Nerve Root Innervation of the Serratus Anterior Muscle
The serratus anterior muscle is innervated by the long thoracic nerve, which arises primarily from the C5, C6, and C7 nerve roots, with the upper portion of the muscle receiving predominant C5 innervation and occasional contributions from C4. 1, 2
Anatomical Organization
The serratus anterior muscle has three distinct parts with specific nerve root contributions:
Upper part: Supplied mainly by the C5 nerve root, with additional contributions from C4, C6, or C7 nerve roots present in 91% of cases (64 of 70 dissections). 2
Middle and lower parts: Innervated by the long thoracic nerve proper, consisting of C6 and C7 nerve roots. 2
The long thoracic nerve passes between the anterior and middle scalene muscles along with other components of the brachial plexus (C5-T1 ventral rami). 3
Functional Implications
The anatomical segmentation correlates with distinct functional roles:
The upper part (C5-dominant) stabilizes rotational motion of the scapula on the thorax during shoulder elevation and traverses in a more posterior direction compared to other parts. 2
The middle part (C6-C7) provides scapular abduction. 2
The lower part (C6-C7) contributes to upward rotation, abduction, and posterior tilting of the scapula. 2
Clinical Relevance
Isolated serratus anterior paralysis manifests as scapular winging and results from long thoracic nerve injury:
The nerve can be compressed by blood vessels and fibrotic tissue as it passes along the thorax. 4
Injury may occur iatrogenically during selective cervical nerve root blocks or interscalene brachial plexus blocks. 1
In upper brachial plexus injuries (C5-C6 root avulsions), the serratus anterior may be affected, contributing to winged scapula and loss of shoulder abduction. 5
Surgical release of the distal segment of the long thoracic nerve shows best outcomes when performed within 12-18 months of paralysis onset. 4