Smooth Muscle

 

In histologic section, it lacks the banding pattern, or striations, seen in the striated and cardiac muscles.

Smooth muscle lines the walls of most hollow organs, including organs of the vascular, gastrointestinal, respiratory, urinary, and reproductive systems.

Clinical Correlation: Smooth muscle is an important therapeutic target because it regulates variables such as blood flow, ventilation of the lungs, and gastrointestinal motility.

  1. Ultrastructure of smooth muscle.

    1. Smooth muscle cells are not striated in appearance (as are skeletal and cardiac muscle) because thin and thick filaments are not organized as sarcomeres.

    2. A network of dense bodies in the cytoplasm of smooth muscle cells serves as attachment points for actin filaments; thick filaments overlap thin filaments in an irregular array

      A two-paneled illustration shows structure of single smooth muscle cells and the coupling mechanism. Panel A shows the morphology of single smooth muscle cells. It comprises of myofilaments and dense bodies. Contraction leads to shrinking of the cells. Panel B shows excitation-contraction coupling in smooth muscle. Ca 2+ ion enters the cell through the calcium channel in the plasma membrane, and then binds with calmodulin molecules. In the complex formed, activated myosin light chain kinase phosphorylates myosin. Furthermore, myosin light chain phosphatase dephosphorylates and inactivates myosin.

       Structure of a smooth muscle cell. No sarcomeres are present; actin filaments are anchored to dense bodies and overlap myosin in an irregular array; no muscle triads are present. B. Excitation contraction coupling in smooth muscle. In most cases, Ca2+ enters the cell from the extracellular fluid (ECF) via voltage-gated Ca2+ channels. Ca2+ binds to calmodulin, resulting in activation of myosin light chain kinase. Phosphorylation of myosin triggers cross-bridge cycling and force development.

    3. The irregular arrangement of actin and myosin allows smooth muscle cells to generate force over a larger range of preloads than is possible in striated muscle.

    4. There is no T tubule or muscle triad structure in smooth muscle; SR is present but it has an irregular arrangement.

    5. There are two general types of smooth muscle:

      1. Visceral smooth muscle is the most common type, in which cells are arranged in large bundles. Cells within a bundle behave as a functional syncytium due to the presence of gap junctions between cells.

      2. Multiunit smooth muscle cells function individually since the cells are not connected by gap junctions. There is:

        • A dense nerve supply from autonomic neurons.

        • Fine control over force development in a manner similar to spatial summation in skeletal muscle (e.g., control over the diameter of the pupil by the iris).

  2. Excitation-contraction coupling in smooth muscle (Figure 1-18B).

    1. The contraction of smooth muscle begins when the intracellular Ca2+ concentration increases.

    2. The source of Ca2+ is usually from the ECF via voltage-gated Ca2+channels when the cell membrane becomes depolarized but can also result from second messenger-stimulated Ca2+ release from the SR.

    3. The regulation of smooth muscle contraction involves myosin phosphorylation because smooth muscle myofilaments do not contain troponins.

    4. The relative activity of the antagonistic enzymes myosin light chain kinase and myosin light chain phosphatase determines whether myosin is activated.

    5. When smooth muscle is stimulated and the intracellular [Ca2+] increases, the result is the formation of Ca2+-calmodulin complexes.

    6. Activated calmodulin stimulates myosin light chain kinase to phosphorylate and thereby activate myosin.

    7. When Ca2+ levels decrease, the muscle relaxes because the activity of myosin light chain phosphatase predominates, causing myosin to become dephosphorylated.

    8. An important feature of some smooth muscles (e.g., sphincters) is the ability to maintain force over long periods by entering a latch bridge state, with slow rates of cross-bridge detachment.

Figure 1-18

A. Structure of a smooth muscle cell. No sarcomeres are present; actin filaments are anchored to dense bodies and overlap myosin in an irregular array; no muscle triads are present. B. Excitation contraction coupling in smooth muscle. In most cases, Ca2+ enters the cell from the extracellular fluid (ECF) via voltage-gated Ca2+ channels. Ca2+ binds to calmodulin, resulting in activation of myosin light chain kinase. Phosphorylation of myosin triggers cross-bridge cycling and force development.

A two-paneled illustration shows structure of single smooth muscle cells and the coupling mechanism. Panel A shows the morphology of single smooth muscle cells. It comprises of myofilaments and dense bodies. Contraction leads to shrinking of the cells. Panel B shows excitation-contraction coupling in smooth muscle. Ca 2+ ion enters the cell through the calcium channel in the plasma membrane, and then binds with calmodulin molecules. In the complex formed, activated myosin light chain kinase phosphorylates myosin. Furthermore, myosin light chain phosphatase dephosphorylates and inactivates myosin.
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B. Smooth Muscle Cells

Mature smooth muscle fibers are spindle-shaped cells with a single central ovoid nucleus. The sarcoplasm at the nuclear poles contains many mitochondria, some RER, and a large Golgi complex. Each fiber produces its own basal lamina, which consists of proteoglycan-rich material and type III collagen fibers.

  1. Myofilaments

      1. Thin filaments. Smooth muscle actin filaments resemble those of skeletal and cardiac muscle. They have associated tropomyosin, but lack troponin. These filaments are stable and are anchored by α-actinin to dense bodies associated with the plasma membrane whose role is analogous to the Z lines of striated muscle.

      1. Thick filaments. Smooth muscle myosin filaments are less stable than those in striated muscle; they are dispersed in the cytoplasm and attach to actin in response to contractile stimuli (IV.D).

      1. Myofilament organization. During contraction, the thick and thin filaments run mostly parallel to the cell's long axis, but they overlap much more than those of striated muscle, accounting for the absence of cross-striations. The greater overlap permits greater contraction. The ratio of thin to thick filaments in smooth muscle is approximately 12:1, and the filament arrangement is less regular and crystalline than in striated muscle (II.B.1.c).

  2. Sarcoplasmic reticulum. Smooth muscle cells contain a sparse and poorly organized sarcoplasmic reticulum that participates in Ca2+ sequestration and release but does not divide the myofilaments into myofibrils. Abundant surface-associated vesicles, caveolae, aid in Ca2+ uptake and release. The small size and slow contraction of these fibers make an elaborate stimulus-conducting system unnecessary; these fibers have no T tubules, dyads, or triads.

  3. Types of smooth muscle fibers. Although similar in morphology, these cells can be classified according to developmental, biochemical, and functional differences.

      1. Visceral smooth muscle derives from splanchnopleural mesenchyme and occurs in the walls of respiratory, digestive, urinary, and reproductive organs. In addition to thick myosin and thin actin filaments, its sarcolemma-associated dense bodies are linked by desmin-containing intermediate filaments. Owing to their poor nerve supply, the cells transmit contractile stimuli to one another through abundant gap junctions, acting as a functional syncytium. Contraction is slow and in waves. Most visceral smooth muscle is classified as unitary smooth muscle. The smooth muscle in the walls of the respiratory airways is more richly innervated and contains multiunit smooth muscle in which individual cells and sheets may contract more independently rather than in waves.

      1. Vascular smooth muscle differentiates in situ from mesenchyme around developing blood vessels. Its intermediate filaments contain vimentin, as well as desmin. Most vascular smooth muscle functions like visceral smooth muscle and is also classified as unitary, although its waves of contraction are more localized and not sustained. The larger blood vessels are characterized by multiunit smooth muscle.

      1. Smooth muscle of the iris. The sphincter and dilator pupillae muscles are unique. Their cells derive from ectoderm and have a rich nerve supply. They are classified as multiunit smooth muscle because the cells can contract individually; they are capable of precise and graded contractions.

C. Smooth Muscle Organization

Unlike striated muscle fibers, which abut end to end, smooth muscle fibers overlap and attach by fusing their endomysial sheaths. The sheaths are interrupted by gap junctions, which transmit the ionic currents that initiate contraction. Smooth muscle fibers form fascicles smaller than those in striated muscle. The fascicles, each surrounded by a meager perimysium, are often organized in layers separated by the thicker epimysial connective tissue. Fibers in adjacent layers may lie perpendicular to one another.

D. Mechanism of Contraction

As in striated muscle, smooth muscle contraction is regulated by an influx of calcium into the cytoplasm. However, smooth muscle lacks troponin. Instead, the calcium binds to calmodulin. The calmodulin–calcium complex binds to and activates a myosin light chain kinase, which phosphorylates and activates the myosin light chain that binds in turn to the actin filaments to initiate contraction. Activated myosin filaments bind the actin filaments and pull them toward and between them. Continued contraction involves the activation and binding of more myosin filaments and further sliding of the actin filaments. The sliding actin filaments pull the attached dense bodies closer together, contracting the cell. Unlike striated muscle fibers, individual smooth muscle fibers may undergo partial peristaltic, or wavelike, contractions. During relaxation, calcium is sequestered causing the myosin filaments to detach from the actin and disperse into the cytoplasm.

E. Initiation of Smooth Muscle Contraction

Like cardiac muscle fibers, smooth muscle fibers are capable of spontaneous contraction that may be modified by autonomic innervation. Motor end-plates are absent. Neurotransmitters diffuse from terminal expansions of the nerve endings between smooth muscle cells to the sarcolemma. Sympathetic (adrenergic) and parasympathetic (cholinergic) endings are present and exert antagonistic (reciprocal) effects. In some organs, contractile activity is enhanced by cholinergic nerves and decreased by adrenergic nerves; in others, the opposite occurs. The binding of these neurotransmitters to their receptors in the smooth muscle cell membrane results in membrane depolarization and the release of calcium from the smooth ER and caveolae into the cytoplasm. The flow of ions from neighboring cells through gap junctions can transmit the contraction stimulus from cell to cell in a wavelike pattern.

Its contraction is slow (often occurring in waves) and involuntary.

V. Response of Muscle to Injury

The response of muscle to injury depends on the muscle type. The wound closure mechanism always involves the proliferation of perimysial and epimysial fibroblasts and the synthesis of connective tissue matrix.

A. Skeletal Muscle

Small, mononucleate satellite cells are scattered in adult skeletal muscles within the basal lamina (endomysium) of mature fibers. Mature skeletal muscle fibers are incapable of mitosis, but the normally quiescent satellite cells can divide after muscle injury (as they do in response to weight bearing and exercise), differentiate into myoblasts, and fuse with existing muscle fibers or more rarely with each other to form new muscle fibers.

B. Cardiac Muscle

Cardiac muscle has little regenerative ability after early childhood. Lesions of the adult heart are repaired by replacement with dense connective tissue scars. Recent studies indicate that mesenchymal stem cells derived from bone marrow can localize to cardiac lesions and participate in their repair.

C. Smooth Muscle

Smooth muscle contains a population of relatively undifferentiated mononucleate smooth muscle stem cells that proliferate and differentiate into new smooth muscle fibers in response to injury. The same mechanism is involved in adding new muscle to the myometrium as the uterus enlarges during pregnancy to accommodate the growing fetus.

 

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Muscle tissue consists of fibers of muscle cells connected together in sheets and fibers. Together these sheets and fibers and known as muscles, and control the movements of an organisms as well as many other contractile functions. There are three different types of muscle. While these muscles differ slightly, they function in a similar way.

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Muscle tissue is composed of cells that optimize the cell property of contractility.

Actin microfilaments and associated proteins generate the forces necessary for the muscle contraction.

Muscle tissue is a specialized tissue found in animals which functions by contracting, thereby applying forces to different parts of the body.

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Skeletal muscle

1.They carry out movements of the body.

2.They support the body.

3.They maintain the posture of the body.

4. Heat generation – Muscles produce heat as a by product of contraction Muscles produce heat as a by product of contraction

Smooth muscle

It is responsible for the contractility of hollow organs, such as blood vessels, the gastrointestinal tract, the bladder.

Cardiac muscle

Cardiac muscle is the muscle of the heart. It is self-contracting, autonomically regulated and must continue to contract in rhythmic fashion for the whole life of the organism. Hence it has special features.

 

 

Muscle tissue  ability to contract. This is opposed to other components or tissues in muscle such astendons or perimysium. It is formed during embryonic development through a process known as myogenesis.[1]

Muscle tissue varies with function and location in the body. In mammals the three types are:skeletal or striated muscle; smooth or non-striated muscle; and cardiac muscle, which is sometimes known as semi-striated. Smooth and cardiac muscle contracts involuntarily, without conscious intervention. These muscle types may be activated both through interaction of the central nervous system as well as by receiving innervation from peripheral plexus orendocrine (hormonal) activation. Striated or skeletal muscle only contracts voluntarily, upon influence of the central nervous system. Reflexes are a form of non-conscious activation of skeletal muscles, but nonetheless arise through activation of the central nervous system, albeit not engaging cortical structures until after the contraction has occurred.[2]

The different muscle types vary in their response to neurotransmitters and endocrine substances such as acetyl-choline, noradrenalin, adrenalin, nitric oxide and among others depending on muscle type and the exact location of the muscle.[3]

Sub-categorization of muscle tissue is also possible, depending on among other things the content of myoglobin, mitochondria, myosin ATPase etc.

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On a recent road trip, a group of medical students came across a farm that advertised particularly meaty goats. The farm consisted of a population of goats with a genetic disorder that resulted in muscle stiffening, and the animals were prone to falling over after being startled. Further investigation of the goat breed uncovered that their genetic mutation delays relaxation in skeletal muscle fibers. Similar mutations can be observed in humans and result in myotonia congenita. Myotonia congenita is associated with mutations in skeletal muscle–expressed Cl– channels. The action potential in skeletal muscle fibers includes which of the following?

Answer

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Structure

Muscle cells are structurally and functionally specialized for contraction. Contraction requires two types of special protein filaments called myofilaments; these include thin filaments containing actin and thick filaments containing myosin.

The length of muscle cells, which sometimes reaches 4 cm, is greater than their width. Muscle cells are, therefore, often called muscle fibers or myofibers.

Special terms applied to muscle include the prefixes sarco and myo.

Muscle tissues are groups of muscle cells organized by connective tissue. This arrangement allows the groups to act together or separately, generating mechanical forces of varying strength.

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