Dominic Stone
Intrinsic muscles are those that are contained within a body part, as opposed to extrinsic muscles, which originate elsewhere. For example, the intrinsic muscles of the tongue are contained completely within the tongue and act to change its shape. Similarly, the intrinsic muscles of the foot reside within the foot, while extrinsic muscles originate in the leg. Intrinsic muscles are also found in the back, hand, and shoulder.
| Characteristics | Values |
|---|---|
| Definition | Intrinsic muscles are those that develop embryologically in the back or in a specific body part, as opposed to extrinsic muscles that originate in another body part. |
| Location | Intrinsic muscles are found in the back, foot, hand, tongue, and shoulder. |
| Function | Intrinsic muscles are responsible for various functions depending on their location, including movement of the hand and fingers, changing the shape of the tongue, and providing stability to the shoulder joint. |
| Examples | Intrinsic back muscles include the iliocostalis, longissimus, spinalis, semispinalis, multifidus, and rotatores. Intrinsic shoulder muscles include the deltoid, teres major, supraspinatus, infraspinatus, subscapularis, and teres minor. Intrinsic hand muscles include the lumbricals and interossei. Intrinsic tongue muscles include the superior longitudinal, transverse, vertical, and inferior longitudinal muscles. Intrinsic foot muscles include the abductor hallucis and lumbricals. |
| Anatomy | Intrinsic muscles can be divided into deep, intermediate, and superficial layers. |
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What You'll Learn
Intrinsic muscles of the tongue
Intrinsic muscles are those that develop embryologically in the body part they are associated with, and are thus described as intrinsic muscles. On the other hand, extrinsic muscles are those that develop outside the body part they are associated with.
The tongue is a muscular organ situated in the oral cavity. It is involved in taste perception, mastication (chewing), deglutition (swallowing), speech, and clearing the oral cavity. The tongue is formed by eight paired muscles, four intrinsic and four extrinsic. The extrinsic muscles are attached to the hyoid bone, mandible, styloid process, soft palate, and pharynx. They are responsible for producing movements of the tongue that the intrinsic muscles cannot, such as protrusion, retraction, retrusion, depression, and elevation.
The four intrinsic muscles of the tongue are:
- Superior longitudinal
- Inferior longitudinal
- Transverse
- Vertical
The intrinsic muscles of the tongue are involved primarily in changing the shape of the tongue, which is essential for speaking. The genioglossus muscle, for example, is a large, thick muscle that contributes significantly to the shape of the tongue. It arises from the mandibular symphysis and inserts onto the body of the hyoid bone and the entire length of the tongue. Bilateral contraction of the genioglossus muscle depresses the central part of the tongue, creates a longitudinal furrow, and protrudes the tongue outside of the mouth. Unilateral contraction, on the other hand, will deviate the tongue contralaterally.
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Intrinsic muscles of the foot
An intrinsic muscle is a muscle that is located within a body part, as opposed to extrinsic muscles, which originate in another body part. For example, intrinsic hand muscles are located within the various hand osseofascial compartments within the anatomic confines of the wrist and phalanges, while extrinsic hand muscles originate in the forearm. Similarly, the intrinsic muscles of the foot are located within the foot, while extrinsic foot muscles arise from the anterior, posterior, and lateral compartments of the leg.
The foot has 29 muscles associated with it, 10 of which are extrinsic and 19 of which are intrinsic. The intrinsic muscles of the foot are important in evaluating and treating lower extremity dysfunction and play a crucial role in the proper function of the foot during sports activities. They are also important in maintaining posture and providing stability to the body during gait.
The intrinsic muscles of the foot can be divided into those situated on the dorsum of the foot and those in the sole of the foot. There are two intrinsic muscles located within the dorsum of the foot: the extensor digitorum brevis and extensor hallucis brevis. There are ten intrinsic muscles located in the sole of the foot, which act collectively to stabilize the arches of the foot and individually to control movement of the digits. The muscles of the plantar aspect are arranged in four layers, with the first layer being the most superficial and located immediately underneath the plantar fascia. The abductor hallucis muscle is located on the medial side of the sole, where it contributes to a small soft tissue bulge. The abductor digiti minimi muscle is located on the lateral side of the foot and is homologous with the abductor digiti minimi of the hand. The second plantar layer contains two muscles: the quadratus plantae and the lumbricals. There are four lumbrical muscles in the foot, each located medial to their respective tendon of the flexor digitorum longus. The third layer contains three muscles: the flexor hallucis brevis, adductor hallucis, and flexor digiti minimi brevis.
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Intrinsic muscles of the hand
The skeletal muscles of the hand are responsible for the movement of the hand and fingers. These muscles are divided into two groups: the extrinsic and intrinsic muscles. The extrinsic muscles are so-called because the muscle belly originates in the forearm. The intrinsic muscles are smaller muscles located within the hand itself and are responsible for fine motor functions.
The intrinsic muscles of the hand develop embryologically in the back and are therefore described as intrinsic muscles. They are important for various hand functions, such as pinch and grip strength. The blood supply to these muscles comes from the ulnar and radial arteries and their branches. The ulnar and radial arteries cross the wrist and send vessels deep and superficially, forming two arches called the superficial and deep palmar arches. The nerves of the intrinsic hand arise from the brachial plexus, a network of nerves from ventral rami between the C5 to T1 nerve roots. The median and ulnar nerves are among those that supply the intrinsic muscles of the hand.
The thenar muscles are three short muscles located at the base of the thumb, producing a bulge known as the thenar eminence. They are responsible for the thumb's fine movements and are innervated by the median nerve. The opponens pollicis is the largest of the thenar muscles and lies underneath the other two. It originates from the tubercle of the trapezium and the associated flexor retinaculum, inserting onto the lateral margin of the first metacarpal.
The hypothenar muscles form the hypothenar eminence, a muscular protrusion on the medial side of the palm at the base of the little finger. These muscles are similar in name and organisation to the thenar muscles and are innervated by the ulnar nerve. The abductor digiti minimi is the most superficial of the hypothenar muscle group. It originates from the pisiform and the tendon of the flexor carpi ulnaris and attaches to the base of the proximal phalanx of the little finger. The flexor digiti minimi brevis lies laterally to the abductor digiti minimi and is responsible for flexing the metacarpophalangeal joint of the little finger.
The interossei muscles are located between the metacarpal bones of the hand and can be divided into two groups: dorsal and palmar. They are responsible for abducting and adducting the fingers, respectively, and also assist the lumbricals in flexing the MCP joints and extending the IP joints. The lumbricals are crucial for finger movement, linking the extensor tendons to the flexor tendons. They originate from the tendon of the flexor digitorum profundus and pass dorsally and laterally around each finger, inserting into the extensor hood. The lateral two lumbricals are innervated by the median nerve, while the medial two are innervated by the ulnar nerve.
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Intrinsic muscles of the back
The muscles of the back can be divided into three groups: superficial, intermediate, and deep (or intrinsic). The superficial and intermediate muscles do not develop in the back and are classified as extrinsic muscles. The deep muscles, on the other hand, develop embryologically in the back and are thus described as intrinsic muscles.
The deep back muscles, also called intrinsic or true back muscles, consist of four layers of muscles: superficial, intermediate, deep, and deepest layers. These muscles lie on each side of the vertebral column, deep to the thoracolumbar fascia. They span the entire length of the vertebral column, extending from the cranium to the pelvis. The deep back muscles act together to provide support and maintain the body's posture, as well as to produce movements of the head, neck, and trunk. The main functions of these muscles are flexion, extension, lateral flexion, and axial rotation of the vertebral column.
The superficial layer is composed of the splenius muscles (spinotransversales muscles), which are the splenius capitis and splenius cervicis. These flat muscles are located on the posterolateral aspect of the neck and the posterior upper thorax, overlying the deep muscles of the neck. The intermediate layer contains the large erector spinae muscles, which are sometimes called the long muscles of the back. This muscle group is the largest of the deep back muscles and lies on either side of the vertebral column between the spinous processes of the vertebrae and the angles of the ribs. There are three intermediate intrinsic back muscles: the iliocostalis, longissimus, and spinalis. Together, these muscles form a column known as the erector spinae. The erector spinae is situated posterolaterally to the spinal column, between the vertebral spinous processes and the costal angle of the ribs.
The semispinalis is the most superficial of the deep intrinsic muscles. It can be divided by its superior attachments into thoracic, cervicis, and capitis. It originates from the transverse processes of C4-T10, with fibres ascending 4-6 vertebral segments, attaching to the spinous processes of C2-T4, and to the occipital bone of the skull. The multifidus is located underneath the semispinalis muscle and is most developed in the lumbar area. It arises from the sacrum, posterior iliac spine, common tendinous origin of the erector spinae, mamillary processes of lumbar vertebrae, transverse processes of T1-T3, and articular processes of C4-C7. The fibres ascend 2-4 vertebral segments, attaching to the vertebral spinous processes.
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Intrinsic muscles of the shoulder
Intrinsic muscles are those that develop embryologically in the back and are thus described as deep muscles. They are covered by deep fascia, which plays a key role in their organisation. The superficial and intermediate muscles do not develop in the back and are classified as extrinsic muscles.
The intrinsic muscles of the shoulder, also known as the scapulohumeral group, originate from the scapula and/or clavicle and attach to the humerus. There are six muscles in this group: the deltoid, teres major, and the four rotator cuff muscles (supraspinatus, infraspinatus, subscapularis, and teres minor).
The deltoid muscle forms the rounded contour of the shoulder and is comprised of anterior, intermediate, and posterior fibres. The anterior (or clavicular) fibres originate from the lateral third of the clavicle. The intermediate (or acromial) fibres originate from the acromion process of the scapula. The functions of the deltoid muscle can be categorised based on the groups of fibres involved: Anterior fibres: shoulder flexion and internal rotation. Intermediate fibres: abduction of the shoulder. Posterior fibres: shoulder extension and external rotation.
The teres major is a thick muscle positioned superiorly to the latissimus dorsi. It originates from the posterior aspect of the inferior angle of the scapula and inserts onto the medial lip of the bicipital groove (intertubercular sulcus) of the humerus. The majority of teres major is innervated by the lower subscapular nerve, and a small portion is innervated by the thoracodorsal nerve (C5-8). The teres major acts to adduct the humerus and assist in extension and internal rotation of the shoulder.
The supraspinatus acts to initiate abduction of the arm, especially between the first 15 degrees of movement, and then assists the deltoid muscle (15-90 degrees of movement). Rotator cuff tendonitis refers to inflammation of the tendons of the rotator cuff muscles, usually occurring secondary to repetitive use of the shoulder joint. The supraspinatus is the muscle most commonly affected. Impingement syndrome is caused by rotator cuff tendonitis and typically presents with pain, weakness, and restricted shoulder movement.
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Frequently asked questions
Intrinsic muscles are those that develop embryologically within a certain body part, as opposed to extrinsic muscles, which develop outside of the body part.
Examples of intrinsic muscles include the hand, foot, back, tongue, and shoulder muscles.
Intrinsic muscles are responsible for various functions depending on their location. For example, the intrinsic muscles of the hand are responsible for pinch and grip strength, while those of the foot help provide stability and optimal function.
