Lesson 1.1.7

1.1.7 Page 3

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You should now realize that communication through the neuron involves a series of action potentials.

 

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action potential: the change in charge that occurs when the gates of the potassium ion channels close and the gates of the sodium ion channels open; a large depolarization event that is conducted along the membrane of a nerve cell or a muscle cell

 

hyperpolarization: the process of generating a membrane potential that is more negative than the normal resting membrane potential

 

overshoot: the situation that results when more potassium ions leak out of the neuron than should because the potassium gates are slow to close; results in hyperpolarization

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When a neuron becomes sufficiently stimulated by a threshold stimulus, the point of stimulation becomes depolarized and the depolarization spreads along the length of the unmyelinated neuron. This depolarization is created by a rapid change in membrane permeability and a corresponding change in the balance of ions maintained at the resting state. Depolarization can be likened to a burning fuse. The flame, like the wave of depolarization progresses along the wick in one direction. However, in the fuse, the wick is burned and cannot be used again. In the neuron, after the wave of depolarization, an immediate recovery called repolarization occurs. To understand the next steps in communication, read pages 374 – 377. Summary notes in your course folder will prove to be invaluable in mastering these concepts. You may wish to create your own illustration of these events and add it to your course folder for review.

 

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Animation 1: Action Potential Propagation in an Unmyelinated Neuron - Action potential video

  • What is the net charge on the outside of a resting neuron?

  • What is the net charge on the inside of a resting neuron?

  • Describe the action potential.

  • What keeps the action potential going along the axon of the neuron?

  • What are localized electrical circuits?

  • Why does the action potential proceed only in one direction?

  • What is an absolute refractory period?

Animation 3: Voltage Gated Channels and the Action Potential - Voltage Gated Channels Video

  • Draw an illustration of the voltage-gated Na+ channel and the inactivation gate.

  • Draw an illustration of the voltage-gated K+ gate.

  • Describe how depolarization occurs and what happens in repolarization.

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The Refractory Period and the All or None Principle

threshold potential: the smallest change in the membrane potential of a cell membrane that is needed to initiate an action potential; approximately - 55 millivolts

 

all or none principle (event): action that occurs either completely or not at all, such as the generation of an action potential by a neuron

 

refractory period: the short time immediately after an action potential in which the neuron cannot respond to another stimulus; period of time it takes to re-establish the net positive charge on the outside of the neuron and the net negative charge on the inside of the neuron, where there are more sodium ions on the outside and more potassium ions on the inside of the neuron.

Throughout this module, you have discovered that normal communication involves periods of interruption. In this case, while repolarization was occurring, there was no chance for communication. Another sodium inrush could not occur. This brief period when no stimulus can elicit a response is called the refractory period. Its duration is usually only one to two milliseconds.

 

The stimulus that begins the influx of sodium ions and starts depolarization has to be of certain intensity in order to open the voltage-gated sodium channels. Once this voltage shift is reached, depolarization begins. It doesn’t matter if the voltage shift is higher because once the gates are open a wave of depolarization occurs. It is just like lighting the wick in our fuse analogy—either it ignites and begins to burn or it does not ignite. Much the same, if the stimulus is not strong enough to open the gates in the neuron membrane, nothing happens. The voltage shift needed to open the sodium gates is called the threshold potential. This ability of the neuron to only respond to a disturbance in electrical charge of a specific threshold value is called the all or none response. Like your ski bindings which either release, or they don’t, the neuron either transmits the wave of depolarization, or it doesn’t. In an all or none response, there are no in-betweens. Like falling dominos, once the wave of depolarization begins, it results in an impulse.

 

To view watch the videos below

All or None Principle

 

Nerve Impulse

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