📝 STANDARD DESCRIPTION:
Shoulder Stabilizer
- Origin: Strictly C6 and C7 (most cranial)
- Course: Short, direct path between supraspinatus/subscapularis → crosses neck of scapula at scapular notch
- Motor ONLY: No sensory fibers
- Targets: Supraspinatus (extends shoulder), Infraspinatus (prevents abduction)
Vulnerability: Winding around neck of scapula like rope over pulley—direct blows or traction crush nerve against bone.
📚 DETAILED DESCRIPTION:
THE DYNAMIC LIGAMENT
The Suprascapular nerve is unique among the 'large six' as being purely motor (no cutaneous distribution). It serves as the primary stabilizer of the shoulder joint through its two target muscles.
STRICT ORIGIN:
Arises ONLY from C6 and C7. Most cranial major nerve of plexus. Short, direct course minimizes stretch but creates compression vulnerability.
ANATOMICAL COURSE:
Courses between supraspinatus and subscapularis muscles, crosses cranial border of scapula at the scapular notch (vestment for passage).
MOTOR FUNCTIONS:
1. Supraspinatus:
• Action: Extends shoulder joint
• Functional Role: Initiates advancement of limb in protraction
2. Infraspinatus:
• Action: Flexes shoulder, laterally rotates humerus
• Critical Secondary Function: Acts as lateral collateral ligament of the shoulder. The massive tendon blends with joint capsule to prevent the humeral head from popping outward (abduction) during weight bearing.
THE PULLEY MECHANISM:
As the nerve winds around the 'neck' of the scapula, it acts like a rope over a pulley. This creates the primary vulnerability:
• Direct Trauma: Blows from gates, hitting chute sides
• Traction Injury: Excessive backward pulling of limb (calving assistance, falls)
• Proximal Humeral Fractures: Displacement at neck of scapula
SWEENEY (Suprascapular Neuropathy):
Pathophysiology: Crush injury → Wallerian degeneration → rapid, profound neurogenic atrophy of both muscles.
Clinical Signs:
• Prominent Spine: Within 1-2 weeks, scapular spine becomes dramatically visible due to supraspinatus/infraspinatus wasting
• 'Shoulder Slip': When placing weight on limb, the shoulder joint exhibits visible lateral excursion (subluxation). The humeral head 'pops' outward because the infraspinatus no longer acts as the dynamic lateral collateral ligament.
• Gait: Circumduction to avoid adduction stress on unstable joint
Prognosis: If axons only stretched (neuropraxia), recovery possible in 2-3 months. If axonotmesis (crush), permanent muscle atrophy results in cosmetic defect though animal may remain functional for breeding.
// COMPARATIVE ANALYSIS
| DOG |
Sweeney rare. Different gait compensation (weight shifted to front). Acromion anatomy different. |
| HORSE |
Classic Sweeney well-documented. Nerve more accessible for surgical decompression at notch. |
CLINICAL RELEVANCE
Sweeney Diagnosis: Key differentiator from brachial plexus injury is PRESERVATION OF SENSATION (pure motor nerve) and ability to bear weight (limb doesn't drag). Atrophy Pattern: Muscle wasting makes spine appear more prominent—classic 'shark fin' appearance on lateral scapula. Pain: Usually non-painful unless acute neuritis present.
