Recent physiological studies of the alimentary autonomic innervation

The optimum pattern of stimulation of the VIP-ergic fibres in the submaxillary gland of the cat has been investigated by comparing the effects of continuous stimulation at 2 Hz for 10 min with those of stimulation at 20 Hz for 1 sec intervals for the same period. Both the fall in submaxillary vascular resistance (SVR) and release of VIP from the gland are significantly increased when the same total number of impulses is delivered in bursts at the higher frequency. Comparison of submaxillary responses to stimulation in 1 sec bursts, over a wide range of frequencies, has shown that, in atropinised cats, the fall in SVR is linearly related to stimulus frequency over the range 2-60 Hz, and maximal at 80 Hz. In addition, the fall in SVR is linearly related to stimulus frequency over the range 2-60 Hz, and maximal at 80 Hz. In addition, the fall in SVR is linearly related to log VIP output from the gland over the whole of the frequency range 2-160 Hz. Stimulation in 1 sec bursts at 10 sec intervals has also shown that certain non-peptidergic autonomic responses are optimal at much higher stimulus frequencies than has hitherto been supposed on the basis of classical studies employing continuous stimulation. It is concluded that it is no longer justified to assume that autonomic nerve fibres are invariably characterised by low natural discharge frequencies or that they necessarily fire at relatively constant rates. The results obtained using bursts of stimuli also show how differential responses can be obtained in the same tissue simply by varying the stimulus frequency and pattern.