Victor Forge
The human hand is a complex system of bones, muscles, nerves, tendons, ligaments, blood vessels, and parts of the lymphatic system. The thumb is unique in its ability to move independently of the other fingers, allowing humans to grasp and manipulate objects with dexterity. This movement is made possible by the muscles that control the thumb, which can be divided into extrinsic hand muscles, with their muscle bellies located in the forearm, and intrinsic hand muscles, with their muscles bellies located in the hand proper. These muscles work together to produce movements such as flexion, extension, adduction, abduction, and opposition of the thumb, which are essential for tasks such as pinching, grasping, and applying pressure.
| Characteristics | Values |
|---|---|
| Number of muscles in the thumb | 9 |
| Location of muscles | Hand and forearm |
| Function of muscles | Flexion, extension, adduction, abduction, and opposition of the thumb |
| Muscle groups | Extrinsic hand muscles, intrinsic hand muscles, thenar eminence, and other muscles |
| Example extrinsic hand muscles | Flexor pollicis longus, extensor pollicis longus, extensor pollicis brevis |
| Example intrinsic hand muscles | Abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, adductor pollicis, first dorsal interosseous muscle |
| Function of opponens muscle | Allow us to cup our hands by bringing the small finger towards the thumb |
| Function of abductor pollicis brevis | Pulls the thumb away from the index finger |
| Function of flexor pollicis brevis | Bends the thumb toward the small finger |
| Function of opponens pollicis | Brings the thumb away from the fingers to grasp objects |
| Function of adductor pollicis | Brings the thumb toward the index finger, provides power for pinching |
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What You'll Learn
The thenar eminence group of muscles
The thumb has nine skeletal muscles located in the hand and forearm, which can be divided into two groups: extrinsic hand muscles and intrinsic hand muscles. The thenar eminence refers to the group of intrinsic hand muscles on the palm at the base of the thumb, also known as the thenar muscle group. This group includes three muscles: the abductor pollicis brevis, the flexor pollicis brevis, and the opponens pollicis. These muscles work together to enable various movements of the thumb, such as abduction, adduction, flexion, and opposition.
The abductor pollicis brevis is the most superficial muscle of the thenar group, originating from the scaphoid tubercle, trapezium bones, and flexor retinaculum. It inserts into the radial sesamoid bone and proximal phalanx of the thumb, allowing the thumb to be pulled away from the index finger. This muscle is innervated by the median nerve (C8 and T1) and receives its blood supply from the superficial palmar branch of the radial artery.
The flexor pollicis brevis is a two-headed muscle that forms the medial aspect of the thenar eminence. The superficial head originates from the flexor retinaculum and the trapezium bone, while the deep head arises from the trapezoid and capitate bones. These heads converge to insert onto the base of the proximal phalanx of the thumb, enabling thumb flexion at the metacarpophalangeal and carpometacarpal joints. The superficial head is innervated by the median nerve, while the deep head receives input from the ulnar nerve (C8-T1).
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 flexor retinaculum, inserting onto the lateral margin of the first metacarpal. This muscle is responsible for opposing the thumb, or bringing it away from the fingers, by medially rotating and flexing the metacarpal on the trapezium. The opponens pollicis is innervated by the median nerve (recurrent branch) and is crucial for our ability to grasp objects.
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The abductor pollicis brevis
The main function of the abductor pollicis brevis is the abduction of the thumb, which is the movement of the thumb at a right angle to the palm. This occurs at the carpometacarpal joint and the metacarpophalangeal joint. The abductor pollicis brevis also facilitates the movement of the thumb towards the fingertips (opposition) and flexion in the metacarpophalangeal joint. These functions are crucial for the hand's proper functioning, such as grasping round objects or performing precise tasks like writing and sewing.
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The flexor pollicis brevis
The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the trapezium, trapezoid, and capitate. The muscle is inserted onto the radial sesamoid bone of the metacarpophalangeal joint. The muscle bellies of the superficial and deep heads travel inferiorly and converge into a single tendon, which contains a sesamoid bone and inserts onto the lateral aspect of the base of the proximal phalanx of the thumb.
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The opponens pollicis
The primary function of the opponens pollicis is to produce opposition of the thumb, which is a complex movement combining flexion, adduction, and medial rotation at the first carpometacarpal joint. This opposition movement allows the tip of the thumb to touch the tips of the other fingers on the same hand, enabling precise and efficient hand movements such as grasping objects or fine gripping.
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The adductor pollicis
The strength of the adductor pollicis muscle can be tested using Froment's sign, where the thumb is pushed against the index finger while the examiner attempts to pull them apart.
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Frequently asked questions
The muscles that control the thumb are located in the hand and forearm. The thumb has its own set of muscles that allow for flexion, extension, adduction, abduction and opposition of the thumb. The muscles acting on the thumb can be divided into two groups: extrinsic hand muscles and intrinsic hand muscles. The extrinsic hand muscles have their muscle bellies located in the forearm, while the intrinsic hand muscles have their muscle bellies located in the hand.
The extrinsic hand muscles that control the thumb include the abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, adductor pollicis, and first dorsal interosseous muscle. The abductor pollicis brevis pulls the thumb away from the index finger, while the flexor pollicis brevis bends the thumb toward the small finger. The opponens pollicis is responsible for pulling the thumb away from the fingers, allowing us to grasp objects. The adductor pollicis brings the thumb toward the index finger and provides power for pinching. The first dorsal interosseous muscle pulls the thumb toward the index finger and provides stability during pinching.
The intrinsic hand muscles that control the thumb include the flexor pollicis longus and brevis muscles. These muscles are responsible for thumb flexion, allowing for grasping and applying pressure with the thumb. The brevis muscle is located in the hand itself and forms part of the thenar pad, while the longus muscle extends deep within the flexor aspect of the forearm.
