Logan Steele
The human body is a complex machine, with various muscles working together to enable movement and function. One key distinction is between intrinsic and extrinsic muscles, which play distinct but complementary roles. Intrinsic muscles are deep, stabilising muscles lying close to joints and the spine, while extrinsic muscles are larger, more superficial muscles generating powerful movements. This topic explores whether extrinsic muscles are voluntary, focusing on their role in the back, tongue, hand, and eye. Understanding these muscles is crucial for both health and movement.
| Characteristics | Values |
|---|---|
| Definition | A group of muscles lying superficially on a structure |
| Location | Closer to the skin's surface and more prominent and visible than intrinsic muscles |
| Function | Generating forceful movements and involved in activities such as lifting, pushing, pulling, and other dynamic actions that require strength and power |
| Primary Movers | Yes |
| Voluntary | Yes |
| Examples | Trapezius, Latissimus Dorsi, Rhomboid Major and Minor, Levator Scapulae, Serratus Posterior Superior and Inferior Muscles |
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What You'll Learn
The extrinsic muscles of the back
The back muscles are divided into two groups: the extrinsic (superficial) back muscles and the intrinsic (deep) back muscles. The extrinsic back muscles are further divided into two groups: the superficial extrinsic muscles of the back and the intermediate extrinsic muscles of the back. The extrinsic muscles of the back include the trapezius, latissimus dorsi, rhomboid major and minor, levator scapulae, and serratus posterior superior and inferior muscles.
The trapezius is the most superficial back muscle and is a broad, flat, and triangular muscle. The muscle fibres connect to the superior nuchal line's medial third, external occipital protuberance, ligamentum nuchae, and apices of the spinous processes of C7 to T12. The fibres then run laterally and attach to the shoulder. The superior fibres descend to connect to the posterior border of the distal third of the clavicle. The middle fibres run horizontally, attaching to the superior portion of the scapular spine. The inferior fibres ascend toward the scapular apex. The trapezius works with other muscles to move the arm and stabilize the shoulder.
The latissimus dorsi is the largest extrinsic back muscle, spanning the entire lateral side of the back. It 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. The muscle fibres then course superolaterally before attaching to the floor of the humeral bicipital groove. The functions of the latissimus dorsi include extending the flexed arm, adducting and internally rotating the arm, and pulling the pectoral girdle backward as a whole when the humerus is fixed against the scapula.
The levator scapulae is a thin, strap-like muscle that descends from the neck to the scapula. It originates from the transverse processes of the C1-C4 vertebrae and attaches to the medial border of the scapula. The rhomboid muscles are the two small quadrangular muscles located between the medial margins of the scapulae. The rhomboid major originates from the spinous processes of the T2-T5 vertebrae and attaches to the medial border of the scapula, between the inferior angle and the root of the spine of the scapula. The rhomboid minor is the smaller of the two rhomboid muscles and is located superiorly. It originates from the C7 and T1 vertebrae and attaches to the medial scapular border near the scapulothoracic joint. The serratus posterior muscles are accessory breathing muscles. The superior pair helps with inspiration, while the inferior pair aids in forced expiration.
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The extrinsic muscles of the tongue
The tongue is all muscle and is composed of eight different muscles that form a flexible matrix. The tongue is made up of two main muscle groups: the intrinsic and extrinsic muscles. The intrinsic muscles are involved in changing the shape of the tongue, which is essential for speaking. The extrinsic muscles, on the other hand, are responsible for moving the tongue in different directions. They are also involved in guiding our speech, eating, and swallowing.
The tongue is involved in several important functions, including digestion, taste perception, and speech. It also plays a role in breathing. The tongue moves food around the mouth, mixes it with saliva, and helps to separate unwanted particles. It also enables the formation of the food bolus during the oral preparatory phase of swallowing. During the oral phase of swallowing, the tongue elevates and sweeps posteriorly to propel the food bolus past the anterior tonsillar pillar, triggering the swallowing reflex.
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The extrinsic muscles of the hand
The muscles included in the anterior forearm with insertion on the hand can be further divided into superficial, intermediate, and deep groups. The majority of the muscles in the group are flexor muscles. The superficial group includes the flexor carpi radialis, palmaris longus, and flexor carpi ulnaris. The flexor carpi ulnaris is innervated by the ulnar nerve, while the flexor carpi radialis and palmaris longus are innervated by the median nerve.
The hypothenar muscles form a muscular protrusion on the medial side of the palm, at the base of the little finger, known as the hypothenar eminence. These muscles are similar in name and organisation to the thenar muscles, which act on the thumb. The thenar muscles form the thenar eminence, a bulge on the palmar surface of the thumb and palm.
The lumbricals are crucial to finger movement, linking the extensor tendons to the flexor tendons. There are four lumbricals in the hand, each associated with a finger. The lateral two lumbricals (of the index and middle fingers) are innervated by the median nerve, while the medial two lumbricals (of the little and ring fingers) are innervated by the ulnar nerve.
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The extrinsic muscles of the eye
There are six extraocular muscles and one muscle that controls movement in the upper eyelid. The four rectus muscles are straight and have a direct path from their origin to the eyeball. They are the superior rectus, medial rectus, inferior rectus, and lateral rectus. The two oblique muscles, on the other hand, take an angular approach to the eyeball. They are the superior oblique and inferior oblique.
The superior rectus muscle is found at the top of the eye and controls upward movement. The medial rectus muscle attaches to the side of the eye closest to the nose and moves the eye inward. The lateral rectus muscle attaches to the side of the eye closest to the temple and allows the eye to move outward. The inferior rectus muscle is located at the bottom part of the eye and allows the eye to move downward.
The superior oblique muscle works like a pulley, starting at the upper back of the eye and extending forward, then threading through a small bony opening on the upper-inner side of the eye socket called the trochlea. After passing through the trochlea, it attaches to the top of the eyeball just behind the superior rectus. The inferior oblique muscle attaches to the lower-inner side of the eye socket, extends laterally, wraps around the bottom of the eye, and attaches just behind the lateral rectus on the far side.
The extraocular muscles are innervated by three cranial nerves: the oculomotor nerve (CN III), the trochlear nerve (CN IV), and the abducens nerve (CN VI). These nerves control the movements of the respective muscles. Damage to one of these nerves can cause paralysis of the respective muscle, altering the resting gaze of the affected eye.
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The extrinsic muscles of the foot
The muscles of the human foot can be divided into two groups: extrinsic and intrinsic muscles. There are 29 muscles associated with the human foot, 10 of which are extrinsic muscles that originate outside the foot but cross the ankle joint to act on the foot. The extrinsic muscles of the foot arise from the anterior, posterior, and lateral compartments of the leg and are responsible for actions such as eversion, inversion, plantarflexion, and dorsiflexion of the foot. They also assist in extending the toes and are innervated by the deep fibular nerve.
The peroneus longus is an extrinsic muscle that originates on the head of the fibula and the upper half of the fibular shaft. It crosses the ankle joint, courses deep into the foot, and inserts on the posterolateral aspect of the medial cuneiform bone and the lateral portion of the base of the first metatarsal. The primary action of this muscle is to evert the foot, plantarflex the ankle, and provide support to the transverse arch of the foot.
The peroneus brevis is another extrinsic muscle that originates on the inferior two-thirds of the lateral fibula and courses posteriorly to the lateral malleolus of the fibula. It inserts on the styloid process of the fifth metatarsal and its primary action is the eversion of the foot and plantarflexion of the ankle.
The peroneus tertius, also known as the peroneus tertius peroneus, is the third and final muscle of the lateral peroneus or fibular muscles. It originates from the middle fibular shaft and inserts on the dorsal surface of the fifth metatarsal. Its primary action is the dorsiflexion, eversion, and abduction of the foot.
The anterior tibialis is the most prominent muscle in the anterior leg and is considered an extrinsic muscle. It originates from the lateral condyle of the tibia and the proximal half to two-thirds of the tibial shaft. It passes under the extensor retinaculum and inserts onto the medial and plantar surfaces of the medial cuneiform and base of the first metatarsal. The anterior tibialis is often visible during dorsiflexion of the foot.
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Frequently asked questions
Extrinsic muscles are a group of muscles that lie superficially on a structure, in contrast to intrinsic muscles, which are deep, stabilizing muscles located within the body, close to the bones and joints.
Some examples of extrinsic muscles include the trapezius, latissimus dorsi, and rectus abdominis (also known as the "abs"). The extrinsic muscles of the tongue include the genioglossus muscle, hyoglossus muscle, styloglossus muscle, and palatoglossus muscle.
Extrinsic muscles are the body's primary movers and are responsible for producing powerful movements such as lifting, pushing, pulling, and reaching. They generate forceful movements and are involved in activities that require strength and power.
The voluntariness of extrinsic muscles depends on their location in the body. For example, the extrinsic muscles of the eyeball are voluntary, while the extrinsic muscles of the back and tongue are involuntary.
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