Smooth Muscle
Smooth muscle gets its name because there are no striations visible in the muscle tissue. In other muscles, dark and light bands are seen under microscopic examination. Nevertheless, smooth muscles do contain different types of filaments. Thick filaments are called myosin and thin filaments are called actin. These filaments are present in other types of muscles, such as striated muscle, in addition to smooth muscle.
The cells of a smooth muscle are long and spindle-shaped. In hollow organs they tend to be arranged in bundles in an outer layer that is oriented with the long axis of the organ, and in a circular pattern around the inner surface.
Smooth muscles are found in internal organs. Examples include the uterus, the bronchi of the lungs, the bladder, and the walls of blood vessels.
The myosin and actin filaments are designed to slide against each other. In smooth and other muscles this is known as the sliding filament model of contraction. The movement of the actin and myosin filaments against one another is an energy-requiring process. This back-and-forth ability allows the muscles to contract. The individual muscle filaments do not become shorter. The process is analogous to the operation of a bicycle air pump, where a central rod slides in and out of the surrounding housing.
The contraction of smooth muscle can be stimulated by the release of chemicals known as neurotransmitters by nearby motor neurons of the autonomic nervous system. Two examples of neurotransmitters are noradrenaline and nitric oxide. Smooth muscle contraction can occur in the absence of motor neurons. For example, if a compound called histamine is released, the smooth muscles lining the air passages will contract. This is the basis of an asthma attack, where the muscle contraction restricts airflow through air passages. Hormones present in the blood also stimulate the contraction of smooth muscles. A well-known example is the contractions of the uterus during childbirth by the hormone oxytocin.
The contraction of smooth muscles is typically slower than the contraction of striated muscle. As well, smooth muscle contraction tends to be maintained for a longer time. Smooth muscle contraction is not intended to provide the explosive power that contraction of a striated muscle provides.
Smooth muscles are not under conscious control. As such, they are also known as involuntary muscles. As an example, the smooth muscles of the bladder contract involuntarily to expel urine. In fact, trying to suppress the contractions requires great concentration and is often uncomfortable.
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