Changes in Sensitivity to Chemical Stimuli 109 



vation. This is found when the effect of acetylcholine is studied on 

 the parotid gland (Stromblad, 19556). Complete parasympathetic 

 postganglionic denervation of the submaxillary gland cannot be 

 carried out, since many of the synapses are situated within the 

 gland; but even after a partial denervation the gland is found 

 to be more supersensitive to acetylcholine and adrenaline than 

 after section of the preganglionic chorda fibres (Emmelin, 

 1960a). Extirpation of the superior cervical ganglion is followed 

 by a distinct supersensitivity in the submaxillary gland, whereas 

 there is scarcely any sensitization at all after section of the 

 preganglionic sympathetic fibres (Emmelin and Engstrom, i960). 

 The exceptions to the law which anticholinesterases and tyramine 

 form have already been discussed. A further exception is the fact 

 that adrenaline has a bigger secretory effect on the parotid gland 

 after pre- than after postganglionic parasympathetic denervation 

 (Stromblad, 19556). The explanation for this has recently been 

 found (Ohlin and Stromblad, 1958) in the fact that the vaso- 

 constrictor effect of adrenaline is greater after post- than after 

 preganglionic denervation, i.e. the vessels react according to 

 Cannon's law, but since large doses of adrenaline are required to 

 cause a secretion from the parotid gland, vasoconstriction will tend 

 to reduce the secretion, and particularly after postganglionic 

 denervation. 



The hypothesis has been put forward that the sensitization is 

 due to lack of secretory impulses (Emmelin and Muren, 19500 

 and b; Emmelin, 1952a and 6, 19606). The following observations 

 may be quoted in support of this view. Administration of a gan- 

 lionic blocking agent for some weeks sensitizes the submaxillary 

 gland to a level similar to that produced by chorda section (Emme- 

 lin, 1959). Parasympatholytic agents, likewise, cause a supersen- 

 sitivity. If the flow of secretory impulses to the gland is reduced by 

 section of some of the afferent fibres of the gustatory reflex arc, a 

 supersensitivity ensues. Administration of secretory agents like 

 pilocarpine, carbamylcholine or adrenaline for some days abolishes 

 the supersensitivity created by denervation. When hypoglossal 

 fibres after cross-suture have reinnervated the gland via the 

 chorda, and secretion is elicited whenever tongue movements are 

 intended, the supersensitivity caused by section of the chorda 

 diminishes. Section of the preganglionic sympathetic fibres does 

 not cause any supersensitivity in the submaxillary gland, as it does 



