Three nerves run into your hand, and each one owns a specific territory of skin and a specific set of muscles.
So the fingers that go numb are not random information — they are the single most useful diagnostic fact you can bring to an appointment. Thumb, index and middle is one nerve. Ring and little is another. The back of the thumb web is a third.
Knowing which one you have narrows the list from twenty conditions to about three.
The median nerve: thumb, index, middle
Skin: palm side of the thumb, index, middle, and the thumb-side half of the ring finger. Plus the fingertips of those same fingers on the back.
Muscles: most of the muscles that bend your fingers, plus the pad of muscle at the base of the thumb that lifts it up away from the palm.
Where it gets squeezed: at the wrist, in the carpal tunnel. Higher up in the forearm, as pronator teres syndrome.
The distinguishing detail: the nerve gives off a branch to the palm BEFORE the carpal tunnel. So carpal tunnel numbs the fingers but usually spares the palm. A numb palm as well points higher up the arm.
The ulnar nerve: ring and little
Skin: little finger and the little-finger half of the ring finger, front and back, plus the edge of the hand.
Muscles: nearly all the small muscles inside the hand — the ones that spread your fingers, pinch strongly and do fine coordination. This is why ulnar problems cost you dexterity and grip, not just sensation.
Where it gets squeezed: at the elbow, in the cubital tunnel — by far the commonest. At the wrist, in Guyon’s canal, often in cyclists.
The distinguishing detail: the nerve gives a branch to the back of the hand before the wrist. Numbness on the BACK of the little-finger side of the hand means the problem is at the elbow, not the wrist. That single observation sorts most ulnar cases.
The ring finger is split down the middle between two nerves. Which half is numb tells you which nerve — and it’s a question almost nobody gets asked.
The radial nerve: back of the thumb web
Skin: the back of the hand on the thumb side, especially the web between thumb and index. No palm territory at all.
Muscles: everything that straightens — the wrist, the knuckles, the thumb. It bends nothing.
Where it gets squeezed:
- in the upper arm, producing a wrist drop.
- In the forearm as radial tunnel syndrome, or as the motor-only PIN palsy.
- At the wrist, as Wartenberg’s syndrome — a tight watch strap being the classic cause.
The three tests, one per nerve
- Median — make an OK sign with thumb and index. A flattened, pinched-looking circle instead of a round one means the deep motor branch is affected
- Ulnar — hold a piece of paper between your thumb and the side of your index finger while someone pulls. If your thumb tip bends to hold on, that’s compensation for ulnar weakness
- Radial — with the forearm supported palm-down, lift the wrist and fingers up against gravity
I test all three on every new nerve patient, on both hands, before I touch a single tunnel. It takes a minute and it stops you treating the wrong nerve.
When the map doesn’t fit
If the numbness doesn’t follow any of these territories, the problem probably isn’t a single nerve in the arm.
- Numbness in a stripe running down the whole arm — a nerve root in the neck, not the hand. See cervical radiculopathy
- All fingers, both hands, symmetrical, worse at night, glove-like — a generalized neuropathy, which is a medical workup rather than a hand one
- Whole hand, following an injury, with burning pain and color change — think CRPS
For the splints involved, see the wrist drop splint and the anti-claw splint.
Before your appointment, work out exactly which fingers and which surfaces are affected, and whether the palm is involved.
That single sentence does more diagnostic work than any scan.
Every article on this site is written using the Closing Window Method: name the window your condition runs on, find where you are in it, and act while it’s still open.