Lect 14 Nervous Tissue Day 2 PDF

CLICKER TIME……………… This type of neuroglial cells function as macrophages, searching for cellular debris to phagocytize A. Astrocytes B. Schwann cells C. Microglia D. Ependymal cells  Myelin Myelin sheath—an insulating layer around a nerve fiber – Formed by...

CLICKER TIME……………… This type of neuroglial cells function as macrophages, searching for cellular debris to phagocytize A. Astrocytes B. Schwann cells C. Microglia D. Ependymal cells  Myelin Myelin sheath—an insulating layer around a nerve fiber – Formed by oligodendrocytes in CNS and Schwann cells in PNS – Consists of the plasma membrane of glial cells 20% protein and 80% lipid Myelin Sheath in PNS Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Schwann cell Axoplasm nucleus Axolemma Neurilemma (c) Myelin sheath Nodes of Ranvier and internodes  Myelin Myelin sheath is segmented ◦ Nodes of Ranvier: gap between segments ◦ Internodes: myelin-covered segments from one gap to the next ◦ Initial segment: short section of nerve fiber between the axon hillock and the first glial cell ◦ Trigger zone: the axon hillock and the initial segment ◦ Play an important role in initiating a nerve signal Diseases of the Myelin Sheath Degenerative disorders of the myelin sheath ◦ Multiple sclerosis ◦ Oligodendrocytes and myelin sheaths in the CNS deteriorate ◦ Myelin replaced by hardened scar tissue ◦ Nerve conduction disrupted (double vision, tremors, numbness, speech defects) ◦ Onset between 20 and 40 and fatal from 25 to 30 years after diagnosis ◦ Cause may be autoimmune triggered by virus CLICKER TIME……………… This type of neuroglial cells function as macrophages, searching for cellular debris to phagocytize A. Astrocytes B. Schwann cells C. Microglia D. Ependymal cells Electrophysiology of Neurons: Electrical Potentials and Currents Electrophysiology—cellular mechanisms for producing electrical potentials and currents ◦ Basis for neural communication and muscle contraction Electrical potential—a difference in the concentration of charged particles between one point and another Electrical current—a flow of charged particles from one point to another ◦ In the body, currents are movements of ions, such as Na+ or K+, through gated channels in the plasma membrane ◦ Gated channels are opened or closed by various stimuli ◦ Enables cell to turn electrical currents on and off Living cells are polarized Resting membrane potential (RMP)—charge difference across the plasma membrane ◦ About −70 mV in a resting, unstimulated neuron ◦ Negative value means there are more negatively charged particles on the inside of the membrane than on the outside The Resting Membrane Potential Resting membrane potential (RMP) exists because of unequal electrolyte distribution between extracellular fluid (ECF) and intracellular fluid (ICF) RMP results from the combined effect of three factors ◦ Ions diffuse down their concentration gradient through the membrane ◦ Plasma membrane is selectively permeable and allows some ions to pass easier than others ◦ Electrical attraction of cations and anions to each other  The Resting Membrane Potential Copyright © The McGraw-Hill Companies, Inc. 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ECF Na+ 145 m Eq/L K+ 4 m Eq/L K+ Na+ channel channel Na+ 12 m Eq/L K+ 150 m Eq/L Large anions that cannot ICF escape cell Na+ concentrated outside of cell (ECF) K+ concentrated inside cell (ICF) http://highered.mcgraw- hill.com/sites/0072495855/student_view0/chapter2/animation  Local Potentials Local potentials—disturbances in membrane potential when a neuron is stimulated Neuron response begins at the dendrite, spreads through the soma, travels down the axon, and ends at the synaptic knobs  Local Potentials When neuron is stimulated by chemicals, light, heat, or mechanical disturbance ◦ Opens the Na+ gates and allows Na+ to rush into the cell ◦ Na+ inflow neutralizes some of the internal negative charge ◦ Voltage measured across the membrane drifts toward zero ◦ Depolarization: case in which membrane voltage shifts to a less negative value ◦ Na+ diffuses for short distance on the inside of the plasma membrane producing a current that travels toward the cell’s trigger zone; this short-range change in voltage is called a local potential  Local Potentials Differences of local potentials from action potentials ◦ Graded: vary in magnitude with stimulus strength ◦ Stronger stimuli open more Na+ gates ◦ Decremental: get weaker the farther they spread from the point of stimulation ◦ Voltage shift caused by Na+ inflow diminishes rapidly with distance ◦ Reversible: when stimulation ceases, K+ diffusion out of cell returns the cell to its normal resting potential ◦ Either excitatory or inhibitory: some neurotransmitters (glycine) make the membrane potential more negative—hyperpolarize it— so it becomes less sensitive and less likely to produce an action potential http://www.wiley.com/college/jenkins/0470227583/anima tions/ch12/index_12_06_01.html  Local Potentials Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Soma Trigger Axon zone Current ECF Ligand Receptor Plasma membrane of dendrite Na+ ICF  Action Potentials Action potential—more dramatic change produced by voltage-regulated ion gates in the plasma membrane ◦ Only occur where there is a high enough density of voltage- regulated gates ◦ Soma (50 to 75 gates per m2 ); cannot generate an action potential ◦ Trigger zone (350 to 500 gates per m2 ); where action potential is generated ◦ If excitatory local potential spreads all the way to the trigger zone and is still strong enough when it arrives, it can open these gates and generate an action potential  Pleasenote Please notethat thatdue due to todiffering differing operating systems, operating systems, some someanimations animations willnot will notappear appear until untilthe the presentation presentation is is viewed in viewed inPresentation Presentation Mode Mode (Slide (Slide Showview). Show view).You Youmay maysee seeblank blank slides slides inthe in the“Normal” “Normal”or or“Slide “SlideSorter” Sorter”views. views. Allanimations All animations will willappear appear after after viewing viewing inPresentation in Presentation Mode Modeand andplaying playing each each animation. Most animation. Mostanimations animations will willrequire require thelatest the latestversion version of ofthe theFlash FlashPlayer, Player, whichis which isavailable available at at http://get.adobe.com/flashplayer. http://get.adobe.com/flashplayer.  Action Potentials Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 +35 Only a thin layer of the cytoplasm next to the cell 3 5 membrane is affected 0 Depolarization Repolarization ◦ In reality, very few ions are Action mV potential involved Threshold 2 –55 Local potential Action potential is often 1 7 called a spike, as it happens –70 Resting membrane 6 Hyperpolarization so fast potential (a) Time  Action Potentials Characteristics of action potential versus a local potential 4 +35 ◦ Follows an all-or-none law ◦ If threshold is reached, neuron 3 5 fires at its maximum voltage 0 Depolarization Repolarization Action ◦ If threshold is not reached, it does mV potential Threshold not fire –55 2 Local ◦ Non-decremental: do not get potential 1 7 weaker with distance –70 Resting membrane 6 Hyperpolarization ◦ Irreversible: once started goes to potential Time completion and cannot be (a) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. stopped Action Potential vs. Local Potential Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 +35 +35 3 Spike 5 0 0 Depolarization Repolarization Action mV mV potential Threshold 2 –55 Local potential 1 7 Hyperpolarization –70 Resting membrane 6 Hyperpolarization potential –70 Time 0 10 20 30 40 50 (a) (b) ms  Sodium and Potassium Channels Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. K+ Na+ K+ channel Na+ channel 35 0 0 mV mV 1 Na+ and K+ channels closed 2 Na+ channels open, Na+ –70 enters cell, K+ channels –70 beginning to open Resting membrane Depolarization begins potential 35 35 0 0 mV mV 3 Na+ channels closed, K+ 4 Na+ channels closed, channels fully open, K+ –70 K+ channels closing –70 leaves cell Depolarization ends, Repolarization complete repolarization begins The Refractory Period Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Absolute Relative refractory refractory period period During an action potential +35 and for a few milliseconds after, it is difficult or impossible to stimulate that 0 region of a neuron to fire mV again Threshold –55 Resting membrane Refractory period—the –70 potential period of resistance to stimulation Time  The Refractory Period Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Absolute Relative Two phases of the refractory period refractory refractory period period ◦ Absolute refractory period +35 ◦ No stimulus of any strength will trigger AP 0 ◦ As long as Na+ gates are open mV ◦ From action potential to RMP ◦ Relative refractory period Threshold ◦ Only especially strong –55 Resting membrane stimulus will trigger new AP –70 potential ◦ K+ gates are still open and any effect of incoming Na+ is opposed by the outgoing K+ Time  The Refractory Period Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Absolute Relative refractory refractory period period The refractory period refers +35 only to a small patch of the neuron’s membrane at one 0 time mV Threshold Other parts of the neuron –55 can be stimulated while the Resting membrane potential small part is in refractory –70 period Time  Signal Conduction in Nerve Fibers For communication to occur, the nerve signal must travel to the end of the axon Unmyelinated fiber has voltage-regulated ion gates along its entire length Action potential from the trigger zone causes Na+ to enter the axon and diffuse into adjacent regions beneath the membrane  Signal Conduction in Nerve Fibers The depolarization excites voltage-regulated gates immediately distal to the action potential Na+ and K+ gates open and close producing a new action potential By repetition the membrane distal to that is excited Chain reaction continues to the end of the axon http://www.youtube.com/watch?v=gSKFqm4pIEI http://www.youtube.com/watch?v=GQl0dwsT4mk http://www.youtube.com/watch?v=Sa1wM750Rvs Signal Conduction in Nerve Fibers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Cell body Axon Signal Action potential ++++–––++ ++++ +++++ ––––+++–––––– –––– – in progress Refractory membrane ––––+++–––––– –––– – Excitable ++++–––++ ++++ +++++ membrane +++++ ++++ –––+ +++ ++ –––––––––+++– –––– – –––––––––+++– –––– – +++++ ++++ –––+ +++ ++ +++++ ++++ ++++ ––– ++ ––––––––––––– +++– – Figure 12.16 ––––––––––––– +++– – +++++ ++++ ++++ ––– ++ Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.  Signal Conduction in Nerve Fibers Voltage-gated channels Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. needed for APs ◦ Fewer than 25 per m2 in myelin-covered regions (internodes) ◦ Up to 12,000 per m2 in nodes of Ranvier Fast Na+ diffusion occurs between nodes ◦ Signal weakens under myelin Na+ inflow at node Na+ diffuses along inside Excitation of voltage- sheath, but still strong (a) generates action potential (slow but non decremental) of axolemma to next node (fast but decremental) regulated gates will generate next action enough to stimulate an potential here action potential at next node Saltatory conduction—nerve signal seems to jump from node to node  Signal Conduction in Nerve Fibers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. + + – – + + + + ++ ++ – – + + – – – – –– –– – – + + – – – – –– –– + + – – + + + + ++ ++ + + + + – – + + ++ ++ – – – – + + – – –– –– – – – – + + – – –– –– + + + + – – + + ++ ++ + + + + + + – – ++ ++ – – – – – – + + –– –– – – – – – – + + –– –– + + + + + + – – ++ ++ Action potential Refractory Excitable (b) in progress membrane membrane Much faster than conduction in unmyelinated fibers 12-32 CLICKER TIME……………… Cells of the nervous system include neurons and neuroglia A. True B. False CLICKER TIME……………… The rising phase of the action potential represents A. The opening of sodium channels B. Influx of sodium into the neuron C. Depolarization D. All of the above CLICKER TIME……………… The division of the nervous system responsible for carrying signals from the CNS to gland and muscle cells that carry out the body’s response, is the A. Sensory division of the PNS B. Motor division of the PNS C. Central nervous system D. Spinal cord CLICKER TIME……………… This type of neuroglial cells function as macrophages, searching for cellular debris to phagocytize A. Astrocytes B. Schwann cells C. Microglia D. Ependymal cells  Synapses A nerve signal can go no further when it reaches the end of the axon ◦ Triggers the release of a neurotransmitter ◦ Stimulates a new wave of electrical activity in the next cell across the synapse Synapse between two neurons ◦ First neuron in the signal path is the presynaptic neuron ◦ Releases neurotransmitter ◦ Second neuron is postsynaptic neuron ◦ Responds to neurotransmitter Structure of a Chemical Synapse Synaptic knob of presynaptic neuron contains synaptic vesicles containing neurotransmitter ◦ Many docked on release sites on plasma membrane ◦ Ready to release neurotransmitter on demand ◦ A reserve pool of synaptic vesicles located further away from membrane Postsynaptic neuron membrane contains proteins that function as receptors and ligand-regulated ion gates 12-38  Structure of a Chemical Synapse Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Microtubules ofcytoskeleton Axon of presynaptic neuron Mitochondria Postsynaptic neuron Synaptic knob Synaptic vesicles containing neurotransmitter Synaptic cleft Neurotransmitter receptor Neurotransmitter Postsynaptic neuron release Presynaptic neurons have synaptic vesicles with neurotransmitter and postsynaptic have receptors and ligand-regulated ion channels Neurotransmitters and Related Messengers Fall into four major categories according to chemical composition ◦ Acetylcholine ◦ Amino acid neurotransmitters ◦ Include glycine, glutamate, aspartate, and -aminobutyric acid (GABA) ◦ Monoamines ◦ Epinephrine, norepinephrine, dopamine (catecholamines) ◦ Histamine and serotonin ◦ Neuropeptides ◦ Table 12.3 P 454 Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.  An Excitatory Cholinergic Synapse Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Cholinergic synapse—employs Presynaptic neuron acetylcholine (ACh) as its neurotransmitter ◦ ACh excites some postsynaptic cells 3 Presynaptic neuron Ca2+  Skeletal muscle 1 2 ACh Na+ 4 K+ 5 Postsynaptic neuron Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. An Excitatory Adrenergic Synapse Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Presynaptic neuron Postsynaptic neuron Neurotransmitter receptor Norepinephrine Adenylate cyclase G protein – – – + + + 1 2 Ligand- 3 5 gated Na+ channels opened cAMP 4 Postsynaptic potential Multiple Enzyme activation possible 6 effects 7 Metabolic Genetic transcription changes Enzyme synthesis CLICKER TIME……………… At a synapse, the neurotransmitter released from a pre-synaptic neuron causes channels to open resulting in an inward current of sodium ions A. Such synapse is said to be excitatory B. Such synapse is said to be inhibitory C. The inward current of sodium will cause a depolarization in the postsynaptic neuron D. The inward current of sodium will cause a local potential that if strong enough it could result in an action potential in the post-synaptic neuron Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. Neural Integration Neural Integration Critical Thinking Questions Why are so many anesthesias (such as ether and chloroform) fat solvents? If you bathed a resting neuron with an excess of potassium ions (for example, more than 40 times higher than normal), what would be the effect? If you could completely stop all sodium ion leakage into the resting neuron, what would be the effect on the cell? Compare the short-term effect of an acetylcholinesterase inhibitor on the heart to its effect on muscles that cause inhalations and exhalations. Use the information you learned in chapters 10 and 11. If 30 typical EPSPs arrived at the trigger zone within a 17-msec period, how many IPSPs will be required to stop the occurrence of an axonic action potential? Assume that each IPSP is 0.5 mV and the neuron has a RMP of – 70mV and a threshold of −55 mV  Critical Thinking Answers Many sensory neurons have a myelin sheath and myelin is primarily one or another phospholipid (p. 444). The potassium ions would now be at a higher concentration in the ECF than in the ICF. The potassium would no longer diffuse out since the ICF concentration was high. Therefore, the resting membrane potential should disappear and the neuron would become non-functional (p. 449; Fig. 12.11 on the same page). The cell would become polarized beyond the normal level (p. 449; Fig. 12.11 on the same page). Acetylcholinesterase inhibitors should allow acetylcholine to remain active in the synaptic gap. Since this neurotransmitter will inhibit cardiac muscles, the heart should slow (p. 461, Table 12.3 on p. 458) but the skeletal muscles (including the inhalation and exhalation muscles [chapters 10 and 11]) will contract more rapidly initially. The acetylcholinesterase inhibitors, if their effect is not blocked, will clearly lead to violent spasms of the skeletal muscles and death. These acetylcholinesterase inhibitors are the active ingredients in modern nerve gases. Only one IPSP is required because the single IPSP would be enough to prevent the reaching of the threshold (Fig. 12.25 on p. 463; pp. 462-463). Clinical Application Question Sam was clearing weeds with a machete and he somehow cut his arm about 10 cm (i.e., 100 mm) proximal to the base of his thumb. About how long at the earliest is it likely to take before he gets feeling back in that part of the thumb? Clinical Application Answer At least 20 days since regeneration takes 3-5 millimeters/day (p. 446, Fig. 12.9 on p. 447 illustrates the process). However, Saladin points out that the process may not restore appropriate function anyway (p. 447).