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 those found in the biceps, where the action potentials terminate at the motor endplate. There, the motor neuron establishes synaptic contact with the muscle fiber and triggers 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. This 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.