AmeriCorps Health Blog

Action Potentials

An action potential is a more dramatic change produced by voltage-regulated ion gates in the plasma membrane. Action potentials occur only where there is a high enough density of voltage-regulated gates. Most of the soma has only 50 to 75 gates per square micrometer (^m2) and cannot generate action potentials. The trigger zone, however, has 350 to 500 gates per ^m2. If an excitatory local potential spreads all the way to the trigger zone and is still strong enough when it arrives, it can open these gates and generate an action potential.

The action potential is a rapid up-and-down shift in membrane voltage. Figure 12.11a shows an action potential numbered to correspond to the following description. Figure 12.12 correlates these voltage changes with events in the plasma membrane.

1. When sodium ions arrive at the axon hillock, they depolarize the membrane at that point. This appears as a steadily rising local potential.
2. For anything more to happen, this local potential must rise to a critical voltage called the threshold (typically about —55 mV). This is the minimum needed to open voltage-regulated gates.
3. The neuron now "fires," or produces an action potential. At threshold, voltage-regulated Na+ gates open quickly, while K+ gates open more slowly. The initial effect on membrane potential is therefore due to Na+. Initially, only a few Na+ gates open but as Na+ enters the cell, it further depolarizes the membrane. This stimulates still more voltage-regulated Na+ gates to open and admit even more Na+. Thus, a positive feedback cycle is created that makes the membrane voltage rise rapidly.
4. As the rising membrane potential passes 0 mV, Na+ gates are inactivated and begin closing. By the time they all close and Na+ inflow ceases, the voltage peaks at approximately +35 mV. (The peak is as low as 0 mV in some neurons and as high as 50 mV in others.) The membrane is now positive on the inside and negative on the outside—its polarity is reversed compared to the RMP.
5. By the time the voltage peaks, the slow K+ gates are fully open. Potassium ions, repelled by the positive intracellular fluid, exit the cell. Their outflow repolarizes the membrane—that is, it shifts the voltage back into the negative numbers. The action potential consists of the up-and-down voltage shifts

Time

Figure 12.11 An Action Potential. (a) Diagrammed with a distorted timescale to make details of the action potential visible. Numbers correspond to stages discussed in the text. (b) On an accurate timescale, the local potential is so brief it is imperceptible, the action potential appears as a spike, and the hyperpolarization is very prolonged.

Time

Spike

HyperpoIarization

0 10

20 30 msec

40 50

Figure 12.11 An Action Potential. (a) Diagrammed with a distorted timescale to make details of the action potential visible. Numbers correspond to stages discussed in the text. (b) On an accurate timescale, the local potential is so brief it is imperceptible, the action potential appears as a spike, and the hyperpolarization is very prolonged.

Saladin: Anatomy & I 12. Nervous Tissue I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition that occur from the time the threshold is reached to the time the voltage returns to the RMP.

1. Potassium gates stay open longer than Na+ gates, so the amount of potassium that leaves the cell is greater than the amount of sodium that entered. Therefore, the membrane voltage drops to 1 or 2 mV more negative than the original RMP, producing a negative overshoot called hyperpolarization.
2. As you can see, Na+ and K+ switch places across the membrane during an action potential. During hyperpolarization, ion diffusion through the membrane and (in the CNS) the removal of extracellular K+ by the astrocytes gradually restores the original resting membrane potential.

At the risk of being misleading, figure 12.12 is drawn as if most of the Na+ and K+ had traded places. In reality, only about one ion in a million crosses the membrane to produce an action potential, and an action potential affects ion distribution only in a thin layer close to the membrane. If the illustration tried to represent these points accurately, the difference would be so

Chapter 12 Nervous Tissue 459

slight you could not see it, indeed the changes in ion concentrations inside and outside the cell are so slight they cannot be measured in the laboratory unless a neuron has been stimulated for a long time. Even after thousands of action potentials, the cytosol still has a higher concentration of K+ and a lower concentration of Na+ than the ECF does.

Figure 12.11a also is deliberately distorted. In order to demonstrate the different phases of the local potential and action potential, the magnitudes of the local potential and hyperpolarization are exaggerated, the local potential is stretched out to make it seem longer, and the duration of hyperpolarization is shrunken so the graph does not run off the page. When these events are plotted on a more realistic timescale, they look like figure 12.11b. The local potential is so brief it is unnoticeable, and hyperpolarization is very long but only slightly more negative than the RMP. An action potential is often called a spike; it is easy to see why from this figure.

Earlier we saw that local potentials are graded, decre-mental, and reversible. We can now examine how action potentials compare on these points.

Na+ and K+ gates closed

350-

Na+ and K+ gates closed

350-

Resting membrane potential

Na+ gate open, Na+ enters cell, > K+ gate beginning to open £

Depolarization begins

Resting membrane potential

Na+ gate closed, K+ gate fully open, K+ leaves cell

Depolarization ends, repolarization begins

Na+ gate closed, K+ gate closing

Repolarization complete

Na+ gate closed, K+ gate closing

Repolarization complete

Figure 12.12 Actions of the Sodium and Potassium Gates During an Action Potential.

Saladin: Anatomy & I 12. Nervous Tissue I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

460 Part Three Integration and Control

• Action potentials follow an all-or-none law. If a stimulus depolarizes the neuron to threshold, the neuron fires at its maximum voltage (such as +35 mV); if threshold is not reached, the neuron does not fire at all. Above threshold, stronger stimuli do not produce stronger action potentials. Thus, action potentials are not graded (proportional to stimulus strength).
• Action potentials are nondecremental. For reasons to be examined shortly, they do not get weaker with distance. An action potential at the end of a nerve fiber will be just as strong as an action potential in the trigger zone up to a meter away.
• Action potentials are irreversible. If a neuron reaches threshold, the action potential goes to completion; it cannot be stopped once it begins.

In some respects, we can compare the firing of a neuron to the firing of a gun. As the trigger is squeezed, a gun either fires with maximum force or does not fire at all (analogous to the all-or-none law). You cannot fire a fast bullet by squeezing the trigger hard or a slow bullet by squeezing it gently—once the trigger is pulled to its "threshold," the bullet always leaves the muzzle at the same velocity. And, like an action potential, the firing of a gun is irreversible once the threshold is reached. Table 12.2 further contrasts a local potential with an action potential, including some characteristics of action potentials explained in the next section.

Was this article helpful?

0 0