RESPONSIVENESS IN VERTEBRATES 



legs on the same side. These movements tending to remove the acid are made 

 possible by the passage of impulses over adjuster neurons which conduct them 

 anteriorly and posteriorly to efferent neurons leading to muscles of the fore 

 and hind legs. The reactions described occur on the side of the animal to 

 which the acid has been applied. If under such conditions the hind leg on 

 this side be held, the muscles of the hind leg on the opposite side will 

 respond to the original stimulus by contracting. This effect is made possible 

 by the presence of adjustor neurons which conduct impulses from one side of 

 the spinal cord to the other and thus bring about bilateral coordination. 



In examples given we have been concerned with isolated reflexes; that is, 

 particular reflex arcs have been discussed as if they were separable from the 

 remainder of the nervous system. Such is obviously not the actual state of 

 affairs. During life great numbers of sensory neurons are conducting impulses 

 to the central nervous system at all times. Within the central nervous system, 

 fibers of a number of different sensory neurons may end on the dendritic 

 processes or cell body of a single sensory or adjustor neuron. Different axonic 

 processes of a single sensory or adjustor neuron may end on different adjustor 

 or motor neurons. It has been reported that the cell body of a single motor 

 neuron in the spinal cord of a mammal may have as many as 1800 end feet of 

 terminal filaments on its surface. Over this apparent maze of pathways pass 

 in orderly fashion the nerve impulses that make possible not only simple reflex 

 actions but higher nervous coordinations as well. 



The factors which determine the smooth function of the nervous system are 

 complex and not entirely understood. In an organ of special sense there are 

 cells that are much more sensitive to a certain kind of change in the environ- 

 ment than to any other. The response of sensory cells to stimuli apparently 

 involves a change in the permeability of their membranes, modifying the ionic 

 or chemical environment of nerve endings in their vicinity. In some way this 

 brings about excitation of the nerve endings so that their membranes exhibit 

 increased permeability to sodium and potassium ions; the movement of ions 

 results in the establishment of an electric current. This excitation builds 

 up in the nerve endings until a nerve impulse of the so-called all-or-none type 

 is touched off and conducted along the nerve fiber in a manner to be described 

 later (p. 112). The more intense the stimulus, the more frequent the initia- 

 tion of the impulses; the frequency varies from 50 to 100 impulses per second. 

 So long as the excitation is adequate, impulses of uniform intensity are 

 established and travel toward the central nervous system and into all processes 

 of the neuron at an undiminished rate. In the terminal filaments in the 

 region of the synapses the arrival of an impulse, lasting about 1/1000 of a 

 second, is probably associated with the transitory production of acetylcholine 

 (p. 115) which increases the permeability of the membrane of the dendrite 

 or nerve-cell body of the neuron with which the synapse occurs. Local 

 responses at the many synapses on adjustor and motor neurons vary in in- 

 tensity and are known as graded responses in contrast to the all-or-none 

 responses of axonic processes. As a result of the phenomenon of facilitation, 



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