108 



THE INDIVIDUAL ORGANISM 



portions. An electric current flows from the passive to the active region, 

 and this causes breakdown of the film in the adjacent region, which 

 thereupon itself becomes active. A wave of excitation is thus propagated 

 throughout the cell. In its wake the nerve fiber is left briefly insensitive, 

 but as soon as oxidation has restored the film, the neuron is ready to 

 respond in the same way to the next, stimulus. The restoration is very 

 swift, so that impulses can follow one another with great rapidity. The 

 rate at which an impulse progresses along an axon or dendrite varies 

 markedly in different types of nerves. It has usually been measured at 

 between 20 and 100 meters per second, the slower rates being more 



symbol for 

 o synapse 



sensory neuron 



on enlarged and 

 semi-realistic detail 

 of a single synapse 



Fig. 7.3. Diagram of seven neurons, showing the cell bodies (containing nuclei), the axons 

 (a), dendrites (d), and synapses between neurons (shaded). {Redrawn from Gates, Ele- 

 mentary Psychology, by permission The Macmillan Company.) 



characteristic of nerve fibers that run to the internal organs. The nature 

 of the nerve impulse is apparently the same in all nerves, regardless of 

 the kind of stimulus that provokes it. 



The synapse. A nerve impulse runs along a chain of neurons like a 

 series of explosions, each neuron "touching off" the next. Transmission 

 occurs at the synapse or place where the axon of one neuron touches the 

 cell body or dendrites of another. Upon excitation the axon produces a 

 tiny amount of a substance, acetylcholine, that sets off an impulse in the 

 second neuron. An impulse can therefore be transmitted across a synapse 

 in only one direction — from axon to cell body or dendrite. The action of the 

 synapse as a one-way valve for impulses is fundamental to the operation 

 of the nervous system. 



