Studies on the nerve endings in the heart

Abstract The hearts of dogs, cats, monkeys and lambs were studied by methylene blue perfusion and/or immersion. The principle types of sensory nerve terminals found in these hearts were end‐nets and complex unencapsulated endings. End‐nets are formed by the anastomoses of the branched dendrites of several apparently different myelinated fibers. Complex unencapsulated endings are discrete much branched endings of medium to large sized fibers and vary greatly in form and size. The epicardium is supplied mainly with complex unencapsulated endings of highly variable form. End‐nets are rare in this location. No sensory endings were demonstrated in the myocardium. Motor terminals in the form of fine unmyelinated nerve fibers were demonstrated on both atrial and ventricular myocardial muscle fibers. The atrial endocardium, particularly in the areas of the openings of the great vessels and on the interatrial septum is the best innervated area of the heart. Endnets, and a wide variety of complex unencapsulated nerve endings occur singly and in combination in these areas. The ventricular endocardium is supplied only with end‐nets. The tricuspid and mitral valve leaflets have both end‐nets and complex unencapsulated endings while the aortic and pulmonary valve leaflets have only end‐nets. The cardiac complex unencapsulated nerve endings (baroreceptors) are morphologically and functionally homologous to complex unencapsulated nerve endings (“stretch” receptors) in other areas of the organism. The detailed structure and function of the end‐net terminals remain undetermined.