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Initiation or Generation of a Nerve

Impulse (Action Potential)

At rest, the voltage-sensitive sodiumand potassium gates in the channels in

the plasma membrane of a nerve cell

are nearly closed and the Na+

!+

pumpmoves " Na+ions to the #$% and & !+

ions to the I$%'

his contributes to the formation of aresting membrane potential of -*

millivolts (m)'


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esting embrane oltage

()


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Initiation of the Nerve Impulse $ontinued.

he sodiumpotassium pump alsocontributes to the resting membrane

potential by pumping out three sodium

ions to the #$% and pumping in t/o

potassium ions to the I$%'

he effect of this process is to ma0e

the outside of the nerve cell positive

compared to the inside of the cell /hich

becomes negative (-* m)'


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Initiation of the Nerve $ell Impulse

$ontinued.

At rest, the I$% of an a1on has a

voltage of about -* m

2hen the membrane of the a1on isproperly stimulated, Na+ions begin to

lea0 into the I$%' his causes the

voltage to change to a less negative

state'

2hen I$% voltage reaches a threshold

of about - 33 m, sodium gates open'


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As sodium gates open, Na+

flo/through sodium channels increases and

4uic0ly changes the voltage from a

resting level of - * m to + "*m' his rapid shift from a negative to a

positive state is called

5#P67AI8AI6N'

At +"* m, the sodium gates close'


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2hen the sodium gates close at +"*m, the depolari9ation process stops'

Interestingly, the + "* m condition

causes the potassium gates to open andallo/s potassium to flo/ from the I$% to

the #$%'

he potassium flo/ 4uic0ly reverses thepotential from + "* m to about -

*m' his is called

#P67AI8AI6N'


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he rapid depolari9ation andrepolari9ation process generates a avoltage pulse pea0 that is called the

A$I6N P6#NIA7 or N##IP:7;#'

he formation of a nerve impulsestimulates the formation of still anothernerve impulse in the ne1t section of thea1on membrane


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Nerve Impulse 5iagram


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6vervie/ of ransmission of Nerve Impulse

Action potentialsynaptic 0nobopening of $a+ channelsneurotransmitter vesicles fuse /ith membrane

release of neurotransmitter into synaptic cleftbinding of neurotransmitter to protein receptor

molecules on receiving neuron membraneopening of ion channelstriggering of ne/ action potential

Neurotransmitter is bro0en do/n by en9ymes< ion channels close -- effect brief and

precise


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=o/ a synapse /or0sNervefiber(axon)

Action

potential

Ca + +

Transmitter

Receptor

Voltage-sensitivechannel

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