Maximus Gage
The human hand is composed of many different bones, muscles, and ligaments that allow for a large amount of movement and dexterity. The muscles of the hand can be divided into two groups: extrinsic and intrinsic. The extrinsic group includes the long flexors and extensors, while the intrinsic group includes smaller muscles located within the hand itself, such as the thenar and hypothenar muscles. Tendons, which are like strong, flexible ropes, connect the muscles to the bones, enabling movement. The fingers have two long flexors located on the underside of the forearm, which attach to the phalanges of the fingers, allowing them to bend. The extensors, located on the back of the forearm, are connected in a more complex way to the dorsum of the fingers and are responsible for straightening the digits. The interossei muscles, consisting of dorsal and palmar groups, are responsible for spreading the fingers away from each other or pulling them together. This intricate network of muscles, tendons, and bones allows the hand to perform a wide range of functions, from precision grip to complex tactile sensations.
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What You'll Learn
The extensor indicis proprius
The primary function of the extensor indicis proprius is the extension of the second digit at the metacarpophalangeal and interphalangeal joints. It acts at these joints to extend the index finger. As it crosses the wrist, this muscle produces a weak extension of this joint.
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Interossei muscles
The interossei muscles are the intrinsic hand muscles situated between the metacarpal bones. They are categorised into two groups: the palmar interossei and the dorsal interossei, each comprising four muscles. These muscles are considered voluntary muscles.
The palmar interossei facilitate finger adduction, pulling our fingers together. They adduct the 1st, 2nd, 4th, and 5th digits about an axis through the midline of the 3rd digit. Adduction occurs at the MCP joints. Additionally, the palmar interossei contribute to flexion at the MCP joints and extension at the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints of their respective digits. The first palmar interosseous originates from the medial, palmar surface of the 1st metacarpal and inserts into the base of the proximal 1st phalanx and the extensor hood. The 2nd palmar interosseous originates from the medial surface of the base of the 2nd metacarpal and inserts into the medial portion of the extensor hood and base of the proximal 1st phalanx. The 3rd and 4th palmar interossei originate from the lateral aspects of the 4th and 5th metacarpals and insert into the lateral portions of their respective extensor hoods and the bases of their corresponding proximal phalanges.
The dorsal interossei enable abduction, allowing us to spread our fingers away from each other. They abduct the digits away from the 3rd digit (away from the axial line) and are bipennate. The first dorsal interosseous muscle is the largest and originates from the 1st and 2nd hand bones. It forms the contour between the thumb and index finger when looking at the top of the hand. The 2nd dorsal interosseous originates from the medial aspect of the 2nd metacarpal and the lateral aspect of the 3rd metacarpal. This muscle inserts into the lateral base of the 3rd proximal phalanx and its extensor hood. The 3rd dorsal interosseous originates from the medial surface of the 3rd metacarpal and the lateral surface of the 4th metacarpal, inserting into the medial base of the 3rd proximal phalanx and the extensor hood of the 3rd digit. The 4th dorsal interosseous originates from the lateral aspect of the 4th metacarpal and the medial side of the 5th metacarpal. This muscle inserts into the lateral base of the 4th proximal phalanx and the extensor hood of the 4th digit.
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Lumbrical muscles
The lumbrical muscles are a set of four intrinsic hand muscles that are attached to the flexor digitorum profundus (FDP) tendons proximally and the digits' extensor expansions distally. They are located deep in the palm and are essential for the fingers' precision grip and fine motor control. Each finger is attached to two lumbricals, except for the thumb, which is associated with only the first lumbrical muscle.
The primary function of the lumbrical muscles is to allow the fingers to straighten, although they can also help bend the MCP joints, which are at the knuckle. The name 'lumbrical' comes from the Latin word for 'earthworm', as they are cylindrical and worm-like, resembling earthworms belonging to the genus Lumbricus. The lumbricals are unusual in that they do not attach to bone. Instead, they attach proximally to the tendons of flexor digitorum profundus and distally to the extensor expansions.
The lumbricals flex the metacarpophalangeal (MCP) joints and extend the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints. By coordinating with other hand muscles, the lumbricals help control the fine and delicate movements of the fingers. The actions of these muscles are essential for gripping objects, typing, and performing various dexterous hand movements.
The lumbricals have a high number of muscle spindles, indicating that they play a role in proprioception and sensory feedback, which is important for precision pinch movements and precise manipulation of objects. The first and second lumbricals (the most radial two) are innervated by the median nerve, while the third and fourth lumbricals (the most ulnar two) are innervated by the deep branch of the ulnar nerve. This is the most common innervation pattern, occurring in 50-60% of individuals.
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Thenar and hypothenar muscles
The human hand is incredibly complex, with a unique combination of muscles that set our species apart from other primates and animals. The movement of opposition, made possible by the fact that more muscles attach to the thumb in modern humans, played an important role in human evolution.
The intrinsic muscles of the hand are located within the hand itself and are responsible for the fine motor functions of the hand. They include the adductor pollicis, opponens pollicis, and abductor pollicis brevis. The thenar and hypothenar muscles are part of the intrinsic muscle group.
Thenar Muscles
The thenar muscles are three short muscles located at the base of the thumb, forming a bulge known as the thenar eminence. They are responsible for the fine movements of the thumb. The median nerve innervates all the thenar muscles. 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 and inserts onto the lateral margin of the first metacarpal. The abductor pollicis brevis is positioned anteriorly to the opponens pollicis. The thumb has one long flexor and a short flexor in the thenar muscle group. The human thumb also has other muscles in the thenar group (opponens and abductor brevis muscle), moving the thumb in opposition, making grasping possible.
Hypothenar Muscles
The hypothenar eminence is formed from the abductor digiti minimi muscle, the flexor digiti minimi brevis muscle, the opponens digiti minimi muscle, and the palmaris brevis muscle. The hypothenar muscle group is formed by three muscles: the abductor digiti minimi, the flexor digiti minimi, and the opponens digiti minimi. 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 in the hand. It originates from the hook of hamate and adjacent flexor retinaculum and inserts into the base of the proximal phalanx of the little finger. The opponens digiti minimi muscle moves the little finger to allow the opposition of the hand in which the pad of the thumb touches the pad of the little finger. The palmaris brevis is a small, thin muscle found in the subcutaneous tissue of the hypothenar eminence. It originates from the palmar aponeurosis and flexor retinaculum and attaches to the dermis of the skin on the medial margin of the hand.
Innervation
The innervation of the hypothenar eminence is via the ulnar nerve. The innervation of the thenar eminence is via the median nerve. When ulnar nerve compromise occurs at the Guyon's canal it causes weakness with abduction and adduction of the fingers, flexion at the MCP and extension at the PIP, and adduction and flexion at the MCP of the thumb. Hypothenar muscles are also affected due to their innervation by the ulnar nerve. In Carpel Tunnel Syndrome (CTS), where median nerve compromise occurs, thenar muscles can show wasting as they are innervated by the median nerve.
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Radial, median, and ulnar nerves
The muscles of the hand can be divided into two groups: the extrinsic and intrinsic muscle groups. The extrinsic muscle groups are the long flexors and extensors, located in the forearm. The intrinsic group refers to the smaller muscles located within the hand itself. The muscles of the hand are innervated by the radial, median, and ulnar nerves from the brachial plexus.
The radial nerve is formed from nerve fibres from all the roots of the brachial plexus. It provides motor innervation to the triceps brachii muscle and innervates the extensor muscles in the forearm. It also provides sensory innervation to the thumb side of the back of the hand, the posterior forearm, and the dorsal surface of part of the hand. The radial nerve can be entrapped at the supinator, a muscle located just below the elbow, causing forearm pain or weakness of finger and thumb muscles.
The median nerve is formed from the C5, C6, C7, C8, and T1 nerve roots of the brachial plexus. It travels along the inside of the arm near the brachial artery and supplies almost all the flexor muscles and all the pronator muscles of the forearm with nerves. It provides motor innervation to the muscles of the anterior compartment of the forearm, as well as the thenar muscles and lateral two lumbricals of the hand. It also provides sensory coverage to the lateral aspect and three and a half digits of the hand. The median nerve is prone to injury, with carpal tunnel syndrome being the most common cause.
The ulnar nerve is formed from the C8 and T1 nerve roots of the brachial plexus. It innervates most of the intrinsic muscles of the hand and two muscles in the anterior forearm. It provides sensory innervation to the little finger and part of the ring finger, the medial palmar area, and the small finger side of the palm and hand. The ulnar nerve is very close to the skin at the elbow, making it easy to bump, and injuries can result in the clawing of the small and ring fingers.
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Frequently asked questions
The muscles in the fingers are connected to tendons, which act like strong, flexible ropes, uniting with the interosseous and lumbrical muscles to form the extensorhood mechanism.
The muscles in the fingers can be divided into two groups: extrinsic and intrinsic muscle groups. The extrinsic muscle groups are the long flexors and extensors. The intrinsic group are the smaller muscles located within the hand itself.
The finger muscles are responsible for movement, dexterity, and tactile sensation. The flexors allow for the bending of the fingers, while the extensors help to straighten them out.
