Analysis of respiratory response patterns following electrical stimulation of the medulla oblongata - abstract

ABSTRACT

The exact location of the "Respiratory Centre" and its possible demarcation into anatomically distinct "inspiratory" and expiratory half centres" have remained the subject of much controversy. In this study the medulla oblongata of 105 cats (1.65-3.40 kg, choralose-urethane anaesthesia) was exposed from the ventral surface, and the effect of the submaximal stimulation (40/sec., 1m sec., 1-4V) on respiration was investigated. The respiratory response pattern was analysed, and group according to the qualitative characteristics into (a) Inspiratory (4 types), (b) expiratory (two types) and (c) Mixed (3 types) effects. The observed response types were then charted on the medullary sections at the points where they were elicited. Taken together, inspiratory responses in the medullary cros sections are seen to be concentrated about three zones (a) 10mm rostral (pontomedullary border) to the Obex level, (b) from the Obex level up to 4mm rostral of it, and (c) 4-9 mm caudal from the Obex. Expiratory reactions may be elicited in a region extending from 8 mm rostral to 2 mm caudal from the Obex. Expiratory effects were constantly 4-6mm rostral to the Obex on the ventrolateral aspect, even at the surface. Inspiratory and expiratory zones are in close contact forming together a rostrocaudally orientated semispiral, suggesting perhaps a functional interaction. The "respiratory centre" (defined here as structures, the stimulation of which results in respiratory arrest in the inspiratory or expiratory positions) may be demarcated into "inspiratory" and "expiratory" half centres, in agreement with Pitts et al. (Am. J. Physiol. 126, 673-688, 1939). It is located laterally in the formatio reticularis at the Obex level. At the transition between these two half centres there is a high degree of overlapping, explaining thereby the findings of Liljestrand (Acta. Physiol Scand. 29 suppl. 106, 321-393, 1953) of an intermingling pool of inpiratory and expiratory neurones. Mixed effects showing an increase in amplitude involving both inspiration and expiration were found concentrated ventrally in both rostral and caudal superficial chemosensitive zones (J. Appl. Physiol. 18, 523-533, 1963 and Pflugers Arch. 312, 189-205, 1969). Extending caudally from the caudal chemosensitive zone, the response is characterized by an inspiratory shift from the end-expiratory position. Ablation studies indicate that this type of response is representative, not of intrinsic respiratory centre activity, but of efferent pathway activity from the respiratory centre (AU)