following its stimulation. joined each superior thyroid artery and was found just cephalad to

Transcription

1 612.44: THE THYROID NERVE IN THE DOG AND ITS FUNCTION. By W. DONALD Ross 1 and V. H. K. MOORHOUSE. From the Department of Physiology, Faculty of Medicine, University of Manitoba. (Received for publication 2nd July 1937.) THE extensive literature dealing with the innervation of the thyroid has been reviewed by Marine [1922], Crawford and Hartley [1925], Smith and Moloy [1930], and, especially from the anatomical viewpoint, by Teitelbaum [1934]. Our present conception of this innervation is largely based on the work of Nonidez [1931]. By careful dissection and serial sections, using foetuses and puppies, Nonidez found a distinct nerve bundle entering the thyroid gland, which received branches from the superior cervical ganglion of the sympathetic and from the superior laryngeal branch of the vagus, these tributaries uniting to form a common trunk, which passed into the gland along the course of the superior thyroid artery and its branches. The research here recorded was directed towards the identification of this nerve bundle in the adult dog and the elicitation of results following its stimulation. ANATOMICAL OBSERVATIONS. Initial dissections were carried out on a specimen preserved with formalin and glycerine, with arterial injection of dye. After the usual exposure the two lobes of the thyroid were dissected, care being taken to avoid severing any nerves. No nerves could be found entering the substance of the gland directly, but a small bundle of nerve fibres joined each superior thyroid artery and was found just cephalad to this vessel. On each side this nerve bundle ended in twigs, which entered the gland along with the arterial branches. The nerve was traced cephalad on each side, as it courses in the fascia between the carotid artery and the larynx, and its upper origin followed, after reflecting muscles and other structures which intervened. The nerve was found to derive fibres from the superior cervical ganglion at its 1 The experiments recorded in this paper were carried out by the first-named author as part of his work for the degree of Bachelor of Science in Medicine of the University of Manitoba. VOL. XXVII., NO

2 210 Ross and Moorhouse junction with the vago-sympathetic trunk, and from the superior laryngeal branch of the vagus, as Nonidez has described. The nerve was sometimes found to receive branches from the vago-sympathetic trunk as well as the above-named branches. Including post-mortem material and the subjects of the operative experiments to be described, the right thyroid nerve was found by dissection in 20 out of 22 dogs, and the left thyroid nerve in 11 out of 13 dogs. In 2 animals neither right nor left nerve could be found; these were small dogs, and the nerve was probably missed because of its insignificant size. The nerve was found most easily in large young dogs, and in later experiments such animals were selected for this work. Search in the fascia between the common carotid artery and the larynx, just cephalad to the superior thyroid artery, usually revealed the nerve. The nerves on both sides can be reached by using a transverse incision of 2-1 inches at the level of the cricoid cartilage. For prolonged electrical stimulation and for the recording of the blood-flow further dissection is required to expose the nerve and the thyroid veins. The origin of ten thyroid nerves was carefully traced. These were found to have a contribution from the superior cervical ganglion in every case. The superior laryngeal branch TUYPOID NEP\/1 of the vagus supplied twigs in nine cases IN TwE DOG out of ten. The vagus, either from the...v vaqu3 Ncrvc ganglion nodosum or from the vago upcrior Cervical sympathetic trunk, gave direct branches Ganqhonlol S4mpathCfC in four cases. One nerve had a com IConnecctions bctween&c~t andvqo. S4mpG MM Trunk munication from the external laryngeal Ncrvcs t. Wa of Carotid nerve, as well as its branch from the Artcrqat Bifurcation superior cervical ganglion and the vago- --- Supcrior Lar4Wnqel sympathetic trunk. Another nerve, whose l/-----twypoid NEP'J( beginning was not traced, had a com- Common Carotid Artcr4municating twig from the external laryngeal '.uperior Thqroid ABcri near the entrance of that nerve into the a'd Larqnqca.1 branch.-thqroid Gland laryngeal muscles. The course of the nerve (Rlqht Lobe) was usually at first posterior to the internal carotid artery, then medial to this vessel Fic. 1.-Mounted dissection showing usual arrangement of and between the common carotid and the thyroid innervation. larynx to the superior thyroid artery. The terminal branchings were fibres which entered the superior lobe of the gland with the branches of the superior thyroid artery. A thorough search was made in all the dissections for nerves apart from this bundle, but none were found. It is possible that some small fibres course to the gland along with other blood-vessels, but it appears that the thyroid nerve constitutes the chief supply. Fig. 1 is a photograph of a mounted and labelled dissection and shows the nerve with its branches as most usually found.

3 The Thyroid Nerve in the Dog and its Function 211 PHYSIOLOGICAL EXPERIMENTS. The rate of blood-flow through the gland was studied in 6 dogs to ascertain the results of stimulation of the nerves. The dogs were FIa. 2.-Perfused thyroid gland. Stimulation of thyroid nerve. FIG. 3.-Stimulation of the thyroid nerve after ergotoxine. The drop record of the right-hand tracing is somewhat faint, and dots have been placed above each drop signal. anawsthetised with morphine and urethane, and the nerves exposed as described. The veins at the lower pole of the gland were cannulated,

4 212 Ross and Moorhouse and the outflow was collected in a vessel filled with a saturated solution of magnesium sulphate, with a drop record of the overflow. Blood pressure was recorded by a mercury manometer. In some experiments the gland was perfused with defibrinated blood at constant pressure and body temperature, with a drop record of the venous outflow. The thyroid nerve and its branches were stimulated with faradic current from a Harvard inductorium with 3 volts in the circuit. Results. Stimulation of the thyroid nerve consistently produced a slowing of the blood-flow through the thyroid both in the intact and perfused gland. This effect was recorded many times, and a typical tracing is shown in fig. 2. The slowing of flow occurred in dogs with intact thyroids independently of changes in the blood-pressure, which were usually insignificant. The constriction was uninfluenced by the injection of atropine or small doses of ergotoxine. With larger dosage FiG. 4.-Stimulation of branch from external laryngeal nerve. indicates the duration of the stimulus. The top tracing of the latter drug stimulation of the thyroid nerve resulted in an acceleration of blood-flow (fig. 3). Constriction effects following the stimulation of the cervical sympathetic have been previously noted by Mason, Markowitz, and Mann [1930], using a plethysmograph method. Stimulation of the vagi did not give consistent results. With the gland intact such stimulation produces passive changes secondary to blood-pressure variations. A dilator response was occasionally recorded on ipsilateral vagus stimulation with the perfused gland. A noteworthy response was obtained in the dog mentioned, in which a communicating branch joined the thyroid nerve from the external laryngeal. Stimulation of this branch caused a slight but definite acceleration of the blood-flow repeatedly (fig. 4). Pilocarpine caused dilatation, and adrenalin a constriction of the vessels. These experiments seem to afford satisfactory evidence that the

5 The Thyroid Nerve in the Dog and its Function 213 thyroid nerve contains both constrictor and dilator fibres. It is possible that the nervous control of the gland, by impulses passing along this nerve, may, through regulation of its blood-flow, play a definite role in the rate of discharge into the general circulation. The results point to the predominance of a sympathetic constrictor effect with the ordinary methods of stimulation. HISTOLOGICAL STUDIES. In 3 dogs the right thyroid nerve was cut, and several weeks later the animals were killed and the thyroids examined histologically. In several experiments all isthmus connexions between the right and left glands were severed, and the right thyroid nerve electrically stimulated for 2 to 3 hours. After this procedure the glands were histologically examined and compared. No significant difference could be detected in the tissue of the glands thus treated. Probably experiments of a much greater duration will be needed to produce demonstrable changes. SUMMARY. The thyroid nerve described by Nonidez in puppies and canine foetuses has been found in adult dogs. It is usually made up of nerve fibres derived from the superior cervical ganglion of the sympathetic and from the superior laryngeal branch of the vagus. The vagus fibres may arise from the ganglion nodosum or the vago-sympathetic trunk, and occasionally from the external laryngeal nerve. The thyroid nerve terminates in, branches which enter the thyroid with the branches of the superior thyroid artery. Stimulation of the thyroid nerve consistently slows the flow of blood through the gland. Acceleration of the blood-flow has been obtained by stimulating the ipsilateral vagus and, in one instance, fibres from the external laryngeal nerve. These results, indicating that the thyroid has a constant constrictor and some dilator innervation, suggest an indirect nervous control of the output of thyroid hormone into the blood-stream. Histological changes in the thyroid, after electrical stimulation of the thyroid nerve for some hours, could not be demonstrated. Section of the nerve, allowing time for degeneration, produced no demonstrable histological change.

6 214 The Thyroid Nerve in the Dog and its Function REFERENCES. CRAWFORD, J. H., and HARTLEY, J. N. J. (1925). J. exp. Med. 42, 179. MARINE, D. (1922). Phy8iol. Rev. 2, 521. MASON, J. B., MARKOWITZ, J., and MANN, F. C. (1930). Amer. J. Physiol. 94, 125. NONIDEZ, J. F. (1931). Amer. J. Anat. 48, 299. SMITH, I. H., and MOLOY, H. C. (1930). Anat. Ree. 45, 393. TEITELBAUM, H. A. (1934). Bull. Sch. Med. Univ. Maryland, 18, 77.