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

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Abstract
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, wihch was distended with a pulsatile pressure. Functional stimulus response curves were obtained by gradually increasing sinus pressure above and decreasing it below a setpoint 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.degree. C (mean 6.8.degree. C) and C fibres between 4 and -1.5.degree. C (mean 1.0.degree. C). Although cooling the sinus nerve to 7.degree. 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.degree. 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.