While most of the processes are similar, there are some notable differences between the actions of skeletal, cardiac and smooth muscle.
Cardiac-muscle cells are striated, and are a lot like skeletal-muscle cells except that in cardiac muscle, the fibers are interconnected. The sarcoplasmic reticulum of cardiac-muscle cells is not as well-developed as that of skeletal-muscle cells. Cardiac-muscle contraction is actin-regulated, meaning that the calcium ions come both from the sarcoplasmic reticulum (as in skeletal muscle) and from outside the cell (as in smooth muscle). Otherwise, the chain of events that occurs in cardiac-muscle contraction is similar to that of skeletal muscle.
Compared to skeletal muscle, smooth-muscle cells are small. They are spindle-shaped, about 50 to 200 microns long and only 2 to 10 microns in diameter. They have no striations or sarcomeres. Instead, they have bundles of thin and thick filaments (as opposed to well-developed bands) that correspond to myofibrils. In smooth-muscle cells, intermediate filaments are interlaced through the cell much like the threads in a pair of "fish-net" stockings. The intermediate filaments anchor the thin filaments and correspond to the Z-disks of skeletal muscle. Unlike skeletal-muscle cells, smooth-muscle cells have no troponin, tropomyosin or organized sarcoplasmic reticulum.
As in skeletal-muscle cells, contraction in a smooth-muscle cell involves the forming of crossbridges and thin filaments sliding past thick filaments. However, because smooth muscle is not as organized as skeletal muscle, shortening occurs in all directions. During contraction, the smooth-muscle cell's intermediate filaments help to draw the cell up, like closing a drawstring purse.
Calcium ions regulate contraction in smooth muscle, but they do it in a slightly different way than in skeletal muscle:
- Calcium ions come from outside of the cell.
- Calcium ions bind to an enzyme complex on myosin, called calmodulin-myosin light chain kinase.
- The enzyme complex breaks up ATP into ADP and transfers the Pi directly to myosin.
- This Pi transfer activates myosin.
- Myosin forms crossbridges with actin (as occurs in skeletal muscle).
- When calcium is pumped out of the cell, the Pi gets removed from myosin by another enzyme.
- The myosin becomes inactive, and the muscle relaxes.
This process is called myosin-regulated contraction.
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