Infraspinatus muscle

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Infraspinatus muscle
Infraspinatus.PNG
Muscles connecting the upper extremity to the vertebral column (posterior view).
Infraspinatus muscle back2.png
Infraspinatus muscle (shown in red) seen from behind.
Latin musculus infraspinatus
Gray's p.441
Origin infraspinous fossa of the scapula
Insertion middle facet of greater tubercle of the humerus
Artery suprascapular and circumflex scapular arteries
Nerve suprascapular nerve
Actions Lateral rotation of arm and stabilizes humerus
TA A04.6.02.008
FMA FMA:32546
Anatomical terms of muscle

In human anatomy, the infraspinatus muscle is a thick triangular muscle, which occupies the chief part of the infraspinatous fossa.[1] As one of the four muscles of the rotator cuff, the main function of the infraspinatus is to externally rotate the humerus and stabilize the shoulder joint.

Structure[edit]

It attaches medially to the infraspinous fossa of the scapula and laterally to the middle facet of the greater tubercle of the humerus.

The muscle arises by fleshy fibers from the medial two-thirds of the infraspinatous fossa, and by tendinous fibers from the ridges on its surface; it also arises from the infraspinatous fascia which covers it, and separates it from the teres major and teres minor.[1]

The fibers converge to a tendon, which glides over the lateral border of the spine of the scapula, and, passing across the posterior part of the capsule of the shoulder-joint, is inserted into the middle impression on the greater tubercle of the humerus.[1] The trapezoidal insertion of the infraspinatus onto the humerus is much larger than the equivalent insertion of the supraspinatus, the reason why the infraspinatus is involved in rotarcuff tears about as frequently as the supraspinatus. [2]

Relations[edit]

The tendon of this muscle is sometimes separated from the capsule of the shoulder-joint by a bursa, which may communicate with the joint cavity.[1]

Innervation[edit]

The suprascapular nerve innervates the supraspinatus and infraspinatus muscles. These muscles function to abduct and laterally rotate the arm, respectively.

Variation[edit]

The infraspinatus is frequently fused with the teres minor.[3]

Function[edit]

The infraspinatus is the main external rotator of the shoulder. When the arm is fixed, it abducts the inferior angle of the scapula. Its synergists are teres minor and the deltoid.[4]

The infraspinatus and teres minor rotate the head of the humerus outward (external, or lateral, rotation); they also assist in carrying the arm backward (extension of the glenohumeral joint).[1]

Additionally, the infraspinatus reinforces the capsule of the shoulder joint.[3]

In animals[edit]

The pectoral muscles — the pectoralis major and pectoralis minor — evolved from a primitive muscle sheet that connected the coracoid to the humerus. In late reptilians and early mammals, this muscle structure was displaced dorsally; while most of its components evolved into the pectoralis major, some fibers eventually attached to the scapula and evolved into the supraspinatus, the infraspinatus, and parts of the subscapularis. [5]

Additional images[edit]

Infraspinatus muscle (shown in red). Animation. 
Muscles on the dorsum of the scapula, infraspinatus is labelled 8 (posterior view). 
Infraspinatus muscle 
Infraspinatus muscle 
Left scapula. Posterior surface. 
Left humerus. Posterior view. 
Infraspinatus muscle 

References[edit]

This article incorporates text from a public domain edition of Gray's Anatomy.

  1. ^ a b c d e Gray's Anatomy, see infobox.
  2. ^ Mochizuki, T.; Sugaya, H.; Uomizu, M.; Maeda, K.; Matsuki, K.; Sekiya, I.; Muneta, T.; Akita, K. (2008). "Humeral Insertion of the Supraspinatus and Infraspinatus. New Anatomical Findings Regarding the Footprint of the Rotator Cuff". The Journal of Bone and Joint Surgery 90 (5): 962. doi:10.2106/JBJS.G.00427. PMID 18451386.  edit
  3. ^ a b Platzer, Werner (2004). Color Atlas of Human Anatomy, Vol. 1: Locomotor System (5th ed.). Thieme. p. 138. ISBN 3-13-533305-1. 
  4. ^ "Infraspinatus". Wheeless' Textbook of Orthopaedics. Retrieved January 2011. 
  5. ^ Brand, R. A. (2008). "Origin and Comparative Anatomy of the Pectoral Limb". Clinical Orthopaedics and Related Research 466 (3): 531. doi:10.1007/s11999-007-0102-6. PMC 2505211. PMID 18264841.  edit, p 541

External links[edit]