Teaching Physics with the Physics Suite Edward F. Redish

The power of a nerve

In the propagation of an electrical pulse along a non-myelinated nerve axon electrical energy stored in the axon's capacitance (i.e., in the separation of charge across the nerve membrane) is discharged and then must be recharged again. Consider a 10 cm axon.

1. The capacitance of an axon per unit length is about 0.3 μF/m and the resting voltage across the membrane is about 70 mV. Estimate the electrical energy required to send a single pulse down the axon.

2. Nerve impulses are short (about 1 ms) and a typical activated rate of sending pulses is 100 pulses/s. Estimate the power in Watts required to maintain teh 10 cm of nerve axon activated.

(Note: The way an axon functions is really more complex than this. We are ignoring the crucial phenomenon of the action potential -- the pulse carrying signals down the axon. Our result just offers an order of magnitude estimate of the energy required.)

RECALL: In units
"m" means milli = 10-3
"μ" means micro = 10-6
"n" means nano = 10-9.

Thus, 750 mF = 750 micro-Farads = 750 x 10-6 F, etc.

NOTE: 1 F = 1 Farad = (1 Coulomb)/(1 Volt)

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