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sarcolemma, T Tubules and sarcoplasmic reticulum excitiation contraction coupling
sarcomere- crossbridge cycling
- -Calcium entry at synaptic knob (a nerve signal is propagated down a motor axon and triggers the entry of calcium into the synaptic knob). Calcium binds to proteins in synaptic vesicle membrane)
- -Release of Ach from synaptic knob (calcium binding triggers synaptic vesicles to merge with the synaptic know plasma membrane and Ach is exocytosed into the synaptic cleft)
- - Binding of Ach to Ach receptor at motor end plate (Ach diffuses across the fluid filled synaptic cleft in the motor end plate to bind with Ach receptors)
Sarcolemma, T Tubules and sarcoplasmic reticulum: excitation contraction coupling
- -development of an end plate potential (EPP) at the motor end plate. (Binding of Ach to ACH receptors in the motor end plate triggers the opening of these chemicals gated ion channels. Na rapidly diffuses into 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 (EPP)- the inside which was negative is now positive)
- -Initiation and propagation of an action potential along sarcolemma and T tubules (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
- -Release of Ca from the sarcoplasmic reticulum (when the action potential reaches the sarcoplasmic reticulum, it triggers the opening of voltage gated CA channels located in the terminal cisternae of the sarcoplasmic reticulum. Ca diffuses out of the cisternae sarcoplasmic reticulum into the sarcoplasm)
Sarcomere- Cross bridge cycling
- -Ca binding (Ca binds to troponin in muscle thin filaments, causing a conformational change in troponin. Troponin changes shape and the entire troponin tropomyosin complex moved. Thus, tropomyosin no longer covers the myosin binding site on actin
- -Crossbrige formations “attach” (myosin heads, which are in the cocked position, binds to the exposed myosin-binding site on actin forming a crossbridge between myosin and actin.
- -Power stroke “pull” (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 Pi are released during this process
- Release of myosin head (ATP binds to ATP binding site on the myosin head, which causes the release of the myosin head from the binding site on actin
- -Reset myosin head (ATP is split into ADP and P by myosin ATPase. This provides the energy to reset the myosin head
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