Responses of Higher Animals: The Nervous System - 453 



nervous system, and do not send fibers into 

 the nerves. Association neurons make contact 

 with other association neurons, as well as 

 with sensory and motor neurons. In fact the 

 interconnections of the association neurons 

 are so extensive throughout the central nerv- 

 ous system that impulses coming in from any 

 sensory nerve can find their way to any motor 

 nerve. However, impulses in all higher ani- 

 mals tend to follow well-defined and localized 

 pathways, which are determined by a variety 

 of factors (see later). 



THE NEURONS OF VERTEBRATE ANIMALS 



The neurons of all higher animals are uni- 

 nucleate cells. Typically each neuron (Fig. 

 25-7) displays one or more dendron fibers, 

 which conduct impulses toward the nucleated 

 cell body, or centron; and each neuron has 

 one (occasionally two or more) well-defined 

 axon fiber, which transmits impulses away 

 from the cell body. Both dendrons and axons 

 may display side branches, or collaterals, and 



DENDRONS 



CENTRON 



OR 

 CELL BODY--^* 



NUCLEUS^ 

 AXONS — 



COLLATERAL 



Fig. 25-7. 

 man. 



A 



MULTIPOLAR 

 TYPE 



Types of neurons, or nerve cells, found in 



UNIPOLAR 

 TYPE 



each fiber terminates in a number of twiglike 

 nerve endings. 



The neurons of vertebrates are essentially 

 comparable to those of invertebrate animals, 

 but the vertebrate nervous system represents 

 a much more intricate complex of reflex arcs, 

 as is shown diagrammatically in Figure 25-8. 



The Synapses and Other Relay Points in the 

 Reflex Arc. One main function of each neu- 

 ron is to relay impulses either to one or more 

 other neurons, or directly to an effector struc- 

 ture. Thus even a relatively simple reflex arc 

 is interrupted by one or more synapses, 

 which are the points of contact between suc- 

 cessive neurons. 



Other types of synaptic junctures are en- 

 countered: at points where receptor cells 

 make contact with the sensory neurons (Fig. 

 25-8), and at points where motor neurons 

 join the muscle fibers or other effector cells. 

 However, the physiological properties of the 

 sensory-neural and myoneural synapses may 

 be somewhat different from those of inter- 

 neural synapses. 



Electronmicrographic studies demonstrate 

 beyond doubt that the synapses are points of 

 discontinuity between the successively ar- 

 ranged cells of the sensory-neural-muscular 

 system. The gap between the membrane at 

 the tip of an axon branch and the membrane 

 of a dendron branch — and this is a typical 

 synapse — measures some 125 ± 25 Angstrom 

 units across. Generally there are a number of 

 microvesicles, called neurosecretory vesicles, 

 situated at the axon terminal, near the syn- 

 aptic membrane. 



The transmission of impulses across a syn- 

 apse involves relay mechanisms that are far 

 from simple. The synapse may act as a re- 

 sistance point at which impulses are damped 

 off completely. Synaptic resistance varies, 

 however, in the different reflex arcs, and con- 

 sequently the synapses play an important 

 role in routing impulses through the nervous 

 system and in localizing responses in the or- 

 ganism. But even more important, perhaps, 

 is that the pattern of impulses in the post- 

 synaptic neuron is usually not a carbon copy 



