Characteristics of C fibre baroreceptors in the carotid sinus of dogs.

1. We compared the pressure‐response characteristics of C fibre and A fibre baroreceptors in the carotid sinus of anaesthetized dogs, recording impulses from the sinus nerve and varying mean pressure in the vascularly isolated sinus, which was distended with a pulsatile pressure. Functional stimulus‐response curves were obtained by gradually increasing sinus pressure above and decreasing it below a set‐point of 100 mmHg. Baroreceptors were identified by a pulsatile discharge synchronous with the pulsations in sinus pressure. A and C fibre baroreceptors were identified by the conduction velocities and blocking temperatures of their axons. 2. The pressure‐response characteristics of C and A fibre baroreceptors differed in several respects. C fibres had a pulsatile firing threshold 50 mmHg higher than that of A fibres (105.8 +/‐ 1.8 and 54.6 +/‐ 2.9 mmHg, respectively), an average maximal sensitivity 35% of that of A fibres (0.39 and 1.12 impulses s‐1 mmHg‐1, respectively), and a maximal frequency (at 220 mmHg) 29% of that of A fibres (24.5 and 84.3 impulses/s, respectively). Although invariably pulsatile at pressures above threshold, the firing pattern of C fibre baroreceptors tended to be more irregular than that of their A fibre counterparts. 3. Impulses were also recorded from C fibres that were stimulated by increasing sinus pressure but had an irregular, non‐pulsatile discharge, a high pressure threshold (averaging 154.1 +/‐ 7.2 mmHg), and a low maximum frequency (10.8 +/‐ 2.4 impulses/s). 4. Cooling the sinus nerve progressively attenuated conduction in both A and C fibres, A fibres being blocked between 12 and 4 degrees C (mean 6.8 degrees C) and C fibres between 4 and ‐1.5 degrees C (mean 1.0 degree C). Although cooling the sinus nerve to 7 degrees C did not block conduction in all A fibres, impulse activity in baroreceptor A fibres at a carotid sinus pressure of 200 mmHg was no greater than that at a pressure of 75 mmHg. By contrast, at 7 degrees C baroreceptor C fibres still provided a signal proportional to sinus pressure. 5. Our results suggest that A and C fibre baroreceptors subserve different reflex functions, the former signalling changes in arterial pressure both above and below the normal set‐point, the latter only changes above. They also suggest that differential cold blockade may be a useful tool to determine the contribution of C fibre baroreceptors to cardiovascular reflexes.