Spatial distribution of various types of bulbar respiratory neurons in the rabbit.

In anesthetized rabbits, the burst activity of 277 single respiratory neurons was recorded extracellularly. The neurons were classified according to their spike incidence distribution within the respiratory cycle and to their response to lung distension or collapse (alpha or beta type). About one third of the neurons found in all animals were located at the level of the caudal end of the promontorium gliosum, widely scattered over the bulbar cross-section. More rostrally the units were located within a dorsal area neighbouring the tractus solitarius, more caudally within a ventral field surrounding the nucleus ambiguus. Most of the inspiratory neurons (the most frequently encountered type) were found within an antero-medial region of the medulla, extending down to about two thirds of its depth. Most of the inspiratory-expiratory units were found within an intermediate zone extending from antero-dorsal to postero-ventral. Most of the expiratory cells were found postero-ventrally. Most of the expiratory-inspiratory neurons (the rarest type) were again found within an antero-medial region at an intermediate depth. The alpha neurons were located in the rostral and the beta units in the caudal part of the medulla. The average proportion of respiratory neurons to non-respiratory units was estimated as about 1:36 and the total number of bulbar respiratory neurons as 500 on each side.