Myofibril Health Dictionary

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Myofibril: From 1 Different Sources

n. one of numerous contractile filaments found within the cytoplasm of *striated muscle cells. When viewed under a microscope myofibrils show alternating bands of high and low refractive index, which give striated muscle its characteristic appearance.
Health Source: Oxford | Concise Colour Medical Dictionary
Author: Jonathan Law, Elizabeth Martin

Muscle

A structure composed of bundles of specialized cells capable of contraction and relaxation to create movement. There are 3 types of muscle: skeletal, smooth, and cardiac.

The skeletal muscles are the most prominent in the body (see muscular system). They are called voluntary muscles because they are under conscious control. Skeletal muscles are composed of groups of muscle fibres arranged in bundles called fascicles. A fibre is made up of longitudinal units called myofibrils, the working units of which are filaments of actin and myosin (two proteins that control contraction). A state of partial contraction is constantly maintained – this is muscle tone.

Smooth muscle is concerned with the movements of internal organs. It is not under conscious control; for this reason, it is also called involuntary muscle. Smooth muscle is made up of long, spindle-shaped cells, and contracts with the same sliding action of actin and myosin as skeletal muscle. This type of muscle is stimulated by the autonomic nervous system; it also responds to hormones and to levels of chemicals in fluid around the muscle.

Cardiac muscle (also called myocardium) is found only in the heart. It is able to contract rhythmically about 100,000 times a day, and has a similar structure to that of skeletal muscle. Contraction is stimulated by the autonomic nervous system, by hormones, and by the stretching of muscle fibres.

The most common muscle disorders are injury and lack of blood supply to a muscle. Rare disorders include muscular dystrophy and myasthenia gravis.... muscle

Fibril

n. a very small fibre or a constituent thread of a fibre (for example, a *myofibril of a muscle fibre). —fibrillar, fibrillary adj.... fibril

Striated Muscle

a tissue comprising the bulk of the body’s musculature. It is also known as skeletal muscle, because it is attached to the skeleton and is responsible for the movement of bones, and voluntary muscle, because it is under voluntary control. Striated muscle is composed of parallel bundles of multinucleate fibres (each containing many myofibrils), which reveal cross-banding when viewed under the microscope. This effect is caused by the alternation of actin and myosin protein filaments within each myofibril (see illustration). According to the ‘sliding filament’ theory, when muscle contraction takes place, the two sets of filaments slide past each other, so reducing the length of each unit (sarcomere) of the myofibril. The sliding is caused by a series of cyclic reactions, requiring ATP, resulting in a change in orientation of projections on the myosin filaments; each projection is first attached to an actin filament but contracts and releases it to become reattached at a different site.... striated muscle

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