A tendon or sinew is a tough band of dense fibrous connective tissue that connects muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tension.
Tendons, like ligaments, are made of collagen. The difference is that ligaments connect bone to bone, while tendons connect muscle to bone. There are about 4,000 tendons in the adult human body.
Structure A tendon is made of dense regular connective tissue, whose main cellular components are special fibroblasts called tendon cells (tenocytes). Tendon cells synthesize the tendon's extracellular matrix, which abounds with densely-packed collagen fibers. The collagen fibers run parallel to each other and are grouped into fascicles. Each fascicle is bound by an endotendineum, which is a delicate loose connective tissue containing thin collagen fibrilsDorlands Medical Dictionary, page 602 and elastic fibers. A set of fascicles is bound by an epitenon, which is a sheath of dense irregular connective tissue. The whole tendon is enclosed by a fascia. The space between the fascia and the tendon tissue is filled with the paratenon, a fatty loose connective tissue.Dorlands Medical Dictionary 2012.Page 1382 Normal healthy tendons are anchored to bone by Sharpey's fibres.
Extracellular matrix The dry mass of normal tendons, which is 30–45% of their total mass, is made of: 60–85% collagen 60–80% collagen I 0–10% collagen III 2% collagen IV small amounts of collagens V, VI, and others 15–40% non-collagenous extracellular matrix components, including: 3% cartilage oligomeric matrix protein, 1–2% elastin, 1–5% proteoglycans, 0.2% inorganic components such as copper, manganese, and calcium.
Although most of a tendon's collagen is type I collagen, many minor collagens are present that play vital roles in tendon development and function. These include type II collagen in the cartilaginous zones, type III collagen in the reticulin fibres of the vascular walls, type IX collagen, type IV collagen in the basement membranes of the capillaries, type V collagen in the vascular walls, and type X collagen in the mineralized fibrocartilage near the interface with the bone.
Ultrastructure and collagen synthesis Collagen fibres coalesce into macroaggregates. After secretion from the cell, cleaved by procollagen N- and C-proteases, the tropocollagen molecules spontaneously assemble into insoluble fibrils. A collagen molecule is about 300 nm long and 1–2 nm wide, and the diameter of the fibrils that are formed can range from 50–500 nm. In tendons, the fibrils then assemble further to form fascicles, which are about 10 mm in length with a diameter of 50–300 μm, and finally into a tendon fibre with a diameter of 100–500 μm.
The collagen in tendons are held together with proteoglycan (a compound consisting of a protein bonded to glycosaminoglycan groups, present especially in connective tissue) components including decorin and, in compressed regions of tendon, aggrecan, which are capable of binding to the collagen fibrils at specific locations. The proteoglycans are interwoven with the collagen fibrils their glycosaminoglycan (GAG) side chains have multiple interactions with the surface of the fibrils showing that the proteoglycans are important structurally in the interconnection of the fibrils. The major GAG components of the tendon are dermatan sulfate and chondroitin sulfate, which associate with collagen and are involved in the fibril assembly process during tendon development. Dermatan sulfate is thought to be responsible for forming associations between fibrils, while chondroitin sulfate is thought to be more involved with occupying volume between the fibrils to keep them separated and help withstand deformation. The dermatan sulfate side chains of decorin aggregate in solution, and this behavior can assist with the assembly of the collagen fibrils. When decorin molecules are bound to a collagen fibril, their dermatan sulfate chains may extend and associate with other dermatan sulfate chains on decorin that is bound to separate fibrils, therefore creating interfibrillar bridges and eventually causing parallel alignment of the fibrils.
Tenocytes The tenocytes produce the collagen molecules, which aggregate end-to-end and side-to-side to produce collagen fibrils. Fibril bundles are organized to form fibres with the elongated tenocytes closely packed between them. There is a three-dimensional network of cell processes associated with collagen in the tendon. The cells communicate with each other through gap junctions, and this signalling gives them the ability to detect and respond to mechanical loading. These communications happen by two proteins essentially: connexin 43, present where the cells processes meet and in cell bodies connexin 32, present only where the processes meet.
Blood vessels may be visualized within the endotendon running parallel to collagen fibres, with occasional branching transverse anastomoses.
The internal tendon bulk is thought to contain no nerve fibres, but the epitenon and paratenon contain nerve endings, while Golgi tendon organs are present at the myotendinous junction between tendon and muscle.
Tendon length varies in all major groups and from person to person. Tendon length is, in practice, the deciding factor regarding actual and potential muscle size. For example, all other relevant biological factors being equal, a man with a shorter tendons and a longer biceps muscle will have greater potential for muscle mass than a man with a longer tendon and a shorter muscle. Successful bodybuilders will generally have shorter tendons. Conversely, in sports requiring athletes to excel in actions such as running or jumping, it is beneficial to have longer than average Achilles tendon and a shorter calf muscle.
Tendon length is determined by genetic predisposition, and has not been shown to either increase or decrease in response to environment, unlike muscles, which can be shortened by trauma, use imbalances and a lack of recovery and stretching. In addition tendons allow muscles to be at an optimal distance from the site where they actively engage in movement, passing through regions where space is premium, like the carpal tunnel.
List of tendons There are about 4,000 tendons in the human body, of which 55 are listed in the following table:
+Sortable table of tendons in the human bodyNameHuman body partFunctionTeres minor tendonsShoulders and armsRotator cuff tendons at the shoulderInfraspinatus tendonsShoulders and armsRotator cuff tendons at the shoulderSupraspinatus tendonsShoulders and armsRotator cuff tendons at the shoulderSubscapularis tendonsShoulders and armsRotator cuff tendons at the shoulderDeltoid tendonsShoulders and armsHelp bend the elbow or rotate the forearmBiceps tendonsShoulders and armsHelp bend the elbow or rotate the forearmTriceps tendonsShoulders and armsHelp bend the elbow or rotate the forearmBrachioradialis tendonsShoulders and armsHelp bend the elbow or rotate the forearmSupinator tendonsShoulders and armsHelp bend the elbow or rotate the forearmFlexor carpi radialis tendonsShoulders and armsHelp bend the wristFlexor carpi ulnaris tendonsShoulders and armsHelp bend the wristExtensor carpi radialis tendonsShoulders and armsHelp bend the wristExtensor carpi radialis brevis tendonsShoulders and armsHelp bend the wristIliopsoas tendonsHips and legsBend backwards and forwards, and when swinging the leg while walkingObturator internus tendonsHips and legsBend backwards and forwards, and when swinging the leg while walkingAdductor longus, brevis and magnus tendonsHips and legsBend backwards and forwards, and when swinging the leg while walkingGluteus maximusHips and legsBend backwards and forwards, and when swinging the leg while walkingGluteus medius tendonsHips and legsBend backwards and forwards, and when swinging the leg while walkingQuadriceps tendons(patellar tendon/ patella)Hips and legsBend or straighten the knee includeHamstring tendonsHips and legsBend or straighten the knee includeSartorius tendonsHips and legsBend or straighten the knee includeGastrocnemius tendonsHips and legsCross the ankle joint and help move the foot up and down, or side to sideAchilles tendonHips and legsCross the ankle joint and help move the foot up and down, or side to sideSoleus tendonsHips and legsCross the ankle joint and help move the foot up and down, or side to sideTibialis anterior tendonsHips and legsCross the ankle joint and help move the foot up and down, or side to sidePeroneus longus tendonsHips and legsCross the ankle joint and help move the foot up and down, or side to sideFlexor digitorum longus tendonsHands and feetHelp to move the fingers and toesInterosseus tendonsHands and feetHelp to move the fingers and toesFlexor digitorum profundus tendonsHands and feetHelp to move the fingers and toesAbductor digiti minimi tendonsHands and feetHelp to move the fingers and toesOpponens pollicis tendonsHands and feetThumbs can move toward and away from the other fingersFlexor pollicis longus tendonsHands and feetThumbs can move toward and away from the other fingersExtensor pollicis tendonsHands and feetThumbs can move toward and away from the other fingersabductor pollicis tendonsHands and feetThumbs can move toward and away from the other fingersFlexor hallucis longus tendonsHands and feetBend and straighten the toesFlexor digitorum brevis tendonsHands and feetBend and straighten the toesLumbrical tendonsHands and feetBend and straighten the toesAbductor hallucis tendonsHands and feetBend and straighten the toesFlexor digitorum longus tendonsHands and feetBend and straighten the toesAbductor digiti minimi tendonsHands and feetBend and straighten the toesOcular tendonsHead, neck and torsoEyes, eyelids and jawLevator palpebrae tendonsHead, neck and torsoEyes, eyelids and jawMasseter tendonsHead, neck and torsoEyes, eyelids and jawTemporalis tendonsHead, neck and torsoEyes, eyelids and jawTrapezius tendonsHead, neck and torsoMove the head and neckSternocleidomastoid tendonsHead, neck and torsoMove the head and neckSemispinalis capitisHead, neck and torsoMove the head and neckSplenius capitis tendonsHead, neck and torsoMove the head and neckMylohyoidHead, neck and torsoMove the head and neckThyrohyoid tendonsHead, neck and torsoMove the head and neckRectus abdominis tendonsHead, neck and torsoTwist and turn the body, maintain posture, or bend and straighten the trunkExternal oblique tendonsHead, neck and torsoTwist and turn the body, maintain posture, or bend and straighten the trunkTransversus abdominis tendonsHead, neck and torsoTwist and turn the body, maintain posture, or bend and straighten the trunkLatissimus dorsi tendonsHead, neck and torsoTwist and turn the body, maintain posture, or bend and straighten the trunkErector spinae tendonsHead, neck and torsoTwist and turn the body, maintain your posture, or bend and straighten the trunk Naming convention for the table:
ColumnExplanationFormattingNamethe name of the tendon in Latininclude/exclude tendon in the name???part of the human bodyWhere it can be found in the human body????FunctionWhat is its purpose in the body???CompositionAn overview of the materials that the tendon is made ofIdeally given in %?
Functions