An Intrinsic Neuromuscular Basis for Mitral Valve Motion in the Dog

The anterior leaflet of the mitral valve of the dog contains blood vessels, nerve fibers, and cardiac muscle in addition to elastic fibers and collagen. When studied in a myograph, the electrically stimulated mitral valve actively developed tension and shortened. Active tension was found to be a function of initial length of the valve and was increased by norepinephrine and decreased by acetylcholine. The presence of neuronally releasable norepinephrine stores in the valve was indicated by responsiveness to tyramine. The negative inotropic response of the mitral valve to acetylcholine was consistent with an atrial origin of the tissue. Possible functional roles for mitral valve muscle and the potential significance of its neural control are discussed.