Steps in the contraction and relaxation of skeletal muscles:
- Nerve impulse, or electrical signal, travels down the nerve to the terminal to cause the release of the neurotransmitter acetylcholine (ACh).
- ACh diffuses across the neuromuscular junction and binds to the receptor sites.
- Stimulation of the receptor sites causes an electrical impulse to form in the muscle membrane. The electrical impulse travels along the muscle membrane and penetrates deep into the muscle through the T-tubular system.
- The electrical impulse stimulates the sarcoplasmic reticulum to release calcium into the sarcomere (a contractile unit of a mofibril) area.
- Calcium allows the actin, myosin, and ATP to interact, causing crossbridge formation and muscle contraction. This process continues as long as calcium is available to the actin and myosin.
- Muscle relaxation occurs when calcium is pumped back into the sarcoplasmic reticulum, away from the actin and myosin. When calcium moves in this way, the actin and myosin cannot interact, and the muscle relaxes.