Biol. 2250
Question 10. A contracting skeletal muscle fiber decreases in length as thick and thin protein filaments within sarcomeres interact to cause sarcomeres to shorten. Tension is exerted on the portions of the skeleton where the muscle is attached to cause movement in the body.
The first physiologic event of skeletal muscle contraction is muscle fiber excitation by a motor neuron. A nerve signal is propagated down a motor axon and triggers the entry of Ca+ into the synaptic knob. Ca+ binds to proteins in synaptic vesicle membrane. Calcium binding triggers synaptic vesicles to merge with the synaptic knob plasma membrane and ACh is exocytose dint the synaptic cleft. ACh diffuses across the fluid filled synaptic cleft in the motor end plate to bind with ACh receptors.
The second physiologic event of muscle contraction is excitation-contraction coupling an event that involves the sarcolemma, T-tubules, and sarcoplasmic reticulum. Binding of ACh to ACh receptors in the motor end plate triggers the opening of these chemically gated ion channels. Na+ rapidly diffuses into and K+ slowly diffuses out of the muscle fiber. The result is a reversal in the electrical charge difference across the membrane of a muscle fiber at the motor end plate, which is called an end plate potential. An action potential is propagated along the sarcolemma and T-tubules. First voltage gated Na+ channels open and Na+ moves in to cause depolarization. Second voltage gated K+ channels open and K+ moves out to cause repolarization. When the action potential reaches the sarcoplasmic reticulum, it triggers the opening of voltage gated Ca2+ channels located in the terminal cisternae of the sarcoplasmic reticulum. Ca2+ diffuses out of the cisternae sarcoplasmic reticulum into the sarcoplasm.
The third physiologic event involves binding of Ca2+ and crossbridge cycling which results in muscle contraction. Ca2+ binds to troponin in muscle thin filaments, causing a conformational change in troponin. Troponin changes shape and the entire troponin-tropomyosin complex is moved thus tropomyosin no longer covers the myosin binding site on actin. Myosin heads, which are in the cocked position, bind to the exposed myosin binding site on actin forming a crossbridge between myosin and actin. The myosin head swivels toward the center of the sarcomere, pulling along the attached thin filament. This motion is called a power stroke. ADP and P1 are released during this process. ATP binds to the ATP binding site on the myosin head, which causes the release of the myosin head from the binding site on actin. ATP is split into ADP and P1 by myosin ATPase. This provides the energy to reset the myosin head.
Skeletal muscle relaxation involves returning all muscle fiber components to their resting state when stimulation form the motor neuron ceases. First event is termination of the nerve signal in the motor neuron which prevents further release of ACh. Acetylcholinesterase in the synaptic cleft continually hydrolyzes ACh to dissociate it from its receptor on the motor end plate and terminate its stimulation of the skeletal muscle fiber. The ACh receptor closes, and both the end plate potential at the motor end plate and the action potential along the sarcolemma and T-tubules ceases. Voltage gated Ca2+ channels in the sarcoplasmic reticulum close. The Ca2+ already released from the sarcoplasmic reticulum is continuously returned into the terminal cisternae by Ca2+ pumps. After cessation of skeletal muscle fiber stimulation, the remaining Ca+ in the sarcoplasm is transported back into storage within the sarcoplasmic reticulum. Troponin returns to its original shape when Ca2+ is removed, and simultaneously the tropomyosin moves over the myosin binding sites on actin. This prevents myosin-actin crossbridge formation. Through the natural elasticity of the muscle fiber, the muscle returns to its original relaxed position, a process facilitated by the release of passive tension that developed in connectin proteins that were compressed during contraction.