CHAPTER IX 



NEUROSECRETION 



One of the most interesting and important developments in 

 physiological work of the last 30 years has been the recognition 

 that certain nerve cells are capable of elaborating and releasing 

 complex organic substances acting as hormones. These cells have 

 become modified to this purpose often to the subordination of their 

 function as neurones. The transmitter substances released at 

 synapses are in general localized in vesicles in these situations and 

 act upon the immediately adjacent nerve and muscle cells. But in 

 the neurosecretory cells the materials are often released directly 

 into the blood system to be transported round the body and act 

 upon some distant organ. Others act directly upon near-by glandu- 

 lar structures inducing them to secrete their own hormones. 



Many of these neurohormone systems are now known, for example 

 the hypothalamus-pituitary axis of vertebrates; and the sinus 

 gland-X gland complex of crustaceans. The effects of the hormones 

 released from these glands have actions upon chromatophores, 

 moulting in insects, migration of eye pigments in crustacean eyes, 

 control of heart beat (pericardial glands in Crustacea), sexual cycles, 

 and regeneration (polychaetes). 



Pioneer workers in this field were the Scharrers (B. and E.). In 

 1937 they wrote a review of the field as it was known at that time, 

 and already neurosecretory granules had been discovered in the 

 cerebral ganglia of L. terrestris. This conclusion was based upon 

 the observation of granules staining with particular reagents and 

 forming vesicles and vacuoles within the nerve cells. However, no 

 further details were forthcoming upon the number and distribution 

 of these cells or whether the contents varied from time to time or 

 under certain known experimental conditions. 



The existence of neurosecretory cells in L. terrestris was con- 

 firmed by Schmid (1947) who obtained evidence for the occurrence 



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