There are three different types of muscle cells in the human body: skeletal, smooth and cardiac. These are classified as either voluntary or involuntary, depending on whether we consciously control their movements. They are further classified by appearance, as either smooth or striated; striated muscle cells when viewed under a microscope have a striped appearance. Muscle cells are highly specialized, each optimally designed to perform its required function. As such, there is variation amongst muscle cells within each category.
Skeletal muscle cells are elongated, forming long fibers. They have multiple nuclei within each cell; this is in contrast to the majority of other cells in our bodies, according to BMH Linguistics. They also contain many mitochondria which are cellular organelles that produce adenosine triphosphate (ATP), the body’s fuel. Smooth muscle cells are short, contain only one nucleus and are not striated in appearance, hence the term "smooth." Cardiac muscle cells are also striated in appearance though they appear less organized into stripes than skeletal muscle cells. These cells may be branched, forming physical connections with many surrounding cells.
According to BMH Linguistics, skeletal muscle cells are the most abundant type in our bodies. These muscle fibers attach to our bones allowing movement in our joints. As well, we use skeletal muscles to maintain our posture. Smooth muscle cells are found lining our internal organs and blood vessels and are responsible for the contraction of organs, such as the bladder. Smooth muscles are considered involuntary. Cardiac muscle cells make up the heart and are responsible for pumping blood throughout our bodies. Cardiac muscle is generally thought of as involuntary.
The University of Leeds lists more than 20 different types of proteins present in muscles. The inclusion, exclusion and amount of each one present will alter the functionality of the cell. The two major proteins, actin and myosin, are present in all three cell classes. The end-to-end arrangement of these two proteins is responsible for the striated appearance of skeletal and cardiac muscle fibers. Smooth muscle, by contrast, contains only half the amount of myosin found in the striated muscle cells.
It is a muscle cell’s ability to contract, or shorten itself, that permits movement. All contraction is dependent on the presence of actin and myosin. According to the University of Leeds, stimulation of actin and myosin bundles causes the proteins to slide toward one another, thereby shortening the fibers. Stimulation may come from a nerve signal, or it may result from the presence of charged molecules or ions being sent out to the muscle cell.
Muscle cell efficiency is important to minimize both excessive heat production as well as daily food requirements. ATP, the body’s energy unit, is largely consumed by muscle cells; the higher the contraction rate, the more ATP that is required to maintain it. Skeletal muscle cells perform tasks at higher contracting rates, using a lot of ATP, although bouts of movement are followed by periods of rest. Cardiac muscle is slower in contracting but constant, and so also requires a high amount of energy use. Smooth muscle generally contracts very slowly and is considered the most efficient of the three muscle cell types, according to the University of Leeds.