Segmental Innervation of the Upper Limb
The spinal segments C5 to T1 collectively innervate the entire upper limb through the brachial plexus. Knowing which segment controls which movement is essential for localising the level of a cord, root, or plexus lesion.
Segmental Control of Movements
| Segment | Movements Controlled | Key Muscles |
|---|---|---|
| C5 | Adduction of shoulder; abduction of shoulder (with C6); flexion of elbow (with C6) | Deltoid, supraspinatus, infraspinatus, teres minor, biceps brachii, brachialis |
| C6 | Abduction of shoulder (with C5); flexion of elbow (with C5); extension of elbow (with C7); pronation and supination; wrist extension (with C7) | Biceps brachii, brachioradialis, supinator, extensor carpi radialis longus |
| C7 | Extension of elbow (with C6); flexion and extension of wrist; flexion and extension of fingers | Triceps brachii, wrist flexors, wrist extensors, finger extensors, finger flexors |
| C8 | Flexion of wrist and fingers (with T1); extension of fingers (with C7) | Flexor digitorum superficialis, flexor digitorum profundus, long finger extensors |
| T1 | All intrinsic muscles of the hand (interossei, lumbricals, thenar, hypothenar) | All palmar and dorsal interossei; thenar and hypothenar muscles |
Key rule: C5/C6 → shoulder and elbow; C6/C7 → wrist; C7/C8 → fingers; T1 → intrinsics.
Segmental Values of Important Reflexes
| Reflex | Nerve | Spinal Segment |
|---|---|---|
| Biceps jerk | Musculocutaneous nerve | C5, C6 |
| Supinator (brachioradialis) jerk | Radial nerve | C5, C6 |
| Triceps jerk | Radial nerve | C6, C7 |
Dermatomes of the Upper Limb
| Segment | Cutaneous Area |
|---|---|
| C5 | Lateral (outer) aspect of the arm — deltoid badge area |
| C6 | Lateral forearm; thumb and index finger |
| C7 | Middle finger |
| C8 | Ring and little fingers; medial forearm |
| T1 | Medial (inner) aspect of the arm |
| T2 | Axilla and medial arm (via intercostobrachial nerve) |
Dermatomes are arranged along the preaxial border (C5→C6 lateral side) and postaxial border (C8→T1 medial side) of the limb.
Clinical Application — Localising Level of Lesion
| Loss | Likely Level |
|---|---|
| Shoulder abduction + elbow flexion lost; wrist/hand intact | C5 (± C6) — Erb’s type |
| Wrist and finger extension lost; intrinsics intact | C7 |
| Intrinsic hand muscles wasted; finger movements lost | T1 — Klumpke’s type |
| Biceps jerk absent; triceps jerk present | C5/C6 lesion |
| Triceps jerk absent | C6/C7 lesion |
Loss of reflexes and myotomal weakness together reliably indicate root level; sensory dermatome loss confirms the segment.