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HANDBOOK OF PHYSIOLOGY ^-^ NEUROPHYSIOLOGY I 



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FIG. 12. Acetylcholine excitation of pressure receptors in the cat's carotid sinus, a: pressure in 

 sinus, 25 mm Hg; injection of 0.5 ml saline, b: same pressure; injection of 0.5 ml of io~' g per ml 

 acetylcholine, c: pressure, iii mm Hg; i.o ml saline, d: same pressure; 1.0 ml acetylcholine 10 » 

 g per ml. Time, o. i sec. All records made 95 sec. after injection. [From Diamond (17).] 



cat are still able to produce a normal frequency- 

 pressure curve when perfused with i per cent hexa- 

 methonium (fig. 13), although the acetylcholine effect 

 is lilocked lay a concentration of io~^ he.xamethonium 

 (17). In the case of the carotid body, the chemical 

 receptors of which appear particularly sensitive to 

 acetylcholine, large doses of blocking agents diminisn 

 the response to low oxygen tensions (61). 



Physostigmine does not affect the response of 

 mechanical receptors in cat's and dog's skin to me- 

 chanical stimulation (13), nor does it alter the 

 pressure-frequency relationship of the pressure re- 

 ceptors of the cat's carotid sinus (17). In two types of 

 chemical receptor, anticholinesterases do enhance the 

 response to the physiological stimulus; thus physo- 

 stigmine and prostigmine increase the activity of 

 chemical receptors of the cat's carotid sinus and 

 prostigmine increases that of chemical receptors in 

 the cat's tongue. 



These results suggest that acetylcholine cannot be 

 an intermediary in the normal process of excitation 

 at many types of receptor. Against this evidence, it 

 has been argued that the blocking agents do not have 

 access to the critical region; however, all these agents 

 block the acetylcholine effect and nicotine is both an 

 exciting and blocking agent. Such arguments can only 

 be valid if it is argued that there is a third region on 

 the sensory pathway that differs from the main part 

 of the neuron in its sensitivity to these substances and 

 from the receptor region in that it is not involved in 

 the production of receptor potentials. There is no 

 evidence that acetylcholine is present in receptors 

 (13), but there is evidence of the presence of cholin- 



esterase in the Pacinian corpuscle (8, 43) and Meiss- 

 ner's corpuscle (8); in the former this appears to be all 

 pseudocholinesterase and its destruction does not 

 appear to effect function in any way during an acute 

 experiment (Diamond, J. & J. A. B. Gray, unpub- 

 lished obser\ationsJi. 



The arguments against the participation of acetyl- 

 choline as an intermediary in the normal process of 

 excitation of some types of receptor do not exclude 

 the possibility that local concentrations of acetyl- 

 choline may modify the excitability of receptors under 

 physiological conditions. There is no evidence for 

 such an action of acetylcholine, but there is evidence 

 that a parallel action can occur with epinephrine. 



Effects 0/ Sympathetic Stimulation and Epmep/inne 



Stimulation of the sympathetic supply to the skin of 

 the frog has been shown to increase the excitability of 

 the cutaneous receptors (67). Stimulation of the sym- 

 pathetic in these preparations increases the response 

 to a standard mechanical stimulus applied to the skin 

 surface; also if the skin is stretched biu not otherwise 

 stimulated mechanically so that there is no discharge 

 in the aflferent fibers, stimulation of the sympathetic 

 may initiate a discharge. These results are paralleled 

 by the application of epinephrine to the skin. The 

 effects of epinephrine and sympathetic stimulation 

 add to those of mechanical stimulation of the skin 

 and the application of currents to it. These results have 

 been obtained in preparations which have been sub- 

 .sequently sectioned and shown to contain no smooth 

 muscle except that associated with the blood vessels 



