Action Potentials

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Action potentials are very different from graded potentials. They are rapid, large alterations in the membrane potential during which time the membrane potential may change 100 mV, from —70 to +30 mV, and then repolarize to its resting membrane potential (Figure 8-18a). Nerve and muscle cells as well as some endocrine, immune, and reproductive cells have plasma membranes capable of producing action potentials. These membranes are called excitable membranes, and their ability to generate action potentials is known as excitability. Whereas all cells are capable of conducting graded potentials, only excitable membranes can conduct action potentials. The propagation of action potentials is the mechanism used by the nervous system to communicate over long distances.

How does an excitable membrane make rapid changes in its membrane potential? How is an action potential propagated along an excitable membrane? These questions are discussed in the following sections.

Ionic Basis of the Action Potential Action potentials can be explained by the concepts already developed for describing the origins of resting membrane potentials. We have seen that the magnitude of the resting membrane potential depends upon the concentration gradients of and membrane permeabilities to different ions, particularly sodium and potassium. This is true for the action potential as well: The action

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Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

PART TWO Biological Control Systems

8. Neural Control Mechanisms

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-Depolarization tstimulus

Hyperpolarization

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Measured at stimulus site

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Measured 1 mm from stimulus site

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Temporal summation

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Spatial summation fx rxfxf:

FIGURE 8-16

Graded potentials can be recorded under experimental conditions in which the stimulus or recording conditions can be varied. Such experiments show that graded potentials (a) can be depolarizing or hyperpolarizing, (b) can vary in size, (c) are conducted decrementally, and (d) can be summed. Temporal and spatial summation will be discussed later in the chapter. [QJ

potential results from a transient change in membrane ion permeability, which allows selected ions to move down their concentration gradients.

In the resting state, the open channels in the plasma membrane are predominantly those that are permeable to potassium (and chloride) ions. Very few sodium-ion channels are open, and the resting potential is there-

FIGURE 8-17

Leakage of charge across the plasma membrane reduces the local current at sites farther along the membrane.

fore close to the potassium equilibrium potential. During an action potential, however, the membrane permeabilities to sodium and potassium ions are markedly altered. (A review of voltage-gated ion channels, Chapter 6, may be helpful at this time.)

The depolarizing phase of the action potential is due to the opening of voltage-gated sodium channels, which increases the membrane permeability to sodium ions several hundredfold (purple line in Figure 8-18b). This allows more sodium ions to move into the cell. During this period, therefore, more positive charge enters the cell in the form of sodium ions than leaves in the form of potassium ions, and the membrane depolarizes. It may even overshoot, becoming positive on the inside and negative on the outside of the membrane. In this phase, the membrane potential approaches but does not quite reach the sodium equilibrium potential (+60 mV).

Action potentials in nerve cells last only about 1 ms and typically show an overshoot. (They may last much longer in certain types of muscle cells.) The

Leakage Charge Across Plasma Membrane

FIGURE 8-17

Leakage of charge across the plasma membrane reduces the local current at sites farther along the membrane.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanism

TABLE 8-4 Differences Between Voltage-Gated Sodium and Potassium Channels

Sodium Body Functions

FIGURE 8-18

The changes during an action potential in (a) membrane potential and (b) membrane permeability (P) to sodium (purple) and potassium (orange) ions.

FIGURE 8-18

The changes during an action potential in (a) membrane potential and (b) membrane permeability (P) to sodium (purple) and potassium (orange) ions.

membrane potential returns so rapidly to its resting level because: (1) the sodium channels that opened during the depolarization phase undergo inactivation near the peak of the action potential, which causes them to close; and (2) voltage-gated potassium channels, which open more slowly than sodium channels, open in response to the depolarization. The timing of these two events can be seen in Figure 8-18b.

Closure of the sodium channels alone would restore the membrane potential to its resting level since potassium flux out would then exceed sodium flux in. However, the process is speeded up by the simultaneous increase in potassium permeability. Potassium diffusion out of the cell is then much greater than the sodium diffusion in, rapidly returning the membrane potential to its resting level. In fact, after the sodium

Compared to voltage-gated potassium channels:

1. Sodium channels open faster in response to a given voltage change.

2. Once activated, sodium channels close more rapidly.

3. Sodium channels inactivate, cycling through an inactive phase.

channels have closed, some of the voltage-gated potassium channels are still open, and in nerve cells there is generally a small hyperpolarization of the membrane potential beyond the resting level (afterhyper-polarization, Figure 8-18a). The differences between voltage-gated sodium and potassium channels are summarized in Table 8-4. Chloride permeability does not change during the action potential.

One might think that large movements of ions across the membrane are required to produce such large changes in membrane potential. Actually, the number of ions that cross the membrane during an action potential is extremely small compared to the total number of ions in the cell, producing only infinitesimal changes in the intracellular ion concentrations. Yet if this tiny number of additional ions crossing the membrane with each action potential were not eventually moved back across the membrane, the concentration gradients of sodium and potassium would gradually disappear, and action potentials could no longer be generated. As might be expected, cellular accumulation of sodium and loss of potassium are prevented by the continuous action of the membrane Na,K-ATPase pumps.

What is achieved by letting sodium move into the neuron and then pumping it back out? Sodium movement down its electrochemical gradient into the cell creates the electric signal necessary for communication between parts of the cell, and pumping sodium out maintains the concentration gradient so that, in response to a new stimulus, sodium will again enter the cell and create another signal.

Mechanism of Ion-channel Changes In the above section, we described the various phases of the action potential as due to the opening and/or closing of voltage-gated ion channels. What causes these changes? The very first part of the depolarization, as we shall see later, is due to local current. Once depolarization starts, the depolarization itself causes voltage-gated sodium channels to open. In light of our discussion of the ionic basis of membrane potentials, it is very easy to confuse the cause-and-effect relationships of this last statement. Earlier we pointed out that an increase in sodium permeability causes mem-

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

8. Neural Control Mechanisms

PART TWO Biological Control Systems

© The McGraw-Hill Companies, 2001

Opening of voltage-gated Na+ channels in membrane

Decreased membrane potential (depolarization)

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Responses

  • elizabeth
    What cells have the ability to generate large changes in their membrane potential?
    8 years ago
  • eleuterio
    Which signal produce rapid long distance communication in the nervous system?
    8 years ago
  • lorena
    How action potentials function?
    7 years ago

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