Chap. 16 CONDUCTION AND COORDINATION— NERVOUS SYSTEM 



287 



rately and in quick succession like bullets from a machine gun. The time 

 between them is the interval of restoration of the electrical charges, called 

 the refractory period because it is the instant when progress is balked for 

 about one to five thousandths of a second. 



Association of Nerve Cells by Synapses 



Synapses are points of contact between nerve cells (Figs. 16.9, 16.10). In 

 passing from cell to cell every impulse must go through a synapse, but it can 

 do so in only one direction. (This is in contrast to the movement of an impulse 

 through the axons and other parts of individual nerve cells which experiment 

 has shown may be either toward the cell body or away from it.) A synapse 

 is a junction of resistance through which impulses pass more slowly than 

 along the nerve fibers. The passage of impulses varies in different synapses 

 and in different physiological conditions of the same synapse. It may be rapid 

 and easy or it may be almost or completely stopped. This is true in the brain 

 when words escape one's memory, then suddenly return. By their selective 

 resistance synapses determine that the proper muscles reply to certain stimuli 

 in an orderly fashion while others remain inactive. They are, at least in part, 

 the basis of the relative quickness of accustomed reaction and thinking and 

 also of relative slowness or nervous fatigue. 



|— REFLEX ARC — , 



I synapse I 



efferent i afferent 

 nG-uron ^_^ neutron 

 (iTLotor)\ ^^ (sen^orij) 



cell bod-Q of aff. neuron 



Fig. 16.9. Synapses, the places of communication between nerve cells. Diagram 

 of the synapse of two nerve cells of the earthworm. A change occurring in a sen- 

 sory cell in the skin is conducted over the axon to its end branches that are inter- 

 twined with the dendrites of the motor cell. From there it passes through the cell 

 body and axon of the motor cell to the muscle. At the synapse the fibers appear 

 continuous but observation has shown that they are only in contact. (After Parker. 

 Courtesy, Ham: Histology, ed. 2. Philadelphia, J. B. Lippincott and Co., 1953.) 



