Stimulate Nerve

FIGURE 6 Extracellular recording of the electrical activity from a nerve. One pair of electrodes is placed so that the nerve can be electrically stimulated while a second pair of electrodes records the neural activity. With weak intensity stimulation (traces 1 and 2), only stimulus artifacts are produced, and no neural activity is observed. As the stimulus intensity to the nerve is increased, the amplitude of the action potential increases (traces 3-5) until a point is reached where further increases in intensity (although producing a larger artifact) do not produce a larger action potential (traces 5 and 6). (Modified from Katz B. Nerve muscle and synapse. New York: McGraw-Hill, 1966.)

(trace 2); however, as the intensity is increased further, a point is reached at which a diphasic action potential is recorded (trace 3, Fig. 6). Note that there is a delay between the stimulus artifact and the diphasic action potential, which is a reflection of the time it takes for the action potential to propagate from its site of initiation (at the stimulating electrodes) to the recording electrodes. (Knowing the time delay and the distance between the stimulating and recording electrodes, the propagation velocity of the action potentials can be calculated.)

An interesting observation occurs when the stimulus intensity is increased still further. Now the amplitude of the extracellular action potential is increased (trace 4, Fig. 6). Increasing the stimulus intensity further produces yet a greater increase in the size of the action potential (trace 5, Fig. 6). Eventually, a point is reached at which increasing the stimulus intensity produces no further increase in the size of the action potential.

At first, these results may seem somewhat paradoxical because they appear to contradict the all-or-nothing law of the action potential. Recall that an action potential not only is propagated in an all-or-nothing fashion but is also initiated in an all-or-nothing fashion. We learned earlier (Fig. 3 in Chapter 4) that an increase in stimulus intensity beyond threshold produces an action potential identical in its amplitude and time course to the action potential produced with a threshold

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