RESPONSIVENESS IN VERTEBRATES 



Coordination with Special Reference 

 to the Internal Environment 



Cellular metabolism, on which life depends, requires a high degree of 

 coordination in the supply of materials and removal of products, whether 

 waste or synthetic. We have discussed in the preceding chapter the major 

 functions of the vertebrate animal that subserve the metabolic requirements 

 of its cells. These varied activities are coordinated in such a way that the 

 environment of individual cells within the body normally varies within narrow 

 limits, no matter what the changes in the external environment may be. If 

 these limits of physiological variation are exceeded, if the regulatory mech- 

 anisms fail or are unable to cope with a major emergency, death results. 



Two different kinds of organs cooperate in the regulation of the internal 

 environment. These are the autonomic nerves and the endocrine glands. 

 Both can alter the chemical environment of individual cells — the autonomic 

 nerves, like other nerves, by means of neurohumors and the endocrine glands 

 by means of the substances they pass into the blood stream. Interactions 

 between these involuntary coordinators occur in many vital processes, and 

 they can be modified in some cases by emotional reactions and conscious 

 nervous responses. 



Regulation by Autonomic Nerves. Gland cells, cardiac muscle, and 

 non-striated muscle of blood vessels, coelomic viscera, and other organs are 

 the effectors served by the division of the peripheral nervous system known as 

 the autonomic nerves. Autonomic nerves are made up of only efferent fibers, 

 two of which are involved in the conduction of any impulse from the central 

 nervous system to an effector. The cell body of the first neuron in the chain 

 of two is located in the central nervous system, and the fiber leaves the 

 central nervous system over the ventral root of a cranial or spinal nerve. 

 This fiber is called the preganglionic fiber because it ends in a ganglion which 

 contains the cell bodies of the postganglionic fibers. Postganglionic fibers 

 of sympathetic nerves extend some distance from the ganglia of the sympa- 

 thetic trunks and those among the viscera to blood vessels, glands, and visceral 

 organs. Since the ganglia of the parasympathetic nerves are located on or 

 near the organs served, their postganglionic fibers are relatively short. 

 Sympathetic and parasympathetic nerves also differ from one another with 

 respect to the kind of neurohumor secreted by the endings of their post- 

 ganglionic fibers. The postganglionic fibers of sympathetic nerves liberate a 

 mixture of two neurohumors known as epinephrine and norepinephrine which 

 are closely related chemically. In this these nerve fibers are unique, since 

 all other nerve fibers, including the postganglionic fibers of the parasympa- 

 thetic nerves, release acetylcholine. This compound is broken down very 

 rapidly and, consequently, an effector responds for a very limited period to an 

 impulse discharged from a parasympathetic nerve. On the other hand, when 

 sympathetic nerves are excited, impulses are discharged not only in the organ 



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