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A tendon or sinew is a tough band of dense fibrous connective tissue that connects skeletal 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.
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 Cartilage 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.
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.
Blood vessels may be visualized within the endotendon running parallel to collagen fibres, with occasional branching transverse anastomosis.
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 Bodybuilding 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.
| +Sortable table of tendons in the human body | ||
| Teres minor tendons | Shoulders and arms | Rotator cuff tendons at the shoulder |
| Infraspinatus tendons | Shoulders and arms | Rotator cuff tendons at the shoulder |
| Supraspinatus tendons | Shoulders and arms | Rotator cuff tendons at the shoulder |
| Subscapularis tendons | Shoulders and arms | Rotator cuff tendons at the shoulder |
| Deltoid tendons | Shoulders and arms | Help bend the elbow or rotate the forearm |
| Biceps tendons | Shoulders and arms | Help bend the elbow or rotate the forearm |
| Triceps tendons | Shoulders and arms | Help bend the elbow or rotate the forearm |
| Brachioradialis tendons | Shoulders and arms | Help bend the elbow or rotate the forearm |
| Supinator tendons | Shoulders and arms | Help bend the elbow or rotate the forearm |
| Flexor carpi radialis tendons | Shoulders and arms | Help bend the wrist |
| Flexor carpi ulnaris tendons | Shoulders and arms | Help bend the wrist |
| Extensor carpi radialis tendons | Shoulders and arms | Help bend the wrist |
| Extensor carpi radialis brevis tendons | Shoulders and arms | Help bend the wrist |
| Iliopsoas tendons | Hips and legs | Bend backwards and forwards, and when swinging the leg while walking |
| Obturator internus tendons | Hips and legs | Bend backwards and forwards, and when swinging the leg while walking |
| Adductor longus, brevis and magnus tendons | Hips and legs | Bend backwards and forwards, and when swinging the leg while walking |
| Gluteus maximus | Hips and legs | Bend backwards and forwards, and when swinging the leg while walking |
| Gluteus medius tendons | Hips and legs | Bend backwards and forwards, and when swinging the leg while walking |
| Quadriceps tendons(patellar tendon/ patella) | Hips and legs | Bend or straighten the knee include |
| Hamstring tendons | Hips and legs | Bend or straighten the knee include |
| Sartorius tendons | Hips and legs | Bend or straighten the knee include |
| Gastrocnemius tendons | Hips and legs | Cross the ankle joint and help move the foot up and down, or side to side |
| Achilles tendon | Hips and legs | Cross the ankle joint and help move the foot up and down, or side to side |
| Soleus tendons | Hips and legs | Cross the ankle joint and help move the foot up and down, or side to side |
| Tibialis anterior tendons | Hips and legs | Cross the ankle joint and help move the foot up and down, or side to side |
| Peroneus longus tendons | Hips and legs | Cross the ankle joint and help move the foot up and down, or side to side |
| Flexor digitorum longus tendons | Hands and feet | Help to move the fingers and toes |
| Interosseus tendons | Hands and feet | Help to move the fingers and toes |
| Flexor digitorum profundus tendons | Hands and feet | Help to move the fingers and toes |
| Abductor digiti minimi tendons | Hands and feet | Help to move the fingers and toes |
| Opponens pollicis tendons | Hands and feet | Thumbs can move toward and away from the other fingers |
| Flexor pollicis longus tendons | Hands and feet | Thumbs can move toward and away from the other fingers |
| Extensor pollicis tendons | Hands and feet | Thumbs can move toward and away from the other fingers |
| abductor pollicis tendons | Hands and feet | Thumbs can move toward and away from the other fingers |
| Flexor hallucis longus tendons | Hands and feet | Bend and straighten the toes |
| Flexor digitorum brevis tendons | Hands and feet | Bend and straighten the toes |
| Lumbrical tendons | Hands and feet | Bend and straighten the toes |
| Abductor hallucis tendons | Hands and feet | Bend and straighten the toes |
| Flexor digitorum longus tendons | Hands and feet | Bend and straighten the toes |
| Abductor digiti minimi tendons | Hands and feet | Bend and straighten the toes |
| Ocular tendons | Head, neck and torso | Eyes, eyelids and jaw |
| Levator palpebrae tendons | Head, neck and torso | Eyes, eyelids and jaw |
| Masseter tendons | Head, neck and torso | Eyes, eyelids and jaw |
| Temporalis tendons | Head, neck and torso | Eyes, eyelids and jaw |
| Trapezius tendons | Head, neck and torso | Move the head and neck |
| Sternocleidomastoid tendons | Head, neck and torso | Move the head and neck |
| Semispinalis capitis | Head, neck and torso | Move the head and neck |
| Splenius capitis tendons | Head, neck and torso | Move the head and neck |
| Mylohyoid | Head, neck and torso | Move the head and neck |
| Thyrohyoid tendons | Head, neck and torso | Move the head and neck |
| Rectus abdominis tendons | Head, neck and torso | Twist and turn the body, maintain posture, or bend and straighten the trunk |
| External oblique tendons | Head, neck and torso | Twist and turn the body, maintain posture, or bend and straighten the trunk |
| Transversus abdominis tendons | Head, neck and torso | Twist and turn the body, maintain posture, or bend and straighten the trunk |
| Latissimus dorsi tendons | Head, neck and torso | Twist and turn the body, maintain posture, or bend and straighten the trunk |
| Erector spinae tendons | Head, neck and torso | Twist and turn the body, maintain your posture, or bend and straighten the trunk |
| Name | the name of the tendon in Latin | include/exclude tendon in the name??? |
| part of the human body | Where it can be found in the human body | ???? |
| Function | What is its purpose in the body | ??? |
| Composition | An overview of the materials that the tendon is made of | Ideally given in %? |
The mechanical properties of the tendon are dependent on the collagen fiber diameter and orientation. The collagen fibrils are parallel to each other and closely packed, but show a wave-like appearance due to planar undulations, or crimps, on a scale of several micrometers. In tendons, the collagen fibres have some flexibility due to the absence of hydroxyproline and proline residues at specific locations in the amino acid sequence, which allows the formation of other conformations such as bends or internal loops in the triple helix and results in the development of crimps. The crimps in the collagen fibrils allow the tendons to have some flexibility as well as a low compressive stiffness. In addition, because the tendon is a multi-stranded structure made up of many partially independent fibrils and fascicles, it does not behave as a single rod, and this property also contributes to its flexibility.
The proteoglycan components of tendons also are important to the mechanical properties. While the collagen fibrils allow tendons to resist tensile stress, the proteoglycans allow them to resist compressive stress. These molecules are very hydrophilic, meaning that they can absorb a large amount of water and therefore have a high swelling ratio. Since they are noncovalently bound to the fibrils, they may reversibly associate and disassociate so that the bridges between fibrils can be broken and reformed. This process may be involved in allowing the fibril to elongate and decrease in diameter under tension. However, the proteoglycans may also have a role in the tensile properties of tendon. The structure of tendon is effectively a fibre composite material, built as a series of hierarchical levels. At each level of the hierarchy, the collagen units are bound together by either collagen crosslinks, or the proteoglycans, to create a structure highly resistant to tensile load. The elongation and the strain of the collagen fibrils alone have been shown to be much lower than the total elongation and strain of the entire tendon under the same amount of stress, demonstrating that the proteoglycan-rich matrix must also undergo deformation, and stiffening of the matrix occurs at high strain rates. This deformation of the non-collagenous matrix occurs at all levels of the tendon hierarchy, and by modulating the organisation and structure of this matrix, the different mechanical properties required by different tendons can be achieved. Energy storing tendons have been shown to utilise significant amounts of sliding between fascicles to enable the high strain characteristics they require, whilst positional tendons rely more heavily on sliding between collagen fibres and fibrils. However, recent data suggests that energy storing tendons may also contain fascicles which are twisted, or helical, in nature - an arrangement that would be highly beneficial for providing the spring-like behaviour required in these tendons.
Several studies have demonstrated that tendons respond to changes in mechanical loading with growth and remodeling processes, much like . In particular, a study showed that disuse of the Achilles tendon in rats resulted in a decrease in the average thickness of the collagen fiber bundles comprising the tendon. In humans, an experiment in which people were subjected to a simulated micro-gravity environment found that tendon stiffness decreased significantly, even when subjects were required to perform restiveness exercises. These effects have implications in areas ranging from treatment of bedridden patients to the design of more effective exercises for astronauts.
Types of tendinopathy include:
Tendinopathies may be caused by several intrinsic factors including age, body weight, and nutrition. The extrinsic factors are often related to sports and include excessive forces or loading, poor training techniques, and environmental conditions.
The three main stages of tendon healing are inflammation, repair or proliferation, and remodeling, which can be further divided into consolidation and maturation. These stages can overlap with each other. In the first stage, inflammatory cells such as neutrophils are recruited to the injury site, along with erythrocytes. Monocytes and macrophages are recruited within the first 24 hours, and phagocytosis of necrotic materials at the injury site occurs. After the release of vasoactive and chemotactic factors, angiogenesis and the cell growth of tenocytes are initiated. Tenocytes then move into the site and start to synthesize collagen III. After a few days, the repair or proliferation stage begins. In this stage, the tenocytes are involved in the synthesis of large amounts of collagen and proteoglycans at the site of injury, and the levels of GAG and water are high. After about six weeks, the remodeling stage begins. The first part of this stage is consolidation, which lasts from about six to ten weeks after the injury. During this time, the synthesis of collagen and GAGs is decreased, and the cellularity is also decreased as the tissue becomes more fibrous as a result of increased production of collagen I and the fibrils become aligned in the direction of mechanical stress. The final maturation stage occurs after ten weeks, and during this time there is an increase in crosslinking of the collagen fibrils, which causes the tissue to become stiffer. Gradually, over about one year, the tissue will turn from fibrous to scar-like.
Matrix metalloproteinases (MMPs) have a very important role in the degradation and remodeling of the ECM during the healing process after a tendon injury. Certain MMPs including MMP-1, MMP-2, MMP-8, MMP-13, and MMP-14 have collagenase activity, meaning that, unlike many other enzymes, they are capable of degrading collagen I fibrils. The degradation of the collagen fibrils by MMP-1 along with the presence of denatured collagen are factors that are believed to cause weakening of the tendon ECM and an increase in the potential for another rupture to occur. In response to repeated mechanical loading or injury, cytokines may be released by tenocytes and can induce the release of MMPs, causing degradation of the ECM and leading to recurring injury and chronic tendinopathies.
A variety of other molecules are involved in tendon repair and regeneration. There are five growth factors that have been shown to be significantly upregulated and active during tendon healing: insulin-like growth factor 1 (IGF-I), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and transforming growth factor beta (TGF-β). These growth factors all have different roles during the healing process. IGF-1 increases collagen and proteoglycan production during the first stage of inflammation, and PDGF is also present during the early stages after injury and promotes the synthesis of other growth factors along with the synthesis of DNA and the proliferation of tendon cells. The three isoforms of TGF-β (TGF-β1, TGF-β2, TGF-β3) are known to play a role in wound healing and scar formation. VEGF is well known to promote angiogenesis and to induce endothelial cell proliferation and migration, and VEGF mRNA has been shown to be expressed at the site of tendon injuries along with collagen I mRNA. Bone morphogenetic proteins (BMPs) are a subgroup of TGF-β superfamily that can induce bone and cartilage formation as well as tissue differentiation, and BMP-12 specifically has been shown to influence formation and differentiation of tendon tissue and to promote fibrogenesis.
Several mechanotransduction mechanisms have been proposed as reasons for the response of tenocytes to mechanical force that enable them to alter their gene expression, protein synthesis, and cell phenotype, and eventually cause changes in tendon structure. A major factor is mechanical deformation of the extracellular matrix, which can affect the actin cytoskeleton and therefore affect cell shape, motility, and function. Mechanical forces can be transmitted by focal adhesion sites, integrins, and cell-cell junctions. Changes in the actin cytoskeleton can activate integrins, which mediate "outside-in" and "inside-out" signaling between the cell and the matrix. G-proteins, which induce intracellular signaling cascades, may also be important, and ion channels are activated by stretching to allow ions such as calcium, sodium, or potassium to enter the cell.
Sinew makes for an excellent cordage material for three reasons: It is extremely strong, it contains natural glues, and it shrinks as it dries, doing away with the need for knots.
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