The contraction of a single muscle fiber begins with an impulse that is generated by the brain and it travels down the nerve to the neuromuscular junction. The impulse creates action potential at the sarcolemma. The action potential in a motor neuron causes the synaptic vesicles in the axon terminal to release the neurotransmitter acetylcholine, or ACH, in the synaptic cleft (“Fundamentals of Anatomy”, 2009, p. 303). The acetylcholine binds to acetylcholine protein receptors on the sarcolemma, which stimulates a muscle impulse.
The muscle impulse, aka action potential, passes in all directions over the sarcolemma (“Muscle Contraction”). Due to this, ion channels that are usually closed open up, allowing sodium to diffuse into the muscle fiber while potassium diffuses out. More sodium enters the fiber than potassium exits resulting in depolarization in the interior of the sarcolemma. Action potential travels along the sarcolemma until it reaches a transverse tubule, then it travels down it resulting in the release of calcium into the sarcoplasmic reticulum (“Fundamentals of Anatomy”, 2009, p. 303).
Depolarization of the t tubules release calcium ions. This will cause Calcium to be released from the terminal cisternae of the sarcoplasmic reticulum. This calcium binds to troponin which is attached to tropomyosin.
Tropomyosin is the long strand that covers the active actin sites. Calcium ions diffuse from the sarcoplasmic reticulum into the sarcoplasm and bind to troponin (“Fundamentals of Anatomy”, 2009, p. 303). When calcium binds with troponin it pulls tropomyosin off of the myosin binding sites of actin. This exposes the binding sites on actin myofilaments and it changes the shape of troponin causing the tropomyosin to move away from the active actin sites.Cross bridges are formed when the myosin heads on the myosin filaments bind to the exposed binding sites on the actin myofilaments by breaking down ATP to ADP (“Fundamentals of Anatomy”, 2009, p. 303).
The cross-bridge pulls actin, which slides over the myosin. Myosin heads release ADP which completes the cross-bridge movement and ATP binds to the myosin head and breaks the cross bridge causing the myosin head to pull the actin filaments inward, which shortens and contracts the muscle. ATP releases energy and the myosin heads pull the actin filaments toward the center of the sarcomere (“Fundamentals of Anatomy”, 2009, p. 303). The Muscle relaxation begins when the ACH is broken down by the enzyme ACHE, which is found in the synaptic cleft.
Another ATP then releases energy that releases the myosin head from the actin and also releases calcium from troponin. Calcium ions move back into the sarcoplasmic reticulum. This allows tropomyosin to block the myosin binding sites on the actin again (“Fundamentals of Anatomy”, 2009, p. 303). Sodium channels are closed and potassium channels open, allowing potassium to diffuse rapidly out of the muscle fiber.
The muscle fiber is no longer stimulated, so the muscle can now relax . The muscle fiber relaxes, ATP breaks down the myosin cross bridges, and the muscle remains ready for future stimulation. This brings the muscle membrane back to its original negatively charged state.