Tonic and reflex functions of medullary sympathetic cardiovascular centers.

IN 1870 it was demonstrated that the maintenance of normal cardiovascular function is dependent upon the integrity of structures which are located in the rostra1 portion of the medulla oblongata of the brain stem. (For studies of Dittmar and of Owsjannikow, see 2.) It was later suggested by Porter (18) that this bulbar system might be subdivided into a “vasoreflex” center and a “vasotonic” center. In an attempt to localize these bulbar centers with greater accuracy, Ranson and Billingsley (20) explored the floor of the fourth ventricle with stimulating electrodes searching for vasomotor reactions. They identified relatively discrete points which yield significant responses to low intensity stimulation:a pressor point in the fovea inferior at the apex of the ala cinerea and a depressor point in the area postrema just lateral to the obex. They also found somewhat variable pressor responses in the region of the facial colliculus which they attributed to Porter’s %asotonic” center. Chen, Lim, Wang, and Yi confirmed the presence of the pressor and depressor points and contended that the pressor point represents a center concerned with the excitation of the entire sympathetic system (6) while the depressor point is concerned with the inhibition of the sympathetic system (10). On the other hand, the concept that these points on the floor of the fourth ventricle represent integrative centers for pressor and depressor functions has been challenged by Scott (21). He found that destruction of these regions by cautery did not interfere with normal pressor and depressor functions except in so far as there was damage to afferent vasomotor pathways. These divergent points of view were resolved by exploring the deeper structures of the medulla, a procedure which was made possible by the introduction of the HorsleyClarke stereotaxic instrument. Orienting fine needle electrodes with the aid of this instrument, Wang and Ranson (22) explored the entire substance of the brain stem from the pons to the decussation of the pyramids. A similar though less extensive exploration has been carried out by Monnier (12). This method revealed extensive pressor and depressor regions within the reticular formation of the medulla which yield maximal or near maximal responses to low intensity stimulation. This evidence served to confirm the contention of Scott that the discrete points on the floor of the fourth ventricle are of little function lasignificance, since they are merely points where the deeper lying centers come close enough to the surface to be stimulated by superficial electrodes.