Smooth muscle tissue is a non-voluntary muscle type named for the fact that its cells do not have a microscopically visible banding or striation pattern. It is found primarily in hollow organs, such as blood vessels and the digestive tract. It also makes up muscles that pull skin hairs erect, focus the eyes, and adjust the diameter of the pupils of the eyes.
Its control mechanisms are more diverse than those of other muscle types. Depolarization of the cell membrane and increases in cytosolic calcium concentration promote contraction. This can be accomplished through many different signaling pathways. Generally this muscle type responds to some combination of stimulation by the autonomic division of the nervous system, hormonal control, its own intrinsic pacemaking muscle cells, and detection of environmental conditions around the smooth muscle cells. Smooth muscle behavior is a crucial part of conditions as diverse as high blood pressure, asthma, and urinary incontinence.
Smooth muscle, like other muscle tissue, is a tissue type that can pull on other structures by the ATP-powered contraction of the smooth muscle cells. Like other muscle cell types, smooth muscle uses interaction between the proteins myosin and actin to do this. Unlike the situation in other kind of muscle cells, the actin and myosin in smooth muscle cells is not arranged in the closely-aligned repeating structures of sarcomeres. Instead, smooth muscle actin and myosin are arranged in a mesh or net within the cell. This mesh is anchored to the cytoskeleton of the cell, and when myosin pulls on actin during their interaction, the entire cytoskeleton of the cell is pulled into a shorter and thicker form, usually with a spiral twist. This pattern of actin and myosin arrangement is not externally visible with a light microscope, so the cells have a homogeneous internal appearance except for the single nucleus. Other muscle types are called striated due to an externally visible geometric arrangement of actin and myosin that gives skeletal and cardiac muscle cells a banded appearance.
Smooth muscle is called an involuntary muscle type because its contraction cannot be triggered by conscious effort. Most smooth muscles do respond to neurotransmitters released from the nervous system, but the part of the nervous system used for this control is the non-voluntary autonomic motor division. For example, you could indirectly manage the smooth muscle in the irises of your eyes by moving from a bright environment to a dimmer one. The light-sensing retinas of your eyes would notify parts of your brainstem about the lighting change. Without your conscious intention, your brainstem can then send autonomic motor directions, by means of nerves and neurotransmitters, to the smooth muscles located in the irises of your eyes. Some iris smooth muscle cells would be instructed to relax, others to contract, and the pupils of your eyes would enlarge. This lets more light reach the retinas of your eyes.
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