The course of the efferent cardiac nerves of the sheep*

Abstract

The purpose of the work to be discussed was to trace the main routes followed by the efferent nerves to the heart in the sheep. Since the thoracic autonomic nervous system of this species is nowhere described in detail, illustrations of some dissections will be presented so that sites of stimulation referred to in the text may be more precisely indicated. METHODSThree Dorset Horn, twelve Clun Forest and twenty-six Welsh Mountain sheep of both sexes were used. Their ages ranged from 1 month to 4 years and their weights from 14-3 to 76-1 kg. Four sheep were anaesthetized with a mixture of chloralose (Hopkins and Williams, 17-0-59-0 mg/kg body weight) and urethane (May and Baker, 170-590 mg/kg) intravenously; during the experiment anaesthesia was maintained by intravenous infusion of 260-7-86 mg of chloralose and 26-0-78-6 mg of urethane/kg/hr. Except for one sheep which was anaesthetized throughout with intravenous pentobarbital (Nembutal, Abbott Laboratories), the remainder, after induction with pentobarbital (16.0-36-4 mg/kg), were anaesthetized by continuous intratracheal cyclopropane (10-150 ml./min) with oxygen (50-300 ml./min) administered from a Boyle's apparatus in a closed circuit with soda-lime for CO2 absorption. In most of the experiments an intravenous transfusion of either plasma substitute or homologous blood (50-200 ml./hr) was maintained throughout.The cervical sympathetic and vagal nerves were first isolated and prepared for subsequent stimulation or dividing, after which one of three surgical procedures was followed: (1) The thoracic vagal trunks and their connexions were exposed by splitting the sternum along the mid line in eight experiments; or (2) parts of the thoracic sympathetic chains and their branches in twenty-five other experiments were exposed by resecting two to nine ribs on one side or both, either by a dorsal approach through the muscular attachments of the scapula or ventrally by dissection of the pectoral muscles; or (3) laminectomy of the seventh cervical to the eighth thoracic vertebrae was performed in five further experiments.These last experiments were performed because the rami communicantes of the middle and upper thoracic sympathetic ganglia are very short and inaccessible, and it was impossible to stimulate them without danger of current spread to the ganglia themselves (see Fig. 1). When exposed the spinal cord was covered with warm liquid paraffin, and moist silk ligatures were * Part of the work presented as a dissertation for a Ph.D. degree to the University of Cambridge.

Vasomotor responses in the hind limbs of foetal and new‐born lambs to asphyxia and aortic chemoreceptor stimulation

“…Cyanide often causes some tachycardia, and always after administration of atropine. While it is not impossible that a few sympathetic fibres may reach the heart in the foetal lamb after doubling up the cervical vagi from the stellate, this cannot be more than a small contribution (Waites, 1957;Colebatch, Dawes, Goodwin & Nadeau, 1965). The fact that cervical vagotomy abolishes the tachycardia observed on injection of cyanide after atropine (Fig.…”

Electrical and mechanical response of arteries to stimulation of sympathetic nerves

“…The cervical sympathetic and vagus nerves were then sectioned at the root of the neck; the nei-ves ran separately from each other at this point in all the sheep used. The sympathetic nerve was identified by the facts that it was smaller than the vagus, and that stimulation of its distal (anterior) cut end caused contraction of the smooth muscle round the eye (Waites, 1957), while stimulation of the central (anterior) cut end of the vagus caused respiratory arrest or coughing. The nerves were then sectioned again under the angle of the jaw (just posterior to the carotid sinus) to prevent these reflexes before the effect of stimulation of the nerves on the common carotid artery was studied.…”

“…In experiments intended to produce degeneration of sympathetic nerve fibres in the artery both the vagus and the sympathetic trunk on the right side were excised for 10-12 cm under pentobarbitone anaesthesia, since the vagus of the sheep may contain a few sympathetic fibres (Waites, 1957). Adventitia was stripped from the ends of the denervated portion of artery to try to sever any nerve fibres reaching it by running in the adventitia.…”

Differences in sensitivity to vasoconstrictor drugs within the wall of the sheep carotid artery

“…Both nerves were removed for a distance of 15-20 cm caudal to the portion of artery to be studied, and the adventitia of the vessel was stripped at the root of the neck to destroy any nerve fibres running in this. The vagus was removed as well as the sympathetic since the vagus in the sheep may contain a few sympathetic fibres (Waites, 1957). Animals were allowed to recover and after 10-42 days they were again anaesthetized and both carotid arteries were removed.…”