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
Only gold members can continue reading. Log In or Register to continue

Content: Junctional sarcoplasmic reticulum (the terminal cisternae) stores Ca 2+ and is the site of Ca 2+ release in response to a muscle fiber action potential. Junctional sarcoplasmic reticulum contains high amounts of calsequestrin, which binds to Ca 2+ and accounts for the high Ca 2+ storage of the sarcoplasmic reticulum. The sarcoplasmic reticulum is so efficient in this role that the muscle fiber loses very little Ca 2+ even after repeated muscle contractions. Junctional sarcoplasmic reticulum is also the site of the Ca 2+ channel responsible for releasing large stores of calcium into the sarcoplasm, the ryanodine receptor, so named for its binding to the plant alkaloid ryanodine. Sarcoplasmic reticulum voltage-gated Ca2+ channels sense the depolarization of the T tubule, interact directly with the ryanodine receptor and induce it to release Ca 2+ from the sarcoplasmic reticulum. 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.