The localization of respiratory regions in the rhombencephalon of the sheep

Abstract

The medulla oblongata of the sheep was explored by means of electrical stimulation in order to localize the points of respiratory reactivity. The stimuli were brief rounded shocks equally spaced at all frequencies employed, of alternating polarity, and of similar shape and voltage. Maximum responses were obtained at a frequency of 250/sec. and of an intensity of 8 V. The current spread of effective density appeared not to exceed 0⋅5 mm. The maximum responses were consistent and reproducible. At such loci inspiratory or expiratory responses could not be reversed by alterations in rate and/or strength of stimuli. The reactive loci were identified histologically. Inspiratory and expiratory apnoeas, maintained for the duration (15 sec.) of the test stimulation, were considered to be indicative of so-called centres, i. e. preponderance of cells of an inspiratory or expiratory nature. Respiratory responses are aggregated in the reticular formation of the medulla oblongata from the level of the acoustic tubercles rostrally to a level just behind the obex caudally. The loci of maximum inspiratory and expiratory apnoeas are separable into clearly defined regions: (1) an inspiratory region which occupies a blunt wedge-shaped volume of the medio-ventral reticular formation, the base of the wedge being at the obex and the truncated end extending to the cephalad margin of the inferior olive; (2) an expiratory region which covers a larger extent than the inspiratory region and lies dorsal, anterior and lateral to it. In the sheep the points of maximal cardio-vascular and respiratory responses are closely interrelated anatomically but are independent functionally. Mean values of respiratory volumes in sheep of 22 to 44 kg. were: tidal air 0⋅20 1., complemental air 0⋅64 1. and supplemental air 0⋅19 1., the last two obtained on optimum stimulation. The calculated vital capacity was 1⋅03 1.