cns physiology lecture (5)

8/13/2019 CNS Physiology Lecture (5)

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the relation of ionic concentrationdifferences to membrane potentials. Itwill be recalled that an electricalpotential across the cell membrane canoppose movement of ions through amembrane if the potential is of properpolarity and magnitude.

A potential that exactlyopposes

movement of an ion is called the Nernstpotentialfor that ion; the equation forthis is the following


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where EMF is the Nernst potential in millivoltson the inside of the membrane. The potential

will be negative (-) for positive ions andpositive (+) for negative ions.


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let us calculate the Nernst potential that willexactly oppose the movement of each of the threeseparate ions: sodium, potassium, and chloride.For the sodium concentration difference shown

142 mEq/L on the exterior and 14 mEq/L on theinterior, the membrane potential that will exactlyoppose sodium ion movement through the sodiumchannels calculates to be +61 millivolts. However,

the actual membrane potential is -65 millivolts, not+61 millivolts. Therefore, those sodium ions thatleak to the interior are immediately pumped backto the exterior by the sodium pump, thusmaintaining the -65 millivolt negative potential

inside the neuron.


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potassium ions, the concentration gradient is120 mEq/L inside the neuron and 4.5 mEq/Loutside. This calculates to a Nernst potentialof -86 millivolts inside the neuron, which is

more negative than the -65 that actuallyexists. Therefore, because of the highintracellular potassium ion concentration,there is a net tendency for potassium ions todiffuse to the outside of the neuron, but thisis opposed by continual pumping of thesepotassium ions back to the interior


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Finally, the chloride ion gradient, 107 mEq/Loutside and 8 mEq/L inside, yields a Nernstpotential of -70 millivolts inside the neuron,which is only slightlymore negative than the

actual measured value of -65 millivolts.Therefore, chloride ions tend to leak veryslightly to the interior of the neuron, butthose few that do leak are moved back to theexterior, perhaps by an active chloride pump


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Keep these three Nernst potentials inmind and remember the direction inwhich the different ions tend to

diffuse because this information isimportant in understanding bothexcitation and inhibition of the

neuron by synapse activation orinactivation of ion channels


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The interior of the neuronal soma contains ahighly conductive electrolytic solution.Therefore, any change in potential in any partof the intrasomal fluid causes an almost

exactly equal change in potential at all otherpoints inside the soma (that is, as long as theneuron is not transmitting an actionpotential).

This is an important principle, because itplays a major role in Summation of signalsentering the neuron from multiple sources,


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The resting membrane potential everywherein the soma is -65 millivolts. a presynapticterminal that has secreted an excitatory

transmitter This transmitter acts on the membrane

excitatory receptor to increase themembrane's permeability to Na+. Because of

the large sodium concentration gradient andlarge electrical negativity inside the neuron,sodium ions diffuse rapidly to the inside ofthe membrane.


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Three states of a neuron: A, Resting neuron, with a normal

intraneuronal potential of -65 millivolts.

B, Neuron in an excited state, with a lessnegative intraneuronal potential (-45millivolts) caused by sodium influx.

C, Neuron in an inhibited state, with a more

negative intraneuronal membrane potential (-70 millivolts) caused by potassium ion efflux,chloride ion influx, or both.


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the resting membrane potential hasincreased in the positive direction from -65to -45 millivolts. This positive increase involtage above the normal resting neuronal

potential-that is, to a less negative value-iscalled the excitatory postsynaptic potential(or EPSP), because if this potential rises highenough in the positive direction, it will elicitan action potential in the postsynapticneuron, thus exciting it. (In this case, theEPSP is +20 millivolts-that is, 20 millivoltsmore positive than the resting value.)


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Discharge of a single presynaptic terminalcan never increase the neuronal potentialfrom -65 millivolts all the way up to -45millivolts.

An increase of this magnitude requiressimultaneous discharge of many terminals-about 40 to 80 for the usual anterior motorneuron-at the same time or in rapid

succession. This occurs by a process calledsummation,


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Effect of Inhibitory Synapses on thePostsynaptic Membrane-InhibitoryPostsynaptic Potential.The inhibitorysynapses open mainly chloridechannels,allowing easy passage ofchloride ions


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We calculated the Nernst potential forchloride ions to be about -70 millivolts. Thispotential is more negative than the -65millivolts normally present inside the resting

neuronal membrane. Therefore, opening thechloride channels will allow negativelycharged chloride ions to move from theextracellular fluid to the interior, which willmake the interior membrane potential morenegative than normal, approaching the -70millivolt level.


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Opening potassium channels willallow positively charged potassiumions to move to the exterior, andthis will also make the interiormembrane potential more negativethan usual. Thus, both chlorideinflux and potassium efflux increasethe degree of intracellularnegativity, which is calledhyperpol riz tion.


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Activation of inhibitory synapses, allowingchloride influx into the cell and/or potassiumefflux out of the cell, with the membranepotential decreasing from its normal value of

-65 millivolts to the more negative value of -70 millivolts. This membrane potential is 5millivolts more negative than normal and istherefore an IPSP of -5 millivolts, whichinhibits transmission of the nerve signalthrough the synapse.


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When an excitatory synapse excites theanterior motor neuron, the neuronalmembrane becomes highly permeable tosodium ions for 1 to 2 milliseconds. Duringthis very short time, enough sodium ionsdiffuse rapidly to the interior of thepostsynaptic motor neuron to increase its

intraneuronal potential by a few millivolts,thus creating the excitatory postsynapticpotential (EPSP


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. Other types of transmittersubstances can excite or inhibit thepostsynaptic neuron for much longer

periods-for hundreds of millisecondsor even for seconds, minutes, orhours.

This is especially true for some of

the neuropeptide types of transmittersubstances


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Excitation of a single presynaptic terminal on thesurface of a neuron almost never excites theneuron. The reason for this is that sufficienttransmitter substance is released by a single

terminal to cause an EPSP usually no greater than0.5 to 1 millivolt, instead of the 10 to 20 millivoltsnormally required to reach threshold for excitation.However, many presynaptic terminals are usuallystimulated at the same time. Even though theseterminals are spread over wide areas of the neuron,

their effects can still summ te;that is, they canadd to one another until neuronal excitation doesoccur.


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: It was pointed out earlier that a change in potential at anysingle point within the soma will cause the potential tochange everywhere inside the soma almost exactly equally.(very high electrical conductivity inside the large neuronal cellbody.)

When the EPSP becomes great enough, the threshold for firingwill be reached and an action potential will develop

spontaneously in the initial segment of the axon. Thiseffect of summing simultaneous postsynapticpotentials by activating multiple terminals onwidely spaced areas of the neuronalmembrane is called sp ti l summ tion.


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Each time a presynaptic terminal fires, the releasedtransmitter substance opens the membrane channelsfor at most a millisecond or so. But the changedpostsynaptic potential lasts up to 15 milliseconds afterthe synaptic membrane channels have already closed.Therefore, a second opening of the same channels can

increase the postsynaptic potential to a still greaterlevel, and the more rapid the rate of stimulation, thegreater the postsynaptic potential becomes. Thus,successive discharges from a single presynapticterminal, if they occur rapidly enough, canadd to one another; that is, they cansummate. This type of summation iscalled tempor l summ tion


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Large Spatial Field of Excitation of theDendrites.The dendrites of the anteriormotor neurons often extend 500 to 1000micrometers in all directions from the

neuronal soma.


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It is also important that between 80and 95 per cent of all the presynapticterminals of the anterior motor

neuron terminate on dendrites, incontrast to only 5 to 20 per centterminating on the neuronal soma.Therefore, the preponderant share of

the excitation is provided by signalstransmitted by way of the dendrites


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Often the summated postsynaptic potential isexcitatory but has not risen high enough to reachthe threshold for firing by the postsynaptic neuron.When this happens, the neuron is said to befacilitated. That is, its membrane potential is

nearer the threshold for firing than normal, but notyet at the firing level.

Consequently, another excitatory signal enteringthe neuron from some other source can then excitethe neuron very easily. Diffuse signals in thenervous system often do facilitate large groups ofneurons so that they can respond quickly andeasily to signals arriving from other sources

cns physiology lecture (5)

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