Experimental baroreceptor function. Curves in four different conditions where the controlling variable was cardiac frequency increased or decreased by parasympathetic (vagus nerves) discharge, in turn driven from the medulla oblongata outflow. If the horizontal scale is turned over, the sigmoid shape becomes more evident. Triangles on the two left panels mark probable operating points. (Modified after Valentinuzzi, Powell et al, 1972, ref , by permission).