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Contraction of smooth muscles

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    Smooth muscles consist of smaller fibers that usually range from 1 to 5 micrometer in diameter and 20 to 500 diameter in length. Attractive forces between myosin and actin cause contraction in smooth muscles.

    Types of smooth muscle:

    The smooth muscle for each organ is distinctive in following ways

    1. Physical diameter
    2. Organization into bundles and sheets
    3. Response to different type of stimuli
    4. characteristics of innervation and
    5. function

    But in general there are two basic types of smooth muscles which are

    1. multi unit smooth muscle
    2. unitary smooth muscle

    Multi unit smooth muscle:

    1. It is composed of discrete, separate smooth muscle fibers
    2. Each fiber operates independently
    3. Innervated by single nerve ending
    4. The outer surface of these Fibers are covered by a thin layer of basement membrane-like substance, a mixture of fine collagen and glycoprotein that helps insulate the separate fibers from one another
    5. Each fiber can contract independently of the others and their control is mainly exerted by nerve signals. In contrast, a major share of control is exerted by non-nervous stimuli.
    6. Example: ciliary muscle of the eye, the iris muscle of the eye, and the piloerecter muscles that cause erection of the hairs when stimulated by the sympathetic nervous system.

    Unitary smooth muscle :

    1. It is also called syncytial smooth muscles or visceral smooth muscle.
    2. Term unitary means hundreds of thousands of smooth muscles that contract together as a single unit, not single muscle.
    3. fiber arranged in sheets or bundles
    4. Their cell membrane adhere to one another at multiple points so that force generated in one muscle fiber can be transmitted to the next
    5. Cells are also joined by many gap junctions through which ions mover freely from one muscle cell to the next so that action potential or simple ion movement can take place from one fiber to the next cause muscle fiber to contract together
    6. Example: present in the walls of the viscera of the body including gastrointestinal tract, bile ducts, uterus and many blood vessels.
    figure smooth muscles unitary and multi

    Reference : John E Hall physiology book

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