The scalene muscles are a group of three pairs of muscles located on the lateral aspect of the neck: the anterior scalene, middle scalene, and posterior scalene. These muscles are innervated by the anterior rami of the fourth, fifth, and sixth cervical spinal nerves (C4–C6).

Functionally, the scalene muscles are integral to the biomechanics of the neck and upper thoracic region, as several major neurovascular structures traverse or are positioned in relation to them. Specifically, the brachial plexus and subclavian artery pass between the anterior and middle scalene muscles, while the subclavian vein and the phrenic nerve pass anterior to the anterior scalene muscle.

Anterior Scalene Muscle

The anterior scalene muscle lies on the lateral aspect of the neck, deep to the sternocleidomastoid. It is the most anterior of the scalene group.

• Origin: Anterior tubercles of the transverse processes of vertebrae C3–C6.
• Insertion: Anterior scalene tubercle of rib 1 and the superior border of rib 1, anterior to the subclavian groove.
• Innervation: Anterior rami of spinal nerves C4–C6.
• Blood Supply: Ascending cervical branch of the inferior thyroid artery.

Function:

• Bilateral contraction: Neck flexion.
• Unilateral contraction:
  
  • Ipsilateral lateral flexion of the neck.
  • Contralateral rotation of the neck.
• Accessory respiration: Stabilizes or elevates the first rib during forced inspiration.

Middle Scalene Muscle

The middle scalene is the largest and longest of the scalene muscles. It has multiple thin muscle bellies arising from the cervical spine that converge into one large belly, which inserts into the first rib.

• Origin: Posterior tubercles of the transverse processes of vertebrae C2–C7.
• Insertion: Superior border of rib 1, posterior to the subclavian groove.
• Innervation: Anterior rami of spinal nerves C3–C8.
• Blood Supply: Ascending cervical branch of the inferior thyroid artery.

Function:

• Unilateral contraction:
  
  • Ipsilateral lateral flexion of the neck.
• Accessory respiration: Stabilizes or elevates the first rib during forced inspiration.

The dorsal scapular nerve and the nerve to the serratus anterior pierce the anterior surface of this muscle.

Posterior Scalene Muscle

The posterior scalene is the smallest and deepest of the scalene muscles. Unlike the anterior and middle scalene, it inserts into the second rib.

• Origin: Posterior tubercles of the transverse processes of vertebrae C4–C6.
• Insertion: External surface of rib 2.
• Innervation: Anterior rami of spinal nerves C6–C8.
• Blood Supply:
  
  • Ascending cervical branch of the inferior thyroid artery.
  • Superficial cervical artery.

Function:

• Unilateral contraction:
  
  • Ipsilateral lateral flexion of the neck.
• Accessory respiration: Stabilizes or elevates the second rib during forced inspiration.

The scalene muscles play important roles in neck movements, posture, respiratory mechanics, and their interaction with surrounding anatomical structures. Their functions can be categorized into biomechanical, respiratory, and neurological aspects, as well as their synergy with other muscles.

Biomechanics aspect

The scalene muscles are essential for both dynamic and static stability of the cervical spine. They facilitate:

Neck flexion and rotation:

• • Bilateral contraction results in neck flexion.
  • Unilateral contraction causes ipsilateral lateral flexion and contralateral rotation.

Postural alignment: These muscles help counterbalance the weight of the head  and optimize  correct posture.

Dynamic stability: The scalenes work with surrounding muscles to stabilize the neck during complex or repetitive movements, such as turning the head or performing tasks requiring precise control of the cervical spine.

Respiratory Functions

The scalene muscles serve as accessory muscles of respiration, particularly during periods of increased respiratory demand:

• During forced inspiration, they elevate the first and second ribs, contributing to the expansion of the thorax and increasing intrathoracic volume.
• They act in coordination with the diaphragm and external intercostals, especially when the diaphragm is fatigued or compromised, as in conditions like asthma or chronic obstructive pulmonary disease (COPD).
• By assisting in thoracic expansion, the scalene muscles improve ventilation and compensate for respiratory distress.

Neurological Implications

The scalene muscles’ proximity to critical neurovascular structures makes their function and dysfunction significant in clinical settings:

• Brachial plexus compression: Overuse or hypertrophy of the scalene muscles can lead to thoracic outlet syndrome, causing symptoms such as radiating pain, numbness, or weakness in the upper extremities.
• Phrenic nerve irritation: The anterior scalene muscle lies adjacent to the phrenic nerve, which can become irritated and affect diaphragmatic movement, compromising respiration.

Synergy with Other Muscles

The scalene muscles do not work in isolation but instead collaborate with other muscles to perform various functions:

• Neck movements: They synergize with the sternocleidomastoid to produce coordinated flexion and rotation of the neck.
• Cervical spine stabilization: Together with muscles like the trapezius, levator scapulae, and deep cervical flexors, the scalene muscles maintain cervical spine stability during movements and static posture.
• Respiratory coordination: The scalenes enhance thoracic expansion in collaboration with the external intercostals during forced inspiration, particularly under conditions of high respiratory demand.

Clinical Relevance

• In cases of respiratory distress, the scalene muscles become visibly active, emphasizing their role as accessory respiratory muscles.
• Tightness or dysfunction in these muscles can also contribute to postural imbalances and neck pain, impacting overall cervical spine mechanics and respiratory efficiency.