What part of the muscle cell releases Ca2+ during contraction?

The part of the muscle cell that releases Ca2+ during contraction is the sarcoplasmic reticulum. This specialized endoplasmic reticulum is essential for muscle function, as it stores calcium ions and releases them into the cytoplasm when a muscle cell is stimulated to contract.

When an action potential travels along the sarcolemma, the membrane potential changes and this signal is sent deep into the muscle fiber through structures known as T-tubules. This electrical signal triggers the sarcoplasmic reticulum to release the stored calcium ions. The increase in intracellular calcium concentration is what ultimately initiates muscle contraction through the interaction of actin and myosin filaments, leading to the sliding filament mechanism.

The other parts mentioned have different roles in the muscle cell: the sarcoplasm is the cytoplasm of the muscle cell where metabolic processes occur, the sarcolemma is the muscle fiber's plasma membrane, and myofibrils are the contractile threads found in striated muscle cells but do not release calcium themselves. Thus, the correct answer regarding the source of Ca2+ release during contraction is indeed the sarcoplasmic reticulum.