Action Potentials on the Sarcolemma Spread to the Interior of the Cell Along the Transverse Tubules
Skeletal muscle cells have a resting membrane potential, as do neurons, and the muscle cell membrane can be depolarized by synaptic transmission at the neuromuscular junction (see Chapter 5).
At this junction, the acetylcholine released by the motor neuron activates nicotinic acetylcholine receptors on the sarcolemma of the muscle cell. The resulting depolarization is sufficient to open enough voltage-gated sodium (Na*) ion channels, also found at the junctional sarcolemma (see Figure 5-1), to trigger a muscle fiber action potential. Thus, it is at the sarcolemma of the neuromuscular junction that muscle fiber action potentials are generated.Once an action potential is generated near the midpoint of the muscle fiber, it spreads in both directions along the length of the fiber by mechanisms similar to action potential spread in unmyelinated neuronal axons. In contrast to those on axons, however, action potentials on the sarcolemma are also transmitted to the interior of the muscle fiber along the T tubules. This allows the action potential to reach the location of the sarcoplasmic reticulum even in the innermost regions of the muscle fiber. The consequences of the action potentials arrival at the location of the sarcoplasmic reticulum are critical for the coupling of excitation (the action potential) with contraction (shortening) of the sarcomeres of the myofibrils.