Serratus Posterior Muscle Anatomy

The serratus posterior muscles comprise two distinct yet functionally related muscles: the serratus posterior superior and the serratus posterior inferior. These thin, flat muscles lie deep within the back, positioned strategically to contribute to respiratory mechanics and thoracic cage stabilization. Despite their relatively modest size, these muscles serve important anatomical and physiological roles that warrant thorough understanding for clinical practice.

Serratus Posterior Muscle Anatomy

Origin & Insertion

The serratus posterior superior originates from the lower portion of the ligamentum nuchae, the spinous processes of the seventh cervical vertebra (C7), and the first two or three thoracic vertebrae (T1-T3). Its fibers course laterally and slightly inferiorly, inserting onto the superior borders of the second through fifth ribs, just lateral to their angles. This muscle lies deep to the rhomboids and is covered by the trapezius and latissimus dorsi muscles. Functionally, the serratus posterior superior elevates the ribs to which it attaches, thereby participating in the mechanics of deep inspiration.

See Also: Serratus Anterior Muscle Anatomy

In contrast, the serratus posterior inferior originates from the spinous processes of the lower two thoracic vertebrae (T11-T12) and the upper two or three lumbar vertebrae (L1-L3), as well as the supraspinous ligaments at these levels. Its fibers extend laterally and superiorly to insert on the inferior borders of the last four ribs (9-12), lateral to their angles. The serratus posterior inferior lies deep to the latissimus dorsi muscle. This muscle functions primarily to pull the lower ribs inferiorly and laterally, thereby preventing their elevation during deep expiration.

Neurovascular Anatomy

Regarding innervation, both serratus posterior muscles receive segmental innervation from the intercostal nerves corresponding to their vertebral levels. The serratus posterior superior is typically innervated by branches from the second through fifth intercostal nerves, while the serratus posterior inferior receives innervation from the ninth through twelfth intercostal nerves. This pattern of innervation reflects their embryological derivation from the ventrolateral components of the myotomes.

The arterial supply to these muscles follows a similar segmental pattern, with contributions from the posterior branches of the intercostal arteries at their respective levels. Venous drainage occurs via corresponding posterior intercostal veins that ultimately drain into the azygos and hemiazygos venous systems.

Histologically, the serratus posterior muscles exhibit the typical features of skeletal muscle, consisting predominantly of slow-twitch (type I) fibers suited for sustained, low-intensity contractions associated with respiratory function. The muscles display a thin, sheet-like architecture with muscle fibers organized in a relatively parallel arrangement, reflecting their simple line of pull.

From a developmental perspective, the serratus posterior muscles arise from the hypaxial portions of the myotomes, specifically from those portions that give rise to the intrinsic muscles of the thoracic wall. Their development is closely linked to the formation of the thoracic cage and the establishment of respiratory mechanics.

Clinical Significance

Clinical significance of the serratus posterior muscles is multifaceted. Dysfunction of these muscles may contribute to alterations in respiratory mechanics, potentially impacting ventilatory function. The serratus posterior superior may become tender in patients with cervicothoracic junction disorders, while the serratus posterior inferior can be implicated in thoracolumbar junction pain syndromes. Additionally, these muscles may serve as anatomical landmarks during surgical approaches to the posterior thoracic wall.

Palpation of the serratus posterior muscles presents a clinical challenge due to their deep location beneath more superficial muscle layers. However, careful assessment during deep inspiration (for the superior muscle) or forced expiration (for the inferior muscle) may allow for indirect evaluation of their function. Electromyographic studies have demonstrated their recruitment during specific respiratory maneuvers, confirming their role in ventilatory mechanics.

In imaging studies, particularly MRI of the thoracic spine, the serratus posterior muscles can be visualized as thin muscular sheets adjacent to the thoracic cage. Their identification may be clinically relevant in the assessment of posterior chest wall pathologies or when planning surgical interventions in this region.

Understanding the anatomy and function of the serratus posterior muscles is essential for comprehensive management of thoracic pain syndromes, respiratory disorders, and surgical planning involving the posterior thoracic wall. Their contribution to respiratory mechanics, while subtle, remains physiologically significant in both normal function and pathological states.

Resources

1. Gray’s Anatomy: The Anatomical Basis of Clinical Practice – https://www.elsevier.com/books/grays-anatomy/standring/978-0-7020-5230-9
2. Atlas of Human Anatomy by Frank H. Netter – https://www.elsevier.com/books/netters-atlas-of-human-anatomy/netter/978-0-323-39322-5