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The interosseous muscles are muscles found near the metacarpal bones that help to control the fingers. There are two types of interosseous muscles: the dorsal interosseous muscles and the palmar interosseous muscles. The dorsal interosseous muscles are located on the back of the hand and are responsible for abducting the index, middle, and ring fingers, while the palmar interosseous muscles are located on the anterior side of the metacarpals and are responsible for adducting the digits. The index finger, in particular, relies on the first dorsal interosseous muscle and the palmar interosseous muscle for strength and mobility. These muscles are innervated by the deep branch of the ulnar nerve, although some individuals exhibit variable innervation.
| Characteristics | Values |
|---|---|
| Innervation | Deep branch of the ulnar nerve |
| Function | Strength and mobility of the index finger |
| Number of Dorsal Interossei | Four |
| Location | Back of the hand |
| Function | Abduct digits away from the middle finger |
| Muscle Type | Voluntary |
| Blood Supply | First dorsal interosseous muscle artery |
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What You'll Learn
The first dorsal interosseous muscle
The dorsal interossei muscles are bipennate, with each muscle arising by two heads from the adjacent sides of the metacarpal bones. The first dorsal interosseous muscle is inserted entirely into the base of its proximal phalanx and the extensor hood there. The radial artery passes between its two heads, from the back of the hand into the palm.
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The palmar interosseous muscle
The palmar interossei are a group of four (sometimes three) intrinsic muscles of the hand. They are unipennate muscles, meaning they attach obliquely to their tendon, with muscle fibres converging to attach to one side of the tendon. The palmar interossei are situated on the palmar surface of the hand, occupying the spaces between the metacarpal bones.
The first palmar interosseous muscle arises from the ulnar side of the base of the first metacarpal bone. Its fibres travel inferiorly to the index finger, inserting via a short tendon onto the medial aspect of the base of the proximal phalanx of the index finger. This muscle adducts the proximal phalanx of the index finger, drawing it towards the middle finger. It also assists in flexion at the metacarpophalangeal joint and extension at the interphalangeal joints.
The second palmar interosseous arises from the entire length of the second metacarpal bone and inserts into the extensor expansion of the index finger. The third palmar interosseous arises from the lateral side of the ring finger extensor hood and proximal phalanx, while the fourth arises from the radial side of the fifth metacarpal bone, inserting into the base of the proximal phalanx of the little finger. The second, third, and fourth palmar interossei adduct the ring and little fingers laterally, away from the middle finger. They also contribute to flexion at the metacarpophalangeal joint and extension at the interphalangeal joints.
The palmar interossei receive their innervation from the deep branch of the ulnar nerve (C8-T1). This nerve supplies the palmar interossei with arterial blood from various arteries, including the deep palmar arch, princeps pollicis artery, and palmar metacarpal arteries. The function of these muscles is to adduct the fingers towards the middle finger and assist in flexion and extension at the metacarpophalangeal and interphalangeal joints.
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The deep branch of the ulnar nerve
The ulnar nerve is a major peripheral nerve of the upper limb. It arises from the brachial plexus within the axilla region and is a continuation of the medial cord, containing fibres from spinal roots C8 and T1. The ulnar nerve gives rise to two muscular branches in the forearm: one to the flexor carpi ulnaris and the second to the ulnar (medial) part of the flexor digitorum profundus.
The ulnar nerve then travels alongside the ulna bone of the forearm into the wrist, where it enters the hand via the ulnar canal (Guyon's canal). In the hand, the nerve terminates by giving rise to superficial and deep branches. The deep branch of the ulnar nerve emerges from the same region as the superficial branch, coursing between the muscles of the hypothenar region into the deep aspect of the palm, alongside the deep palmar branch of the ulnar artery.
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The dorsal interossei
The middle digit has two dorsal interossei insert onto it, while the first digit (thumb) and the fifth digit (little finger) have none. Each finger is provided with two interossei (palmar or dorsal), except the little finger, where the abductor digiti minimi muscle replaces one of the dorsal interossei. The first dorsal interosseous muscle is larger than the others and is inserted entirely into the base of its proximal phalanx and the extensor hood. The second dorsal interosseous muscle arises from the medial aspect of the second metacarpal and the lateral aspect of the third metacarpal, inserting into the lateral base of the second phalanx and extensor hood of the second digit. The third dorsal interosseous muscle originates from the medial portion of the third metacarpal and the lateral portion of the fourth metacarpal, with insertion at the medial base of the third phalanx and extensor hood of the third digit. The fourth dorsal interosseous muscle arises from the lateral aspect of the fourth metacarpal and the medial side of the fifth metacarpal, inserting into the lateral base of the fourth phalanx and extensor hood of the fourth digit.
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Variable innervation
The first dorsal interosseous muscle (FDI) is usually innervated by the deep branch of the ulnar nerve. However, as was first noted by Sunderland in 1946, some individuals have variable innervation of the FDI. This variation in innervation was later confirmed by an electrophysiological study, which found that the FDI could also be innervated by the median nerve. This study included 211 patients who had undergone peripheral nerve surgery and found that the variable innervation of the FDI was more common in patients with cubital tunnel syndrome (CuTS), especially those of a younger age.
The variable innervation of the FDI in CuTS patients may be related to the injury of the ulnar nerve, which can lead to denervation of the FDI and muscular atrophy. The study also found that the incidence of variable innervation was higher in the CuTS group, with a rate of 30.51%. This variation in innervation may lead to a better prognosis for CuTS patients as the muscular branches that grow from the median nerve can reinnervate the FDI, improving muscular atrophy to some degree.
Electromyography was used to determine FDI variation during surgery, and a hand function instrument was used to estimate the pinch strength between the thumb and index finger in the CuTS patients. The results showed that the sensitivity was 72.22% and the specificity was 72.50%. This indicates that the variable innervation of the FDI could be detected with a high degree of accuracy using electromyography.
In addition to the median nerve, Okwueze discovered that in about a quarter of the population, the FDI could also be innervated by the posterior interosseous nerve and the superficial branch of the radial nerve. This further highlights the variable innervation of the FDI and the potential for compensation by other nerves in the event of injury or disease.
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Frequently asked questions
Interosseous muscles are muscles found near the metacarpal bones that help control the fingers.
Interosseous muscles enable flexion at the metacarpophalangeal joints and extension at the interphalangeal joints. They also abduct the digits away from the axial line (the middle finger).
The interosseous muscles are innervated by the deep branch of the ulnar nerve.
Yes, there are two types of interosseous muscles: palmar interosseous muscles and dorsal interosseous muscles.
The dorsal interossei are intrinsic muscles of the hand. They are located on the dorsum of the hand and can be palpated between the metacarpal heads.
