154 KINETIC HORMONES — II 



has been reported to have opposite effects upon the same muscle 

 at different dosages; this suggests that the other cases might be 

 accounted for by different threshold levels of sensitivity. It seems 

 more likely, however, that there may be some specificity in the 

 muscles themselves, akin to the differentiation of many visceral 

 muscles into those which are sensitive to sympathetic stimulation 

 and those which are inhibited by it, but are usually sensitive to 

 parasympathetic stimulation (Rosenblueth, 1950). 



Otherwise the apparent lack of inhibitor hormones may partly 

 be because muscles and myoepithelial cells give an all-or-none 

 reaction to stimulation and remain indefinitely quiescent in its 

 absence ; no intermediate state of contraction of any given cell can 

 be achieved by a balance betw^een stimulating and inhibiting 

 factors, such as can arrest the pigment in a chromatophore at any 

 position between the extremes of dispersion and concentration. 

 Graded responses might seem more probable for glands; but it 

 may well be that a similar all-or-none reaction to that in muscles 

 holds good also for the individual secreting cells within the gland, 

 and that changes in rate of secretion depend upon the number of 

 cells induced to release their secretory products into the gland 

 lumen. 



When kinetic hormones control chromatophores the effect is 

 usually rather slow ; but the rate differs in different organisms and 

 may depend chiefly upon the concentration of the hormone 

 reaching the effector in any given case. In achieving and main- 

 taining a protective background response, or in adapting an eye to 

 changes in daylight intensity, a slow response may be an advantage 

 in preventing sudden and conspicuous changes. 



Muscles and glands often show rapid responses to kinetic 

 hormones, and may complete their reactions in a matter of minutes 

 or even seconds, like the response to heart-accelerators (§ 3.111), 

 or to oxytocin inducing milk "let-down" (§ 3.114), or to ACTH 

 stimulating the release of ACH (§ 4.231). In such cases the only 

 limitation on the speed of reaction seems to be the time that the 

 hormone takes to pass in the circulation from its source to the 

 effector. The effector, once stimulated, can react as quickly to a 

 hormone as to a nerve impulse. In the case of rhythmically con- 

 tracting muscles, like those of the heart or those causing gut 



