Interactive Shoulder Part 2 - Muscle Structure

EDITOR’S NOTE: The following is a small sample of the Interactive Shoulder CD in the Primal Pictures 3-D Anatomy CD ROM series.

Deltoids

• Proximal Attachments. The deltoid is a thick, powerful muscle that covers the shoulder joint and upper humerus. It arises from the anterior border of lateral third of clavicle, the lateral border of acromion and the lower lip of scapular spine, as well as from the fascia over infraspinatus muscle.
• Distal Attachments. It inserts on the V shaped deltoid tuberosity half way down the lateral aspect of the shaft of humerus.
• Nerve Supply. Deltoid is innervated by the axillary nerve (C 5 ,6).
• Actions. It is a powerful abductor of the humerus. Abduction is, however, initiated by supraspinatus. The anterior portion of deltoid contributes to flexion of the humerus with the posterior portion to extension. When supraspinatus is torn, the upward pull of deltoid results in superior subluxation of the humeral head. The acromial part of the deltoid is multipennate. Tendinous septa arising from the acromion and the deltoid tuberosity interdigitate, with short muscle fibers extending between the septa. This gives the muscle a short but powerful pull.

Latissimus Dorsi

Latissimus dorsi is a large flat muscle that passes between trunk and humerus, acting on both the shoulder joint and shoulder girdle.

• Proximal Attachments. It arises from spines and supraspinous ligaments of the lower six thoracic vertebrae deep to trapezius and from thoracolumbar fascia, by which it is attached to the spines of all lumbar and sacral vertebrae. In addition, it arises from posterior iliac crest and the lower three or four ribs. It passes upwards and laterally, gaining a small slip from the inferior angle of scapula.
• Distal Attachment. The muscle converges to form a flat "strap like" tendon, which winds around the inferior border of teres major to reach its humeral attachment in the floor of the intertubercular groove of the humerus. The lower border of the tendon unites with the tendon of teres major for a short distance. A bursa lies between the two.
• Nerve Supply. The thoracodorsal nerve (C 6 ,7 ,8).
• Actions. Latissimus dorsi retracts the shoulder girdle and is a powerful extensor of the flexed humerus (i.e., climbing). It also adducts and internally rotates the humerus. It is an accessory muscle of respiration, particularly violent expiration. Latissimus dorsi and teres major form the posterior fold of the axilla.

Trapezius

The trapezius is a broad flat triangular muscle that lies superficially at the back of neck and upper trunk, the pair forming the "trapezium" from which it derives its name.

• Proximal Attachments. It arises from the superior nuchal line of occipital bone, the external occipital protuberance, ligamentum nuchae, the spines of seventh cervical and all thoracic vertebrae and the intervening supraspinous ligaments.
• Distal Attachments. The upper fibers pass down and laterally to insert in the posterior aspect of lateral clavicle. The middle fibers pass horizontally to the medial border of acromion and upper lip of scapula spine. The lower fibers ascend to the tubercle of scapula spine. The upper fibers form the posterior border of the posterior triangle of the neck.
• Nerve Supply. Trapezius receives motor innervation from the spinal accessory nerve, which enters from the posterior triangle. Fibers from the ventral rami of C3 and C4 are thought to be sensory.
• Actions. The upper fibers elevate the scapula during such tasks as carrying a weight. The middle fibers produce bracing of the shoulder girdle. The lower fibers depress the medial aspect of the scapula and shoulder (i.e., when using the hands to rise from sitting). The action of upper and lower fibers together tend to rotate the scapula so that the glenoid fossa points upwards and forwards.