The Muscles

     It is the muscles that allow for movement. Muscle cells are the only cell type in our body that quickly constrict. There are three types of muscles: the skeletal muscles and the cardiac muscle which are striated muscle, and smooth muscles which do not show stria. Although the characteristics of these three types of muscle fibers will be discussed in the section on muscle fibers, only the striated muscles will be considered in this section. The muscles constitute approximately 40% of our weight.

    Most of the striated muscles are skeletal muscle, and as their name indicates, are related to the bones, the skeleton. Other striated muscles are the cutaneous muscles which, upon contraction, move the skin; this is the case of the muscles that make us smile and wink. There are also the accessory muscles such as those that control the movement of the eyes. Striated muscles are said voluntary muscles; we can voluntarily control their contraction hence some movements. Cardiac muscle and smooth muscles (such as those of the intestine) are involuntary muscles and we have no control on their contractions.

    As you can judge from the figure on the left, the muscles are quite numerous, and this is only a superficial view. The muscles also have a variety of forms: they may be circular as those around the eyes, flat like those of the stomach, or fusiform as most of the muscles of our arms and legs.

    Our muscles are made of muscle fibres that could be as long as muscle itself, but measure only 10 to 80 micron in diameter. The term 'striated' muscles comes from the fact that internal arrangement of the contractile unit, the myofibrils, shows dark and light bands all aligned. These bands delimitate contractile units, where proteins (mainly the actin and myosin) are organizing to pull on support points and thus cause contraction.

Three contractile units, at rest and contracted.