20.7: Muscle Contraction
A voluntary skeletal muscle contraction begins in the brain—as a conscious effort from the frontal lobe to the primary motor cortex—before activating an alpha motor neuron located in the anterior horn of the spinal cord.
The signal continues down a nerve to the specific muscle fiber, such as the biceps, where the action potentials terminate at the motor end plate—where the motor neuron establishes synaptic contact with a muscle fiber—and trigger the release of the neurotransmitter, acetylcholine, which diffuses across the synaptic cleft and binds to receptors.
As a result, the sarcolemma becomes more permeable to sodium ions, resulting in more action potentials that spread along its external surface and into the interior of the muscle fiber through transverse, or T, tubules, which triggers the release of calcium ions from the sarcoplasmic reticulum into the myofibrils.
This release of calcium initiates actin-myosin cross-bridge activity and the observation of the muscle shortening and contracting.