158 KINETIC HORMONES — II 



4.322 Nervous control of secretory cells of Jiervous origin 



Many hormone-secreting cells derived from the nervous system 

 are neurosecretory cells. These seem always to retain an intimate 

 contact with it (§ 2.11); positive evidence of nervous control of 

 the release of secretion from these cells is limited to a few cases 

 only, but it may well be true of all. The main doubt is as to how the 

 control is achieved ; perhaps it is only a question of degree which 

 determines at what point in a series of neurones the transmission of 

 a stimulus from one to another ceases to be a nerve impulse, with 

 a minute release of the appropriate chemical substance at the nerve 

 endings, and becomes the release of a microscopically visible 

 amount of neurosecretion, passing down the axon and activating 

 the cells with which the axon is in contact. Technically the two 

 are hard to separate, since section of the axon stops the flow of the 

 chemical as completely as it stops the passage of the nerve impulse. 

 Since in either case activation only passes to the immediately 

 adjacent neurosecretory cell, a hormone (in the sense of a substance 

 circulating in the blood) is not produced, though a neurohormone 

 might be. Nervous stimulation of hormone secretion from the 

 neurosecretory cell is most likely; but in any case the process 

 remains distinct from the stimulation of gland cells by endocrino- 

 kinetic hormones in the circulation (§ 4.323). 



All too often the action of kinetic hormones has only been shown 

 in extracts, and the effect of the nervous system upon their natural 

 secretion has been neglected. Yet there is often evidence, presump- 

 tive or actual, for an action of the nervous system on the effectors, 

 although they are under the intermediate control of kinetic 

 hormones; for they produce responses that are so closely related 

 to environmental factors that it seems as though the nervous 

 system must be involved (Table 17). Examples of this are to be 

 seen in both crustaceans and vertebrates, the chromatophores of 

 which respond to changes of background that can only stimulate 

 hormone secretion indirectly through the eyes (§3.222); or in 

 CarausiuSy the pigment cells of which only respond to the effect 

 of moisture on the skin, if the ventral nerve cord is intact (§ 3.121). 



The neurosecretory cells in the brain of insects, which are the 

 sources of the endocrinokinetic hormones stimulating secretion by 



