INTRODUCTION 807 



A. Introduction 



1. Definition 



The nervous system serves to integrate the various parts of the animal into 

 a functional whole, and also to relate the animal with its environment. It 

 consequently is specialized to detect changes in the environment (irritability) 

 and to conduct (transmit) the impulses aroused by the environmental change 

 to distant parts of the organism. The environmental change provides the 

 stimulus, the protoplasmic property of irritability detects the stimulus, and 

 transmission of impulses thus aroused makes it possible for the animal to 

 respond once the impulse reaches the responding mechanism. This series of 

 events is illustrated well in less complex animal forms such as an ameba. In 

 this organism, the stimulus aroused by an irritating environmental change is 

 transmitted directly to other parts of the cell, and the ameba responds by 

 a contraction of its protoplasm away from the source of irritation. On the 

 other hand, the complex structure of the vertebrate animal necessitates an 

 association of untold numbers of cells, some of which are specialized in the 

 detection of stimuli, and others transmit impulses to a coordinating center, 

 from whence still other cells convey the impulses to specialized effector (re- 

 sponding) structures (fig. 352A). 



2. Structural and Functional Features 

 a. The Morphological and Functional Unit of the Nervous System 



There are two opposing views regarding the morphological and functional 

 unit of the nervous system. One view, widely championed, postulates that this 

 unit is a specialized cell called the neuron. The neuron is a distinct cellular 

 entity, having a cell body containing a nucleus and a central mass of cyto- 

 plasm from which extend cytoplasmic processes of various lengths (fig. 352B). 

 The nervous system is made up of many neurons in physiological contact 

 with each other at specialized functional junctions known as the synapses 

 (fig. 352A). The synapse represents an area of functional contact specialized 

 in the conduction of impulses from one neuron to another. However, it is not 

 an area of morphological fusion between neurons. Each neuron, according 

 to this view, originates from a separate embryonic cell or neuroblast of ecto- 

 dermal origin, and each develops a definite polarity, i.e. impulses normally 

 pass in one direction to the cell body and from thence distad to the area of 

 synapse. 



A contrary, older view is the reticular or nerve-net theory. This theory as- 

 sumes that the nerve cells and their processes are a continuous mass of proto- 

 plasm or syncytium in which the "cell bodies" are local aggregations of a 

 nucleus and a cytoplasmic mass. The entire controversy between this and the 

 neuron theory revolves around the "synapse area." The neuron doctrine as- 



