Cardiac muscles are striated, while smooth muscles are not.
Cardiac Muscles: Cardiac muscle exhibits cross striations formed by alternating segments of thick and thin protein filaments. Like skeletal muscle, the primary structural proteins of cardiac muscle are actin and myosin. They are mononucleated with abundant mitochondria and their size is smaller than the skeletal muscle fibres. In some of the cardiac muscles, Sarcoplasmic reticulum and T-tubules are present while in some these may be absent.
Cardiac muscles are different from the skeletal muscles in two functions, Cardiac muscles contract automatically to squeeze the walls of the heart inward. During contraction the cell membrane of the cardiac muscle fibres undergoes electric changes, known as action potentials. After the completion of an action potential, the cell membrane remains in a refractory state in order to provide relaxation to muscles. This refractory period is required for the normal rhythmic contraction of the heart.
Smooth Muscles: Smooth muscles do not exhibit transverse striations like skeletal and cardiac muscles. Smooth muscle fibres are much smaller than skeletal muscle fibres. Smooth muscle is responsible for the contractility of hollow organs, such as blood vessels, the gastrointestinal tract, the bladder, or the uterus. Smooth muscles are present in the iris of the eye and skin. However, the speed of smooth muscle contraction is only a small fraction of that of skeletal muscle. Smooth muscles are small, spindle shaped with single nucleus having actin and myosin filaments in its cytoplasm.
The contraction mechanism in smooth muscles is similar to the sliding filament type of contraction as found in the skeletal muscles. They exhibits varying spontaneous rhythmic contractions, which need not be stimulated through the nerves, however, can be modified by nerves as well as by hormones such as epinephrine and norepinephrine. It simulates slowly to a response that is why it utilizes very less amount of energy.
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