Title: Muscle Membrane, T Tubules, and Sarcoplasmic Reticulum | Neupsy Key
Headings: Muscle Membrane, T Tubules, and Sarcoplasmic Reticulum
Muscle Membrane, T Tubules, and Sarcoplasmic Reticulum
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Content: Free sarcoplasmic reticulum constitutes the remainder of the intrafiber sarcoplasmic reticulum membrane system. It is not associated with the T tubule system and functions in Ca 2+ reuptake into the sarcoplasmic reticulum. It has a large number of Ca 2+ adenosine triphosphatase (ATPase) pumps on its surface that function in the energy-dependent reuptake of sarcoplasmic Ca 2+ and therefore plays a role in muscle relaxation rather than excitation and contraction. A motor nerve action potential propagates down the motor nerve axon to the nerve terminal, causing release of acetylcholine (ACh) into the synaptic cleft of the neuromuscular junction. After binding of acetylcholine to the postsynaptic muscle membrane acetylcholine receptor (AChR), there is generation of an end-plate potential. Suprathreshold end-plate potentials generate muscle fiber action potentials. These propagate along the sarcolemma, depolarizing the muscle membrane sequentially as it travels along the membrane. The action potential continues down the T tubules. Depolarization of the T tubule membrane activates the voltage-gated Ca 2+ channel, thus activating the ryanodine receptor, releasing large amounts of Ca 2+ from the sarcoplasmic reticulum into the sarcoplasm. This leads to cross-bridge formation between thin and thick filaments, activation of the cross-bridge cycle, sliding of the thick and thin filaments past one another, sarcomere shortening, and eventually muscle contraction.