Effects of local nerve cooling on conduction in vagal fibres shed light upon respiratory reflexes in the rabbit

Abstract

In ten vagus nerves the effect of local cooling on the compound action potential was studied in the temperature range of 34 to 0 degrees C in spontaneously breathing, anaesthetized rabbits. The mean temperature at which the myelinated (A) fibres were completely blocked, was 10.2 +/- 2.4 degrees C (mean +/- S.D.). In nine nerves, local vagus cooling to 0 degrees C failed to block all non-myelinated (C) fibres. In one nerve, total blocking occurred at 2.0 degrees C. We conclude that in the rabbit, the earlier found increase in tonic activity of the diaphragm following lung inflation or deflation during bilateral local vagus cooling to a temperature between 8 and 0 degrees C is due to afferent impulses in vagal C fibres.

The vagus is inhibitory of insulin secretion under fasting conditions

“…Portal hyperglycemia reduces the vagal hepatic afferent activity (33) that is supposed to be responsible for insulin secretion through a depression of the inhibitory tone of sympathetic pancreatic efferents (9). Alternatively, vagal neurons with different thermal sensitivity toward vagal cooling have been demonstrated as follows: unmyelinated neurons are blocked at temperatures lower than for myelinated neurons, i.e., 2-4°C vs. 6-7°C, respectively (10,11,35,43). However, unmyelinated neurons with lower blocking temperature, i.e., 0°C, have been described in the lung (7,35).…”

“…Alternatively, vagal neurons with different thermal sensitivity toward vagal cooling have been demonstrated as follows: unmyelinated neurons are blocked at temperatures lower than for myelinated neurons, i.e., 2-4°C vs. 6-7°C, respectively (10,11,35,43). However, unmyelinated neurons with lower blocking temperature, i.e., 0°C, have been described in the lung (7,35). It is therefore possible that a temperature of 2°C left conduction for a minority of unmyelinated neurons.…”