The Shoulder Joint – Anatomy, Nerve Supply, Function

The shoulder joint, also known as the glenohumeral joint, is a ball and socket joint with the most extensive range of motion in the human body. The muscles of the shoulder have a wide range of functions, including abduction, adduction, flexion, extension, internal and external rotation. [rx] The central bony structure of the shoulder is the scapula, where all of the muscles interact. At the lateral aspect of the scapula is the articular surface of the glenohumeral joint, the glenoid cavity.  The glenoid cavity is peripherally surrounded and reinforced by the glenoid labrum, shoulder joint capsule, supporting ligaments, and the myotendinous attachments of the rotator cuff muscles.  The muscles of the shoulder play a critical role in providing stability to the shoulder joint.  The primary muscle group that supports the shoulder joint is the rotator cuff muscles.  The four rotator cuff muscles include the supraspinatus, infraspinatus, teres minor, and subscapularis. [rx][rx][rx]

Structure of The Shoulder Joint

The upper extremity is attached to the axial skeleton by way of the sternoclavicular joint.  The three joints of the pectoral girdle are the sternoclavicular joint, coracoclavicular joint, and the acromioclavicular joint.  The bones of the pectoral girdle are the clavicle, scapula, and humerus.  The clavicle is positioned immediately superior to the first rib.  The distal aspect of the clavicle articulates with the acromial process and coracoid process of the scapula, forming the acromioclavicular joint and coracoclavicular joints, respectively.  The most important structural ligaments of the shoulder joint are the glenohumeral ligaments and the coracoacromial ligament.[rx]

The scapula is a flat bone with multiple muscular attachments.  The glenoid fossa serves the articulating function with the humeral head at the lateral angle of the scapula.  The glenohumeral joint is the point of articulation between the humerus and the scapula and thoracic cavity, with the latter occurring through the scapulothoracic articulation.  The scapula connects to the clavicle via the coracoclavicular joint and the acromioclavicular joint.

The coracoclavicular joint is strengthened by the coracoclavicular ligament that unites the undersurface of the clavicle to the coracoid process of the scapula.  The acromioclavicular joint is at the lateral aspect of the clavicle and does not provide much structural support to the shoulder joint.  The coracoid process, the acromion process, and coracoacromial ligament provide peripheral reinforcement for the shoulder joint along with the muscles of the shoulder.  The shoulder muscles and peripheral structures of the shoulder function to increase the structural integrity of the shoulder joint.[rx]

Blood Supply of The Shoulder Joint

The arterial supply to the upper extremity originates from the subclavian artery.  These vessels exist on both sides of the body to provide upper extremity blood supply.  Both arteries receive their blood supply from the arch of the aorta.  On both sides of the body, the subclavian artery branches include the vertebral artery, internal thoracic artery, thyrocervical trunk, and dorsal scapular artery.

The subclavian artery becomes the axillary artery once it reaches the lateral border of the first rib.

There are three parts to the axillary artery, with each portion having arterial branches to supply the muscles of the shoulder.  Multiple arteries branch from the axillary artery including the superior thoracic artery, thoracoacromial artery, circumflex humeral artery, and the lateral thoracic artery.  The subscapular artery is a division of the third part of the axillary artery.  The subscapular artery gives off the circumflex scapular artery and the thoracodorsal artery.  In general, the muscles of the shoulder receive vascular supply by named arteries associated with the muscles that they supply.[rx]

Efferent lymphatic vessels arise from the distal upper extremity and pass through the shoulder.  Additionally, axillary lymph nodes contribute to efferent lymphatic vessels in the region of the shoulder and pass proximally through the shoulder.  Deep lymphatic vessels accompany superficial lymphatic vessels.  The deep lymphatic vessels drain lymph from the joint capsule, tendons, and nerves.  The lymphatics of the shoulder and axillary region are drained by the subclavian lymphatic trunk.  On the right, the subclavian trunk drains into the right lymphatic duct.  On the left, the subclavian trunk drains into the thoracic duct.[rx]

Nerves Supply of The Shoulder Joint

The upper and lower branches of the subscapular nerve innervate the subscapularis muscle.  The suprascapular nerve innervates the supraspinatus and infraspinatus muscles. The posterior branch of the axillary nerve supplies the teres minor.  The axillary nerve also innervates the deltoid muscle.  The nerve supply to the trapezius is by the spinal accessory nerve/11th cranial nerve with some direct branches from cervical plexus.  Innervation to levator scapula is by C3-C5.  The nerve supply to the rhomboids is the dorsal scapular nerve.  The nerve supply to serratus anterior is the long thoracic nerve. The pectoralis major muscle receives its nerve supply via the medial and lateral pectoral nerves.[rx]

The Major Movements At The Glenohumeral Joint 

• Abduction – upward lateral movement of humerus out to the side, away from the body, in the plane of the scapula
• Adduction – downward movement of humerus medially toward the body from abduction, in the plane of the scapula
• Flexion – the movement of humerus straight anteriorly
• Extension – the movement of humerus straight posteriorly
• External rotation – the movement of humerus laterally around its long axis away from the midline
• Internal rotation – the movement of humerus medially around its long axis toward the midline
• Horizontal adduction (transverse flexion) – the movement of the humerus in a horizontal or transverse plane toward and across the chest
• Horizontal abduction (transverse extension) – the movement of the humerus in a horizontal or transverse plane away from the chest

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