The arrival of an action potential in the T-tubule activates the voltage sensitive dihydropyridine receptors (Ca²⁺ channels) in the membrane.  These in turn, induce transient opening of the ryanodine sensitive Ca²⁺ channels in the SR causing Ca²⁺ to be dumped from the terminal cisternae into the cytosol, and raising the concentration from <10^-7 M to >10^-4 M.  The Ca²⁺ combines with TnC and causes exposure of the strong binding sites on the actin filament, initiating contraction.  As the action potential passes, the ryanodine channels close.  The released Ca²⁺ is soon bound to the calbindin (parvalbumin) in the cytosol, which has a higher affinity for Ca²⁺  than does TnC, but which combines with Ca²⁺  more slowly since the Mg²⁺ to which it is bound, must first be displaced.  The Ca²⁺ is then transferred to the Ca²⁺-Mg²⁺-ATPase in the SR and pumped back into the lumen of the cisternae.  Calcium ion concentration then, rises and falls as a brief pulse in response to the arrival of an AP in the muscle fibre.

This single pulse of Ca²⁺ released by a single action potential causes a brief contraction then relaxation, known as a twitch. The duration of this twitch varies in different muscle types, and in different types of skeletal muscle fibres.

A motor unit is a motor neuron plus all the muscle fibres which it innervates.  When an AP travels down the axon it reaches all the fibres in the motor unit, and all will normally be activated.  The neuron and all its fibres thus act as a unit.  Failure at the mammalian nerve-muscle junction is rare.

The fibres belonging to a motor unit are not normally clumped together, but are interspersed among those from other motor units.  The nature of the motor neuron and its firing pattern influences the characteristics of the muscle fibres in the motor unit.