junctional feet

Knob-like structures in muscle fibres connecting transverse tubules (T-tubules) with the sarcoplasmic reticulum. It has been hypothesized that the junctional feet undergo a conformational change on the arrival of an electrical impulse at the T-tubule, causing adjacent calcium channel proteins to open. This allows calcium ions to diffuse inside the muscle fibre, enabling a muscle action to take place (see sliding filament theory).