COORDINATION AND CONTROL: (1) THE NERVOUS SYSTEM 109 



Ganglia. A ganglion is a group of nerve-cell bodies held together 

 with a small amount of connective tissue. Ganglia are found in connec- 

 tion with the autonomic nervous system and are present as part of the 

 dorsal root of each spinal nerve. Ganglia are outside the spinal cord and 

 brain. 



Nerve centers or "nuclei." These are groups of nerve-cell bodies 

 within the spinal cord or brain. 



SENSATION AND THE SENSES 



The central nervous system is constantly receiving impulses from a 

 large variety of sources. In addition to the sensory impressions received 

 from the organs of sense — smell, taste, hearing, sight, and balance — there 

 are a number of general sense receptors distributed to all surfaces of the 

 body, where they are located in the dermis of the skin. This latter group 

 includes receptors for touch, pressure, heat, cold, and pain. Another group 

 of receptors ending in the muscles, joints, and tendons bring in kinesthetic 

 information that has to do with the stress, position, and tension of the 

 various body parts. Still other receptors in the visceral system give rise 

 to impulses that do not ordinarily affect consciousness. 



Each of these numerous kinds of receptors, however it be stimulated, 

 can convey only its own particular type of information. Stimulation of a 

 touch receptor can produce only a sensation of touch ; stimulation of the 

 retina or optic nerve can produce only a sensation of light, even though 

 it is an electric needle or a knife that has produced the stimulation. Since 

 all nerve impulses appear to be identical, we are forced to conclude that 

 the impulses received over any given receptor neuron can be interpreted 

 in only one specific way; that, however this neuron is stimulated, the 

 nerve impulse that results can produce only the type of sensation nor- 

 mally received by its receptor. If the point of a rather blunt needle is 

 brought gently against the skin, only a touch receptor will be stimulated; 

 if it is pressed harder, pressure receptors will also be stimulated, and a 

 sense of pressure is combined with or supersedes the sensation of touch; 

 if the needle is still more strongly pressed against the finger, the some- 

 what less sensitive pain receptors are stimulated, and a sensation of pain 1 

 results. We know, of course, from everyday experience that various 

 objects may stimulate two or more types of sense receptors at one time, 

 as when the handling of a small piece of ice gives rise to sensations of both 

 cold and touch. 



1 The nerve endings, which, when stimulated, produce a sensation of pain, are 

 numerous and widely distributed over the surface of the body but end nakedly. 

 Touch, pressure, cold and heat receptors, on the other hand, end in capsules (or, in 

 the case of some touch receptors, at the bases of the hairs) that appear to make them 

 much more sensitive to their specific sources of stimulation. 



