Neurophysiological investigations of medullary chemosensitive areas of respiration

Abstract

The ventrolateral surface of the medulla has been designated as the location of the central chemoreceptors for respiration by Mitchell et al. (1963a, b; 1965). This area is presumably sensitive to the [H +] of CSF. Microelectrode recordings of unitary neuronal activity in the area during changes in pH of the CSF perfusing the area failed to show changes in firing rates correlated with changes in pH of perfusing CSF. Electrical stimulation of the ventral surface also failed to effect any changes in respiration. Recordings of unit activity of respiratory center neurons showed that respiratory center activity increased rapidly with application of acid CSF to the ventral surface and decreased with application of local anesthetic. It was found that respiration and respiratory center activity persisted in spite of the fact that the super-ficial “chemoreceptor areas” were anesthetized by local anesthetic. Bilateral lesioning of the superficial area of the medulla designated as chemosensitive resulted in apnea; however, it also resulted in ischemia of the lateral portions of the medulla and respiratory centers. By calculation it was shown that substances applied to the ventrolateral surfaces of the medulla would diffuse into the small arterioles of the subarachnoid space and thusly, be rapidly transported deep into the medulla and respiratory centers. It is considered feasible to suppose that substances which stimulate or depress respiration when introduced into the CSF produce their effects more directly at deep neurons after diffusion into the arterial supply to the medulla.