Electrophysiological and structural studies on the heart muscle of the lobster Homarus americanus.

The electrophysiological properties and structure of heart muscle fibres of the lobster myocardium have been investigated. Discontinuities were observed in the expected decrement of electrotonic potentials in a study of the passive membrane properties of the muscle. Injection of Procion yellow revealed transverse barriers to dye movement, identified in electron micrographs as intercellular junctions. Unlike the intercalated discs of the vertebrate myocardium, which these regions superficially resemble, nexus regions of 'close' or 'gap' junctions are absent. The intercellular cleft is generally 400 A in width, locally reduced to 75-100 A The functional implications of these findings are discussed. Cardiac myoneural junctions, which include regularly repeating structures extending into the synaptic cleft from the postsynaptic membrane, are described.