Morphological observations on superficial medullary CO2--chemosensitive areas

Physiological investigations have indicated that the ventrolateral surface of the medulla oblongata is involved in the chemical drive to respiration. In this investigation, light and electron microscopic investigations of the 3 chemosensitive regions reveal the following. (1) Evaginations of the ventral surface abut the overlying pia mater thereby delimiting discrete compartments; invaginations of the surface delimit wide cisternae lined with basement membrane. Neuronal elements with numerous synapses, were found scattered among astrocytic processes of the marginal glia in intermediate and caudal chemosensitive areas Microvasculature are conspicuously absent from the marginal glia. Intramedullary vessels are surrounded by perivascular spaces and the endothelium shows zonulae occludentes at cell junctions. (2) Horseradish peroxidase (HRP) applied to the ventral surface diffused throughout the interstitial and perivascular compartments, into synaptic clefts and neuronal soma. Diffusion of HRP into blood vessels was blocked at zonulae occludentes. Following intravenous injection of HRP, no reaction product was found outside cerebral vasculature in chemosensitive areas. (3) In spontaneously breathing cats, 2% procaine applied to the caudal chemosensitive area resulted in respiratory depression which began with the second breath. It is proposed, that substances which stimulate or depress respiration, when applied to the ventral medullary surface, produce their effects on superficial neurons located in the intermediate and caudal chemosensitive areas after diffusion through interstitial spaces.