Sympathetic Innervation 55 



in the sympathetic trunk keeps the vessels of the submaxillary 

 gland in a state of contraction. If the flow of blood through the 

 submaxillary gland of the cat is recorded in an acute experiment 

 it can be shown that section of the cervical sympathetic trunk con- 

 siderably increases the blood flow. Now, it can often be demon- 

 strated that to restore the preoperative level of vascular tone it is 

 necessary to stimulate the peripheral stump of the sectioned trunk 

 using an impulse rate, which causes a slow flow of saliva 

 (Emmelin and Engstrom, 1960a). Since the same degree of tone 

 was maintained physiologically, presumably by impulses of the 

 same rate, without any secretion, this observation may suggest that 

 separate secretory fibres are present in the trunk. The physiological 

 discharge rate in vasoconstrictor fibres seems to be of the order of 

 1 to 3 impulses per second under "resting conditions" (see Folkow, 

 1955). Impulses of such a rate, artificially applied to the cervical 

 sympathetic trunk will fairly regularly evoke a flow of saliva ; a very 

 slow secretion has even been observed in one cat on stimulation of 

 the sympathetic trunk at a frequency of 10 per minute (Emmelin 

 and Engstrom, 19600). It would be interesting to know the lowest 

 rate of sympathetic stimulation causing a change in the membrane 

 potential of the single gland cell. It also seems desirable to see 

 whether an intense vasoconstriction can be elicited reflexly in a 

 salivary gland without any secretion. In a series of experiments on 

 cats the sympathetic vasoconstrictors were activated by bleeding. 

 In order to avoid interference from blood-carried adrenaline and 

 the mechanical effect of the bleeding the submaxillary gland was 

 perfused separately by blood from the carotid artery of another 

 cat. Bleeding was found to cause a pronounced vasoconstriction in 

 the gland, but no secretion. Electrical stimulation of the sym- 

 pathetic trunk using a frequency which gave a similar constriction 

 invariably evoked a flow of saliva (Emmelin and Engstrom, 1959 

 and 1960a). 



More work may be required to settle the question raised by the 

 investigations of Kuntz and Richins. It seems safe, however, to 

 conclude that sympathetic fibres under fairly physiological con- 

 ditions may stimulate the secretory cells of many salivary glands, 

 even if no fibres end in anatomical contact with these cells. The 

 fact that the maximal rate of secretion obtainable on sympathetic 

 stimulation in the cat's submaxillary gland is about as big as that 

 caused by injected adrenaline or noradrenaline suggests that the 



