COORDINATION AND CONTROL: (1) THE NERVOUS SYSTEM 



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Suppose that the pain receptor is in the skin of the right forefinger 

 and that the axon of the motor neuron ends in a muscle that retracts this 

 finger. Any stimulus, a pinprick, for instance, that stimulates the pain 

 receptor will initiate a nervous impulse at that point. This sweeps up 

 the dendrite of the sensory nerve, passes on into its axon, and makes a 

 synapse with the dendrite of the ad just or neuron. The nerve impulse 

 now moves across this neuron and into its axon and makes a second 

 synapse with the dendrite of the motor neuron ; it passes into the axon of 

 this neuron and thus reaches the muscle fibers, which immediately con- 

 tract to jerk the finger away from the pin. The whole process, from pin- 

 prick to jerk, requires but a small fraction of a second, but it has involved 

 all the events enumerated above. 



Fig. 7.6. Diagram of a simple reflex arc. (a) Axon; (d) dendrite. 



Any chain of neurons leading from a receptor to an effector (in a mus- 

 cle or gland) is a reflex arc; and the action that results from a stimulus 

 received through such an arc is a reflex action. Many reflex arcs — hence 

 much reflex action — are far more complicated than the one described 

 above. They may involve many neurons and traverse long distances 

 through the brain and spinal cord. Figure 7.6 includes only three neurons 

 (actually many thousands would be shown in any thin slice from the 

 cord), but it will be noted that the tips of all the axons are finely branched 

 and that both the adjustor and efferent neurons have a number of den- 

 drites. This branching provides for many possible synapses, other than 

 the two that are shown. It is also obvious that the greater the number of 

 connecting or adjustor neurons involved the greater will be the possible 

 variety of synapses among different neurons. 



If we exclude the autonomic system and the connections with sensory 

 receptor cells, all synapses occur within the central nervous system — i.e. 



