Extracellular calcium is required for the contraction of which types of muscle cells?

Extracellular calcium is indeed essential for the contraction of both cardiac and smooth muscle cells. In cardiac muscle, calcium ions from the extracellular fluid enter the cells through specialized channels during depolarization, triggering a release of additional calcium from the sarcoplasmic reticulum, which leads to muscle contraction. This calcium-induced calcium release mechanism is crucial for the effective contraction of the heart, ensuring that it pumps blood efficiently.

Smooth muscle cells also rely heavily on extracellular calcium for their contraction process. Calcium enters these cells through voltage-gated and receptor-operated channels, leading to interactions between the calcium ions and proteins within the cells that initiate contraction. This is particularly important in regulating functions such as blood vessel constriction and peristalsis in the digestive tract.

In contrast, while skeletal muscle contraction does involve calcium, it primarily utilizes calcium released from the sarcoplasmic reticulum rather than relying on extracellular calcium to the same extent as cardiac and smooth muscle cells. Therefore, the presence of extracellular calcium is a critical factor for both cardiac and smooth muscle contractions, making this the correct choice.