Tubular system excitability: an essential component of excitation-contraction coupling in fast-twitch fibres of vertebrate skeletal muscle.

The tubular (t-) system is the main interface between the myoplasm and the extracellular environment and is responsible for the rapid inward spread of excitation from the sarcolemma to the inner parts of the skeletal muscle fibre as well as for signal transfer to the sarcoplasmic reticulum to release Ca2+ that, in turn, activates the contractile apparatus. In this review, I explore the insights provided by the mechanically skinned muscle fibre preparation to the better understanding of the importance of the t-system excitability in determining the force response under physiologically relevant conditions. In the mechanically skinned muscle fibre, the t-system seals off after is physically separated from the sarcolemma and its excitability can be investigated by electrical stimulation under controlled conditions. Parameters that can be assessed include the threshold for action potential generation, specific electrical resistance and time constant of the tubular wall, quantity of charge transferred during an action potential, refractory period, length constant and velocity of excitation propagation. Results obtained with mechanically skinned fibres from fast-twitch muscles show that decreased t-system excitability does not necessarily translate into reduced force output, but for any particular set of physiologically relevant conditions there is a level below which a further decrease in t-system excitability markedly decreases the force output. There are several built-in mechanisms linked to the metabolic/energetic state of the muscle fibre which prevent complete action potential failure in the t-system, thus allowing the muscle to respond to nerve stimulation, even if the response becomes markedly attenuated.