Skeletal Muscle

Function

The tissue most commonly thought of as muscle is skeletal muscle. Skeletal muscles cover your skeleton, giving your body its shape and movement. They are attached to your skeleton by strong, springy tendons or are directly connected to rough patches of bone. Skeletal muscles are under voluntary control, which means you consciously control what they do.

Just about all body movement, from walking to nodding your head, is caused by skeletal muscle contraction. Your skeletal muscles function almost continuously to maintain your posture, making one tiny adjustment after another to keep your body upright. Skeletal muscle is also important for holding your bones in the correct position and prevents your joints from dislocating. Some skeletal muscles in your face are directly attached to your skin. The slightest contraction of one of these muscles changes your facial expression.

Structure

A muscle is a bundle of many cells calledmyocytes, or muscle fibers. You can think of muscle fibers as longcylinders, and compared to othercellsin your body, muscle fibers are quite big. They are from about 1 to 40 microns long and 10 to 100 microns in diameter. For comparison, a strand of hair is about 100 microns in diameter, and a typical cell in your body is about 10 microns in diameter.

A muscle fiber contains manymyofibrils, which are long cylinders of muscleproteins. Some muscle cells are so long they can run the entire length of the muscle. These proteins allow a muscle cell to contract.

Thecell membraneof a skeletal muscle fiber, thesarcolemma, is the site ofaction potentialconduction and helps trigger muscle contraction. Within this cell membrane are various organelles. Interestingly, muscle fibers are multi-nucleated (have more than one nucleus).

Muscle cells have numerous mitochondria, more than other types of cells.

Thesarcoplasmic reticulum (SR)is a variation of the smooth endoplasmic reticulum. It is able to store manyionsin solution that the cell will need at a later time. When a cell needs to do something immediately, it doesn’t make sense to search the environment for extra ions that may or may not be floating around. It is easier to have them stored in a pack for easy use. For example, when you are running around and your muscle cells are active, they need calcium (Ca) ions. The SR can release those ions immediately. When you are resting, they are able to store them for later use.

Structure-Function Connection:

  1. Why do you think skeletal muscle cells have more mitochondria than other cells?
  2. Why do you think skeletal muscles are large and cylindrical shaped? How would that help its function?
  3. The endoplasmic reticulum is an organelle used for transport in the cell. Why do you think skeletal muscle has a very prominent endoplasmic reticulum?
  4. What purpose might multiple nuclei serve to a skeletal muscle cell?