Serratus posterior muscles
The serratus posterior muscles are two paired muscles located in the upper and lower back. They include:
- The serratus posterior superior muscle
- The serratus posterior inferior muscle
Together, these muscles comprise the intermediate layer of the extrinsic musculature of the back.
The serratus posterior muscles extend obliquely from the vertebral column to the rib cage. The main function of these muscles is to facilitate the act of respiration; the serratus posterior superior muscle elevates the ribs, while the serratus posterior inferior muscle depresses the ribs. These actions are particularly important in forced respiration.
|Serratus posterior superior||
Origin: Nuchal ligament, spinous processes of vertebrae C7-T3
Insertion: Superior borders of ribs 2-5
Innervation: 2nd-5th Intercostal nerves
Function: Elevates ribs
|Serratus posterior inferior||
Origin: Spinous processes of vertebrae T11-L2
Insertion: Inferior borders of ribs 9-12
Innervation: Anterior rami of spinal nerves T9-T12 (a.k.a. 9th-11th Intercostal nerves + subcostal nerve)
Function: Depresses ribs/ Draws ribs inferoposteriorly
This article will discuss the anatomy and function of the serratus posterior muscles.
Origin and insertion
The serratus posterior muscles run from the spinous processes of vertebrae to the ribs, which is why they are sometimes referred to as the spinocostal muscles.
- The serratus posterior superior muscle arises as a thin tendinous sheet from the nuchal ligament and spinous processes of vertebrae C7-T3, as well as the associated supraspinous ligaments. The muscle fibers extend inferolaterally towards the lateral aspect of the posterior thoracic cage. The insertion of the muscle takes the shape of four finger-like projections, each of which inserts onto the posterosuperior aspect of ribs 2 to 5, lateral to their angles.
- The serratus posterior inferior muscle originates from the spinous processes of vertebrae T11-L2 and their supraspinous ligaments. From there, it ascends anterolaterally to insert onto the inferoposterior aspect of ribs 9 to 12, just lateral to their angles.
The serratus posterior inferior muscle lies superficial to the erector spinae muscle group and the thoracolumbar fascia and deep to the latissimus dorsi muscle. It is pierced by the lateral branches of the T7-T12 spinal thoracic nerves.
Serratus posterior superior is innervated by the intercostal nerves T2-T5.
Serratus posterior inferior is innervated by the anterior rami of the T9-T11 spinal nerves, as well as the subcostal nerves (T12).
Both muscles receive their vascular supply from the posterior intercostal arteries. Serratus posterior inferior receives additional supply from the subcostal artery and upper lumbar arteries.
The function of the serratus posterior muscles is still largely debated, however, the general consensus is that they play a role in respiration. The serratus posterior superior elevates the ribs and thus supports inspiration (accessory muscle of inspiration). In contrast, the serratus posterior inferior helps during expiration by depressing the ribs (accessory muscle of expiration).
The disproportionate use of the serratus posterior superior and other back muscles (e.g. cradling the phone between ear and shoulder) can result in a condition known as the scapulocostal syndrome. It is characterized by pain and paresthesia along the medial border of scapula radiating to the neck, chest and upper extremities. Particularly, the pain felt in the little finger during palpation of the muscle is one of the classic signs. The pain can be increased by lifting objects with the outstretched hands or other activities that cause the scapula to exert pressure on the serratus posterior superior muscle.
Clinically, the scapulocostal syndrome is easily mistaken for a lesion of the spinal nerves of the cervical vertebral column (cervical radiculopathy) which typically presents with numbness, muscle weakness and further neurological deficits (e.g. weak or absent reflexes). As a differential diagnosis, arthritis or rotator cuff rupture should also be considered.