MEDICAL PHYSIOLOGY

Detailed explanation-1: -Muscle contraction usually stops when signaling from the motor neuron ends, which repolarizes the sarcolemma and T-tubules, and closes the voltage-gated calcium channels in the SR. Ca++ ions are then pumped back into the SR, which causes the tropomyosin to reshield (or re-cover) the binding sites on the actin strands.

Detailed explanation-2: -Ca++ ions are then pumped back into the SR, through the process of active transport, which requires ATP. The lack of Ca++ ions causes the tropomyosin to reshield (or re-cover) the binding sites on the actin strands, allowing the actin (thin) and myosin (thick) interaction to relax, ending the cross-bridge cycle.

Detailed explanation-3: -Stimulation of the muscle fibre, causes a wave of depolarisation to pass down the t-tubule, and the SR to release calcium ions into the sarcoplasm. Calcium is pumped back up into the SR to lower calcium ion concentration in the sarcoplasm, to relax the muscle (turn off contraction).

Detailed explanation-4: -Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, allowing the muscle cell to relax. The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum in the sarcoplasm.

Detailed explanation-5: -Calcium is released from the sarcoplasmic reticulum into the the sarcoplasm. It binds the troponin molecules on the thin filaments, causing the strands of tropomyosin to shift, exposing the myosin-binding sites on the thin filaments.