Presentation on theme: "Chapter 12 Nervous Tissue"— Presentation transcript:
1 Chapter 12 Nervous Tissue Copyright 2009 John Wiley & Sons, Inc.
2 Overview of the Nervous System The nervous system, along with the endocrine system, helps to keep controlled conditions within limits that maintain health and helps to maintain homeostasis.The nervous system is responsible for all our behaviors, memories, and movements.The branch of medical science that deals with the normal functioning and disorders of the nervous system is called neurology.Copyright 2009 John Wiley & Sons, Inc.
3 Major Structures of the Nervous System Copyright 2009 John Wiley & Sons, Inc.
4 Overview of Major Structures Twelve pairs of cranial nerves.Thirty-one pairs of spinal nerves emerge from the spinal cord.Ganglia, located outside the brain and spinal cord, are small masses of nervous tissue, containing primarily cell bodies of neurons.Enteric plexuses help regulate the digestive system.Sensory receptors are either parts of neurons or specialized cells that monitor changes in the internal or external environment.Copyright 2009 John Wiley & Sons, Inc.
5 Functions of Nervous System Sensory function: to sense changes in the internal and external environment through sensory receptors.Sensory (afferent) neurons serve this function.Integrative function: to analyze the sensory information, store some aspects, and make decisions regarding appropriate behaviors.Association or interneurons serve this function.Motor function is to respond to stimuli by initiating action.Motor(efferent) neurons serve this function.Copyright 2009 John Wiley & Sons, Inc.
6 Nervous System Divisions Central nervous system (CNS)consists of the brain and spinal cordPeripheral nervous system (PNS)consists of cranial and spinal nerves that contain both sensory and motor fibersconnects CNS to muscles, glands & all sensory receptorsCopyright 2009 John Wiley & Sons, Inc.
7 Structure of a Multipolar Neuron Copyright 2009 John Wiley & Sons, Inc.
8 Histology of the Nervous System: Neurons Functional unit of nervous systemHave capacity to produce action potentialselectrical excitabilityCell bodysingle nucleus with prominent nucleolusNissl bodies (chromatophilic substance)rough ER & free ribosomes for protein synthesisneurofilaments give cell shape and supportmicrotubules move material inside celllipofuscin pigment clumps (harmless aging)Cell processes = dendrites & axonsCopyright 2009 John Wiley & Sons, Inc.
9 Structural Classification of Neurons Copyright 2009 John Wiley & Sons, Inc.
10 Copyright 2009 John Wiley & Sons, Inc. Axonal TransportCell body is location for most protein synthesisneurotransmitters & repair proteinsAxonal transport system moves substancesslow axonal flowmovement in one direction only -- away from cell bodymovement at 1-5 mm per dayfast axonal flowmoves organelles & materials along surface of microtubulesat mm per daytransports in either directionfor use or for recycling in cell bodyCopyright 2009 John Wiley & Sons, Inc.
11 Sensory receptors that are dendrites of unipolar neurons Copyright 2009 John Wiley & Sons, Inc.
12 Copyright 2009 John Wiley & Sons, Inc. CNS NeuronsCopyright 2009 John Wiley & Sons, Inc.
13 Copyright 2009 John Wiley & Sons, Inc. Neuroglia of the CNSMost common glial cell typeEach forms myelin sheath around more than one axons in CNSAnalogous to Schwann cells of PNSCopyright 2009 John Wiley & Sons, Inc.
14 Copyright 2009 John Wiley & Sons, Inc. Neuroglia of the PNSCells encircling PNS axonsEach cell produces part of the myelin sheath surrounding an axon in the PNSCopyright 2009 John Wiley & Sons, Inc.
15 Myelinated and unmyelinated axons Schwann cells myelinate (wrap around) axons in the PNS during fetal developmentSchwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheathTube guides growing axons that are repairing themselvesCopyright 2009 John Wiley & Sons, Inc.
16 Myelinated and unmyelinated axons Copyright 2009 John Wiley & Sons, Inc.
17 Organization of the Nervous System Copyright 2009 John Wiley & Sons, Inc.
18 Subdivisions of the PNS Somatic (voluntary) nervous system (SNS)neurons from cutaneous and special sensory receptors to the CNSmotor neurons to skeletal muscle tissueAutonomic (involuntary) nervous systemssensory neurons from visceral organs to CNSmotor neurons to smooth & cardiac muscle and glandssympathetic division (speeds up heart rate)parasympathetic division (slow down heart rate)Enteric nervous system (ENS)involuntary sensory & motor neurons control GI tractneurons function independently of ANS & CNSCopyright 2009 John Wiley & Sons, Inc.
19 Electrical Signals in Neurons Neurons are electrically excitable due to the voltage difference across their membraneCommunicate with 2 types of electric signalsaction potentials that can travel long distancesgraded potentials that are local membrane changes onlyIn living cells, a flow of ions occurs through ion channels in the cell membraneCopyright 2009 John Wiley & Sons, Inc.
20 Overview of Nervous System Functions Copyright 2009 John Wiley & Sons, Inc.
21 Copyright 2009 John Wiley & Sons, Inc. Types of Ion ChannelsLeakage (nongated) channels are always opennerve cells have more K+ than Na+ leakage channelsas a result, membrane permeability to K+ is higherexplains resting membrane potential of -70mV in nerve tissueLigand-gated channels open and close in response to a stimulusresults in neuron excitabilityVoltage-gated channels respond to a direct change in the membrane potential.Mechanically gated ion channels respond to mechanical vibration or pressure.Copyright 2009 John Wiley & Sons, Inc.
22 Ion channels in plasma membrane Copyright 2009 John Wiley & Sons, Inc.
23 Resting Membrane Potential Negative ions along inside of cell membrane & positive ions along outsidepotential energy difference at rest is -70 mVcell is “polarized”Resting potential exists becauseconcentration of ions different inside & outsideextracellular fluid rich in Na+ and Clcytosol full of K+, organic phosphate & amino acidsmembrane permeability differs for Na+ and K+greater permeability for K+inward flow of Na+ can’t keep up with outward flow of K+Na+/K+ pump removes Na+ as fast as it leaks inCopyright 2009 John Wiley & Sons, Inc.
24 Resting Membrane Potential Copyright 2009 John Wiley & Sons, Inc.
25 Factors that contribute to resting membrane potential Copyright 2009 John Wiley & Sons, Inc.
26 Copyright 2009 John Wiley & Sons, Inc. Graded PotentialsSmall deviations from resting potential of -70mVhyperpolarization = membrane has become more negativedepolarization = membrane has become more positiveThe signals are graded, meaning they vary in amplitude (size), depending on the strength of the stimulus and localized.Graded potentials occur most often in the dendrites and cell body of a neuron.Copyright 2009 John Wiley & Sons, Inc.
27 Hyperpolarized/Depolarized Graded Potential Copyright 2009 John Wiley & Sons, Inc.
28 Copyright 2009 John Wiley & Sons, Inc. Graded potentials in response to opening mechanically-gated channels or ligand-gated channelsCopyright 2009 John Wiley & Sons, Inc.
29 Stimulus strength and graded potentials Copyright 2009 John Wiley & Sons, Inc.
30 Copyright 2009 John Wiley & Sons, Inc. SummationCopyright 2009 John Wiley & Sons, Inc.
31 Generation of Action Potentials An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization).During an action potential, voltage-gated Na+ and K+ channels open in sequence (According to the all-or-none principle, if a stimulus reaches threshold, the action potential is always the same.A stronger stimulus will not cause a larger impulse.Copyright 2009 John Wiley & Sons, Inc.
32 Copyright 2009 John Wiley & Sons, Inc. Action PotentialsCopyright 2009 John Wiley & Sons, Inc.
33 Stimulus strength and Action Potential generation Copyright 2009 John Wiley & Sons, Inc.
34 Copyright 2009 John Wiley & Sons, Inc. Changes in ion flow during depolarizing and repolarizing phases of Action PotentialCopyright 2009 John Wiley & Sons, Inc.
35 Copyright 2009 John Wiley & Sons, Inc. Depolarizing PhaseChemical or mechanical stimulus caused a graded potential to reach at least (-55mV or threshold)Voltage-gated Na+ channels open & Na+ rushes into cellin resting membrane, inactivation gate of sodium channel is open & activation gate is closed (Na+ can not get in)when threshold (-55mV) is reached, both open & Na+ entersinactivation gate closes again in few ten-thousandths of secondonly a total of 20,000 Na+ actually enter the cell, but they change the membrane potential considerably(up to +30mV)Positive feedback processCopyright 2009 John Wiley & Sons, Inc.
36 Copyright 2009 John Wiley & Sons, Inc. Repolarizing PhaseWhen threshold potential of -55mV is reached, voltage-gated K+ channels openK+ channel opening is much slower than Na+ channel opening which caused depolarizationWhen K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops)K+ outflow returns membrane potential to -70mVIf enough K+ leaves the cell, it will reach a -90mV membrane potential and enter the after-hyperpolarizing phaseK+ channels close and the membrane potential returns to the resting potential of -70mVCopyright 2009 John Wiley & Sons, Inc.
37 Copyright 2009 John Wiley & Sons, Inc. Refractory periodPeriod of time during which neuron can not generate another action potentialAbsolute refractory periodeven very strong stimulus will not begin another APinactivated Na+ channels must return to the resting state before they can be reopenedlarge fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possibleRelative refractory perioda suprathreshold stimulus will be able to start an APK+ channels are still open, but Na+ channels have closedCopyright 2009 John Wiley & Sons, Inc.
38 Continuous versus Saltatory Conduction Continuous conduction (unmyelinated fibers)step-by-step depolarization of each portion of the length of the axolemmaSaltatory conductiondepolarization only at nodes of Ranvier where there is a high density of voltage-gated ion channelscurrent carried by ions flows through extracellular fluid from node to nodeCopyright 2009 John Wiley & Sons, Inc.
39 Copyright 2009 John Wiley & Sons, Inc. Propagation of an Action Potential in a neuron after it arises at the trigger zoneCopyright 2009 John Wiley & Sons, Inc.
40 Factors that affect speed of propagation Amount of myelinationAxon diameterTemperatureCopyright 2009 John Wiley & Sons, Inc.
41 Speed of impulse propagation The propagation speed of a nerve impulse is not related to stimulus strength.larger, myelinated fibers conduct impulses faster due to size & saltatory conductionFiber typesA fibers largest (5-20 microns & 130 m/sec)myelinated somatic sensory & motor to skeletal muscleB fibers medium (2-3 microns & 15 m/sec)myelinated visceral sensory & autonomic preganglionicC fibers smallest ( microns & 2 m/sec)unmyelinated sensory & autonomic motorCopyright 2009 John Wiley & Sons, Inc.
42 Signal Transmission at the Synapse 2 Types of synapseselectricalionic current spreads to next cell through gap junctionsfaster, two-way transmission & capable of synchronizing groups of neuronschemicalone-way information transfer from a presynaptic neuron to a postsynaptic neuronaxodendritic -- from axon to dendriteaxosomatic -- from axon to cell bodyaxoaxonic -- from axon to axonCopyright 2009 John Wiley & Sons, Inc.
43 Signal transmission at the chemical synapse Copyright 2009 John Wiley & Sons, Inc.
44 Copyright 2009 John Wiley & Sons, Inc. Chemical SynapsesAction potential reaches end bulb and voltage-gated Ca+ 2 channels openCa+2 flows inward triggering release of neurotransmitterNeurotransmitter crosses synaptic cleft & binding to ligand-gated receptorsthe more neurotransmitter released the greater the change in potential of the postsynaptic cellSynaptic delay is 0.5 msecOne-way information transferCopyright 2009 John Wiley & Sons, Inc.
45 Signal transmission at a chemical synapse Copyright 2009 John Wiley & Sons, Inc.
46 Excitory and Inhibitory Postsynaptic Potentials The effect of a neurotransmitter can be either excitatory or inhibitorya depolarizing postsynaptic potential is called an EPSPit results from the opening of ligand-gated Na+ channelsthe postsynaptic cell is more likely to reach thresholdan inhibitory postsynaptic potential is called an IPSPit results from the opening of ligand-gated Cl- or K+ channelsit causes the postsynaptic cell to become more negative or hyperpolarizedthe postsynaptic cell is less likely to reach thresholdCopyright 2009 John Wiley & Sons, Inc.
47 Ionotropic and Metatropic Receptors Copyright 2009 John Wiley & Sons, Inc.
48 Removal of Neurotransmitter Diffusionmove down concentration gradientEnzymatic degradationacetylcholinesteraseUptake by neurons or glia cellsneurotransmitter transportersProzac = serotonin reuptake inhibitorCopyright 2009 John Wiley & Sons, Inc.
49 Three Possible Responses Small EPSP occurspotential reaches -56 mV onlyAn impulse is generatedthreshold was reachedmembrane potential of at least -55 mVIPSP occursmembrane hyperpolarizedpotential drops below -70 mVCopyright 2009 John Wiley & Sons, Inc.
50 Copyright 2009 John Wiley & Sons, Inc. SummationIf several presynaptic end bulbs release their neurotransmitter at about the same time, the combined effect may generate a nerve impulse due to summationSummation may be spatial or temporal.Copyright 2009 John Wiley & Sons, Inc.
51 Copyright 2009 John Wiley & Sons, Inc. Spatial SummationSummation of effects of neurotransmitters released from several end bulbs onto one neuronCopyright 2009 John Wiley & Sons, Inc.
52 Copyright 2009 John Wiley & Sons, Inc. Temporal SummationSummation of effect of neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuronCopyright 2009 John Wiley & Sons, Inc.
53 Copyright 2009 John Wiley & Sons, Inc. SummationCopyright 2009 John Wiley & Sons, Inc.
54 Copyright 2009 John Wiley & Sons, Inc. NeurotransmittersBoth excitatory and inhibitory neurotransmitters are present in the CNS and PNS; the same neurotransmitter may be excitatory in some locations and inhibitory in others.Important neurotransmitters include acetylcholine, glutamate, aspartate, gamma aminobutyric acid, glycine, norepinephrine, epinephrine, and dopamine.Copyright 2009 John Wiley & Sons, Inc.
55 Copyright 2009 John Wiley & Sons, Inc. NeurotransmittersCopyright 2009 John Wiley & Sons, Inc.
56 Neurotransmitter Effects Neurotransmitter effects can be modifiedsynthesis can be stimulated or inhibitedrelease can be blocked or enhancedremoval can be stimulated or blockedreceptor site can be blocked or activatedAgonistanything that enhances a transmitters effectsAntagonistanything that blocks the action of a neurotranmitterCopyright 2009 John Wiley & Sons, Inc.
57 Small-Molecule Neurotransmitters Acetylcholine (ACh)released by many PNS neurons & some CNSexcitatory on NMJ but inhibitory at othersinactivated by acetylcholinesteraseAmino Acidsglutamate released by nearly all excitatory neurons in the brain ---- inactivated by glutamate specific transportersGABA is inhibitory neurotransmitter for 1/3 of all brain synapses (Valium is a GABA agonist -- enhancing its inhibitory effect)Copyright 2009 John Wiley & Sons, Inc.
58 Copyright 2009 John Wiley & Sons, Inc. Neural CircuitsA neuronal network may contain thousands or even millions of neurons.Neuronal circuits are involved in many important activitiesbreathingshort-term memorywaking upCopyright 2009 John Wiley & Sons, Inc.
59 Copyright 2009 John Wiley & Sons, Inc. Neural CircuitsCopyright 2009 John Wiley & Sons, Inc.
60 Copyright 2009 John Wiley & Sons, Inc. Regeneration & RepairPlasticity maintained throughout lifesprouting of new dendritessynthesis of new proteinschanges in synaptic contacts with other neuronsLimited ability for regeneration (repair)PNS can repair damaged dendrites or axonsCNS no repairs are possibleCopyright 2009 John Wiley & Sons, Inc.
61 Damage and Repair in the Peripheral Nervous System When there is damage to an axon, usually there are changes, called chromatolysis, which occur in the cell body of the affected cell; this causes swelling of the cell body and peaks between 10 and 20 days after injury.By the third to fifth day, degeneration of the distal portion of the neuronal process and myelin sheath (Wallerian degeneration) occurs; afterward, macrophages phagocytize the remains.Retrograde degeneration of the proximal portion of the fiber extends only to the first neurofibral node.Regeneration follows chromatolysis; synthesis of RNA and protein accelerates, favoring rebuilding of the axon and often taking several months.Copyright 2009 John Wiley & Sons, Inc.
62 Copyright 2009 John Wiley & Sons, Inc. Repair within the PNSAxons & dendrites may be repaired ifneuron cell body remains intactschwann cells remain active and form a tubescar tissue does not form too rapidlyChromatolysis24-48 hours after injury, Nissl bodies break up into fine granular massesCopyright 2009 John Wiley & Sons, Inc.
63 Copyright 2009 John Wiley & Sons, Inc. Repair within the PNSBy 3-5 days,wallerian degeneration occurs (breakdown of axon & myelin sheath distal to injury)retrograde degeneration occurs back one nodeWithin several months, regeneration occursneurolemma on each side of injury repairs tube (schwann cell mitosis)axonal buds grow down the tube to reconnect (1.5 mm per day)Copyright 2009 John Wiley & Sons, Inc.
64 Neurogenesis in the CNS Formation of new neurons from stem cells was not thought to occur in humansThere is a lack of neurogenesis in other regions of the brain and spinal cord.Factors preventing neurogenesis in CNSinhibition by neuroglial cells, absence of growth stimulating factors, lack of neurolemmas, and rapid formation of scar tissueCopyright 2009 John Wiley & Sons, Inc.
65 Copyright 2009 John Wiley & Sons, Inc. End of Chapter 12Copyright 2009 John Wiley & Sons, Inc. All rights reserved. Reproduction or translation of this work beyond that permitted in section 117 of the 1976 United States Copyright Act without express permission of the copyright owner is unlawful. Request for further information should be addressed to the Permission Department, John Wiley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for distribution or resale. The Publishers assumes no responsibility for errors, omissions, or damages caused by the use of theses programs or from the use of the information herein.Copyright 2009 John Wiley & Sons, Inc.