Skeletal Muscle Cell
A skeletal muscle cell is a type of striated muscle that is responsible for voluntary movement at the expense of energy. The cells are unique because they are multinucleates- they contain more than one nucleus per cell. The reason for this is the fusion of myoblasts in developing embryos via specific cell-to-cell adhesion molecules. This fusion is also responsible for their relatively large size.
They have a very high level of organization in order for them to contract efficiently. Each muscle is made up of many muscle fibres. These fibres are made of numerous myofibrils which are in turn composed of sarcomeres. These sarcomeres are made up of a variety of proteins that are organised into filaments, most notably the thick and thin filaments. The thick filament is made of myosin and the thin filament is made of numerous proteins such as actin, tropomyosin and troponin. It is these proteins that are the main contractile apparatus in the cell via the cross-bridge cycle.
Their contraction involves the attachment of calcium ions to troponin causing a conformational shape change. This removes tropomyosin from the myosin head binding site and allows the head to bind. The power stroke then occurs as ATP is hydrolysed and the head of the myosin moves, sliding the filaments along one another. The head detaches at the expense of another ATP molecule and repeats the cycle. It is this that creates the shortening of the muscle and hence the contraction required.