The Extensor King
- Origin: C7, C8, T1, T2 (caudal plexus)
- Vulnerable Zone: Musculospiral groove (caudal humerus)
- Motor: Triceps (elbow extension), all carpal/digital extensors
- Sensory: Craniolateral forearm, dorsal manus, digits III & IV

Lesion Patterns:
High (Proximal): Dropped elbow (triceps paralysis), cannot bear weight
Low (Distal): Knuckled fetlock only (triceps spared), can bear weight if manually placed

THE ANTI-GRAVITY BATTERY

The Radial nerve is clinically the most significant nerve in the forelimb—the primary defense against gravity, powering anti-gravity muscles that convert the limb into a rigid column supporting half a ton of mass.

ORIGIN & PATHWAY:
Arises from caudal plexus (C7-T2), separates between subscapularis and teres major, then enters its defining anatomical feature: the Musculospiral Groove.

THE GROOVE - CRITICAL VULNERABILITY:
The nerve winds spirally around caudal/lateral humeral shaft, lying directly against periosteum, covered only by lateral head of triceps. This creates the 'perfect storm' for compressive neuropathy:

• Recumbency Trauma: When heavy cows lie on hard concrete or are strapped to tilt tables, ribcage weight compresses triceps → crushes radial nerve against bone
• Fractures: Humeral shaft fractures commonly injure the nerve in this groove

MOTOR SUPPLY:

Proximal Branches (Before Groove):
• Triceps Brachii (Long, Lateral, Medial, Accessory heads)
• Tensor Fasciae Antebrachii
• Anconeus
Function: Extension of elbow—the single most critical movement for standing. If the elbow cannot extend, the limb collapses.

Distal Deep Branches (After Groove):
• Extensor Carpi Radialis
• Common Digital Extensor
• Lateral Digital Extensor
• Ulnaris Lateralis (carpal extensor in flexed position)
Function: Extension of carpus and digits—prepares hoof to land flat and bear weight.

SENSORY DISTRIBUTION:
Superficial branch provides cutaneous sensation to craniolateral forearm and dorsal surface of manus, specifically digits III and IV. Vital for sensing 'knuckling' or dorsal placement.

LESION HIERARCHY:

High (Proximal) Lesion: Above triceps branches. Sign: 'Dropped Elbow'—olecranon drops ventrally. Carpus and fetlock flex due to reciprocal tension. Toe drags. Cannot bear weight. Limb appears 'too long.'

Low (Distal) Lesion: In musculospiral groove. Sign: 'Knuckled Fetlock'—triceps spared (can lock shoulder), but distal extensors paralyzed. Animal stands with carpus/fetlock partially flexed. Crucial Distinction: If examiner manually places hoof flat and pushes knee back, cow CAN bear weight because triceps maintains column.