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Supraspinatus muscle
Template:Muscle infobox The supraspinatus (plural supraspinati, from Latin supraspinatus) is a relatively small muscle of the upper arm that runs from the supraspinatous fossa superior of the scapula (shoulder blade) to the greater tubercle of the humerus. It is one of the four rotator cuff muscles and also abducts the arm at the shoulder. The spine of the scapula separates the supraspinatus muscle from the infraspinatus muscle, which originates below the spine.
Contents
Origin
The supraspinatus muscle arises from the [[supraspinous fo], a shallow depression in the body of the scapular above its spine. The supraspinatus muscle tendon passes laterally beneath the cover of the acromion. Research in 1996 showed that the postero-lateral origin was more lateral than classically described.[1]
Insertion
The supraspinatus tendon is inserted into the most superior facet of the greater tubercle of the humerus.
The distal attachments of the three rotator cuff muscles that insert into the greater tubercle of the humerus can be abbreviated as SIT when viewed from superior to inferior (Supraspinatus, Infraspinatus, and Teres minor).
The acronym SITS regarding the rotator cuff muscles is completed by including the Subscapularis muscle, which unlike the other rotator cuff muscles attaches to the lesser tubercle of the humerus.[2]
Innervation
The supraspinatus muscle is supplied by the suprascapular nerve (C5 and C6), which arises from the superior trunk of the brachial plexus and passes laterally through the posterior triangle of the neck and through the scapular notch on the superior border of the scapula. After supplying fibers to the supraspinatus muscle, it supplies articular branches to the capsule of the shoulder joint.
This nerve can be damaged along its course in fractures of the overlying clavicle, which can reduce the person’s ability to initiate the abduction.
Action
Contraction of the supraspinatus muscle leads to abduction of the arm at the shoulder joint. It is the main agonist muscle for this movement during the first 10-15 degrees of its arc. Beyond 30 degrees the deltoid muscle becomes increasingly more effective at abducting the arm and becomes the main propagator of this action.
The supraspinatus muscle is one of the musculotendinous support structures called the rotator cuff that surround and enclose the shoulder. It helps to resist the inferior gravitational forces placed across the shoulder joint due to the downward pull from the weight of the upper limb.
The supraspinatus also helps to stabilize the shoulder joint by keeping the head of the humerus firmly pressed medially against the glenoid fossa of the scapula.
For USMLE step 1, know that the supraspinatus initiates abduction of the shoulder. Without a functioning supraspinatus, the physician must start abducting the patient's arm and eventually the patient will be able to finish abduction if the deltoid is functional, which is common because the supraspinatus is innervated by the suprascapular nerve from the superior/upper trunk of the brachial plexus. The deltoid is innervated more distally by the axillary nerve, which arises from the posterior cord of the brachial plexus.
Controversy about Action
A 1963 study, "Function of the Supraspinatus Muscle and its Relation to the Supraspinatus syndrome - An experimental in Man" by B Van Linge and J D Mulder in Leiden, Holland showed that the supraspinatus does not cause the abduction of the first 30 degrees, as widely accepted, but rather is a synergist muscle. This means that it assists the deltoid, but is not, by itself the abducting muscle. In this study, the supraspinatus was anaesthetised, and the deltoid muscle was still able to complete a full range of abduction, however was not able to sustain an isometric contraction for more than one minute. From this study, it is suggested that the "true" action of the supraspinatus is to hold the capsule in position to allow greater functional strength and stamina of the deltoid muscle.[3]
Additional images
- Shoulderjoint.PNG
Diagram of the human shoulder joint
- Gray203.png
Left scapula. Dorsal surface.
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Left humerus. Anterior view.
- Gray521.png
The scapular and circumflex arteries.
- Gray808.png
The right brachial plexus with its short branches, viewed from in front.
- Gray810.png
Suprascapular and axillary nerves of right side, seen from behind.
- Gray818.png
The suprascapular, axillary, and radial nerves.
References
- ↑ [1][2]
- ↑ Template:MedicalMnemonics
- ↑ Journal of Bone and Joint Surgery, Vol 45B, No. 4, November 1963, pp 750–754
External links
- Template:MuscleLoyola
- supraspinatus+muscle at eMedicine Dictionary
- GoogleBody - Supraspinatus muscle
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hr:Nadgrebeni mišić id:Otot supraspinatushe:השריר הרום קוצי la:Musculus supraspinatus hu:Tövis feletti izom nl:Musculus supraspinatus ja:棘上筋 pl:Mięsień nadgrzebieniowy pt:Músculo supraespinhal ru:Надостная мышца sl:Nadgrebenčnica sv:Supraspinatus