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The human hand is a complex structure, with several muscles, tendons, and nerves working together to enable a wide range of finger positions and movements. The index finger is no exception, and its movement is controlled by a combination of muscles, tendons, and nerves. The median nerve, for instance, plays a critical role in wrist, hand, and finger movements, including the index finger. The lumbrical muscles, arising from the deep flexor, also play a key role in finger straightening and bending the MCP joints. The interossei muscles, located between the bones of the hand, help spread or pull our fingers together. The flexor digitorum profundus, located deep in the forearm, enables the bending of the index finger, while the flexor digitorum superficialis arises from the medial epicondyle (elbow bone) and contributes to finger movement.
| Characteristics | Values |
|---|---|
| Muscles controlling the index finger | Lumbrical muscles, flexor digitorum profundus, flexor digitorum superficialis, interossei muscles, thenar muscles, opponens pollicis, median nerve |
| Function | Contraction, extension, flexion, abduction, adduction, rotation |
| Control | Somatic or voluntary nervous system, spinal cord, brain stem |
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What You'll Learn
The role of the lumbrical muscles
The lumbrical muscles are four short intrinsic muscles of the hand located between the metacarpal bones, deep to the palmar fascia. The word 'lumbrical' is derived from the Latin word 'lumbricidae', which means earthworm, reflecting the muscles' worm-like appearance. The lumbricals are unique in that they have their origin and insertion in tendons, attaching proximally to the tendons of the flexor digitorum profundus and distally to the extensor expansions. Unlike other muscles, they do not attach to bone.
The lumbrical muscles flex the fingers at the metacarpophalangeal (MCP) joints, allowing the fingers to straighten, and they also help bend the MCP joints, which are at the knuckle. They contribute to interphalangeal joint extension by acting as deflexors of the proximal interphalangeal joint. The lumbricals enable complex movement of the hand, such as holding a pen, and contribute to the hand's general dexterity. They are also involved in sensory feedback, which is important for precision pinch movements and precise manipulation of objects.
The lumbricals are numbered 1-4 from the most radial/lateral to the most ulnar/medial. Each lumbrical muscle originates from one or two adjacent tendons of the flexor digitorum profundus muscle. Lumbricals 1 and 2 are usually unipennate, with their fibres arising obliquely from one tendon, while lumbricals 3 and 4 are bipennate, arising from two adjacent tendons. Lumbrical 1 arises from the radial side and palmar surface of the tendon of the index finger, while lumbrical 2 arises from the same side and surface of the tendon of the middle finger.
The lumbrical muscles are highly specialised in terms of their architectural properties, with a small cross-sectional area but long fibre length. Their unique properties indicate that they are important for fast, alternating movements and fine-tuning digit motion. The high number of muscle spindles in the lumbricals suggests that they play a role in proprioception, which is crucial for the hand's complex movements and dexterity.
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The median nerve
After giving off these branches, the median nerve enters the hand via the carpal tunnel, where it divides into two more branches: the recurrent branch and the palmar digital nerves. The recurrent branch innervates the thenar muscles, while the palmar digital nerves innervate the palmar surface and fingertips of the lateral three and a half digits.
Compression of the median nerve within the carpal tunnel can result in carpal tunnel syndrome (CTS), causing numbness, tingling, and pain.
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Interossei muscles
The interossei muscles are intrinsic hand muscles situated between the metacarpal bones. They are categorised into two groups: palmar interossei and dorsal interossei, each consisting of four muscles. These muscles are involved in the complex movement of the hand and fingers.
The palmar interossei muscles are unipennate muscles that originate from the metacarpals of the respective digit to which they are associated. They facilitate adduction of the 1st, 2nd, 4th, and 5th digits towards the 3rd digit (the middle finger). This movement occurs around an imaginary axis through the midline of the 3rd digit, often referred to as the axial line. Adduction facilitated by the palmar interossei occurs at the MCP (metacarpophalangeal) joints. Additionally, these muscles 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 muscle is often considered rudimentary, with some sources classifying it as part of either the adductor pollicis or the flexor pollicis brevis.
The dorsal interossei muscles are bipennate and are the most dorsally situated of all intrinsic hand muscles. They are associated with abduction of the 2nd, 3rd, and 4th fingers, pulling them away from the 3rd digit (the middle finger). The dorsal interossei muscles receive their supply from various arteries, including the dorsal and palmar metacarpal arteries, the radial artery, and the princeps pollicis artery, among others.
Both the palmar and dorsal interossei muscles contribute to flexion of the MCP joints and extension of the interphalangeal joints. They receive innervation from the deep branch of the ulnar nerve. This nerve controls the interossei muscles, and its injury may significantly impair hand functions, particularly finger abduction and adduction.
The interossei muscles, along with the lumbrical muscles, play a crucial role in controlling the index finger. The lumbrical muscles enable the fingers to straighten and can also assist in bending the MCP joints. The interossei muscles, particularly the dorsal interossei, help spread the fingers away from each other, while the palmar interossei pull the fingers together. This complex interplay of muscles allows for precise control of the index finger, enabling tasks such as microsurgery and playing musical instruments.
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The hypothenar muscle group
The hypothenar muscles are the three short muscles of the medial (ulnar) palmar compartment of the hand. They form a noticeable fleshy prominence on the medial side of the palm called the hypothenar eminence. All of the hypothenar eminence muscles, except for the palmaris brevis, are innervated by the deep branch of the ulnar nerve (C8, T1). The palmaris brevis is supplied by the superficial branch of the ulnar nerve (C8, T1). The function of the palmaris brevis muscle is to tighten the palmar aponeurosis and to contribute to the strength of the grip.
The hypothenar muscles are similar to the thenar muscles in both name and organisation. The thenar muscle group is found at the base of the thumb, forming the muscle bulk on the thumb side of the hand. It is comprised of three muscles: the abductor pollicis brevis, the flexor pollicis brevis, and the opponens pollicis. The abductor pollicis brevis pulls the thumb away from the index finger, and the flexor pollicis brevis bends the thumb toward the small finger. The opponens pollicis performs one of the most important functions of the human hand: the ability to bring the thumb away from the fingers so that we can grasp objects.
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The thenar muscle group
The thenar muscles are three short muscles located at the base of the thumb. The muscle bellies produce 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 the most superficial muscle of the thenar group. It originates from the tubercles of the scaphoid and trapezium bones as well as the flexor retinaculum. From here, the muscle fibres extend laterally and distally, ending in a tendon that inserts onto the lateral aspect of the base of the proximal phalanx. The abductor pollicis brevis muscle receives its innervation via the recurrent (thenar) branch of the median nerve. Its blood supply is provided mainly by the superficial palmar branch of the radial artery.
The flexor pollicis brevis is the most medial of the thenar muscles. It arises from two muscle heads (superficial and deep) which are separated by the tendon of the flexor pollicis longus. The superficial head originates from the flexor retinaculum and the tubercle of the trapezium bone, while the deep head originates from the trapezoid and capitate bones. From here, the heads run side by side in a lateral direction to insert onto the base of the proximal phalanx.
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Frequently asked questions
The lumbrical muscle is the main muscle that controls the index finger.
The lumbrical muscle's main function is to allow the fingers to straighten, although it can also help bend the MCP joints.
The index finger is also controlled by the interossei muscles, which are located between the metacarpal bones of the hand. There are four dorsal and three palmar interossei muscles. The first dorsal interosseous muscle forms the contour between the thumb and index finger.
The median nerve controls movement and feeling in the forearm, wrist, hand, thumb, and fingers.
Damage to the median nerve can cause carpal tunnel syndrome, which results in finger pain, wrist pain, and numbness.
