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VERTEBRATE LIFE AND ORGANIZATION 



Figure 29.7. The transmission of a nerve impulse. (Modified after Guyton.) 



trie current is a flow of electrons; the nerve impulse a wave of depolari- 

 zation involving changes in the permeability of the plasma membrane 

 and the movement of ions. The electric current derives its energy from 

 the difference in potential at the opposite ends of the wire; the nerve 

 impulse, from chemical changes that take place in each part of the 

 neuron. In this respect, the transmission of a nerve impulse is analogous 

 to the burning of a fuse: the powder in a given part of the fuse provides 

 the energy for the burning of the fuse in that region. 



As the nerve impulse passes along the neuron, the membrane be- 

 comes repolarized at the site of stimulation, and a wave of repolariza- 

 tion spreads along the neuron. However, it takes a measurable period 

 of time for the chemical changes responsible for repolarization to occur. 

 During the period that the membrane is depolarized, a second impulse 

 cannot be transmitted along the neuron, but as soon as the membrane 

 is repolarized, another impulse can proceed. The very brief period, one 

 or two milliseconds, during which the neuron is recovering is known as 

 the refractory period. The presence of such a period means, of course, 

 that a neuron can transmit only a limited number of impulses per unit 



