Assume that a small battery is inserted and its negative pole is connected to the stimulating electrode. With the switch open, a resting potential of -60 mV is recorded. As a result of closing the switch, however, the negative pole of the battery is connected to the stimulating electrode, which tends to artificially make the inside of the cell more negative relative to the external solution. There is a slight downward deflection of the recording trace.
If this experiment is repeated using a slightly larger battery, more current flows into the cell, and a larger increase in the negativity of the cell is recorded. Larger batteries produce even greater increases in the potential. Any time the negativity of the cell interior is increased, the potential change is known as a hyperpo-larization. The membrane is more polarized than normal.
Now consider the consequences of repeating this experiment with the positive pole of the battery connected to the stimulating electrode. Turning on the switch now makes the inside of the cell artificially more positive than the resting potential. The polarized state of the membrane is decreased. Increasing the size of the battery produces a greater decrease in the negativity of the cell, and over a limited range the resultant potential is a graded function of the size of the stimulus that is used to produce it. Any time the interior of the cell becomes more positive, the potential change is known as a depolarization. These hyperpolarizations and depolarizations that are artificially produced are known as electrotonic, graded, or passive potentials (some additional features of electrotonic potentials are discussed later in Chapter 5); however, the point to note here is that, within a limited range of stimulus intensities, hyperpolarizing and depolarizing electrotonic potentials are graded functions of the size of the stimulus used to produce them.
An interesting phenomenon occurs when the magnitude of the battery used to produce the depolarizing potentials is increased further. As the size of the battery and thus the amount of depolarization is increased, a critical level is reached, known as the threshold, wherein a new type of potential is produced that is different in its amplitude, duration, and form from the depolarizing pulse used to produce it. This new type of potential change elicited when threshold is reached is known as the action potential, which is elicited in an all-or-nothing fashion. Stimuli below threshold fail to elicit an action potential; stimuli at threshold or above threshold successfully elicit an action potential. Increasing the stimulus intensity beyond threshold produces an action potential identical to the action potential produced at the threshold level. In this experiment, the duration of the depolarization is so short that only a single action potential could be initiated. If the duration is longer, multiple action potentials are initiated, and their frequency depends on the stimulus intensity. This is simply a restatement of the all-or-nothing law of action potentials presented earlier. Below threshold, no action potential is elicited; at or above threshold, an all-or-nothing action potential is initiated. Increasing the stimulus intensity still further produces the same amplitude action potential; only the frequency is increased.
Not only are action potentials elicited in an all-or-nothing fashion, but, as described in Chapter 5, they also propagate in an all-or-nothing fashion. If an action potential is initiated in the cell body, it will propagate along the nerve axon and eventually invade the synaptic terminals and initiate a process known as synaptic transmission (see Chapter 6). Unlike action potentials, electrotonic potentials do not propagate in an all-or-nothing fashion. Electrotonic potentials do spread but only for short distances (see Chapter 5).
There are several interesting features of the action potential. One is that the polarity of the cell completely reverses during the peak of the action potential. Initially, the inside of the cell is -60 mV with respect to the outside but, during the peak of the action potential, the potential reverses and approaches a value +55 mV inside with respect to the outside. The region of the action potential that varies between the 0-mV level and its peak value is known as the overshoot. Another interesting characteristic of action potentials is their repolarization phase (the return to the resting level). The action potential does not immediately return to the resting potential of -60 mV; there is a period of time when the cell is actually more negative than the resting level. This phase of the action potential is known as the undershoot or the hyperpolarizing afterpotential.
As indicated earlier, nerve potentials are the vehicles by which peripheral information is coded and propagated to the central nervous system; motor commands initiated in the central nervous system are propagated to the periphery by nerve action potentials, and the action potential is the first step in the initiation of muscular contraction.
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