Action Potential 2

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The Action Potential
Graphics are used with permission of:
Pearson Education Inc., publishing as Benjamin Cummings (http://www.aw-bc.com)

** If this is not printed in color, it is suggested you color code the ion channels and ions as you go through this topic. Ions channels and ions should be color coded as follows: Red: Sodium ion channels and sodium ions

Blue: Potassium ion channels and potassium ions

Page 1. Introduction
• Neurons communicate over long distances by generating and sending an electrical signal called a nerve impulse, or action potential.

Page 2. Goals
• To understand that rapid changes in permeability of the neuronal membrane produce the action potential. • To recognize that altering voltage-gated ion channels changes membrane permeability. • To understand the movement of sodium and potassium ions during the action potential. • To examine refractory periods.

• To learn about conduction velocity.

Page 3. The Action Potential: An Overview
• The action potential is a large change in membrane potential from a resting value of about -70 millivolts to a peak of about +30 millivolts, and back to -70 millivolts again. • The action potential results from a rapid change in the permeability of the neuronal membrane to sodium and potassium. The permeability changes as voltage-gated ion channels open and close. • In the following pages we will study step-by-step the changes that occur as an action potential is generated and then propagated down the axon.

Page 4. The Action Potential Begins at the Axon Hillock
• The action potential is generated at the axon hillock, where the density of voltage-gated sodium channels is greatest. • The action potential begins when signals from the dendrites and cell body reach the axon hillock and cause the membrane potential there to become more positive, a process called depolarization. • These local signals travel for only a short distance and are very different from action potentials. We will study them in a separate module that covers synapse.

Page 5. During Depolarization Sodium Moves into the Neuron
• As the axon hillock depolarizes, voltage-gated channels for sodium open rapidly, increasing membrane permeability to sodium. • Sodium moves down its electrochemical gradient into the cell. • On the diagram on the top of the next page, color code the ion channels. Label the ion channels as follows from top to bottom: • Potassium passive channel.

• Sodium voltage-gated channel.
• Sodium voltage-gated channel.
• Potassium voltage-gated channel.
• Sodium passive channel.
• Sodium voltage-gated channel.
• Describe what happens to the sodium voltage-gated channel when the membrane is depolarized:

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Page 6. Threshold
|• If the stimulus to the axon hillock is great enough, the neuron depolarizes|• Record the membrane potential as you work through this page: | |by about 15 millivolts and reaches a trigger point called threshold. | | |• At threshold, an action potential is generated. Weak stimuli that do not |[pic] | |reach threshold do not produce an action potential. Thus we say that the | | |action potential is an all-or-none event. | | |• Action potentials always have the same amplitude and the same duration. |...
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