VAGUS 

 NERVE 



Fig. 25-9. Diagram of Otto Loewi's experiment, dem- 

 onstrating the neurohumor, acetylcholine. Stimula- 

 tion of the vagus nerve of the upper heart soon 

 retarded the beating of the lower heart as well. 



flows from one heart to the other. Similarly, 

 excitation of the accelerator (sympathetic) 

 nerve to the upper specimen accelerated the 

 beat in both hearts. This indicated the lib- 

 eration of another neurosecretion, or excita- 

 tory substance. 



Subsequent work resulted in the isolation 

 and identification of the cardiac decelerating 

 factor. It is a fairly simple nitrogenous com- 

 pound, namely, acetylcholine. Acetylcholine 

 is secreted by the terminal axon branches in 

 many parts of the nervous system. It serves 

 as the excitatory agency (1) at the myoneural 

 synapses of all motor neurons, (2) at all syn- 

 apses in the parasympathetic (p. 465) system, 

 and (3) at the synapses of the preganglionic 

 neurons of the whole sympathetic (p. 466) 

 nervous system. Acetylcholine is probably 

 produced also at many synapses within the 



Responses of Higher Animals: The Nervous System - 455 



central nervous system, that is, at the syn- 

 apses joining the successive association neu- 

 rons of various reflex arcs. Collectively, 

 therefore, such neurons, which are excited 

 by acetylcholine, are said to be cholinergic. 



Acetylcholine is liberated, probably, from 

 the microvesicles of the axon tips, into the 

 synaptic gaps. Here it contacts the dendritic 

 membranes, causing excitation. Such excita- 

 tion is short-lived, however. A special enzyme, 

 cholinesterase, soon initiates a hydrolytic 

 breakdown of the excitatory substance into 

 acetate and choline which are inactive. 



The heart accelerator factor has also been 

 identified. It is noradrenaline (p. 406), for- 

 merly referred to as sympathin. Noradrena- 

 line has a more restricted action as a synaptic 

 transmitter. It is produced by the postgangli- 

 onic neurons of the sympathetic system and, 

 perhaps, by some neurons in the brain stem 

 (p. 459). Neurons that are excited by nor- 

 adrenaline are referred to as adrenergic. 



In addition to acetylcholine and noradren- 

 aline, at least three other synaptically active 

 substances are indicated by current research. 

 These are serotonin (5-hydroxytryptamine), 

 factor S, and factor I. Serotonin may act as 

 a synaptic transmitter in some neurons within 

 the central nervous system; factor S (sensory 

 factor) appears to be involved in the forma- 

 tion of the generator potential (p. 456) in re- 

 ceptors generally; and factor I (inhibition 

 factor) appears to mediate the inhibitory ef- 

 fects of certain nerve cells in the central 

 nervous system. 



FUNCTIONS OF NERVE CELL BODIES 

 AND DENDRITES 



Previously it was thought that the sole 

 function of the cell body, or centron, was to 

 nourish its connected fibers (dendrites and 

 axons). The nucleus, of course, lies in the 

 cell body, and if an axon or dendrite is di- 

 vorced from its centron it begins to degener- 

 ate after several days. In fact, following the 

 pathway of such degeneration has enabled 

 neuroanatomists to trace out many of the 



