Arm Abductor Muscles – Anatomy, Nerve Supply, Function

Structure of Arm Abductor Muscles

The ability to abduct the arm is a crucial contributor to the full range of motion of the arm. Four different muscles control this action: supraspinatus, deltoid, trapezius, and serratus anterior. The supraspinatus is the primary muscle for the abduction of the arm to 15 degrees. The deltoid controls abduction from 15 to 90 degrees. The trapezius and serratus anterior coordinate with each other and the scapula to facilitate abduction of the arm upwards of 90 degrees.[1]

Blood Supply of Arm Abductor Muscles

Blood supply of the supraspinatus

• The suprascapular artery delivers blood to the supraspinatus muscle.

Blood supply of the deltoid

• The posterior circumflex humeral artery and the deltoid branch of the thoracoacromial artery are the vascular sources for the deltoid.

Blood supply of the trapezius

• The transverse cervical? artery provides vascular supply to the trapezius.

Blood supply of the serratus anterior

• The circumflex scapular artery is the blood supply to the serratus anterior.

Nerves of Arm Abductor Muscles

Innervation of the supraspinatus

• The neural supply of the supraspinatus is by the suprascapular nerve (C5, C6) from the upper trunk of the brachial plexus.

Innervation of the deltoid

• The neural supply of the deltoid is via the axillary nerve (C5, C6) from the posterior cord of the brachial plexus.

Innervation of the trapezius

• The neural supply of the trapezius is the spinal accessory nerve (C1-C5). C3 and C4 are responsible for proprioception of the trapezius.

Innervation of the serratus anterior

• The neural supply of the serratus anterior is the long thoracic? nerve (C5-C7) which originates from the roots of the brachial plexus.

Muscles of Arm Abductor Muscles

Supraspinatus

The supraspinatus muscle originates from the supraspinous fossa of the scapula, passes under the acromion, and inserts on the superior facet of the greater tubercle of the humerus. It is responsible for the initiation of arm abduction and is in control of the motion up to the first 15 degrees of abduction. Past 15 degrees, it assists the deltoid with the abduction of the arm up to 90 degrees. Additionally, the supraspinatus contributes to shoulder joint stability by providing resistance to gravitational forces acting on the joint and maintaining contact between the head of the humerus and the glenoid fossa.

Deltoid

The deltoid, aptly named after the Greek letter delta, is a triangular-shaped muscle found over the glenohumeral joint and is composed of three different heads: anterior, lateral, and posterior. The anterior head originates from the anterior surface of the lateral third of the clavicle. The lateral head originates from the superior surface of the acromion process. The posterior head originates from the posterior border of the spine of the scapula. All heads of the deltoid come together to insert on the deltoid tuberosity of the humerus. The deltoid is the primary muscle responsible for the abduction of the arm from 15 to 90 degrees. It also serves as a stabilizer of the humeral head, especially in instances of load carrying.[2]

Trapezius and Serratus Anterior

The trapezius is a large, superficial muscle of the back that divides into three functional parts: descending (superior), middle, and ascending (inferior). The superior fibers of the trapezius originate from the medial third of the superior nuchal line, spinous process of C7, external occipital protuberance, and nuchal ligament?; they converge and insert on the posterior portion of the lateral third of the clavicle. The middle fibers also originate from the spinous process of C7, as well as the spinous processes of T1-T3, and insert upon acromion and the spine of the scapula. The inferior fibers originate from the spinous processes of T4-T12 and converge near the scapula in the form of an aponeurosis. The trapezius functions to laterally rotate, elevate, and retract the scapula. It can also function to extend and laterally flex the neck if the scapula is kept in a fixed position.

The serratus anterior is a saw-shaped muscle originating from the upper eight ribs and inserting upon the inner medial border of the scapula. Its primary function is to laterally rotate and protract the scapula.

The trapezius and serratus anterior muscles work in tandem to coordinate rotation and movement of the scapula to accommodate the full range of motion of the arm. Specifically, they facilitate abduction of the arm from 90 degrees and further upwards.