Smooth muscle

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Smooth muscle (also known as visceral muscle due to the locations in which they are present [1]) is one of the three main types of muscle tissue that exist in the human body [2] . Smooth muscle is under involuntary control and is innervated by the autonomic nervous system. It can also be stimulated without the use of nerves, this is termed Pharmomechanical Coupling. In this type of stimulation an agent (usually hormone) is used to cause contraction rather than an action potential. The contractions that occur due to other agents and not action potentials are former when paracrine agents are deteced, acidity changes, oxygen concentration changes, osmolarity changes, and ion composition alters. This can cause contractions in localised areas to oppose the changes in homeostasis.[3] One example of how smooth muscle cells contribute to homeostasis is the way in which they contract in cold conditions to erect hairs to produce a layer of insulation for the body[4]. When certain hormones bind to Ca2+ mobilising receptors on the sarcolemma inositol 1,4,5-trisphosphate (InsP3) is produced. This secondary messenger opens InsP3-gated Ca2+ channels allowing an influx of calcium ions. In turn this influx causes ryanodine receptors (RYR) to open allowing an even greater influx of calcium ions [5].

Smooth muscle lines the walls of hollow internal organs such as the bladder and intestine [6], organs of this type are known as viscera. Smooth muscle cells are composed of myosin myofilaments dispersed throughout the muscle cell cytoplasm and filaments of actin held together in contractile bundles of 12-15. Each actin bundle overlaps the myosin filament on one side, and converges to attach to a dense body on the other side. A single myosin filament is overlapped by two seperate actin contractile bodies, each of which are attached to a different dense body. These dense bodies are attached to intermediate filaments. These intermediate fillaments exist between dense bodies, connecting them, and they are anchored by dense plaque-like bodies [7]. The contractile filament bundles of actin and myosin are loosely arranged in a diagonal fashion, in different directions around the perimeter of the smooth muscle cell. This arrangement of fibres causes the muscle cell to become globular upon contraction [8]. Smooth muscle cells are fusiform in shape meaning that they are wide in the middle with tapered ends, they also have only a single nucleus. Smooth muscle cells do not contain the sarcomeres found in skeletal and cardiac muscle and therefore appear unstriated under a light microscope [9]. Smooth muscle cells are unstriated because there is no regular arrangement of actin and myosin filaments.They contain only a few sarcoplasmic reticula, instead using extracellular calcium as the source of calcium ions which initiate contraction [10].

There are two types of smooth muscle cell, multi- unit and single unit smooth muscle. Single-unit smooth muscle cells are connected by gap junctions that electrically connect cells to one another, so contract as a single unit. These can be found in the intestinal tract, the skin, and the walls of small arteries, veins and hollow organs[11]. Multi-unit cells lack gap junctions, so are not linked electrically. They must be stimulated independently, which allows fine control of contractions by selective activation of individual muscle cells. Multi-unit cells can be found in the eye [12]. Smooth muscle cell contracts in different direction because there is no regular arrangement of its contractile proteins and this is important in the movement of the intestine.

When a smooth muscle contracts Ca2+ ions enter the muscle fibre and bind to calmodulin (the secondary messenger in this process) [13]. In smooth muscle, Ca2+ ions move into the cytosol from both the sarcoplasmic reticulum and the extracellular fluid[14]. Smooth muscle cells do not contain troponin, like in skeletal muscle cells, so Ca2+ binds to calmodulin when the muscle cell contracts[15]. This calcium - calmodulin complex removes the caldesmon from the actin sites where myosin will attach.  An enzyme called a myosin light chain kinase (MLCK) is activated. MLCK is responsible for phosphorylating myosin filaments so that it can form cross-bridges with actin filaments. Relaxation occurs when the myosin is dephosphorylated by myosin phosphatase (MP) removing it from actin [16]. This process is relatively slow because it relies on the diffusion of calcium ions over large distances, maximum contraction is often nearly a second long and uses very little ATP. This is important so that smooth muscle doesn’t fatigue during sustained periods of activity [17]. Because smooth muscle cells do not fatigue they are able to constantly function. The rate of ATP splitting determines the rate of muscle contraction. In smooth muscle myosin, the rate of ATPase activity is 10 to 100 slower than in skeletal muscle myosin. It must also be noted that nerve stimulation can cause a excitation or inhibition of the muscle cell[18]. The amount of MLCK present in the muscle cell depends on the amount of Ca2+-calmodulin complexes, which is dependent on Ca2+ levels - this means contraction is a graded response[19].

Unlike in other types of muscle cell, smooth muscle is capable of not only hypertrophy but also mitosis. This allows organs such as the pregnant uterus to grow by adding new myoctyes as well as enlarging existing ones. It also means that injured smooth muscle can repair itself faster.[20]


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  20. Saladin, Kenneth (2012) Anatomy and Physiology, 6th edition, p430
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