The Action Potential Propagates Itself Along the Neuron

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    The action potentialpropagates itself along the

    neuron

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    An action potential is a localized

    electrical event

    a change from a neurons resting

    potential at a specific point.

    To function as a signal, this localevent must travel along the neuron.

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    A nerve signal starts out as one action

    potential, generated on the axon near the cell

    body of the neuron.

    The effect of this action potential is liketipping the first of a row of standing dominoes

    The first domino does not travel along the

    row, but its fall is relayed to the end of the

    row, one domino at a time.

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    The three (nampak 2) parts of Figure

    show the changes that occur in part of

    an axon at three successive times as a nerve signal passes from left to

    right.

    As we saw, all the ion movementsassociated with a particular action

    potential occur at one place on the axon

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    left. (1) When this region of the axon

    (blue) has its Na+ channels open,

    Na+ rushes inward (blue arrows),and an action potential is generated.

    This corresponds to the upswing of

    the curve on the graph in figure

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    (2) When that same region has its K+

    channels open, K+ diffuses out of the

    axon ; at this time, its Na+ channels areclosed and inactivated, and we would

    see the downswing of the action

    potential initiated in part 1. (3) A short time later, we would see no

    signs of an action potential at this spot

    because the axon membrane here hasreturned to its resting potential.

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    Now let's see how these events lead to

    the domino effect" of a nerve signal.

    In part 1 of the figure. The blue arrowspointing sideways within the axon

    indicate local spreading of the electrical

    changes caused by the in Na+ ionsassociated with the first action potential

    .

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    These changes trigger the opening of Na+

    channels in the membrane just to the right of

    the action potential.

    As a second action potential is generated, asindicated blue region in part 2. In the same

    way, a third action potential is generated in

    part 3, and each action potential generates

    another all the way down the axon.

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    So why are action potentials propagated in

    only one direction along the axon (left to right

    in the figure)?As the blue arrows indicate, local electrical

    changes do spread in both directions in the

    axon.

    However, these changes cannot open Na+

    channels and generate an action potential

    when Na+ channels are inactivated. Thus, an

    action potential cannot be generated in theregions where K+ is leaving (green in the

    figure) and Na+ channels are still inactivated.

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    So we see that a nerve signal, also known as

    a potential, propagates itself in one direction

    by the electrical changes it produces in theneuron membrane.

    A potential is a bit of coded information that

    can travel from one end of a neuron to

    another.

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    What does this tell how a nervous systemactually works?

    If you rap your finger on a desk, for instance,

    the contact is a stimulus that trigger actionpotentials in the tips of sensory neurons inyour skin.

    The action potentials propagate along the

    axon. Carrying the information (that yourfinger has hit a hard object) into your centralnervous system

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    Action potentials are all-or-none events; that

    is the same no matter how strong or weak the

    stimulus that triggers them.

    How, then, do action potentials relay different

    intensities of information to your central

    nervous system?

    It is the frequencyof action potentials thatwith the intensity of stimuli.

    If you rap your finger hard against the desk,

    your CNS receives many more potentials

    per millisecond than after a soft tap.

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    Once your central nervous system receives

    information in the form of action potentials, it

    can process information and formulate a

    response to it. The nervous system depends on the sensory

    neurons passing their signal to other neurons

    in the CNS.