Depressor reflexes from medullated and non‐medullated fibres in the rabbit's aortic nerve

Abstract

It is commonly believed that the reflex fall in systemic blood pressure which results from electrical stimulation of the aortic nerve is due to excitation of the barosensory fibres. However, Douglas & Schaumann (1956) concluded from their experiments on the reflex effects of electrical stimulation of the cat's aortic nerve that this nerve must contain additional depressor afferents smaller than any barosensory fibres that have been recognized in it. These small afferents are not easily characterized in the cat because its aortic nerve is enclosed in the vagus throughout the greater part of its course. In the rabbit, on the other hand, the aortic nerve runs freely in the neck and is thus particularly suitable for experiments to correlate the reflex depressor effects obtained on stimulation of this nerve with its fibre components. Our study has revealed two distinct groups of afferents in the aortic nerve of the rabbit, both of which reflexly lower the blood pressure. The first group is composed of fastconducting fibres and contains the known barosensory afferents. The second group is made up of much smaller fibres which appear to be non-medullated, and whose reflex depressor effects are long-lasting and powerful. Non-medullated afferents have not previously been demonstrated in the buffer nerves.At present we do not know the nature of their receptors. METHODSAdult lop-eared rabbits were used and anaesthetized with 1-6-1-8 g/kg urethane given as a 25% solution into the marginal ear vein. Some animals required a further injection of the anaesthetic during the experiment. The right and left carotid arteries were tied, and the two vagi and both sympathetic nerves were sectioned. The right or left aortic nerve was identified as it joined one or other superior laryngeal nerve, and dissected downwards and cut a few centimetres from this junction. The freed nerve was placed on a pair of silver-silver chloride electrodes (about 3-4 mm inter-electrode distance) for stimulation, and kept in a liquid paraffin bath made by suitable * British Council Scholar.

Non‐medullated fibres in the saphenous nerve which signal touch

“…They found that stimulation of the C fibres caused fused reflex muscular contractions at much lower frequencies than did excitation of the 8 fibres. The second finding is that of and Douglas, Ritchie & Schaumann (1956) who obtained analogous results with the non-medullated and medullated afferent fibres in the rabbit's vagus and aortic nerves. They found that the depressor reflexes associated with the C fibres were obtained at low frequencies of stimulation, often less than 1 shock/sec; such frequencies were much below those required to produce the depressor reflexes associated with the A fibres.…”

The effects of cyclopropane, halothane and ether on central baroreceptor pathways

“…In our experience, involving a total of some sixty experiments, there was never any increase in arterial pressure on aortic nerve stimulation, which agrees with the observations of Mott (1963) and Douglas et al (1956), and contrasts with the effects of sinus nerve stimulation which in the rabbit excites respiration and often increases arterial pressure. Electrical stimulation of the aortic nerve in the rabbit appears to be a convenient method of eliciting pure baroreceptor reflexes.…”

“…Douglas et al (1956) showed that in the rabbit electrical stimulation of the aortic nerve produced progressively greater reductions in arterial pressure as the applied voltage was increased. The series showed two phases, attributable to stimulation of low threshold A fibres, and of C fibres the threshold of which was higher than the maximal A fibre stimulus.…”

“…ANAESTHETICS AND BARORECEPTOR PATHWAYS 539 confirmed in the present experiments, and it was shown further that the inhibition of preganglionic cervical sympathetic activity which is associated with aortic nerve stimulation also has two phases. Figure 1, from one experiment, shows the responses of arterial pressure and sympathetic discharge to two stimulation frequencies; the second rise in the magnitude of the effect begins at the threshold for C fibre excitation, and is much more marked at the lower frequency of stimulation (Douglas et al 1956). T. J. BISCOE AND B.…”

“…The rabbit was selected for experiment because the available evidence suggests that the aortic (depressor) nerve in this species contains many baroreceptor afferents but few or no chemoreceptor fibres (Douglas, Ritchie & Schaumann, 1956). A short account of some of the results has been published elsewhere (Millar & Biscoe, 1964) and a demonstration was given to the Physiological Society (Biscoe & Millar, 1964 b).…”