An action potential is the change in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell. An action potential occurs when a neuron sends information down an axon, away from the cell body. A threshold is the minimum amount of stimulation needed to start a neural impulse (you know, the electrical impulses that travel throughout your body carrying important information). Action potentials generated by neural impulses are “all or nothing,” meaning the signal reaches the threshold for communication or it doesn’t. No signal is stronger or weaker than another. Depolarization increase in positive charge inside the plasma membrane caused by a depolarizing graded potential causes increasing numbers of voltage- gated Na+ channels to open rapidly.
The equilibrium potential for potassium is negative, so it wants the charge across the membrane to be negative. So, by all of these potassium ions leaving, that makes the membrane potential more negative, and that process is called repolarization In many cells, a period of hyperpolarization, or after potential, exists following each action potential. The after potential exists because the voltage-gated k+ channels open and close more slowly than the voltage-gated Na+ channels and remains open for a slightly longer time than it takes to bring the membrane potential back to its original resting levels.
Refractory Period Once an action potential is produced at a given point on the plasma membrane, the sensitivity of that area to further stimulation decreases for a time called refractory period. •The absolute refractory period places a limit on the rate at which a neuron can conduct impulses, and the relative refractory period permits variation in the rate at which a neuron conducts impulses. Such variation is important because it is one of the ways by which our nervous system recognizes differences in stimulus strength
Action Potential Frequency
The number of action potentials produced per unit of time in response to a stimulus. A subthreshold stimulus is any stimulus not strong enough to produce a graded potential. Therefore, no action potential is produced. A threshold stimulus produces a graded potential that is just strong enough to reach threshold and cause the production of a single action potential. A submaximal stimulus includes all stimuli between threshold and maximal stimulus strength. For submaximal stimuli, the action potential frequency increase in proportion to the strength of the stimulus because the size of a graded potential increase with stimulus strength. .
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