Learning objective

By the end of this lesson, the student should be able to identify the scalene muscles, describe their attachments, actions, and innervation, and explain their key anatomical relationships and clinical relevance, particularly in respiration and regional anaesthesia.

The scalene muscles are three paired muscles—anterior, middle, and posterior—located on the lateral aspect of the neck. They form part of the floor of the posterior triangle of the neck and play an important role in neck movement and respiration.

Collectively, the scalenes act as accessory muscles of respiration, elevating the upper ribs during forced inspiration, and contribute to flexion and lateral flexion of the cervical spine.

Anterior scalene

The anterior scalene lies deep to the sternocleidomastoid muscle on the lateral side of the neck.

Attachments:
Originates from the anterior tubercles of the transverse processes of C3–C6 and inserts onto the scalene tubercle of the first rib.
Actions:
Elevates the first rib; unilateral contraction causes ipsilateral lateral flexion of the neck, while bilateral contraction produces neck flexion.
Innervation:
Anterior rami of C5–C6.

Middle scalene

The middle scalene is the largest and longest of the scalene muscles. It consists of multiple slips arising from the cervical spine that converge into a single muscle belly.

Attachments:
Originates from the posterior tubercles of the transverse processes of C2–C7 and inserts onto the superior surface of the first rib.
Actions:
Elevates the first rib and produces ipsilateral lateral flexion of the neck.
Innervation:
Anterior rami of C3–C8.

Posterior scalene

The posterior scalene is the smallest and deepest of the three muscles and is unique in inserting into the second rib.

Attachments:
Originates from the posterior tubercles of the transverse processes of C5–C7 and inserts onto the second rib.
Actions:
Elevates the second rib and assists with ipsilateral lateral flexion of the neck.
Innervation:
Anterior rami of C6–C8.

Anatomical relationships

The scalene muscles are closely related to several vital neurovascular structures:

The brachial plexus and subclavian artery pass between the anterior and middle scalene muscles.
The subclavian vein runs anterior to the anterior scalene, crossing it horizontally.
The phrenic nerve descends vertically on the anterior surface of the anterior scalene.

These relationships make the scalene muscles essential landmarks in both anatomy and clinical practice.

Clinical relevance

Interscalene block

Because the brachial plexus passes between the anterior and middle scalene muscles, this space is used to perform an interscalene block for upper limb surgery. Local anaesthetic is injected between these muscles, typically at the level of the cricoid cartilage, to provide regional anaesthesia.

Accessory muscles of respiration

The scalene muscles elevate the first and second ribs, increasing intrathoracic volume during inspiration. In healthy individuals, they are not required for normal breathing. However, visible or palpable activation of the scalenes is an important clinical sign of respiratory distress, indicating increased work of breathing.