Dominic Stone
The human body is a complex system of muscles, bones, and nerves. One group of muscles, known as extrinsic muscles, play a crucial role in our movement and bodily functions. These muscles are characterised by their origin and insertion points, with extrinsic muscles originating outside a specific body part but inserting and acting within it. For example, the extrinsic muscles of the hand originate in the forearm but insert into the hand, allowing for a wide range of motions. Similarly, the extrinsic muscles of the back and shoulder, such as the trapezius, latissimus dorsi, and levator scapulae, control the movement of the scapula and humerus, enabling actions like reaching, lifting, and pulling. Understanding the anatomy of extrinsic muscles is not only essential for comprehending human movement but also for medical procedures, as knowledge of these muscles guides precise instrument placement during surgery.
| Characteristics | Values |
|---|---|
| Definition | Extrinsic muscles are those that originate in one part of the body and attach to another part |
| Location | The extrinsic muscles of the shoulder originate from the trunk and attach to the bones of the shoulder (clavicle, scapula, or humerus). They are located in the back and are also known as the superficial back muscles. |
| Superficial extrinsic muscles of the shoulder | Trapezius, Latissimus dorsi |
| Trapezius | Broad, flat, and triangular muscle. The muscles on each side form a trapezoid shape. |
| Trapezius function | Elevates the scapula and rotates it during abduction of the arm. |
| Trapezius attachments | Originates from the skull, nuchal ligament, and the spinous processes of C7-T12. The fibres attach to the clavicle, acromion, and the scapula spine. |
| Latissimus dorsi | The largest extrinsic back muscle, spanning the entire lateral side of the back. |
| Latissimus dorsi attachments | Originates from the lateral sides of the spinous processes of T7 to T12, thoracodorsal fascia, posterior iliac crest, and proximal third of ribs 9 through 12. |
| Other extrinsic back muscles | Rhomboid major and minor, levator scapulae, serratus posterior superior and inferior muscles |
| Tongue extrinsic muscles | Genioglossus, hyoglossus, styloglossus, palatoglossus |
| Eye extrinsic muscles | Three pairs of taut, elastic muscles that constantly balance the pull of the others |
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What You'll Learn
The extrinsic muscles of the shoulder
There are two superficial extrinsic muscles: the trapezius and the latissimus dorsi. The trapezius is a broad, flat, and triangular muscle that forms a trapezoid shape. It originates from the skull, nuchal ligament, and the spinous processes of C7-T12 and attaches to the clavicle, acromion, and scapula spine. The upper fibres of the trapezius elevate and rotate the scapula during abduction of the arm, the middle fibres retract the scapula, and the lower fibres pull the scapula inferiorly.
The latissimus dorsi originates from the lower part of the back and covers a wide area. It arises from the spinous processes of T7-T12, iliac crest, thoracolumbar fascia, and the inferior three ribs. The fibres converge into a tendon that attaches to the intertubercular sulcus of the humerus. The latissimus dorsi extends, adducts, and medially rotates the upper limb.
The deep layer of extrinsic muscles includes the levator scapulae and the rhomboid muscles (major and minor). The levator scapulae is a small, strap-like muscle that begins in the neck and descends to attach to the scapula. It originates from the transverse processes of the C1-C4 vertebrae and attaches to the medial border of the scapula. The levator scapulae elevates the scapula and assists in rotating it.
The rhomboid muscles originate from the spinous processes of the vertebrae and attach to the medial border of the scapula. The rhomboid major is located inferiorly to the levator scapulae and inserts into the scapula in an inferior position. It retracts and rotates the scapula and works in conjunction with other muscles to ensure that the scapula is attached to the ribcage. The rhomboid minor sits between the levator scapulae and rhomboid major and is situated superiorly to the rhomboid major. It retracts and rotates the scapula.
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The extrinsic muscles of the back
The extrinsic back muscles can be further divided into two groups: the superficial extrinsic muscles of the back and the intermediate extrinsic muscles of the back. The trapezius, one of the superficial extrinsic muscles, is the most superficial of all the back muscles. It is a large, broad, flat, triangular muscle that forms a trapezoid shape on the posterosuperior trunk. The muscle fibres of the trapezius connect to the shoulder, with the superior fibres connecting to the posterior border of the distal third of the clavicle, and the middle fibres attaching to the superior portion of the scapular spine. The trapezius works with other muscles to move the arm and stabilise the shoulder.
The latissimus dorsi is another superficial extrinsic muscle. It is the largest extrinsic back muscle, spanning the entire lateral side of the back. The muscle fibres converge into a tendon that attaches to the intertubercular sulcus of the humerus. The latissimus dorsi adducts, extends, and medially rotates the humerus. The levator scapulae and the rhomboid major and minor are also superficial extrinsic muscles. The levator scapulae is a small, thin, strap-like muscle that begins in the neck and descends to attach to the scapula. The rhomboid muscles are two small quadrangular muscles located between the medial margins of the scapulae.
The serratus posterior superior and serratus posterior inferior muscles are the intermediate extrinsic muscles of the back. These muscles attach to the ligamentum nuchae, vertebral spinous processes, and ribs.
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The extrinsic muscles of the tongue
The four extrinsic muscles of the tongue are:
- Genioglossus: This muscle arises from the mandibular symphysis and inserts into the hyoid bone or the full length of the tongue. It protrudes, depresses, and retracts the tongue, pulling its tip back and down. The genioglossus is innervated by the hypoglossal nerve (CN XII).
- Hyoglossus: The hyoglossus originates from the hyoid bone and inserts into the side of the tongue. It depresses and retracts the tongue, and its contraction results in the retrusion and depression of the tongue's lateral margin. The hyoglossus is also innervated by the hypoglossal nerve (CN XII).
- Styloglossus: This muscle originates at the styloid process and temporal bone and inserts into the side of the tongue. It retracts and elevates the tongue. The styloglossus is innervated by the hypoglossal nerve (CN XII).
- Palatoglossus: The palatoglossus arises from the palatine aponeurosis and inserts across the tongue. It elevates the posterior portion of the tongue, aiding in swallowing and preventing the spill of saliva from the vestibule into the oropharynx. Unlike the other three extrinsic muscles, the palatoglossus is innervated by the vagus nerve (CN X).
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The extrinsic muscles of the hand
- Extensor carpi radialis longus (ECRL)
- Extensor carpi radialis brevis (ECRB)
- Extensor digitorum (ED)
- Extensor digiti minimi (EDM)
- Extensor carpi ulnaris (ECU)
- Abductor pollicis longus (APL)
- Extensor pollicis brevis (EPB)
- Extensor pollicis longus (EPL)
- Extensor indicis (EI)
The ECRL and ECRB form the lateral compartment, with their muscle fibres ending at the upper third and mid-forearm respectively. They then continue as flat tendons along the lateral border of the radius, beneath the APL and EPB. The ED divides into four tendons, which, along with the EI tendons, pass through the fourth tendon compartment of the dorsal carpal ligament. On the back of the hand, the ED tendons diverge to follow the fingers, while the EI tendon joins the ulnar side of one of the ED tendons along the back of the index finger. The EDM follows a similar path as the EI, but it follows the ED tendon along the little finger.
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The extrinsic muscles of the eye
The four recti muscles are the superior rectus, inferior rectus, lateral rectus, and medial rectus. They are named after their straight paths, originating from the common tendinous ring, a fibrous ring of connective tissue that surrounds the optic nerve where it connects to the orbit. The superior rectus controls upward movement, the inferior rectus controls downward movement, the lateral rectus controls outward movement, and the medial rectus controls inward movement.
The two oblique muscles are the superior oblique and inferior oblique. Unlike the recti muscles, the oblique muscles do not originate from the common tendinous ring. Instead, they attach angularly to the eye and have separate origins. The superior oblique originates from the sphenoid bone, which is part of the eye socket, and passes through a pulley (trochlea) before attaching to the top of the eyeball, just behind the superior rectus. The inferior oblique attaches to the lower-inner side of the eye socket, extends away from the nose, wraps around the bottom of the eye, and attaches just behind the lateral rectus.
The six extrinsic muscles of the eye work in pairs, with one muscle's movement being controlled or balanced by its partner in the same eye. They are also supplied mainly by branches of the ophthalmic artery, either directly or indirectly.
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Frequently asked questions
Extrinsic muscles are those that originate in the trunk or torso and attach to the bones of the shoulder, such as the clavicle, scapula, or humerus. They are also found in the back, tongue, leg, and eye.
Some examples of extrinsic muscles include the trapezius, latissimus dorsi, rhomboid major and minor, levator scapulae, and serratus posterior superior and inferior muscles.
Intrinsic muscles are deep postural muscles that act as the foundation of good posture. Extrinsic muscles, while necessary for movement, should not be relied upon for long-term posture maintenance. Imbalances between these muscle groups can lead to postural issues, joint wear and tear, and an increased risk of injury.
You can strengthen your extrinsic muscles through various exercises, including yoga, Pilates, and stability training. It is important to maintain proper form during these exercises and to gradually increase the difficulty as your muscles become stronger.
