Download pdf - The Action Potential Propagates Itself Along the Neuron

7/28/2019 The Action Potential Propagates Itself Along the Neuron

1/14

The action potentialpropagates itself along the

neuron


2/14

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.


3/14

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.


4/14


5/14

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


6/14

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


7/14

(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.


8/14

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

.


9/14

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.


10/14

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.


11/14

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.


12/14

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


13/14

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.


14/14

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.