The cells that form the muscle tissue are elongated and, for this reason, they are also called muscle fibers.
Be careful not to confuse the term muscle fibers, which are cells, with connective tissue fibers, which are not cells but formed solely by the association of several macromolecules.
There are three types of muscle tissues, each composed of cells with distinct morphological and functional characteristics> All have contractile properties, that is, they may shorten their lenghth and returns to their regular lenghth. The distribution of each type in the body is also characteristic of each type.
All muscle fibers have in common:
– the presence of a large amount of contractile proteins, primarily myosin and actin associated to other proteins. The way in which these proteins are organized differs among the three types of muscle cells.
– the capacity to generate movement or tension as a result of their contraction. As these cells are elongated, they can undergo significant longitudinal shortening, generating movement. When the cells contract without shortening, they generate tension.
Due to the peculiar organization of contractile proteins in two of the types of muscle fibers, these are called striated muscle tissue. Why? When viewed under a microscope, their fibers exhibit transverse bands along the cytoplasm.
In the third type of muscle fiber, the arrangement of contractile proteins is quite different, and the cells do not show transverse striations. This type is called smooth muscle tissue.
Main features of muscle cells:
– Skeletal striated muscle – its contraction is voluntary , it constitutes the body’s musculature attached to bones, as well as some muscles not attached to bones, such as those present in the skin (for instance: mimic muscles), the orbicularis oculi, and a few others.
– Cardiac striated muscle tissue – its contraction is involuntary, they constitue muscle the heart – the myocardium – and is also found in the initial portion of the aorta.
– Smooth muscle tissue – its contraction is involuntary and constitutes the muscles of the viscera and of the walls of blood vessels.