Parasympathetic Secretory Innervation 41 



secretion, may under special experimental conditions act as constrictor 

 or inhibitory fibres. 



2. Nerve fibres in contact with certain structures may under 

 special conditions affect other structures as well, through diffusion 

 of chemical mediators. Secretory nerves may thus influence the 

 vascular bed, and vasomotor fibres the secretory cells. It is not 

 unreasonable to suppose a diffusion of the chemical transmitters. A 

 piece of cotton-wool, soaked in a solution of acetylcholine and 

 placed on the surface of a salivary gland, causes a lively secretion ; 

 that this is not due to absorption of the drug into the general cir- 

 culation is shown by the fact that the other salivary glands do not 

 respond. Such a diffusion has in fact been assumed to take place 

 by Cannon and Rosenblueth (1937) when considering the possi- 

 bility that only certain "key cells" are supplied by nerve fibres. 

 It is, however, not known whether under physiological conditions 

 the transmitters are able to influence distant effectors. 



3. Stimulation of secretory fibres may indirectly lead to vaso- 

 dilatation because vasodilator agents are released from the activated 

 secretory cells. 



4. Stimulation of vasoconstrictor fibres may reduce the supply 

 of blood to such an extent that a secretion is diminished or stopped. 



5. Stimulation of motor nerve fibres may lead to expulsion of 

 saliva, thereby simulating a secretory effect. 



At our present state of knowledge it seems safe to state that the 

 parasympathetic nerves supply the salivary glands with fibres which 

 under ordinary conditions are secretory, and that the sympathetic 

 nerves carry vasoconstrictor fibres. Whether motor and secretory 

 sympathetic fibres exist will be discussed below, whereas problems 

 concerning vasomotor fibres will be dealt with in Chapter VII. 



The parasympathetic secretory innervation. All the salivary glands, 

 both the three big pairs and the small glands in the oral mucosa, 

 seem to receive their main secretory innervation from the para- 

 sympathetic system. A single electrical shock applied to the para- 

 sympathetic nerve affects the secretory cells. This had already been 

 seen in the external electrogram of the submaxillary gland by 

 Bradford (1888); Rosenblueth, Forbes and Lambert (1933), using 

 concentric needle electrodes, made the same observation. In his 

 recent experiments with microelectrodes inserted from the surface 

 of the submaxillary gland into single cells Lundberg (1955, 1958) 

 has been able to show that single shock stimulation of the chorda 



