Direct nervous pathways between the cervical vagus and the aortic or superior laryngeal nerves traversed by baroreceptor, ‘pulmonary stretch’, and other fibres in the cat

Abstract

Only brief reports have appeared concerning carotid sinus baroreceptor fibres which conduct orthodromic impulses peripherally in the aortic nerve (Holmes, 1954;Bianconi & Raschi, 1959) and a vagal cardiac branch (Green, 1959) of the cat. The finding of a similar fibre in the cervical vagus of a dog (Jewett, 1964) led to the study to be described here. In the cat, besides these fibres, other baroreceptor fibres and some 'pulmonary stretch' fibres were found to conduct impulses peripherally in the aortic nerve. Electrical stimulation gave results which might be explained by assuming that these 'sensory' fibres do not pass to the central nervous system. METHODSNineteen cats weighing between 1-6 and 5-5 kg were anaesthetized with intra-peritoneal pentobarbitone sodium, 30-40 mg/kg body wt. With the cat supine the trachea was cannulated low in the neck. To facilitate dissection the block of tissue containing the pretracheal musculature, trachea, oesophagus, larynx, and lower pharynx was removed after nerves and vessels along the sides had been sectioned. To record respiration through changes in intra-thoracic oesophageal pressure, the oesophageal stump was cannulated with a polyethylene catheter inserted far enough to lie within the thorax. An unheated (28-33°C) liquid paraffin pool (sp.gr. 0.850) was formed within supported skin flaps; beneath its surface the vagus could be dissected readily to above the nodose ganglion. The nodose ganglion as well as the aortic nerve was identified by dissecting centrally along the superior laryngeal nerve. All recordings were made from the central segmentst (either at the cut end or along the nerve) of cut whole nerves from which the connective tissue had been removed with fine jeweller's forceps. Recording electrodes were AgCl-coated Ag wire of 0-025 in. diameter and stimulating electrodes were the same or of Pt; electrodes were positioned by Prior manipulators. Each of two square-wave stimulators could be made to fire at any given point on a freerunning oscilloscope sweep; the repetition frequency was usually either 50/sec or 100/sec. Recording electrode potentials were differentially amplified by a battery-powered Tektronix 122 pre-amplifier leading to a Tektronix 502 dual-beam oscilloscope. The high-frequency filters attenuated to half value at 1 kc and low-frequency filter attenuated to more than * Present address: Department of Physiology, University of California Medical Center, San Francisco, California 94122, U.S.A. t The term 'segment' is used to avoid the confusion arising from the usual terminology: a cut nerve has four ends, but only two segments.

Action potentials in parasympathetic and sympathetic efferent fibres to the trachea and lungs of dogs and cats

“…In the absence of more detailed study it is suggested that the results with group IV fibres and some of the unclassified fibres are consistent with the suggestion by Jewett (1964b) that there is a system of afferent nerves having collateral branches and/or peripheral synaptic connexions. The ones described here would have unknown efferent actions on the lungs and trachea.…”

“…The behaviour of group IV fibres was so characteristic of vascular afferent nerves (baroreceptor fibres), that it was most surprising to find that activity was often abolished by block of cervical vagal conduction. The patterns of response to various stimuli which changed blood pressure showed clearly that the fibres were not coming from systemic arterial baroreceptors, which may possibly send fibres down the vagus nerves (Holmes, 1954;Bianconi & Raschi, 1959;Green, 1959;Jewett, 1964b), and the activity could usually be correlated best with central venous or atrial pressures, although this was not established. The timing in relation to the cardiac cycle, the usual inspiratory increases in frequency, the changes on lung inflation by positive pressure (augmentation of discharge 85 J. G. WIDDICOMBE for the pulmonary fibre in an open-chested dog, and inhibition in closedchested animals), and the responses to tracheal occlusion, are all consistent with most of the fibres arising from receptors on the low-pressure vascular compartments of the chest.…”

Activity of single efferent fibres in the cervical vagus nerve of the dog, with special reference to possible cardio‐inhibitory fibres

“…This suggests that the fibres may become smaller as they descend, which might explain the difficulties in demonstrating histologically that there are myelinated efferent fibres in the vagal cardiac branches (Heinbecker, 1931;Heinbecker & O'Leary, 1933;Daly & Evans, 1953;Evans & Murray, 1954;Mizeres, 1955Mizeres, , 1957Agostoni, Chinnock, Daly & Murray, 1957). Type I fibres are probably not the same as those reported by Okada et al (1961b) since two Type I fibres were not affected by superior vena caval infusions; furthermore, there is a possibility that the fibres reported by these authors are not cardio-inhibitory (see Jewett, 1964).…”

“…One Type I fibre had a higher conduction velocity inthe proximal vagus than in the distal vagus, which has been reported for some single fibres in the vagus nerve (Iggo, 1958;Paintal, 1962). This suggests that the fibres may become smaller as they descend, which might explain the difficulties in demonstrating histologically that there are myelinated efferent fibres in the vagal cardiac branches (Heinbecker, 1931;Heinbecker & O'Leary, 1933;Daly & Evans, 1953;Evans & Murray, 1954;Mizeres, 1955Mizeres, , 1957Agostoni, Chinnock, Daly & Murray, 1957 Jewett, 1964).…”

“…Fibres similar to Type VI have been found descending the aortic nerve of the cat by Holmes (1954), Raschi (1959), andJewett (1964). Possibly the Type VI fibre was also descending amongst the baroreceptors of the right 'aortic' nerve since there was (as is frequent) no separate aortic nerve in the mid-cervical region in this animal.…”