Postganglionic Neurones 61 



The secretory response to sympathetic stimulation differs from 

 that to parasympathetic stimulation in being more or less short- 

 lasting. This is well illustrated in Figs. 3.6 and 3.7. During constant 

 stimulation of the sympathetic the flow of saliva decreases ; some- 

 times the rate may temporarily increase again, but on the whole it 

 tends to decline and often to stop completely. This is obviously not 

 because the secretory cells (or their receptive mechanism) are ex- 

 hausted, for a long-lasting, regular flow can be maintained by con- 

 tinuous injection of adrenaline or noradrenaline. Nor is this decrease 

 in rate due to failure of synaptic transmission, for it is seen when 

 the postganglionic fibres are stimulated also. The pronounced vaso- 

 constriction evoked by stimulation may play a role; the temporary 

 increase in secretion sometimes seen always accompanies a vaso- 

 dilatation, which for a short period replaces the constriction. The 

 experiment of Fig. 3.7 may point in the same direction. It shows 

 that after priscol the sympathetic constriction is almost abolished, 

 and now the secretory response is somewhat bigger than before 

 priscol. The figure shows also, however, that even with priscol 

 the sympathetic secretion is not well maintained ; consequently the 

 decrease in rate cannot wholly be due to constriction. Probably the 

 stores of the adrenergic transmitter get used up more quickly than 

 they can be replaced ; it should be noticed that the experiments are 

 usually carried out with unphysiologically high stimulation fre- 

 quencies. With a low rate of stimulation the secretion can, in fact, 

 often be well maintained for a long time (Emmelin and Engstrom, 

 1960a). A factor contributing to the initial rapid flow may be the 

 simultaneous stimulation of sympathetic motor fibres. 



In man, stimulation of the sympathetic trunk in the neck causes 

 a secretion of saliva from the submaxillary, but not from the parotid 

 gland (Folkow and Laage-Hellman, i960). Similarly, adrenaline 

 injected into a salivary duct in man, evokes a flow of saliva from 

 the submaxillary, but not from the parotid gland (Emmelin and 

 Stromblad, 1954). In human salivary glands the amine oxidase 

 activity is remarkably high (Stromblad, 1959). 



THE POSTGANGLIONIC NEURONES OF THE SECRETORY 



NERVES 



Recent experiments suggest that the postganglionic neurones 

 may have an action on the secretory cells apart from the one due 

 to impulses from the central nervous system relayed across the 



