Cable Properties of Neurons

If a pair of stimulating electrodes produces a depolarization that is too weak to cause the opening of voltage-regulated Na+ gates—that is, if the depolarization is below threshold (about -55 mV)—the change in membrane potential will be localized

The Nervous System: Neurons and Synapses 165

Absolute refractory period

(due to inactivatec Na+ chann

Relative refractory period

(due to continued outward diffusion els) of K+)

^-^

Time (milliseconds)

Time (milliseconds)

■ Figure 7.16 Absolute and relative refractory periods. While a segment of axon is producing an action potential, the membrane is absolutely or relatively resistant (refractory) to further stimulation.

to within 1 to 2 mm of the point of stimulation. For example, if the stimulus causes depolarization from -70 mV to -60 mV at one point, and the recording electrodes are placed only 3 mm away from the stimulus, the membrane potential recorded will remain at -70 mV (the resting potential). The axon is thus a very poor conductor compared to a metal wire.

The term cable properties refers to the ability of a neuron to transmit charges through its cytoplasm. These cable properties are quite poor because there is a high internal resistance to the spread of charges and because many charges leak out of the axon through its membrane. If an axon had to conduct only through its cable properties, therefore, no axon could be more than a millimeter in length. The fact that some axons are a meter or more in length suggests that the conduction of nerve impulses does not rely on the cable properties of the axon.

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