THE NEURON AND ITS CONDUCTING PATHS 



109 



The Function of the Neuron. — We shall see later on that the cell- 

 body is the nutritive center of the neuron, because its destruction 

 entails the disintegration of all of its prolongations. Its purpose 

 is to produce the nerve impulse and to convey it to distant parts. The 

 arrangement in each neuron, however, is such that it can conduct in 

 only one direction, namely from the dendrites to the axon. It pos- 

 sesses, therefore, a distinct polarity, 

 the former prolongations being the 

 avenues by which the nerve impulse 

 is received and the latter the path 

 by which it is conveyed to other parts. 

 The general arrangement of the 

 neuron, therefore, depends in a large 

 measure upon the connections which 

 it must establish with neighboring 

 nerve-cells for functional purposes. 



Neurons are usually designated as 

 afferent or sensory and as efferent or 

 motor. The former conduct impulses 

 from the periphery to the center and 

 the latter from the center to the peri- 

 phery. Moreover, since several neu- 

 rons of each type are always required 

 to cover large distances, they are 

 commonly arranged in series and are 

 then differentiated from one another 

 by characterizing them as neurons of 

 the first, second, third, and so forth 

 order. Just how many of them are 

 required to unite two widely sepa- 

 rated points of the nervous system 

 differs greatly. Thus it is said that 

 some of the efferent neurons of the 

 spinal cord attain a length of 0.5-1.0 

 m., so that the distance between the 

 cortex of the cerebrum and the foot 

 may be covered by no more than two 

 neurons, their relay station being 

 situated in the anterior horn of the 

 gray matter of the lumbar cord. On the afferent side, the path is 

 less direct and hence, a more frequent relaying is made necessary. 

 . Thus, a sensory impulse generated in the foot, generally requires three 

 or four consecutive neurons for its passage into the cerebrum. 



Reflex Action. — The simplest relationship between these afferent 

 and efferent neurons is presented by the so-called reflex circuit 

 which permits of the occurrence of the simplest possible reaction, 

 known as the reflex act. The responses executed with the help of 



Fig. 62. — M, motor neuron; S, 

 sensory neuron; M, motor end- 

 organ; 5, sensory end-organ; A, axis 

 cylinder; Af/S, myelin sheath; iV, neuro- 

 lemma; C, collateral; CB, cell-body; 

 D, dendrites; Nu, nucleus and nu- 

 cleolus; R, sensory terminals. 



