Cardiovascular and Respiratory Responses to Cooling of the Medulla Oblongata of the Cat 1

In a previous study (1), one of us demonstrated that warming certain neural elements in the lateral reticular formation of the medulla oblongata of the cat produced tachycardia, slight hypertension, and respiratory inhibition. The present investigation is an attempt to determine whether these neural elements also respond to cooling.

Materials and Methods. Twenty cats were anesthetized by intraperitoneal injection of urethane (0.8–1.0 g/kg). Rectal temperature was maintained at about 38° by an electric heater controlled automatically by a Klixon 5CT3 proportional thermoregulator. The trachea was cannulated and both vague nerves were dissected free of surrounding tissue. The right femoral artery was cannulated and the arterial pressure was monitored with a Statham P23AC transducer. Heart rate was monitored with a Grass 5P4 tachograph unit which was triggered by the arterial pulses. Respiratory movements were monitored with a chest pneumograph, connected to a Statham P23BC transducer. All recordings were made on a Grass 5B polygraph.

The head of the cat was placed in a stereotaxic instrument. The medulla oblongata was cooled as follows:

1. For general cooling the dorsal and ventral surfaces of the medulla were irrigated with cold physiological saline (25-20°). For irrigation of the dorsal surface of the medulla, the cat was placed in a prone position, a portion of the occipital bone was removed, and caudal part of the cerebellum was retracted to expose the dorsal surface of the medulla. For irrigation of the ventral surface of the medulla, the cat was placed in a supine position, the muscles around the pharynx and larynx were dissected free, and part of the basilar portion of the occipital bone and the underlying dura were removed to expose the ventral surface of the medulla.