Fine Structure of Myotendinous Junctions of Ventricular Papillary Muscles of the Cat (Felis Domestica) and Bat (Myotis Lucifugus)

The ultrastucture of the myotendinous junction (MTJ) of the cardiac papillary muscle was studied in cat and bat. In both species findings were similar for either ventricle. Toward the tip of the papillary muscle, myocytes are oriented in parallel array and each cell is narrowed in diameter. Longitudinal or oblique segments of intercalated discs became more prevalent than transverse segments. The distal ends of the last myocytes exhibited finger-like invaginations of the sarcolemma. The lateral sarcolemmae were longitudinally creased, causing a cavernous appearance to the sarcoplasm. Subsarcolemmal cisternae and spherical microparticles were found near the sarcolemma. The basal lamina of myocytes was thick (about 0.7 μ) and was bound to numerous microfibrils. These microfibrils (170 Å wide) were enmeshed with collagen fibrils from the chorda and appeared to connect muscle and tendon. Myofilaments of the last sarcomeres terminated into subsarcolemmal dense mats at distal or lateral margins of narrowing myocytes. All margins of even the terminal myocytes could be penetrated with lanthanum. In papillary muscle tips the nerve fiber varicosities were filled with small mitochondria, of a type suggesting sensory neuroreceptors. In the MTJ of cat hearts narrowing myocytes terminated with occasional P cells which were not organized into multicellular clusters. In bat hearts the P cells did occasionally cluster in the MTJ, and in attached valve leaflets. P cells in the MTJ suggest possible local automaticity, decrement in conduction velocity of excitation and, perhaps, moderation of the jerk during papillary muscle contraction.