First Palmar Interosseous (Hand)
Hand · Upper Limb
The first palmar interosseous is a small, unipennate muscle on the palmar surface of the hand, arising from the palmar surface of the second metacarpal. It inserts into the extensor expansion and base of the proximal phalanx of the index finger. It adducts the index finger toward the axial line of the hand, following the mnemonic PAD, meaning Palmar ADduct, and, through the extensor expansion, flexes the metacarpophalangeal joint while extending the interphalangeal joints. It is supplied by the deep branch of the ulnar nerve (C8, T1).
Origin, Insertion, Action & Nerve
Origin
Palmar surface of the 2nd metacarpal (unipennate)
Insertion
Extensor expansion and base of proximal phalanx of the index finger
Action
Adducts the index finger toward the axial line of the hand (PAD); flexes MCP and extends IP joints
Nerve
Deep branch of ulnar nerve (C8, T1)
Attachments explained
The First Palmar Interosseous (Hand) is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Palmar surface of the 2nd metacarpal (unipennate)
Insertion
Extensor expansion and base of proximal phalanx of the index finger
Action & function
When the First Palmar Interosseous (Hand) contracts, it produces the following movements: Adducts the index finger toward the axial line of the hand (PAD); flexes MCP and extends IP joints.
As part of the Hand group of the Upper Limb, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The First Palmar Interosseous (Hand) receives its nerve supply from the Deep branch of ulnar nerve (C8, T1). This nerve carries fibres from spinal segment(s) C8, T1.
Because a muscle can only contract when its nerve is intact, injury to the Deep branch of ulnar nerve or to its spinal roots (C8, T1) can weaken or paralyse the First Palmar Interosseous (Hand), impairing the movements it normally produces (adducts the index finger toward the axial line of the hand (PAD)). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the First Palmar Interosseous (Hand) (the OIANS method)
Pair the mnemonics to keep the interossei straight: PAD means Palmar ADduct, the exact opposite of the dorsal DAB. So this muscle pulls the index finger back toward the middle-finger axis, undoing what the first dorsal interosseous does. Being unipennate and arising from a single metacarpal, it is slimmer than its dorsal counterparts.
Because palmar and dorsal interossei together let the fingers spread and close, learning them as antagonistic pairs is the fastest route. Note that all of them, palmar and dorsal alike, share the deep branch of the ulnar nerve and the same knuckle-flexing, finger-straightening action.
Frequently asked questions
Where is the First Palmar Interosseous (Hand) located?
The First Palmar Interosseous (Hand) is a muscle of the Hand group, located in the Upper Limb.
What is the origin of the First Palmar Interosseous (Hand)?
Palmar surface of the 2nd metacarpal (unipennate)
What is the insertion of the First Palmar Interosseous (Hand)?
Extensor expansion and base of proximal phalanx of the index finger
What movements does the First Palmar Interosseous (Hand) produce?
Adducts the index finger toward the axial line of the hand (PAD); flexes MCP and extends IP joints
What nerve supplies the First Palmar Interosseous (Hand)?
Deep branch of ulnar nerve (C8, T1)
Is the First Palmar Interosseous (Hand) free to study in OIANS?
Yes. The First Palmar Interosseous (Hand) is free in OIANS: its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory, and because it belongs to the Upper Limb it is also covered by the free Quiz and Flashcard decks. The other regions, the "All Muscles" deck, and the Progress and Explore tools are unlocked with a one-time Lifetime purchase.
Related muscles
Study the First Palmar Interosseous (Hand)
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