F

Action potential

Action potential

Microelectrode outside fiber

Resting membrane potential

Stretch muscle

Em ■

r|

V

V

100msec

20mV

1 msec p-r the microelectrode is outside the sensory axon, both the microelectrode and the reference point are in the ECF, and the voltmeter records no voltage difference. When the electrode is inserted into the sensory fiber, however, it measures the voltage difference between the inside and outside of the neuron, the membrane potential. As expected from the discussion in Chapter 5, the membrane potential of the sensory fiber is about -70 mV.

When the muscle is stretched (Figure 6-1b), the membrane potential in the sensory fiber undergoes a dramatic series of rapid changes. After a small delay, the membrane potential suddenly jumps transiently in a positive direction (a depolarization) and actually reverses in sign for a brief period. When the potential returns toward its resting value, it may transiently become more negative than its normal resting value. The transient jump in potential is called an action potential, which is the long-distance signal of the nervous system. If the stretch is sufficiently strong, it might elicit a series of several action potentials, each with the same shape and amplitude, as illustrated in Figure 6-1c.

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