Question

Contraction of skeletal muscle and smooth muscle cells both involve Ca₂⁺ ions. How does the use of Ca differ between the two cell types?

1) The direction of Ca movement (into or out of the cell) is different
2) Skeletal muscles require higher concentrations of Ca while smooth muscle cells require lower concentrations
3) Ca activates microtubules in one and microfilaments in the other
4) The cells have different binding partners for Ca
5) They both use Ca the same exact way

Answer 1

Skeletal muscle contraction is initiated by Ca2+ binding to troponin, while in smooth muscle, Ca2+ binds to calmodulin, activating myosin kinase for contraction. Skeletal muscle uses a sarcoplasmic reticulum for Ca2+ storage, whereas smooth muscle relies more on the entry of extracellular Ca2+ through sarcolemma channels.

Step-by-step explanation:

The use of Ca2+ ions in skeletal and smooth muscle contraction differs primarily in their interaction within the cells. In skeletal muscle, Ca2+ binds to troponin to initiate muscle contraction by exposing actin’s myosin binding sites. In contrast, in smooth muscle cells, Ca2+ binds to calmodulin, activating myosin light chain kinase, which phosphorylates myosin heads, thus initiating contraction.

In smooth muscle, the influx of extracellular Ca2+ through calcium channels in the sarcolemma contributes the most to the contraction process. Whereas, in skeletal muscle, Ca2+ is released from the sarcoplasmic reticulum via voltage-dependent calcium channels when the muscle is stimulated to contract. After the contraction, Ca2+ is pumped back into the sarcoplasmic reticulum or out of the cell, this step is ATP-dependent and helps muscles to relax.