Junctional feet and particles in the triads of a fast-twitch muscle fibre.

Structural details of junctional feet in triads of fish muscle are described. These feet have a less dense central core and contact both sarcoplasmic reticulum and T-tubule membranes at tetragonally disposed sites. The distribution of intramembraneous particles differs at the junctional T-membrane, and the junction is asymmetric.