scalenus

Scalenus, or scalene muscles, are a group of three paired muscles located on the lateral aspect of the neck. The three muscles are the anterior scalene (scalenus anterior), the middle scalene (scalenus medius), and the posterior scalene (scalenus posterior). They are traditionally described as accessory muscles of respiration and as dynamic elements of neck movement.

All three scalene muscles originate from the transverse processes of the cervical vertebrae. The anterior scalene typically originates from the transverse processes of C3 to C6 and inserts on the first rib at the scalene tubercle. The middle scalene originates from various cervical transverse processes and also inserts on the first rib. The posterior scalene originates from the lower cervical transverse processes and inserts on the second rib. Collectively, they form a muscular partition between the neck and the upper thorax.

The scalene muscles elevate the first and second ribs during deep inspiration, aiding ventilation. They also function to flex and laterally bend the neck. Innervation is via branches of the ventral rami of cervical nerves, with individual patterns varying among the three muscles.

Relations to neurovascular structures are clinically important. The brachial plexus lies between the anterior and middle scalene muscles, while the subclavian artery passes in relation to the scalenes (being divided into parts by the anterior scalene). The subclavian vein lies anterior to the anterior scalene. The scalene muscles are involved in thoracic outlet syndrome when hypertrophied or when extra ribs or anomalies compress nearby structures.

Clinically, they are relevant in interscalene nerve blocks for shoulder and upper limb anesthesia and can be implicated in scalene-related pain or vascular symptoms.