CHAPTER XC1I 

 THE RECEPTORS 



Having reviewed the fundamental conditions of conduction in nerve 

 fibers and in the reflex arc, we are now in a position to consider the 

 arrangements of neurons which form the basis of the principal aspects 

 of central nervous activity. We will consider the afferent part of the 

 reflex arc, the activity of which gives rise to the discharge of impulses 

 over motor neurons in reflex activity, and to the phenomena of sensa- 

 tion and discrimination which determine the nature of volitional acts. 

 Disturbances in these arrangements give rise to the sensory symptoms of 

 nervous disease. Such a consideration must start with the study of the 

 receptors or sense organs. 



The Evolution of Specialized Receptors 



The receptor is a cell specialized in such a way as to be excited by 

 minute changes in the condition of its environment. As a result of 

 such excitation reflex actitity is set up which causes the animal to re- 

 spond, usually in a way which is to its advantage, and sensations are 

 produced which give information concerning the nature and position of 

 the stimulus and its probable consequences. The primitive type of re- 

 ceptor, which appears in the coelenterates and occurs generally in most 

 invertebrates consists of an epithelial cell from which a fiber extends 

 into the central nervous system. Such an arrangement persists in the 

 olfactory sense organs in man. With the development of longer nerve 

 trunks in the vertebrate nervous system the cell body is no longer found 

 at the termination of the afferent fiber, but has taken up a position 

 nearer to the central nervous system, in the spinal ganglion. The fibers 

 of such an afferent neuron terminate peripherally in a number of fine 

 branches which extend along the cells of the epithelium, forming a sense 

 organ known as a free nerve termination. The sense of pain in man 

 can be definitely associated with receptors of this type. The highest 

 degree of sensitivity cannot be attained by such an arrangement. For 

 this purpose special cells in the skin become modified into receptors, and 

 about these terminate the fibers of the sensory nerves. Thus one cell 

 serves as a receptor, while the other is concerned primarily with con- 

 ducting the disturbance set up in the receptor to the central nervous 

 system. The receptors of the ear, the eye, the sense of taste and probably 

 other cutaneous sensations are of this type. (Fig. 212.) 



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