Responses of Higher Animals: The Nervous System - 457 



nerve that consists mainly of efferent axons, 

 passing toward some muscle. When such a 

 nerve is excited — as by pinching — a series of 

 action potentials can be detected by an oscil- 

 loscope. These impulses pass not only out- 

 ward toward the muscle (causing it to con- 

 tract); but they also pass inward toward the 

 spinal cord. However, the backflow of im- 

 pulses dies out as soon as it reaches the spinal 

 cord. At the spinal cord the impulses en- 

 counter the synaptic junctures between the 

 motor neurons and the other neurons of the 

 central nervous system (Fig. 25-8), and these 

 synapses are not able to conduct in a central 

 direction. 



Apparently the dendrons represent the "re- 

 ceptive" portion of a neuron. The dendrons 

 of a postsynaptic neuron may be excited by 

 impulses coming from presynaptic axons, but 

 axons cannot be excited by impulses coming 

 from the dendrons. 



Synaptic Resistance: Localization of Re- 

 sponses. Even when impulses approach a 

 synapse in the proper direction, they may or 

 may not be transmitted, depending on a 

 variety of factors. The resistance to the pas- 

 sage of impulses is extremely variable from 

 synapse to synapse, and in any one synapse 

 from time to time. Impulses entering the 

 central nervous system from a stimulated 

 sense organ tend to follow certain paths that 

 carry the impulses to some limited and lo- 

 calized group of muscles or other effectors. 

 In other words, certain reflex arcs transmit 

 impulses more readily than others, and con- 

 sequently the application of a particular 

 stimulus frequently results in the perform- 

 ance of one or more definitely localized re- 

 flex acts. However, the forces and factors that 

 determine synaptic transmission are very 

 complex and not fully understood at present. 



Synaptic Summation. A single stimulus, 

 applied to an afferent nerve, may fail to 

 evoke any response, even though the stimulus 

 is not too weak to excite the nerve itself. But 

 the same or even a weaker stimulus may 

 evoke the response, if repeated a number of 

 times in rapid succession. This phenomenon 



of synaptic summation indicates that a series 

 of impulses leads to the accumulation of 

 some force or substance, which finally may 

 overcome a synaptic block. The success of a 

 summation depends upon the exact timing 

 of the succession of impulses arriving at the 

 synapses, and this fact is important in deter- 

 mining the reflexes of the intact animal. 

 Ordinarily when a sense organ is stimulated, 

 it discharges not one, but a volley of impulses 

 into the afferent nerve; and in the volley, the 

 frequency of the impulses increases steadily 

 as the strength of the stimulus is raised. Thus 

 a single weak stimulus, applied to a sense 

 organ, may fail to evoke a reflex, whereas a 

 stronger stimulus, even when applied just 

 once, may be entirely successful. 



Synaptic Fatigue. A prolonged and con- 

 tinuous repetition of the same stimulus sooner 

 or later results in a failure of the response. 

 This block is not necessarily due to a fatigue 

 of the synapses of the particular reflex arc, 

 since the receptor cells themselves may not 

 continue to discharge impulses if the stimu- 

 lation is continued unduly. But even when 

 the sensory nerve is excited directly by a 

 continuous series of stimuli, a block is finally 

 effected by synaptic fatigue. Synapses are 

 much more quickly fatigued than myoneural 

 junctures, which, in turn, are fatigued much 

 sooner than a muscle; nerve fibers proper are 

 practically unfatigable. Fatigue of the syn- 

 apses, like that of other tissues, is a reversible 

 process, and the synaptic block disappears 

 during periods of rest, if an adequate supply 

 of oxygen is available. 



Facilitation: Habit, Memory, Learning. 

 Assuming the fatigue is avoided, the mere 

 transmission of a series of impulses across the 

 synapses of a given reflex arc results in an 

 enduring increase in the conductivity of this 

 arc to the passage of subsequent impulses. 

 Thus it is always easier for an animal to 

 repeat a given response than to perform the 

 action for the first time. This effect, which is 

 called facilitation, is especially characteristic 

 of the brain synapses of higher animals — 

 particularly in higher vertebrates, such as 



