Muscle is defined as soft tissue that is found in most animals, and it is one of the four basic animal tissues, along with the nervous tissue, connective, and epithelium tissue. There exist three types of muscle, of which skeletal and cardiac muscles are striated, whereas the smooth muscle is not.
Muscle Structure and Organization
Smooth muscle has spindle-shaped cells with a diameter of 5 to 10 m and a length of 50 to 250 m. These cells possess a central and a single nucleus. Surrounding the nucleus and throughout most of the cytoplasm are the thin (actin) filaments and the thick (myosin). Tiny projections, which originate from the myosin filament, are believed to be cross-bridges. Actin to the myosin filaments ratio (nearly 12 to 1) is twice that observed in the striated muscle and therefore may provide a greater opportunity for a cross-bridge to attach and generate the force in smooth muscle. An increased probability for the attachment can, in part, account for the ability of a smooth muscle to generate, having far less myosin, greater or comparable force compared to the striated muscle.
Smooth muscle varies from striated muscle in its lack of any apparent organization of myosin and actin contractile filaments into the discrete contractile units known as sarcomeres. Research has revealed that a sarcomere-like structure can nonetheless exist in smooth muscle. Such a type of sarcomere-like unit would be composed of actin filaments, which are anchored to dense amorphous bodies in the cytoplasm and dense plaques as well on the cell membrane.
The below representation shows the muscle cell structure (muscle structure).
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The protein -actinin, which is located in the Z lines of striated muscle, where the actin filaments are known to be connected, is responsible for these dense areas. Therefore, the force generated by the myosin cross-bridges attached to actin can be transmitted via actin filaments to dense bodies and, after that, through neighbouring contractile units, which ultimately terminate on the cell membrane.
The relaxed, smooth muscle cells possess a smooth cell membrane appearance, but upon any contraction, large membrane blebs (also called eruptions) form as the result of inwardly directed contractile forces, which are applied at the discrete points on the membrane of muscle. These particular points are presumably the dense plaques on the cell membrane, where the actin filaments attach. As an isolated cell shortens, it does so in the manner of a corkscrew-like.
It’s also been proposed that the contractile proteins in smooth muscle are helically orientated within the muscle cell in order for a single cell to shorten in a unique way. This resultant helical arrangement agrees with the earlier speculation, in which the contractile apparatus in smooth muscle can be arranged at slight angles relative to the cell’s long axis. Such contractile protein arrangement could contribute to the enhanced force-generating ability and slower shortening velocity of smooth muscle.
The contractile proteins interact to generate the force that should be transmitted to the tissue, where the individual smooth muscle cells are embedded. Smooth muscle cells do not contain the tendons present in the striated muscles, which allow for the transfer of muscular force for skeleton operating. Smooth muscles, on the other hand, are usually embedded in a rich connective tissue matrix that unites the smooth muscle cells in the tissue to form a bigger functional unit.
The cell’s inner organelles are involved in energy synthesis and calcium storage. Mitochondria are located most frequently near the cell nucleus and at the cell’s periphery. As in the striated muscles, these mitochondria are linked to ATP production. The sarcoplasmic reticulum is involved in intracellular calcium storage. As in the striated muscle, this intracellular membrane system plays an essential role in defining whether or not the contraction takes place by regulating the intracellular calcium concentration.
Types of Muscle
There exist 3 types of muscle, of which skeletal and cardiac muscles are striated, whereas the smooth muscle is not. Muscle action may be classified as being voluntary or involuntary. Smooth and cardiac muscles contract without conscious thought, and they are termed involuntary. And the skeletal muscles contract upon command. Skeletal muscles, in turn, may be divided into slow and fast-twitch fibres.
Initiation of Contraction
Smooth muscle cells contract in response to hormonal or neurological stimulation, resulting in an increase in intracellular calcium due to calcium entering through membrane channels or being released from intracellular storage sites. The elevated calcium level in the cell cytoplasm results in force generation. However, the intracellular calcium level rise initiates contraction through the mechanism that varies substantially from that in the striated muscle.
The myosin cross-bridges in striated muscle are prevented from attaching to actin by the troponin-tropomyosin system molecule’s presence on the actin filament. In the case of smooth muscle, although tropomyosin is available, troponin is not; that means an entirely different regulatory scheme operates in the smooth muscle. The contractile system’s regulation in the smooth muscle is linked to myosin filament, and regulation in striated muscle is generally linked to the actin filament.