Comprehensive Anatomy of the Shoulder and Arm Muscles and Intermuscular Spaces

Overview of the Shoulder Region Muscles

The shoulder region is composed of a complex arrangement of six primary muscles that facilitate a wide range of motion at the glenohumeral joint. These muscles include the deltoid, the teres major, and the four muscles of the rotator cuff: the supraspinatus, the infraspinatus, the teres minor, and the subscapularis. Each of these muscles plays a specific role in provide stability to the joint and enabling movements such as abduction, rotation, flexion, and extension of the humerus.

Detailed Anatomy of the Deltoid Muscle

The deltoid muscle is the large, triangular muscle covering the shoulder joint. Its anatomy is typically divided into three distinct parts based on their specific points of origin. The anterior part, sometimes referred to as the clavicular part, originates from the lateral one-third of the clavicle. The acromial part, or the middle fibers, originates from the acromion of the scapula, specifically the lateral margin of the acromion. The spinal part, which forms the posterior portion of the muscle, originates from the crest of the spine of the scapula.

All three parts of the deltoid converge to insert into the deltoid tuberosity, which is located on the middle of the lateral aspect of the shaft of the humerus. The muscle is innervated by the axillary nerve, which carries fibers from the $C5$ and $C6$ spinal nerves. The action of the deltoid is multifaceted depending on which fibers are contracting. The anterior fibers are responsible for the flexion and medial rotation of the humerus. The middle fibers are the primary abductors of the arm, specifically facilitating abduction within the range of $8^\circ$ to $90^\circ$. The posterior fibers are responsible for the extension and lateral rotation of the humerus.

Anatomical Structures Situated Deep to the Deltoid

There are numerous anatomical structures located underneath the deltoid muscle, including bones, joints, ligaments, bursae, muscles, vessels, and nerves. The underlying bones include the upper end of the humerus and the coracoid process of the scapula. The primary joint protected by the deltoid is the shoulder or glenohumeral joint, and the significant ligament in this region is the coracoacromial ligament. Several bursae are situated around the shoulder joint to reduce friction, including the subscapular bursa, the subacromial or subdeltoid bursa, and the infraspinatus bursa.

A large number of muscle insertions and origins are found deep to the deltoid. Insertions include those of the pectoralis minor, pectoralis major, teres major, latissimus dorsi, subscapularis, supraspinatus, infraspinatus, and teres minor. Origins found in this area include the long head of the biceps, the short head of the biceps, the coracobrachialis, and both the long and lateral heads of the triceps. The vascular supply in this deep region is provided by the anterior and posterior circumflex humeral vessels. The primary nerve found here is the axillary nerve. Furthermore, the quadrangular and triangular subscapular intermuscular spaces are also located beneath the deltoid coverage.

Teres Major and the Rotator Cuff Complex

The teres major muscle originates from the lateral border of the scapula. It inserts into the bicipital groove of the humerus, and its insertion is notes as being similar in clinical relevance to the insertions of the latissimus dorsi and pectoralis major. The teres major is innervated by the lower subscapular nerve. Its primary actions include the extension, adduction, and medial rotation of the humerus, paralleling the actions of the latissimus dorsi.

The rotator cuff comprises four muscles that wrap around the humeral head to stabilize the shoulder. These include the supraspinatus, which elevates the shoulder joint out to the side; the infraspinatus, which externally rotates the shoulder joint; the teres minor, which also externally rotates the shoulder joint; and the subscapularis, which allows the humerus to move freely during the elevation of the arm. These muscles are essential for maintaining the integrity of the glenohumeral joint during movement.

Muscles of the Arm: Anterior Compartment

The arm is defined as the anatomical region between the shoulder and the elbow. It is composed of four muscles, with three located in the anterior compartment and one located in the posterior compartment. The anterior compartment muscles include the brachialis, the coracobrachialis, and the biceps brachii.

The brachialis muscle originates from the anterior surface of the humerus, specifically the distal half of the bone. It inserts into the coronoid process and the tuberosity of the ulna. It is primarily innervated by the musculocutaneous nerve ($C5$ and $C6$), though a small lateral part of the muscle receives innervation from the radial nerve ($C7$). Its primary action is flexion at the elbow joint.

The biceps brachii has two heads of origin. The short head originates from the tip of the coracoid process of the scapula, while the long head originates from the supraglenoid tubercle of the scapula. The muscle inserts into the tuberosity of the radius and the fascia of the forearm via the bicipital aponeurosis. Its actions are to supinate the forearm and, when the forearm is supine, to flex the forearm. It is innervated by the musculocutaneous nerve ($C5$ and $C6$).

The coracobrachialis muscle originates from the coracoid process of the scapula and inserts into the medial surface or mid-shaft of the humerus. It is innervated by the musculocutaneous nerve and acts to flex or adduct the shoulder joint. A clinical symptom of a strained coracobrachialis is tenderness in the upper arm.

Triceps Brachii: The Posterior Compartment

The posterior compartment of the arm contains the triceps brachii muscle, which consists of three heads. The long head originates from the infraglenoid tubercle of the scapula. The lateral head originates from the proximal portion of the posterior surface of the humerus. The medial head originates from the distal half of the posterior humerus. All three heads converge to insert into the olecranon process of the ulna. The primary action of the triceps brachii is the extension of both the humerus and the ulna.

Intermuscular Spaces of the Shoulder and Arm

There are several critical intermuscular spaces in the shoulder and arm through which vital nerves and vessels pass. The quadrangular space is bounded by the surgical neck of the humerus, the long head of the triceps brachii, the teres minor, and the teres major. The structures passing through the quadrangular space include the axillary nerve and the posterior circumflex humeral artery and vein. This space is often highlighted in the context of the nerve to the teres minor and the superior and inferior branches of the axillary nerve, including its cutaneous branches.

The triangular space is another important region, where the circumflex scapular artery can be found. Additionally, the triangular interval is an area through which the radial nerve and the deep brachial artery (profunda brachii) pass, located between the long and lateral heads of the triceps and the teres major muscle. Other structures in the general vicinity of these spaces include the suprascapular artery and suprascapular nerve, which pass through or near the suprascapular notch near the supraspinatus and infraspinatus muscles.

Clinical Correlation and Testing

Understanding the anatomy of the arm and shoulder is vital for clinical diagnosis. Conditions such as rotator cuff tendonitis involve inflammation of the tendons of the SIT (Supraspinatus, Infraspinatus, Teres minor) and subscapularis muscles. Reflex testing is used to assess spinal nerve integrity: the biceps reflex is used for testing the $C6$ spinal nerve, while the triceps reflex is used to test the $C7$ spinal nerve. Other clinical issues include biceps tendon rupture and the practice of steroid injections in bodybuilders, which often involve the large muscle masses of the arm or deltoid.