Tendon
From Freepedia
- Tendon is also the name of a commune in the Vosges département in France.
- Tendon is also the abbreviated word of Tenpura-Donburi.
- Sinew is also a part of a structure or system that provides support and holds it together.
A tendon or sinew is a tough band of fibrous connective tissue that connects muscle to bone. They are similar to ligaments except that ligaments join one bone to another.
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Composition
Image:Tenocyte nuclei.jpgTendons are composed mainly of water, type-I collagen and cells called tenocytes. Minor fibrillar collagens, fibril-associated collagens and proteoglycans are present in small quantities and are critical for tendon structure. Most of the strength of tendon is due to the parallel, hierarchical arrangement of densely-packed collagen fibrils. Tenocytes are specialised fibroblasts responsible for the maintenance of collagen structure.
Anatomy
The origin of a tendon is where it joins to a muscle. Collagen fibers from within the muscle organ are continuous with those of the tendon. A tendon inserts into bone at an enthesis where the collagen fibres are mineralised and integrated into bone tissue.
Tenocytes produce collagen molecules which aggregate end-to-end and side-to-side to produce collagen fibrils. Fibril bundles are organised by tenocytes to form fibres. Collagen fibres coalesce into macroaggregates. Groups of macroaggregates are bounded by connective tissue endotendon and are termed fascicles. Groups of fascicles are bounded by the epitendon and peritendon to form the tendon organ.
Blood vessels may be visualised within the endotendon running parallel to collagen fibres, with occasional branching transverse anastomoses.
The internal tendon bulk is thought to contain no nerve fibres, however the epi- and peritendon contain nerve endings, while Golgi tendon organs are present at the junction between tendon and muscle.
Physiology
Tendons were previously thought to be metabolically inert ropes. In fact tenocytes are active cells capable of altering their gene expression profile in response to environmental cues. Tissue deformations experienced in daily life cause fluid flow, strain and shear that activate mechanotransduction pathways within tenocytes. Stretch and voltage activated channels are present on the surface of tenocytes. Gap junction proteins link adjacent tenocytes, allowing direct intercellular communication to occur. Cytokines take part in extracellular signaling between tenocytes and between tenocytes and other cell types.
Exercise causes increased blood flow, oxygen demand, collagen synthesis and matrix metalloproteinase activity. Increases in gene transcription and post-translational modification of extracellular matrix proteins are part of dynamic acute and chronic responses to mechanical loading.
Compressive stress causes tendon to become fibrocartilage.
Tendonitis/Tendinosis/Tendinopathy
Chronic overuse of tendons leads to microscopic tears within the collagen matrix, which gradually weakens the tissue. Swelling in a region of microdamage or partial tear can be detected visually or by palpation. Increased water content and disorganised collagen matrix in tendon lesions may be detected by ultrasonography or magnetic resonance imaging.
Due to their highly specialised ultrastructure and slow collagen turnover, tendons and ligaments are very slow to heal if injured, and rarely regain their original strength. Partial tears heal by the rapid production of disorganised type-III collagen, which is inferior in strength to normal tendon. Recurrence of injury in the damaged region of tendon is common.
Treatment of tendon injuries is largely palliative. Non-steroidal anti-inflammatory drugs combined with rest and gradual return to exercise is a common therapy. Return to function may be accelerated by the injection of stem cells. Completely ruptured tendons may be sutured together with or without grafted material.
The Achilles tendon is a particularly large tendon connecting the heel to the muscles of the calf. It is so named because the mythic hero Achilles was said to have been killed due to an injury at this spot. Achilles tendonitis is a common injury, particularly in sports that involve lunging and jumping.
A veterinary equivalent to Achilles tendonitis is bowed tendon, tendonitis of the superficial digital flexor tendon of the horse.
Sinew was also widely used in the medieval times as a form of ancient elastic.
See also
References
Tendons & Ligaments in Journal of Musculoskeletal & Neuronal Interactions; 2005, 5(1): Eds. Lyritis GP & Jee WSS
