Final answer:
The sarcoplasmic reticulum stores and releases calcium ions, which is crucial for muscle contraction. Smooth muscle cells have a smaller sarcoplasmic reticulum but still rely on calcium ions for contraction.
Step-by-step explanation:
The sarcoplasmic reticulum (SR) plays a crucial role in muscle contraction by storing and releasing calcium ions. When a muscle is stimulated to contract, an action potential travels along the sarcolemma and into the T-tubules. This signal triggers the release of calcium ions from the SR, which bind to the protein troponin and initiate the sliding of actin and myosin filaments, resulting in muscle contraction.
Smooth muscle cells have a smaller sarcoplasmic reticulum compared to skeletal muscle cells. However, they still rely on calcium ions for contraction. In smooth muscle cells, calcium ions can enter the cell through channels in the sarcolemma. These calcium ions can then bind to calmodulin and activate contraction.