Tendon

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. 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 fibrils 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. 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 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 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:

Name	Human body part	Function
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	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

Naming convention for the table:

Column	Explanation	Formatting
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 %?