The upper subscapular nerve is a small peripheral nerve that originates from the posterior cord of the brachial plexus. It typically receives contributions from the C5 and C6 spinal nerve roots. This nerve primarily supplies motor innervation to the upper part of the subscapularis muscle, which is involved in the movement of the shoulder joint.
Location
The upper subscapular nerve arises from the posterior cord of the brachial plexus in the axilla. It is situated near the thoracodorsal and lower subscapular nerves. After its origin, it travels a short course along the posterior wall of the axilla, reaching the subscapularis muscle, which is located on the anterior surface of the scapula (shoulder blade). It terminates by innervating the superior portion of the subscapularis muscle.
Anatomy
Origin
The upper subscapular nerve originates from the posterior cord of the brachial plexus. It receives nerve fibers primarily from the C5 and C6 spinal roots. It is one of the three subscapular nerves arising from the posterior cord, the others being the lower subscapular nerve and the thoracodorsal nerve.
Course
Proximal Course
After originating from the posterior cord in the axillary region, the upper subscapular nerve takes a short course posteriorly. It remains close to the posterior wall of the axilla and runs along the posterior surface of the shoulder region.
Pathway to Subscapularis
The nerve travels directly to the subscapularis muscle, specifically targeting the upper portion of the muscle. It does not course through a long or complicated pathway but instead takes a relatively direct route from its origin to its target muscle.
Relations
- Proximity to Other Nerves: The upper subscapular nerve runs in close relation to the lower subscapular nerve and thoracodorsal nerve, all of which arise from the posterior cord. These three nerves are critical for innervating the posterior muscles of the shoulder and upper back.
- Adjacent Structures: It is located near the subscapular vessels, which supply blood to the subscapularis muscle. The nerve lies deep within the axillary space, posterior to the axillary artery and vein.
Branching
The upper subscapular nerve does not have significant branching beyond its target muscle, as it primarily serves to innervate the upper part of the subscapularis muscle.
Termination
The upper subscapular nerve terminates by entering the upper fibers of the subscapularis muscle, a large, triangular muscle that covers the anterior surface of the scapula. Its motor fibers are distributed in this region, ensuring proper innervation for the muscle’s superior portion. Unlike some nerves in the brachial plexus, the upper subscapular nerve has a limited area of innervation and remains focused on the superior part of the subscapularis.
Function
The primary function of the upper subscapular nerve is to provide motor innervation to the upper portion of the subscapularis muscle, a key muscle in the shoulder joint. This nerve plays an essential role in facilitating specific movements of the shoulder, particularly in stabilizing and rotating the humerus (upper arm bone).
Motor Innervation of the Subscapularis Muscle
The upper subscapular nerve innervates the superior fibers of the subscapularis muscle, which is one of the four muscles that make up the rotator cuff. The subscapularis covers the anterior surface of the scapula (shoulder blade) and is crucial in shoulder movement and stabilization.
Functions of the Subscapularis Muscle (via Upper Subscapular Nerve)
- Medial (Internal) Rotation of the Arm
- The upper subscapular nerve enables the subscapularis muscle to medially rotate the humerus. This movement is essential for turning the arm inward, such as when placing the hand behind the back or across the body.
- The subscapularis muscle is the most powerful medial rotator of the shoulder, and the upper subscapular nerve contributes significantly to this strength.
- Stabilization of the Shoulder Joint
- The subscapularis, innervated by the upper subscapular nerve, helps stabilize the glenohumeral joint (shoulder joint) by keeping the head of the humerus securely in the shallow socket of the scapula (glenoid fossa). This stabilization is critical during arm movements, particularly when the arm is lifted or rotated.
- By maintaining stability, the subscapularis prevents the head of the humerus from dislocating or moving excessively during dynamic shoulder movements.
Role in Shoulder Movements
The upper subscapular nerve plays a crucial role in allowing the subscapularis muscle to assist with several important shoulder movements:
- Internal Rotation of the Arm
- When rotating the arm toward the body’s midline, such as turning the palm toward the body or reaching behind the back, the upper subscapular nerve engages the subscapularis muscle to mediate this movement. This function is vital for daily activities such as tucking in a shirt or reaching for objects on the opposite side of the body.
- Assistance in Shoulder Adduction
- Although primarily a rotator, the subscapularis muscle also assists in adduction, where the arm is drawn closer to the body. The upper subscapular nerve contributes to this secondary role, enabling movements such as bringing the arm down from an outstretched position.
- Dynamic Stability of the Shoulder
- Through its innervation of the upper part of the subscapularis, the upper subscapular nerve helps the muscle act as a stabilizer for the shoulder joint during complex upper limb movements. This stability is crucial in actions that require coordinated shoulder rotation, such as throwing or lifting.
Contribution to Rotator Cuff Function
As part of the rotator cuff, the subscapularis, under the control of the upper subscapular nerve, plays a significant role in maintaining shoulder stability and controlling humeral head movement. The rotator cuff works synergistically with other muscles (supraspinatus, infraspinatus, and teres minor) to allow for smooth and controlled shoulder movements.
Clinical Significance
The upper subscapular nerve is clinically important due to its role in innervating the upper part of the subscapularis muscle, a key component of the rotator cuff. Damage to this nerve can result in weakness or dysfunction of the subscapularis muscle, affecting shoulder movements and stability.
Common Causes of Injury
- Surgical Procedures: The nerve is at risk of injury during surgeries involving the axilla or shoulder, such as brachial plexus surgeries, shoulder arthroscopy, or trauma repairs.
- Trauma: Direct trauma to the posterior cord of the brachial plexus or the axillary region can lead to upper subscapular nerve damage.
Clinical Manifestations
- Weakness in Internal Rotation: Damage to the nerve may result in weakened medial (internal) rotation of the arm, making it difficult to perform tasks like reaching behind the back or across the body.
- Shoulder Instability: Since the subscapularis contributes to shoulder stability, damage to the upper subscapular nerve may lead to instability in the shoulder joint, increasing the risk of dislocation or injury during movement.
Diagnostic and Treatment Considerations
Diagnosis of upper subscapular nerve injury often involves physical examination, electromyography (EMG), and imaging studies. Treatment may include physical therapy to strengthen surrounding muscles, nerve repair, or surgical intervention depending on the severity of the injury. Early detection and treatment are essential to prevent long-term dysfunction in shoulder movement and stability.
