- Slides: 5
Muscle Contraction • Muscle contractions begin with a neuron stimulating a muscle fiber (cell) or several muscle fibers. • The neuron releases a neurotransmitter (Acetylcholine) ACh into the synaptic cleft. • The ACh binds with protein receptors on the sarcolemma (cell membrane) of the muscle fiber. • If enough ACh is released and binds with the receptors it causes the sodium (Na) channels on the muscle fiber to open and Na floods in. This generates an action potential and causes the muscle fiber to become polarized.
Muscle Contraction • When the Na+ floods in it causes the Sarcoplasmic Reticulum (SR) to release Calcium (Ca) which it stores. • Calcium is very important as it moves the regulatory proteins Troponin and Tropomyosin out of the way. • Calcium binds directly to Troponin which then moves Tropomyosin off the protein Actin. • With the regulatory proteins out of the way, Myosin heads (crossbridge) can now bind to binding sites on Actin.
Muscle Contraction • Myosin heads then pull Actin at the attachment site, they detach and reattach at another site further down the actin filament. This shortens the sarcomere. This occurs all along the muscle fiber. • In order for the myosin head to detach and be able to reattach to another site on actin, ATP energy is needed. • When the stimulus ends and the neuron is no longer releasing ACh into the synaptic cleft the muscle must return to the resting state.
Returning the Muscle to the Relaxed State • The left over ACh in the Synaptic Cleft is broken down by enzymes. • Ca+ are reabsorbed into the Sarcoplasmic Reticulum. • Na+ is pumped out of the muscle fiber by the Na+K+ pump. • Now the muscle is back to its relaxed or resting state.