Cellular Biology

The Structure and Function of Muscle Cells

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"The Structure and Function of Muscle Cells"
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The two principal functions of muscles are to produce movement and to maintain posture. Muscle tissues have the ability to shorten or contract.

Because a muscle cell's length is much greater than its width, they are frequently referred to as muscle fibres.

There are three types of muscle tissue in the human body: Skeletal muscle tissue, smooth muscle tissue and cardiac muscle tissue.

*Skeletal muscle tissues are attached to the skeleton and are controlled by the nervous system for voluntary movement. Examples are biceps, triceps and abdominal muscles.

*Smooth muscle tissue contracts without conscious control. Its spindle-shaped fibers form sheaths and make up the walls of hollow organs such as the stomach, intestines, and bladder. The tissue's involuntary movements are slow, so contractions last longer than those of other muscle tissue, and fatigue is rare. Depending on the sort of arrangement of the smooth muscle the function varies: if arranged in a circle inside an organ, contraction constricts the cavity inside the organ, if arranged lengthwise, contraction shortens the organ.

*Cardiac muscle tissue is found only in the heart and is also known as myocardium. It is similar to smooth muscle. Cardiac muscle tissue's contractions occur through the autonomic nervous system (involuntary control). They do not have nerves supplying impulses; instead, an electrical impulse passes along a certain route of the heart, causing the cardiac muscle to contract.

Muscle fibers share most of the structures as the rest of the body's cells, but with a few extra components. The following description is based on skeletal muscle fibres.

+Sarcomere - the basic contractile unit of the muscle.

+Sarcoplasmic reticulum - membranous system of tubules and cisternae (reservoirs for calcium ) that forms a closely meshed network.

+Transverse tubules - portion of the sarcoplasmic reticulum that stores and releases calcium during contraction and relaxation.

+Nuclei - brain of the cell. Muscle fibres contain many nuclei.

+ Mitochondria - the powerhouse of the cell, which provides the cell with chemical energy.

+ Golgi complex - modifies and processess newly produced proteins inside the cell.

+Lysosomes - small vesicles located in the sarcoplasm that contain a large number of enzymes.

+ Sarcoplasm -the substance that fills the cell, similar to cytoplasm of other cells.

+Myoneural junction - where motor nerve endings terminate on the sarcolemma.

+Motor end plate - structure present at the myoneural junction that forms a small mound on the surface of the muscle fibre

+ Sarcolemma - the membrane surrounding the muscle fiber.

The following gives an idea of the hierarchy of muscle organization:

1. Myofilaments - are visible only with the electron microscope; composed primarily of actin and myosin. Actin and myosin are alternating filaments of protein within the sarcomere, which are the basis for the mechanism of contraction of muscle fibres.

2. Myofibrils - visible with the light microscope, lie parallel to the long axis of the cell; composed of bundles of overlapping myofilaments.

3. Muscle fibre - filled with hundreds of myofibrils, which are oriented parallel to each other and to the long axis of the muscle fibre.

4. Muscle fascicle. Collection of muscle fibres forms a whole muscle.

The theory of contraction is known as the Interdigitating Filament Model of Muscle Contraction, or the Sliding Theory of Muscle Contraction.

It explains that the actin and myosin within the sarcomere slide past one another, shortening the entire length of the sarcomere. In order to slide past one another, the Myosin heads will interact with the Actin filaments and bend to pull past the Actin. Thus the muscle contracts as a whole.

Smooth muscle tissue and Cardiac muscle tissue contract in a similar manner.

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