Hypothermia Due to Localized Lesions in Midbrain of Rabbit.

Abstract

These lesions were produced by injections of chromic acid under ether or urethane anesthesia and subsequent gross and microscopic studies made of the brain structures injured. Lesions in one limited area were always followed by a gradual fall in body temperature from a normal of 38° or 39° to 28° or 26° C. within 3 to 5 hours. Injection at any other point so far studied was without marked temperature effect. Accompanying the hypothermia was a gradual decrease in muscular tonus and at 26° C. almost complete atonia. The animal in this condition appears lifeless but if properly cared for can be kept for a number of days.

These results were repeatedly obtained by the injections (some 100) which have been made to date. The microscopic studies are at present in progress.

From the brains already examined it appears that the effective lesion is in the central portion of the superior quadrigemina, 2 to 3 mm. from the aqueduct and within a few millimeters of (but never including) the red nucleus. Since the puncture may be made from a dorsal or lateral direction with the same effect, the superficial midbrain structures are excluded. Within the line of injury are the central gray matter and several nuclei and fiber tracts, notably the nuclei of the third and fourth cranial nerves, portions of the superior cerebellar peduncle, and the posterior longitudinal bundle. It is impossible as yet to say which of these when injured is responsible for the hypothermia or hypotonus resulting, but I feel reasonably sure that some or all of the fibers in the posterior longitudinal bundle are involved in every typical case. This bundle is made up largely of ascending fibers from the vestibular nuclei (probably all 3 nuclei) which are commonly agreed to be necessary for the maintenance of normal tonus. Dysfunction of these nuclei always causes atonia perhaps by removal of an inhibition to the normal myotatic reflex inhibition of the muscle itself. The bundle also includes descending fibers from the interstitial and other less known nuclei of the hypothalamus.