428 



ORGAN SYSTEMS OF MAN 



impulses that cause the sweat glands to 

 pour their secretion over the skin, and thus 

 cool the body. Thousands of these pathways 

 are continually carrying messages to and 

 from parts of the body, most of them un- 

 known to the owner. 



The autonomic nervous system is divided 

 into two rather distinct parts: the thora- 

 columbar (sympathetic) which is com- 

 posed of the double chain of ganglia already 

 noted; and the craniosacral (parasympa- 

 thetic) which originates in the posterior 

 portion of the brain (midbrain and me- 

 dulla) and the sacral region (Fig. 16-24). 

 The thoracolumbar system has short pre- 

 ganglionic fibers and generally long post- 

 ganglionic ones. The opposite is true of the 

 craniosacral system, and in fact, the ganglia 

 lie in some cases, such as the heart, em- 

 bedded within the organ itself. These two 

 systems send nerve fibers to all of the organs 

 operating involuntarily so that each has a 

 double innervation. However, the two sys- 

 tems produce opposite effects. For example, 

 if the nerve from the thoracolumbar system 

 to the heart is stimulated, the beat is accele- 

 rated, but if the nerve from the craniosacral 

 system is stimulated the beat is slowed 

 down. Stimulation of one nerve may cause 

 excitation in one organ and inhibition in 

 another, thus stimulation of the vagus ( cra- 

 niosacral ) accelerates the heart but inhibits 

 the stomach. The value of such a mechanism 

 is obvious. It is the interaction of these two 

 systems that regulates the flow of blood to 

 various parts of the body, differing with 

 each condition in which the animal finds 

 itself. It also causes the pupil of the eye to 

 dilate or constrict, depending on the amount 

 of light that is needed for vision. These 

 and hundreds of other routine jobs the 

 body does quietly and efficiently and en- 

 tirely without the knowledge of the owner. 

 This system has certainly taken the drudg- 

 ery out of operating the body, and has left 

 for the higher centers the job of getting the 

 whole organism in a position to obtain food 

 or to do the many other things that are es- 



sential for life. If the central nervous system 

 were burdened with the job of operating 

 this machinery, little else could be done. It 

 would be like requiring the President of the 

 country to see that the proper amount of 

 water flows through a certain aqueduct in 

 New York City. 



Biologists have been bothered by the 

 problem as to why impulses arriving in an 

 organ via either part of the autonomic sys- 

 tem cause acceleration or inhibition even 

 though the nerves appear to be identical, 

 and what the mechanism is that differenti- 

 ates between them. Some ingenious experi- 

 ments have been performed to give an 

 answer to these perplexing questions. It has 

 been shown that a chemical (neurohumor) 

 is secreted at the point of juncture between 

 the nerve endings and the organ innervated. 

 Furthermore, the chemical is different for 

 the two divisions of the autonomic system, 

 which is what one might expect if the 

 action is the opposite. Stimulation of the 

 craniosacral system, for example, produces 

 acetylcholine at the nerve endings, and 

 stimulation of the thoracolumbar produces 

 sympathin. Thus, in order to complete the 

 mission of delivering a message to a muscle 

 or gland, a physical and a chemical action 

 must take place. A substance known as 

 choline esterase is present, which counter- 

 acts the neurohumor and thus prevents 

 cumulative effects. The action of sympathin 

 is much like that of adrenalin, the secre- 

 tion from the medullary portion of the 

 adrenal glands (endocrine) and its action 

 is antagonistic to acetylcholine. It has no 

 inhibitor and must be destroyed by oxida- 

 tion some time after it is formed. 



The discovery of specific neurohumors 

 in the autonomic nervous system that 

 bridge the gap between nerve endings and 

 muscle has naturally led scientists to postu- 

 late that perhaps the same mechanism 

 causes the bridging of the gaps between 

 neurons in the central nervous system. Im- 

 pulses passing to the synapse differ both in 

 frequency and duration from those leaving 



