Anterior Scalene Muscle

Overview

The anterior scalene muscle is one of the three paired scalene muscles located in the lateral part of the neck. It plays a vital role in both respiration and movement of the cervical spine. This muscle acts as an accessory muscle of inspiration by elevating the first rib and also contributes to flexion and lateral bending of the neck. Its anatomical position, lying close to important neurovascular structures such as the brachial plexus and subclavian artery, makes it clinically significant in various thoracic outlet and neck pathologies.

Location

The anterior scalene muscle is located in the lateral cervical region of the neck, deep to the sternocleidomastoid muscle. It originates from the anterior tubercles of the transverse processes of cervical vertebrae C3 to C6 and descends obliquely downward and laterally to insert onto the scalene tubercle on the superior surface of the first rib. It lies anterior to the middle scalene and posterior to the sternocleidomastoid. The subclavian vein passes anterior to it, while the subclavian artery and brachial plexus pass posteriorly between the anterior and middle scalene muscles — a region known as the interscalene triangle.

Structure

The anterior scalene is a long, slender, strap-like muscle composed primarily of skeletal muscle fibers. Its detailed structural features include:

• Origin: Anterior tubercles of the transverse processes of the third to sixth cervical vertebrae (C3–C6).
• Insertion: Scalene tubercle on the inner border of the first rib and the ridge anterior to the groove for the subclavian artery.
• Shape: Narrow at the top, broadens as it descends to attach to the rib.
• Relations:
  • Anteriorly: Covered by the sternocleidomastoid and crossed by the phrenic nerve, subclavian vein, and transverse cervical and suprascapular arteries.
  • Posteriorly: Related to the brachial plexus and subclavian artery.
  • Medially: Adjacent to the longus colli muscle and vertebral artery.
  • Laterally: Borders the middle scalene muscle, with the interscalene space between them.
• Innervation: Direct muscular branches from the anterior rami of cervical spinal nerves C4 to C6.
• Blood supply: Supplied by branches of the ascending cervical artery (a branch of the inferior thyroid artery) and small twigs from the thyrocervical trunk.

Function

The anterior scalene muscle contributes to both respiratory and postural functions. Its actions include:

• Elevation of the first rib: During inspiration, it elevates the first rib, assisting in expanding the thoracic cavity. This is its primary role as an accessory muscle of inspiration.
• Neck flexion: Acting bilaterally, it flexes the cervical spine, bringing the chin toward the chest.
• Lateral flexion: Acting unilaterally, it bends the neck to the same side (ipsilateral lateral flexion).
• Stabilization: Helps stabilize the cervical spine during head and neck movements.

Physiological Role(s)

• Respiratory assistance: Plays a key role as an accessory inspiratory muscle, especially during deep or labored breathing, such as in exercise or respiratory distress.
• Postural support: Maintains cervical spine alignment and posture by working synergistically with other neck muscles.
• Neurovascular protection: Its position provides a protective muscular boundary for the subclavian artery and brachial plexus as they pass into the upper limb.
• Movement coordination: Functions in coordination with the middle and posterior scalene muscles to produce smooth neck and upper thoracic motion.

Clinical Significance

• Scalene syndrome / Thoracic outlet syndrome (TOS): Compression of the subclavian artery or brachial plexus between the anterior and middle scalene muscles can result in pain, numbness, and weakness in the upper limb. This may be due to hypertrophy, fibrotic changes, or anatomical variations of the scalenes.
• Phrenic nerve irritation: The phrenic nerve crosses the anterior surface of the anterior scalene; muscle spasm or inflammation can irritate the nerve, leading to referred pain or hiccups.
• Trigger points and myofascial pain: Chronic strain or poor posture can cause tightness and trigger points in the anterior scalene, leading to neck stiffness and radiating shoulder or arm pain.
• Anesthetic landmark: The anterior scalene is an important anatomical landmark for performing interscalene brachial plexus blocks used in regional anesthesia for shoulder and upper limb surgeries.
• Accessory breathing muscle overuse: In patients with chronic obstructive pulmonary disease (COPD) or respiratory distress, the anterior scalene may become overactive and hypertrophied due to constant recruitment for breathing support.