Measuring the Longdistance Signal in Neurons

What kind of signal carries the message along the sensory neuron in the patellar reflex? As described in Chapter 1, the signals in the nervous system are electrical signals, and to monitor these signals it is necessary to measure the changes in electrical potential associated with the activation of the reflex. This can be done by placing an intracellular microelectrode inside the sensory axon to measure the electrical membrane potential of the neuron. A diagram illustrating this kind of experiment is shown in Figure 6-1. A voltmeter is connected to measure the voltage difference between point a, at the tip of the microelec-trode, and point b, a reference point in the ECF. As shown in Figure 6-1b, when

Figure 6-1 An example of an action potential in a neuron. (a) An experimental arrangement for recording the membrane potential of a nerve cell fiber. (b) Resting membrane potential and an action potential recorded by a microelectrode inside the sensory neuron of the patellar reflex loop. (c) A series of action potentials in a single stretch-receptor sensory fiber during stretch of the muscle. The lower trace shows a single action potential on an expanded time scale to illustrate its waveform in more detail.

Voltage-sensing microelectrode

Sensory nerve fiber

Voltage-sensing microelectrode

Sensory nerve fiber

Microelectrode penetrates fiber

Microelectrode penetrates fiber

Em 0

Microelectrode outside fiber

Resting membrane potential

-100

Stretch muscle

Time

Muscle length

0 0

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