Ch. 12 notes ALL SECTIONS
Ch. 12 notes ALL SECTIONS BIOL 3455.001
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Date Created: 11/06/15
Ch.12 - Neural Tissue 12.1 - Anatomical/functional divisions of nervous system neuron basic functional unit of nervous system neuroglia aka glial cells; are supporting cells; preserve physical/biochemical structure of neural tissue • provide supportive framework for neural tissue • act as phagocytes • help regulate composition of interstitial fluid • outnumber neurons anatomical divisions: CNS and PNS CNS brain & spinal cord; site of higher function ex: learning, intelligence, emotion, memory • integrating, processing, coordinating sensory data & motor commands PNS deliver sensory info to CNS & carry motor commands to peripheral tissues/systems • nerve fibers bundles of axons carry sensory info/motor commands in PNS functional divisions of PNS: afferent and efferent afferent division bring sensory info to CNS from receptors receptor detect changes in environment or respond to stimuli efferent division carry motor commands from CNS to glands, muscles, etc. effector target organ that responds by doing something efferent division include somatic and autonomic divisions somatic nervous system (SNS) voluntary contractions/skeletal muscles; movement under conscious control autonomic nervous system (ANS) aka visceral motor system involuntary control, ex: regulate smooth/cardiac muscle, gland secretions • sympathetic division accelerate heart rate, inc. BP & adrenaline, “fight or flight” response • parasympathetic division slows heart rate, low BP, “rest” response (figure: Martini, Nath, & Bartholomew 387) 12.2 - Nerve cells and intracellular communication multipolar neuron most common type of neuron in CNS structure include cell body (soma), axon, dendrites, and telodendria (terminal branches) perikaryon cytoplasm surrounding nucleus neurofilaments and neurotubules part of cytoskeleton of perikaryon neurofibrils bundles of neurofilaments Nissl bodies regions stain darkly and contain free ribosomes and clusters of RER (rough endoplasmic reticulum) • gray matter areas containing cell bodies are given gray color by nissl bodies neurons lack centrioles neural stem cells inactive except in nose and hippocampus (store memories) dendridic spines 8090% of neuron’s surface area axoplasm cytoplasm of axon axolemma aka lemma/husk specialized portion of plasma membrane that surround axoplasm collaterals side branches presynaptic cell sends message and includes axon terminal postsynaptic cell receives message presynaptic cell release neurotransmitters neuromuscular junction synapse btw neuron and muscle cell neuroglandular junction neuron control activity of gland cell kinesin/dynein molecular motors in which materials travel length of axon on neurotubules axoplasmic transport movement of materials btw cell body and axon terminals “slow stream” and “fast stream” transport mechanism anterograde flow flow of materials from cell body to axon terminal by kinesin retrograde flow materials transported from axon terminal toward cell body by dynein • rabies retrograde flow carry virus into CNS Clinical Case Structural classificaiton of neurons classified by anaxonic, bipolar, unipolar, multipolar A. anaxonic neurons many small dendrites & no axon located in brain & special sense organs functions not very well known B. bipolar neurons relay info about sight, smell, hearing from receptor cells to other neurons small and fairly rare 2 processses dendrite & axon C. unipolar neuron aka pseudounipolar neuron most sensory neurons are unipolar longest axons carry sensation from tip of toes to spinal cord D. multipolar neuron 2 or more dendrites & single axon most common neurons in CNS ex: motor neurons longest axon carry motor command from spinal cord to muscles of toes Figure 124 (Martini, Nath, and Bartholomew 391) Functional Classification sensory neurons, motor neurons, and interneurons 1) Sensory neuron or afferent neuron deliver sensory info to CNS are unipolar neurons somatic sensory neuron monitor outside world & our position w/in it (Martini, Nath, and Bartholomew 390) visceral sensory neuron monitor internal conditions Sensory receptors interoceptors, exteroceptors, proprioceptors • interoceptors monitor digestive/respiratory/cardiovascular/urinary/reproduc. systems sensation of pain, deep pressure, stretching • exteroceptors info of external environment in form of touch, temp., pressure, taste, smell, sight, equilibrium, hearing • proprioceptors monitor position/movement of skeletal system 2) Motor neuron efferent neuron carry signals from CNS to peripheral effectors body contain half a million motor neurons efferent fibers axons traveling away from CNS 3) Interneuron distribute sensory info & coordinate motor activity; also part of higher functions about 20 billion and outnumber other types of neurons most in brain and spinal cord • Tip: SAME SensoryAfferentMotorEfferent 12-3: Neuroglia CNS Neuroglia make up half the volume of nervous system 4 types of CNS neuroglia: ependymal cells, astrocytes, oligodendrocytes, microglia A) Ependymal cells produce, monitor, circulate CSF these cells line central canal & ventricles ependyma simple cuboidal to columnar epithelium; lack basement membrane 3 types of ependymal cells: ependymocytes, tanycytes, & CSFproducing cells • ependymocytes contain cilia/microvilli that help in circulation of CSF • tanycytes nonciliated w/microvilli on surface transport substances btw CSF & brain found in one brain ventricle B) Astrocytes largest, most numerous in CNS maintain bloodbrain barrier secrete chemicals that maintain permeability of capillary endothelial cells structural support regulate ion/nutrient/gas concentrations absorb/recycle neurotransmitters repair damaged neural tissue C) Oligodendrocytes wrap axons with myelin sheath tie clusters of axons together white matter made up of myelinated axons gray matter made up of unmyelinated axons D) Microglia phagocytic cells (engulf waste, pathogens, etc.) smallest & least numerous PNS Neuroglia ganglia, satellite cells, schwann cells A) ganglia cell bodies of neurons that are clustered in masses B) satellite cells regulate environment around neurons C) Schwann cells aka neurilemma cells form myelin sheath around axons Injuries & the Neural Response schwann cells play part in repairing damaged nerves in PNS Wallerian degeneration “axon distal to injury site degenerates & macrophages migrate into area to clean up debris” (Martini, Nath, and Bartholomew 398) membrane potential or transmembrane potential resting membrane potential varies from moment to moment depending on activity of cell; unstimulated limited regeneration occur in CNS because: • more axons involved • astrocytes produce scar tissue that can prevent axon growth across damaged area • astrocytes release chemicals that block regrowth of axons (Martini, Nath, and Bartholomew 398) • Clinical Study: tumors primary CNS tumors originate in CNS 75% of CNS tumors are primary tumors result from divisions of abnormal neuroglia rather than from divisions of abnormal neurons secondary CNS tumors arise from metastasis (spread) of cancer cells that originate elsewhere nueorons inc. in # until age 4 12-4: Membrane Potential of Cell membrane potential & resting membrane potential are 2 characteristic physiological features of all cells graded potential localized change in resting mem. potential; decrease w/distance from stimulus simplest form of information processing is integration of stimuli at level of the individual cell • 3 important concepts of mem. potential: 1) ECF & cytosol differ in ionic composition ECF —> high Na+/Cl cytosol —> high K+/negatively charged proteins 2) Selectively permeable membranes leak channels membrane channels that are always open 3) Membrane permeability varies by ions current movement of charges to eliminate potential difference resistance how much membrane restrict ion movement Electrochemical Gradient equilibrium potential no net movement of particular ion across membrane eq. potential of K+ is about 90mV, resting potential is 70mV eq. potential of Na+ is +66mV electrochemical gradient sum of chemical & electrical forces acting on ion across plasma membrane; electrochem. gradient is form of potential energy passive channels are leak channels are always open (important in establishing resting mem. potential) active channels are gated channels open/close in response to specific stimuli • 3 classes of gated channels: 1) chemically (ligand) gated 2) voltagegated 3)mechanically gated chemically gated channels most abundant on dendrites/cell body of neuron voltagegated channels abundant on axon mechanically gated channels located on dendrites of sensory neurons mechanically gated channels open in response to distortion of membrane Graded Potentials depolarization - shift from resting membrane potential toward more positive potential local current - movement of + charges parallel to inner/outer surface of membrane repolarization - process of restoring normal resting membrane potential after depolarization hyperpolarization - inside of cell becomes negative; loss of + ions 12-5: Action Potentials All-or-none Principle - stimulus either triggers action potential or none at all • generation of action potentials: 1. depolarization to threshold 2. activation of gated Na+ channels & rapid depolarization 3. inactivation of gated Na+ channels & activation of gated K+ channels 4. closing of gated K+ channels refractory period membrane doesn ’t respond normally to additional depolarizing stimuli from time action potential begins until normal resting mem. potential has stabilized absolute refractory period membrane can ’t respond to further stimulation from moment Na+ channels open at threshold until Na+ channel inactivation ends (Martini, Nath, & Bartholomew 407) (first part of refractory period; lasts 0.41.0 sec) relative refractory period begins when Na+ channels regain normal resting condition & continues until membrane potential stabilizes at resting levels depolarization results from influx of Na+ & repolarization involve loss of K+ sodiumpotassium pump returns intracellular & extracellular conc. to prestimulation levels NaK pump use ATP NaK pump exchange 2 extracellular K+ ions for 3 intracellular Na+ ions & one molecule of ATP is broken down to ADP (Martini, Nath, & Bartholomew 407) ATPase protein that gets energy to pump ions by splitting phosphate group from ATP molec. to form ADP propagation flow of charge similar to that in a conductor continuous propagation —> unmyelinated axons saltatory propagation —> myelinated axons • Continuous propagation along unmyelinated axon: ac. potential move across surface of membrane in series of tiny steps 1. as action potential develops at initial segment, mem. potential here depolarize to +30 mV 2. as Na+ ions entering at initial segment spread away from open gated channels, graded depolarization brings 2nd segment to threshold 3. action potential occurs in 2nd segment while 1st segment begins repolarization 4. as Na+ ions entering at 2nd segment spread laterally, graded depolarization brings membrane in 3rd segment to threshold & cycle is repeated (Martini, Nath, & Bartholomew 410) • Saltatory propagation along myelinated axon 1. action potential occurs at initial segment 2. local current produces graded depolarization that brings axolemma at next node to threshold 3. action potential develops at node (2nd segment) 4. local current produces graded depolarization that brings axolemma at node (3rd segment) to threshold (Martini, Nath, & Bartholomew 411) Table 123 (Martini, Nath, & Bartholomew 412) 126: Axon diameter and propagation speed axon diameter and myelin affect propagation speed larger diameter = lower resistance to ion movement classify axons into 3 groups based on diameter, myelination, & propagation speed: 1. Type A fibers largest myelinated axons (420 μm) carry ac. potentials at speed up to 120 m/s carry sensory info (position, balance, light touch/pressure) to CNS 2. Type B fibers smaller myelinated axons (24 μm) 18 m/s carry info to and from CNS temp, pain, general touch/pressure carry instruction to smooth/cardiac muscle, glands, peripheral effectors 3. Type C fibers unmyelinated (less than 2 μm) 1 m/s same functions as Type B fibers 1/3 of all axons carrying sensory info are myelinated most sensory info received from Type C fibers 127: Synapses and neuron communication electrical synapse direct physical contact btw cells • pre & postsynaptic membranes locked together at gap junctions • connexons integral membrane proteins • connexons form pores that permit ionsto pas btw cells • elec. synapses rare in CNS and PNS chemical synapse involves neurotransmitter • cells not directly coupled • arriving ac. potential may or may not release enough neurotransmitter to bring postsynaptic neuron to threshold • most abundant type of synapse excitatory neurotransmitters cause depolarization & promote generation of ac. potentials inhibitory neurotransmitters cause hyperpolarization & suppress generation of ac. potentials effect of neurotransmitter on postsynaptic membrane depends on properties of receptor, not on nature of neurotransmitter (Martini, Nath, Bartholomew 414) Cholinergic Synapse cholinergic synapses release ACh (ex: neuromuscular junction) ACh in axon terminal is packaged in synaptic vesicles • Cholinergic synapse events: 2. action potential arrives & depolarizes axon terminal 3. extracellular Ca+ ions enter axon terminal triggering exocytosis of ACh 4. ACh binds to receptors & depolarizes the postsynaptic membrane • ACh receptors are chemically gated channels —> inc. permeability to Na+ • depolarization is graded potential 4. ACh is removed by AChE • depolarization ends as ACh broken down into acetate & choline by AChE (hydrolysis) • axon terminal reabsorbs choline from synaptic cleft & use it to resynthesize ACh (Martini, Nath, & Bartholomew 416) synaptic delay occurs btw arrival of ac. potential at axon terminal & the effect on the postsynaptic membrane delay bc of Ca+ influx and neurotransmitter release ACh molecules are recycled synaptic fatigue resynthesis/transport mechanisms not able to keep up with demand for neurotransmitter (Martini, Nath, & Bartholomew 416) 128: Neurotransmitters & Neuromodulators categories of neurotransmitters: biogenic amines, amino acids, neuropeptides, dissolved gases • norepinephrine (NE) in brain and ANS adrenergic synapse release NE excitatory, depolarizing effect on postsynaptic membrane • dopamine CNS neurotransmitter either inhibitory (control movements) or excitatory effects Parkinson ’s disease rigidity/stiffness resulting from damaged dopamineproducing neurons cocaine inhibit removal of dopamine from some synapses —> result in “high” • serotonin effect attention & emotional states antidepressant drugs inhibit reabsorption of serotonin by axon terminals —> relieve symptoms of depression • GABA inhibitory effect; 20% of synapses release GABA functions incompletely understood in CNS, GABA release reduces anxiety nitric oxide generated by axon terminals that innervate smooth muscle in walls of b.vessels in PNS & at synapses in brain (Martini, Nath, & Bartholomew 417) carbon monoxide function as neurotransmitter neuromodulator alter rate of neurotransmitter release by the presynaptic neuron or change postsynaptic cell ’s response to neurotransmitters neuromodulator act by binding to receptors in pre/postsynaptic membranes & activating cytoplasmic enzymes neuropeptides small peptide chains synthesized & release by axon terminal neuromodulators called opioids similar to opium 4 classes of opioids: 1) endorphins 2) enkephalins 3) endomorphins 4) dynorphins opioids relieve pain inhibit release of substance P at synapses that relay pain sensations (Martini, Nath, & Bartholomew 417) neuromodulators: • have longterm effects that are slow to appear • trigger responses that involve steps & intermediary compounds • may affect either or both pre or postsynamtic membrane • can be released alone or along w/neurotransmitter neurotransmitters/neuromodulators fall into 3 functional groups: • compounds that have direct effect on mem. potential (open/close gated ion channels; ionotropic effect; ex: ACh) • compounds that have indirect effect on mem. potential (metabotropic) • lipidsoluble gases that exert their effects inside cell ionotropic effect when neurotransmitters alter ion movement across membrane metabotropic effect involve changes in metabolic activity of postsynaptic cell compounds that have indirect effect on membrane potential work through intermediaries known as second messengers (Martini, Nath, & Bartholomew 420) second messenger ions/molec. that are produced or released inside cell when first messenger binds to receptor first messenger neurotransmitter delivers message to receptors on plasma mem. or w/in cell G protein enzyme complex coupled to membrane receptor link btw first & second messenger involves G protein adenylate cyclase enzyme converts ATP to cyclicAMP (cAMP) cAMP second messenger that may open membrane channels, activate intracellular enzymes, or both, depending on nature of postsynaptic cell (Martini, Nath, & Bartholomew 420) NO & CO important neurotransmitters bc they can diffuse thru lipid membranes and bind to enzymes (figures: Martini, Nath, & Bartholomew 417418) 129: Excitatory & Inhibitory Stimuli net effect on mem. potential of cell body (esp. at axon hillock) determines how neuron responds from moment to moment axon hillock integrates excitatory/inhibitory stimuli affecting cell body/dendrites this integration process, which determines rate of action potential generation at initial segment, is simplest level ofinformation processing (Martini, Nath, & Bartholomew 421) excitatory & inhibitory stimuli are integrated thru interactions btw postsynaptic potentials Postsynaptic Potentials postsynaptic potentials are graded potentials that develop in postsynaptic membrane in response to neurotransmitter (Martini, Nath, & Bartholomew 421) 2 types of postsynaptic potentials develop at neurontoneuron synapses: 1. excitatory postsynaptic potential (EPSP) graded depolarization caused by arrival of neurotransmitter at postsynaptic membrane • results from opening of chemically gated membrane channels that lead to depolarization of plasma membrane (ex: binding of ACh is EPSP) 2. inhibitory postsynaptic potential (IPSP) graded hyperpolarization of postsynaptic membrane • result from opening of chemically gated K+ channels • during hyperpolarization, neuron is inhibited bc largerthanusual depolarizing stimulus needed to bring mem. potential to threshold Summation summation integrates effects of all graded potentials that affect one portion of plasma mem. 2 forms of summation: temporal and spatial summation • temporal summation addition of stimuli occurring in rapid succession at a single synapse that is active repeatedly (Martini, Nath, & Bartholomew 421) • spatial summation occurs when simultaneous stimuli applied at different locations have a cumulative effect on the mem. potential; multiple synapses active simultaneously facilitated when neuron whose mem. potential shifts closer to threshold the larger the degree of facilitation, the smaller is the additional stimulus need to trigger ac. potential (Martini, Nath, & Bartholomew 422) axoaxonic synapse axon to axon synapse occurs btw axons of 2 neurons • can either dec. or inc. rate of neurotransmitter release at presynaptic membrane presynaptic inhibition release of GABA inhibits opening of voltagegated Ca+ channels in axon terminal —> reduces amount of neurotransmitter (NT) released when ac. potential arrive —> reduces effects of synaptic activity on postsynaptic membrane presynaptic facilitation activity at axoaxonic synapse inc. amount of NT released when ac. potential arrives at axon terminal —> this increase enhances/prolongs NT’s effect on postsynaptic membrane (Martini, Nath, & Bartholomew 423) • serotonin involved in presynaptic facilitation & voltagegated Ca+ channels remain open longer in serotonin’s presence degree of sensory stimulation or strength of motor response is proportional to the frequency of action potentials the longer the initial segment of axon hillock remains above threshold the more ac. potentials it produces frequency of ac. potentials depends on degree of depolarization above threshold the greater the degree of depolarization the higher the frequency of ac. potentials ac. potentials can be generated at max rate when relative refractory period has been completely eliminated • maximum theoretical frequency of ac. potentials is established by duration of absolute refractory period. (figure: Martini, Nath, & Bartholomew 425) • Clinical Study: (Martini, Nath, & Bartholomew 395) demyelination progressive destruction of myelin sheaths in CNS & PNS • loss of sensation & motor control —> regions numb & paralyzed • caused by exposure to heavymetal ions diphtheria result from bacterial infection • diphtheria toxin damage schwann cells & destroy myelin sheaths in PNS • result is sensory/motor problems that can lead to paralysis multiple sclerosis recurrent incidents of demyelination that affects axons in optic nerve, brain, & spinal cord • partial loss of vision, problems w/ speech, balance, general motor coordination and bowel/bladder control • MS 1.5 times higher in women GuillainBarre syndrome autoimmune disorder characterized by demyelination of peripheral nerves • weakness/tingling of legs that spread to arms; inc. in severity —> paralysis • virus triggers syndrom • most patients fully recover (figure: Martini, Nath, & Bartholomew 425) Works Cited Martini, Frederic, Judi Lindsley Nath, and Edwin Bartholomew. Fundamentals of Anatomy & Physiology. Tenth ed. San Francisco: Pearson Education, 2015. Print.
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