Upper limb
Introduced| English | Meaning |
|---|---|
| brachial plexus | brachial plexus |
| musculocutaneous nerve | The brachial plexus nerve to the front arm flexors, including biceps. |
| axillary nerve | The nerve to deltoid and teres minor, at risk in surgical-neck fractures of the humerus. |
| radial nerve | The nerve to the arm and forearm extensors; injury causes wrist drop. |
| median nerve | The nerve through the carpal tunnel to most forearm flexors and the thenar muscles. |
| ulnar nerve | The nerve behind the medial epicondyle to most intrinsic hand muscles; injury causes a claw hand. |
A circuit you can read
- The arm is wired by one plexus with five main nerves, and each nerve has a movement you can test and a patch of skin you can touch.
- Learn the circuit and a single bedside test tells you which wire is cut.
The brachial plexus
- The brachial plexus 臂丛 starts as five roots, C5 to T1. They merge into trunks, split into divisions, regroup as cords and end as nerves.
- The layers matter because they explain injury patterns: a root injury and a nerve injury look different at the bedside.
Put the parts of the brachial plexus in order, from the spinal cord outwards.
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Five roots (C5–T1) merge into three trunks, split into six divisions, form three cords and end as the terminal nerves.
How many spinal roots form the brachial plexus?
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C5, C6, C7, C8 and T1.
Five nerves, five jobs
- Musculocutaneous nerve 肌皮神经: bends the elbow. Axillary nerve 腋神经: lifts the arm sideways. Radial nerve 桡神经: straightens the elbow, wrist and fingers.
- Median nerve 正中神经: precision grip and thumb opposition. Ulnar nerve 尺神经: the small hand muscles that spread and close the fingers.
Match each nerve to the movement that tests it.
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One movement per nerve makes the examination quick and the lesion easy to name.
Why lesions follow movements
- The limb bud rotates as it grows, so the skin strips (dermatomes) spiral: C5–C6 along the thumb side, C8–T1 along the little-finger side.
- The plexus remixes the roots so each nerve serves one set of movements. So a nerve lesion shows a movement pattern, not a spinal-segment pattern.
A cut median nerve causes weakness and numbness that match a single spinal segment.
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The median nerve carries fibres from several roots (C5–T1), so its lesion follows the nerve's movements and skin patch, not one segment.
Worked example: reading the circuit
- Wrist drop: radial nerve, often at the spiral groove of the humerus ("Saturday-night palsy"). Weak thumb opposition with thenar wasting: median nerve at the carpal tunnel.
- Claw hand: ulnar nerve. Each has a one-line test: extend the wrist, touch thumb to little finger, spread the fingers.
After a fracture of the middle of the humerus, a patient cannot lift the wrist. Which nerve is injured?
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The radial nerve winds around the shaft of the humerus in the spiral groove, so a mid-shaft fracture can catch it, causing wrist drop.
Worked example: the nerve you can feel
- (Fictional patient) Pain at the inner elbow running to the little finger, worse when holding a phone. The ulnar nerve passes behind the medial epicondyle, in a groove you can feel on your own arm.
- Lying just under the skin there makes it easy to examine (tapping it causes tingling) and easy to trap. Anatomy you can feel is anatomy you keep.
Where does the ulnar nerve run at the elbow?
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Its shallow course behind the medial epicondyle is why it is easy to tap, and easy to compress.
Three common slips
- Dermatomes as straight stripes. They spiral along the limb because of its rotation.
- Testing a muscle by name. Test a movement (extension), then think which muscle and nerve.
- Learning the plexus for its own sake. Its layers exist to explain injury patterns.
Carpal tunnel syndrome mainly causes which sign?
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The median nerve passes through the carpal tunnel; it supplies the thenar muscles and the skin of the radial three and a half fingers.
Recap
- C5–T1 → trunks → divisions → cords → five terminal nerves.
- Radial = extend, median = oppose, ulnar = spread.
- The ulnar groove is the reminder that superficial anatomy is clinical anatomy.