LOCALIZATION AND ORGANIZATION OF RESPIRATORY NEURONS IN THE BRAIN-STEM OF THE TOAD, WITH REFERENCE TO ACTIVITIES OF SLOW MOTOR SYSTEM

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Abstract
The brain stem of toads was explored with a tungsten microelectrode in order to locate the somata of respiratory neurons, with simultaneous application of electro-myography to the submaxillary muscle. Small respiratory neurons for the gorge respiration were located in a limited region underneath the striae acousticae. The region is near the margin of the floor of the fourth ventricle at the level of the root of the vestibular nerve and at the depth of about 1.2-1.5 mm from the dorsal surface of the brain. Their activity was characterized by continuous discharges of low spikes of about 100 [mu]V in amplitude with fluctuating intervals, by which the ventilation movement of the submaxillary muscle was put to action. During the ventilation movement (gorge respiration), slow muscle responses were exclusively recorded from the submaxillary muscles. Therefore the small neurons which caused the responses were assumed to belong to the slow motor system. Large neurons for pulmonary respiration were found more extensively around the region of the small neurons, reaching the optic lobe frontally and the obex caudally. Large spike bursts of about 300 [mu]V in amplitude due to activities of the large neurons were always preceded by several low spike discharges of about 85/sec in frequency, which seems to be caused by an increased excitability of the small neurons. Then twitch (fast) responses occurred in the submaxillary muscles, always preceded by a short train of high frequent slow muscle responses. Activities of their large neurons were measured to irradiate at about 0.2 to 0.25m/sec along synaptic relays between them.

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