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3.5C). In contrast to saltatory conduction, in unmyelinated axons, action potentials are conductedas slower waves of depolarization as they are propagated alongthe axon.nation and poor balance; depression; impaired speech; memoryproblems; visual problems; altered sensory perception; pain;fatigue; and bowel, bladder, and sexual dysfunctions, resultingfrom the deficits in neural conduction caused by damage to myelinsheaths and underlying axons in the central nervous system. Diagnosis is based on clinical findings, MRI evidence of demyelinatinglesions of the brain and spinal cord, and characteristic oligoclonalbands of γ-globulins in cerebrospinal fluid (CSF).Cerebrospinal fluid electrophoresisNormal123456Pre- Albumin -1 -2 Trans- albuminferrin123456Multiple sclerosis:-globulin elevatedOligoclonal bandsComputed recordingsNormalMultiple sclerosis12Prealbumin Albumin3-14-25Transferrin6-globulinMultiple Sclerosis: Diagnostic Tests—Spinal Fluid CSF samples obtained by lumbar puncturedisplay characteristic elevation of γ-globulins (oligoclonal bands) upon gel electrophoresis in approximately90% of patients.Nerve and Muscle PhysiologySYNAPTIC TRANSMISSIONA synapse is a site at which an electrical response in one cellis transmitted to another cell.
This can occur between somecell types through electrical synapses, when electrical currentpasses directly from one cell to another through gap junctions. Cardiac myocytes and some types of smooth musclecommunicate through this mechanism (some neurons communicate both through electrical and chemical transmission).At electrical synapses between cells, intramembrane proteinscalled connexons form channels that allow flow of ionsbetween cells, thus allowing current transmission.At a chemical synapse, release of a neurotransmitter by aneuron results in electrical stimulation of the presynaptic cell.A wide variety of neurotransmitters are found in variousregions of the central and peripheral nervous system (Table3.2).
The morphology of a chemical synapse is illustrated inFigure 3.6. The sequence of events in transmission at this typeof synapse is:1. Depolarization of the neuron reaches the synapticbouton (axon terminal), where it causes opening of Ca2+channels.2.
The influx of Ca2+ results in the release of neurotransmitter, stored in presynaptic vesicles, into the synapticcleft (the gap between the cells).3. The transmitter diffuses across the synaptic cleft andbinds to specific membrane receptors on the postsynaptic membrane.Table 3.2334. Binding of the neurotransmitter produces a changein the membrane potential of the postsynapticmembrane.Depending on the presynaptic neuron involved (and thus, thespecific neurotransmitter released), the chemical transmissionmay result in either an excitatory postsynaptic potential(EPSP; depolarization) or an inhibitory postsynaptic potential (IPSP; hyperpolarization), as a result of Na+ or Cl− influxat the postsynaptic membrane, respectively (Fig.
3.7). Chemical transmission of this type is unidirectional between a presynaptic fiber and a postsynaptic cell. A given neuron typicallyreceives synaptic input from multiple inhibitory and excitatory neurons; therefore, generation of an action potentialdepends on summation of these multiple inputs. An actionpotential may occur as a result of temporal summation whena series of impulses is generated by an excitatory presynapticfiber, or as a result of spatial summation of local potentialsgenerated by multiple excitatory fibers in the postsynaptic cell(Fig. 3.8). Synaptic inhibition may be either presynaptic orpostsynaptic; some inhibitory fibers act directly on an excitatory fiber (presynaptic inhibition), while others producepostsynaptic inhibition (Fig.
3.9). Summation of local potentials in the postsynaptic membrane is dependent on the spaceconstant and time constant of the membrane.NEUROMUSCULAR JUNCTIONMotor neurons (motoneurons) are efferent nerves that originate in the central nervous system and communicate withSummary of Some Neurotransmitters and Where within the Central and Peripheral NervousSystem They Are FoundTransmitterLocationTransmitterLocationAcetylcholineNeuromuscular junction,autonomic endings andganglia, CNSGasNitric oxideCNS, GI tractBiogenic aminesNorepinephrineDopamineSerotoninSympathetic endings, CNSCNSCNS, GI tractAmino acidsγ-Aminobutyric acid (GABA)GlutamateCNSCNSPurinesAdenosineAdenosine triphosphate (ATP)CNSCNSPeptidesβ-EndorphinsEnkephalinsAntidiuretic hormonePituitary-releasing hormonesSomatostatinNeuropeptide YVasoactive intestinal peptideCNS, Central nervous system; GI, gastrointestinal.(Reprinted with permission from Hansen J: Netter’s Atlas of Human Physiology, Philadelphia, Elsevier, 2002.)CNS, GI tractCNSCNS (hypothalamus/posteriorpituitary)CNS (hypothalamus/anteriorpituitary)CNS, GI tractCNSCNS, GI tract34The Nervous System and MuscleDendriteAxonNodeMyelin sheathDendritesNumerous boutons (synaptic knobs) of presynaptic neuronsterminating on a motor neuron and its dendritesAEnlarged section of boutonAxon (axoplasm)AxolemmaMitochondriaGlial processSynaptic vesiclesSynaptic cleftPresynaptic membrane (densely staining)Postsynaptic membrane (densely staining)Postsynaptic cellBFigure 3.6 Morphology of Synapses An action potential in one neuron is transmitted to anotherneuron or an effector tissue at the synapses.
The terminal regions of axons branch and form synapticboutons. The myelination is lost where the bouton is in close contact with a dendrite or cell body of anotherneuron (A). Neurotransmitters are released into the synaptic cleft when vesicles of the synaptic bouton fusewith the presynaptic membrane (B). The released neurotransmitters diffuse and bind to receptors on thepostsynaptic membrane, resulting in excitatory or inhibitory effects at the postsynaptic membrane.skeletal muscle fibers at specialized synapses known as motorendplates or neuromuscular junctions (Fig. 3.10). Among theseveral types of motor neurons, the most common is the αmotor neuron.
Branches of an α-motor neuron may formmultiple neuromuscular junctions in depressions in the sarcolemmae (muscle cell membranes) of muscle fibers; each αmotor neuron may thus innervate multiple muscle fibers,although each fiber is innervated by only one α-motor neuron.An α-motor neuron and the fibers it innervates is called amotor unit. All the motor neurons innervating a muscle arecollectively known as a motor neuron pool.The synaptic boutons of α-motor neurons are rich in vesiclescontaining the neurotransmitter acetylcholine.
As is the case forother neurons, an action potential in an α-motor neuron produces Ca2+ influx at the synaptic bouton, which results in releaseof neurotransmitter from vesicles of the synaptic bouton. Vesicular release of neurotransmitter is quantal in nature, and is conceptualized as the release of packets (quanta) of neurotransmitter,the quanta being defined by the response to the neurotransmitter contained in a single vesicle. At the postsynaptic membraneof a neuromuscular junction, acetylcholine binds to a specificsubtype of acetylcholine receptor known as the nicotinicNerve and Muscle Physiology⎧⎪⎪⎨⎪⎪⎩⎧⎪⎪⎨⎪⎪⎩ExcitatoryInhibitorySynaptic vesiclesin synaptic boutonPresynaptic membraneNaTransmitter substancesKSynaptic cleftPostsynaptic membraneWhen impulse reaches the excitatory synaptic bouton, itcauses release of a transmitter substance into the synapticcleft.
This increases permeability of the postsynapticmembrane to Na and K. More Na moves into thepostsynaptic cell than K moves out, due to greaterelectrochemical gradient.ClAt the inhibitory synapse, transmitter substance released by animpulse increases permeability of the postsynaptic membraneto Cl.Synaptic boutonResultant net ionic current flow is in a direction that tends todepolarize the postsynaptic cell. If depolarization reachesfiring threshold, an impulse is generated in the postsynapticcell.Resultant ionic current flow is in the direction that tends tohyperpolarize the postsynaptic cell.
This makes depolarizationby excitatory synapses more difficult—more depolarizationis required to reach the threshold.6570Potential0481216msecCurrent flow and potential changePotential (mV)Potential (mV)Current7004msec81216Potential75CurrentCurrent flow and potential changeFigure 3.7 Chemical Synaptic Transmission Synaptic transmission may result in either excitatoryor inhibitory effects at the postsynaptic membrane. In excitatory transmission (left), a depolarizing changein local potential (excitatory postsynaptic potential; EPSP) is produced. In inhibitory transmission (right), ahyperpolarizing change in local potential (inhibitory postsynaptic potential; IPSP) is produced.
An actionpotential occurs when the temporal and spatial summation of local potentials reaches the threshold foraction potential generation (see Fig. 3.8).3536The Nervous System and MuscleExcitatory fibersmVmVExcitatory fibers–70–70AxonAxonInhibitory fibersInhibitory fibersA. Resting state: motor nerve cell shown with synaptic boutons ofexcitatory and inhibitory nerve fibers ending close to itB. Partial depolarization: impulse from one excitatory fiber has causedpartial (below firing threshold) depolarization of a motor neuronExcitatory fibersmV–70–70AxonAxonInhibitory fibersInhibitory fibersC.
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