How are skeletal muscles stimulated to contract?

Skeletal muscles are stimulated to contract by a process known as excitation-contraction coupling, which is a series of events that links the arrival of an electrical signal at the neuromuscular junction to the generation of force within the muscle fiber. Here is an overview of the steps involved:

1. Action Potential: When a motor neuron is stimulated, it generates an action potential, which is an electrical impulse that travels along its axon.

2. Neuromuscular Junction: The action potential reaches the neuromuscular junction, which is the synapse between the motor neuron and the muscle fiber.

3. Neurotransmitter Release: At the neuromuscular junction, the motor neuron releases a neurotransmitter called acetylcholine (ACh) into the synaptic cleft, the space between the neuron and the muscle fiber.

4. Binding of ACh to Receptors: Acetylcholine binds to specific receptors on the muscle cell membrane, known as nicotinic acetylcholine receptors. This binding causes the receptors to open and allow sodium ions (Na+) to enter the muscle cell.

5. Depolarization: The influx of sodium ions leads to depolarization of the muscle cell membrane, meaning the inside of the cell becomes more positive relative to the outside.

6. Excitation-Contraction Coupling: The depolarization of the muscle cell membrane triggers excitation-contraction coupling. This process involves the release of calcium ions (Ca2+) from the sarcoplasmic reticulum, the muscle cell's internal calcium store.

7. Calcium Binding: Calcium ions bind to receptors on the surface of the sarcoplasmic reticulum, causing conformational changes that result in the release of more calcium ions into the muscle cell cytoplasm.

8. Calcium-Induced Calcium Release: The initial release of calcium ions triggers a process called calcium-induced calcium release, where calcium ions bind to receptors on the surface of the sarcoplasmic reticulum, leading to the release of even more calcium ions, amplifying the calcium signal.

9. Calcium and Troponin: Increased calcium levels in the cytoplasm bind to a protein called troponin, which is part of the troponin-tropomyosin complex. This binding causes conformational changes that expose the myosin-binding sites on actin filaments.

10. Cross-Bridge Formation: The exposed myosin-binding sites on actin filaments allow the formation of cross-bridges between the thick (myosin) filaments and the thin (actin) filaments within the muscle cell.

11. Muscle Contraction: The formation of cross-bridges triggers the power stroke of the muscle contraction cycle, where the myosin heads bind to the actin filaments, pivot, and pull the thin filaments toward the center of the sarcomere, the basic unit of muscle contraction. This sliding of the filaments causes the muscle to shorten and generate force.

The continued stimulation of the muscle by the motor neuron and the subsequent release of calcium ions maintain the cross-bridge formation and the sliding of filaments, resulting in sustained muscle contraction. When the motor neuron stops firing, the calcium ions are pumped back into the sarcoplasmic reticulum, the cross-bridges detach, and the muscle relaxes.

This sequence of events ensures precise control and coordination of skeletal muscle contractions, allowing for various movements and actions of the human body.