the cytoskeletal tracts for axonal transport slow anterograde actually moves at fast speeds but just makes frequent stops

neuromodulators

hormones, neuropeptides, and other messengers that modify synaptic transmission in groups of neurons -have long-term effects on entire groups of neurons -can adjust or modulate activity by increasing the release of neurotransmitters by presynaptic neurons, adjust sensitivity of postsynaptic neurons to neurotransmitters, or alter the rate of neurotransmitter reuptake and activate secondary messenger pathways i.e. the lateral geniculate nucleus of the thalamus receives input from brainstem and releases a variety of neuromodulators (i.e. Nitric oxide) which alter the excitability of thalamic neurons and ultimately affect behavior states such as sleep/wake cycle and attention

location of white and gray matter in CNS

in spinal cord: gray matter (integration centers) are in the center of the cord and white matter is around it -makes sense bc white matter is myelinated and sending and receiving signals so it needs go be on the perimeter of the spinal cord in brain: white matter is in center and is surrounded by gray matter -makes sense bc spinal cord connects to middle of brain and this is where info is sent and received while integration can occur around the perimeter

degenerative disorders of myelin sheath

include multiple sclerosis and Tay Sachs disease both deal with some set of conditions causing nerve conduction to be disrupted and causing problems with associated body systems of those nerves MS: damaged myelin sheath and nerve fiber causes nerve signal conduction to be disrupted -believed to be autoimmune disorder triggered by virus or other non-genetic trigger Tay Sachs: Eastern European Jewish infants lack lysosomal enzyme that breaks down glycolipid GM2, so it accumulates in lysosomes, causing them to bulge and burst, causing cell death. Infant dies by age 3 or 4.

ganglion

knot-like swelling in a nerve where cell bodies of peripheral neurons are concentrated

Local Potentials (LPs)

localized changes in membrane potential that occur when a neuron is stimulated stimulus can be chemical, light, or mechanical; could be neurotransmitter in synapse or stimulus at receptor a chemical (ligand) binds to receptors on dendrites and opens ligand-gated sodium channels, allowing Na+ to flow into cell and depolarize cell, making it more positive -this creates a short-range or localized change in potential -if LP can make it to the trigger zone, it will cause voltage-gated ion channels to open there and cause an action potential These LPs are: graded (vary in voltage depending on stimulus), decremental (get weaker as they spread out bc of ion leakage and cytoplasmic resistance), reversible (cell quickly returns to RMP when stimulus ceases), and can be either excitatory (make membrane more positive) or inhibitory (make membrane more negative)

unmyelinated cells

many nerve fibers in both CNS and PNS are unmyelinated in CNS, unmyelinated cells in gray matter are wrapped in astrocytes even unmyelinated fibers are enveloped in Schwann cells in PNS, just not wrapped around completely -single Schwann cell harbors anywhere from 1-12 nerve fibers in grooves of its surface -Schwann cell wraps around nerve fiber just once (neurilemma) PNS motor functions that require very quick conduction (like ANS sympathetic division) would be completely myelinated, but this takes up lots of room, so other not so important functions may be unmyelinated and just have single neurilemma wrapping from Schwann cell white matter appears white from myelination (high lipid content), so gray matter represents unmyelinated nerve fibers -"fast" myelinated fibers are white and "slow" unmyelinated fibers are gray (integration centers) makes sense considering location of white and gray matter in brain

Astrocytes

most abundant glial cells in CNS; cover entire brain surface and most non synaptic regions of neurons in gray matter; very diverse functions; have long cytoplasmic processes associated w/ neurons and blood capillaries; functions in creation of blood-brain barrier and aids in structural integrity Functions: -cover brain surface and nonsynaptic regions of neurons in gray matter -form supportive framework -perivascular feet stimulate blood capillaries to form tight, protective seal that is the blood-brain barrier -secrete nerve growth factors that regulate nerve development -communicate electrically with neurons -regulate composition of ECF; absorb neurotransmitters and K+ released by neuron to prevent them from reaching excessive levels in ECF -form hardened scar tissue when neuron damaged and fill space formerly occupied by damaged neurons (astrocytosis or sclerosis)

regeneration of nerve fibers

nerve fibers in PNS are more susceptible to injury bc they aren't protected in way CNS neurons are -damaged PNS neuron can regenerate if its soma remains and at least some neurilemma is intact Process: 2) Nerve fibers is injured and distal portion dies (cut off from signals and axonal transport pf proteins and nutrients/waste exchange). Distal Schwann cells also die and macrophages clean up debris. 3) Soma experiences deformities (enlarged and nucleus off to side) bc it is no longer receiving growth factors and is "blind"; cut off from info. ER breaks up. Some neurons may die at this stage. Axon stump exhibits multiple growth processes at location of cut. Muscle cell that was being innervated by this nerve experiences denervation atrophy, where it begins shrinking in response to cut off nerve supply. 4) a regeneration tube is formed from Schwann cells, basal lamina, and neurilemma near injury site. a growth process from axon stump eventually finds its way into tube and grows rapidly, while other growth processes are retracted. Nerve growth factors tell axon where to grow (where tube is). 5) regeneration tuve guides growing sprout back to original target cells and reestablish synaptic contact 6) contact is reestablished and soma returns to original state. reinnervated muscle fiber grows back to original size. not a perfect process. may fail. may connect to incorrect muscle fiber. takes up to 2 years to recover. usually some degree of functional deficit even after that. Schwann cells and endoneurium/neurilemma are required for regeneration. CNS lacks these and thus cannot regenerate (but they are more protected from injury)

Multiple sclerosis:

oligodendrocytes and myelin sheath of CNS deteriorate and are replaced with harden scar tissue -esp ages 20-40 -lesions on brain and spinal cord nerve conduction becomes disrupted, with effects depending on part of CNS involved -double vision, blindness, speech defects, neurosis, tremors, numbness patients experiencing variable cycles of milder and worse symptoms until they eventually become bedridden cause is unknown; believed to be autoimmune disorder triggered by a virus or other non-genetic trigger in genetically susceptible individuals m -HLA-DRB1 is gene that helps immune system identify host proteins from foreign proteins -Il-7R is gene that produces receptors from membrane proteins, but variations in gene may cause it to produce receptors inside cell instead of in membrane, making them useless (receptors usually function in growth, division, and survival of cells) affects neuronal function because blood-brain barrier is broken down, myelin sheath and nerve fiber are damaged, and signals are hard to transmit No known cure, life expectancies vary; Kopeny says may be fatal but most have normal life expectancies

myelination in CNS

oligodendrocytes myelinate nerve fibers in immediate vicinity in CNS they are stationary/anchored and can't wrap around nerve fibers, so they push new layers under old ones bc they can't migrate around neurons myelination spirals inward toward center of nerve fiber (centripetal myelination) and places new layers under old ones, pushing old layers up no neurilemma or endoneurium present (bc of centripetal myelination toward center)

Refractory periods

periods of resistance to new stimulation Absolute refractory period: lasts from threshold to RMP (right before hyperpolarization). **Lasts as long as Na+ channels are open and then deactivated.** -No stimulus of any strength will trigger new AP Relative refractory period: lasts for period of hyperpolarization, until RMP reestablished. **Lasts this long bc K+ channels are still open so even if cell is stimulated and Na+ flows in, K+ will just flow out to balance it out. Thus, a very strong stimulus would be required to get the cell to threshold** -Only very strong stimuli will trigger new AP. Na+ channels are "reactivated" during relative phase but most stimulation causing Na+ inflow would be cancelled out by K+ outflow, since K+ channels still open. Refractory periods ensure that APs travel in one direction and don't flow back to soma

dendrites

primary site for receiving signals from other neurons more dendrites means the cell is adapted for receiving more info more dendrites means the cell is better at integrating info and making decisions

Action Potential (AP)

rapid depolarization then repolarization in membrane voltage, initiated at trigger zone and propagated down the axon The initial segment functions as the trigger zone bc it has extremely high density of voltage-gated ion channels, so that if a LP can make it to the initial segment with a high enough voltage, it will cause the initial segment's many ion channels to open and cause an action potential AP's are not graded like LP's but follow an all-or-none law (don't get confused w/ twitches) -above threshold, stronger stimuli don't produce stronger action potentials APs also differ from LPs because they are nondecremental, irreversible, and always begin w/ depolarization

two types of receptor cells

receptor cell could be sensory neuron itself (same cell that has ability to possibly generate an AP) or it could be an epithelial sensory receptor cell (which would release a neurotransmitter that binds to and stimulates the actual sensory neuron)

olfactory receptors neurons

receptor cells that initiate the sense of smell a large gene family of approx. 1K different genes gives rise to an equivalent number of olfactory receptor types -receptors are located on olfactory receptor cells, occupying a small area in the upper part of the nasal epithelium and detecting inhaled odorant molecules each olfactory receptor cell has just 1 types of odorant receptor which can detect only a limited number of odorant substances -olfactory receptor cells are highly specialized for a few odors each cells send thin nerve processes directly to glomeruli in olfactory bulb of brain; glomeruli are organized by receptor type they receive signals form olfactory bulb then sends info to other parts of the brain to be integrated and make a conscious experience of smell

denervation atrophy

shrinkage of paralyzed muscle when nerve innervating it is injured Muscle begins shrinking (atrophy) when distal nerve is degenerating due to injury. Muscle is cut off from nerve signals at this point. Muscle returns to original size if and when nerve regenerates and reestablishes contact with muscle.

axons

single axon originates at axon hillock; can be myelinated or unmyelinated Axon collaterals: little branchings off an axon that travel back to earlier neurons in a neural pool and can restimulate them to keep a signal going Terminal rearborization: extensive complex of fine branches at axon's distal end -each branch ends in axon terminal and forms synapse w/ next cell -synaptic vesicles in axon terminal contain neurotransmitters -functional significance is that it allows a nerve to transmit signals very quickly and possibly to multiple target cells

Diversity in neurotransmitter action and effect

some neurotransmitters are excitatory (depolarizing) and some are inhibitory (hyper polarizing) for some, the effect they have depends on the kind of receptor present on the postsynaptic cell some can open ligand-gated ion channels, while other can act through secondary messengers, like cAMP or intracellular ligands that bind to ion-gated channels from inside cell single neurotransmitter could be excitatory on one cell and inhibitory on another cell, depending on the receptors present on those cells

satellite cells

surround somas in ganglion of PNS provide insulation around soma and regulate chemical environment of neurons

Excitatory Cholinergic synapse

synapse that employs ACh as its neurotransmitter, which excites some cells like skeletal muscles while inhibiting some like cardiac muscle much faster than excitatory adrenergic synapse but doesn't have advantage of signal amplification that adrenergic synapses have Nerve signal arrives at axon terminal and opens calcium channels. Calcium enters axon and signals exocytosis of synaptic vesicles, releasing ACh into synaptic cleft. Empty vesicles drop back into cytoplasm to be refilled with ACh while reserve vesicles make their way down the axon to the active sites to release their ACh as well. ACh diffuses across cleft and binds to receptors on ligand-gated ion channels and opens them, causing Na+ to enter cell and K+ to leave. This causes depolarization across plasma membrane, creating a postsynaptic local potential which could possibly induce AP at trigger zone. -This is ACh effect on neuron; don't get confused with ACh effect on muscle fiber.

Inhibitory GABA-ergic synapse

synapse that employs GABA as its neurotransmitter works by same method as ACh, binding to ion channels and causing changes in membrane potential; release and binding are same as ACh method, except that GABA's receptor is chloride channel that opens and allows Cl- to flow into cell, hyperpolarizing the cell and thus inhibiting it, making an action potential less likely

glioblastoma

tumor composed of developing glial tissue, possibly astrocytes most common and most malignant glial tumor abnormal and numerous blood vessels supplying tumor and tumor easily spreads throughout brain tissue symptoms can include headaches, nausea, vomiting, personality changes, and slowing of cognitive function

neuron types

unipolar, bipolar, multipolar, anaxonic unipolar: have just 1 process leading away from soma, which branches into a T w/ peripheral dendrites and central fiber -skin neurons that carry signals to spinal cord for touch and pain -one end has dendrites and other end connects straight to CNS bipolar: 1 axon and 1 dendrite -olfactory, retina, and ear cells multipolar: 1 axon and multiple dendrites; most common type -most of brain and spinal cord neurons anaxonic: multiple dendrites but no axon; communicate locally through dendrites and do NOT produce APs -found in brain, adrenal medulla, and retina, where they help in visual processes like perception of contrast

general organization of a spinal nerve

very similar to that of a muscle individual neurons are organized into fascicles, which are organized into full spinal nerve w/ blood vessels endoneurium, like endomysium, surrounds individual axons/neurons perineurium, like perimysium, surrounds fascicles epineurium, like epimysium, surrounds entire nerve single neuron in a nerve fiber may be either afferent or efferent, but nerve as a whole may contain both afferent and efferent neurons

white matter and gray matter

white matter appears white from myelination (high lipid content), so gray matter represents unmyelinated nerve fibers -"fast" myelinated fibers are white (afferent/efferent) and "slow" unmyelinated fibers are gray (integration centers) makes sense considering location of white and gray matter in brain