AP propagation to the semitendinosus muscle. Action potential (AP) along the nerve tree activating several motor units of the semitendinosus muscle. The timing reference given in each picture is the time since the stimulation at the beginning of the sciatic nerve and the direction of action potential propagation in both the nerve and the muscle is indicated by corresponding arrows. (a) AP has reached the proximal endplate of the muscle and begins depolarising fibres. (b) AP has reached the distal endplate of the muscle and continues down the nerve. APs continue to propagate along fibres at the proximal endplate. (c) AP depolarisation of muscle fibres spreads from both endplates.