Scalene muscles

Origin and insertion

The scalenus anterior muscle is the anteriormost of the three scalene muscles. It originates from the anterior tubercles of transverse processes of the vertebrae C3-C6. The muscle takes an inferior, almost vertical, course towards the thoracic cage. It gives off a single flat tendon, that inserts onto the scalene tubercle and superior border of first rib, just anterior to the groove for subclavian artery.

The scalenus anterior muscle sometimes gives off a few muscular slips that insert into the suprapleural membrane. These slips are known as the scalenus minimus (pleuralis) muscle, or the Albinus muscle or Sibson muscle. It is a normal anatomical variant present within 7-46% of people. When present, the scalenus minimus muscle arises from the anterior border of the transverse process of C7 vertebra. It inserts onto the inner border of the first rib behind the groove for subclavian artery, with a few slips also attaching to the pleural dome. Due to its pleural attachment, it is believed that this muscle tenses the pleural dome.

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Relations

The scalenus anterior muscle is found in the lateral part of the neck, anterior to the subclavian artery and posterior to the phrenic nerve. It is related to many structures of the neck;

• Posteriorly, it is related to the suprapleural membrane, pleura, roots of the brachial plexus and the subclavian artery.
• Anteriorly to the muscle are the:
  • Clavicle, subclavius, sternocleidomastoid and omohyoid muscle
  • Lateral part of the carotid sheath, transverse cervical, suprascapular and ascending cervical arteries, subclavian vein, prevertebral fascia
  • C3 to C5 spinal roots which form the phrenic nerve right on the anterior surface of the scalenus anterior.

The scalenus anterior muscle forms the anterior border of the interscalene (scalene) triangle. The posterior border of this space is formed by the scalenus medius, while the inferior border is comprised by the first rib. This space is important as it is traversed by the trunks of the brachial plexus and the third segment of the subclavian artery.

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Innervation

The scalenus anterior receives its nerve supply from the anterior rami of the spinal nerves C4-C6.

Blood supply

The blood supply to all scalene muscles comes from the ascending cervical branch of inferior thyroid artery. The inferior thyroid artery itself is an indirect branch of the subclavian artery via the thyrocervical trunk.

Function

The scalenus anterior muscle acts on the joints between the cervical vertebrae. Depending on the position of the head and the type of muscle contraction, the scalenus anterior can produce the following movements:

• When the costal attachment is fixed, bilateral contraction of the muscles causes the flexion of the neck.
• When the head is the anatomical position (upright and facing anteriorly), unilateral contraction of the muscle causes ipsilateral flexion of the neck (to the same side).

When their attachment on the vertebral column is fixed, the scalenus anterior muscles have the ability to fix or lift the first rib superiorly. Combined with the contraction of the external intercostal muscles which elevate the ribs 2-12, this action of scalenus anterior expands the antero-posterior diameter of the thorax. However, the muscle acts this way only during forced respiration, which makes it an accessory respiratory muscle, rather than a primary respiratory muscle.

Origin and insertion

The scalenus medius muscle is the largest and the longest of the scalenes. It originates from the transverse processes of axis (C2), transverse processes of atlas (C1), and the posterior tubercles of transverse processes of the vertebrae C3-C7.

The muscle fibres extend posterolaterally to form a tendon, which inserts into the superior border of the first rib. The insertion site is located posterior to the groove for subclavian artery and anterior to the tubercle of first rib.

Relations

The anterolateral surface of the scalenus medius is covered by the sternocleidomastoid muscle. The lower half of the muscle is crossed by the omohyoid muscle, clavicle, subclavian artery and anterior rami of the cervical spinal nerves. The anterior surface of the muscle is pierced by the dorsal scapular nerve and the nerve to serratus anterior muscle.

These nerves emerge on the lateral surface of the muscle and continue towards the rhomboids and serratus anterior, which they supply, respectively. The posterolateral surface of the muscle is related to the levator scapulae and the scalenus posterior muscle.

Innervation

The innervation to the scalenus medius is delivered by the anterior rami of cervical spinal nerves C3-C8.

Blood supply

Just like the scalenus anterior, the scalenus medius receives its blood supply from the ascending cervical branch of the inferior thyroid artery.

Function

The contraction of the scalenus medius muscle produces strong ipsilateral flexion of the neck. In the same manner, as the scalenus anterior, this muscle helps to stabilize or raise the 1st rib during respiration, when its vertebral attachment is fixed.

Origin and insertion

The scalenus posterior is the smallest of the scalene muscles. Its fibers originate from the posterior tubercles of transverse processes of cervical vertebrae C5-C7.

The muscle extends posterolaterally and tapers into a thin tendon, which inserts into the external surface of the 2nd rib. The site of insertion is located posterior to the attachment of the serratus anterior muscle. Sometimes the fibres of the scalenus posterior can be blended with the fibres of the scalenus medius.

Innervation

The scalenus posterior muscle is innervated by the anterior rami of spinal nerves C6-C8.

Blood supply

Just like the other scalenes, scalenus posterior receives its vascular supply from the ascending cervical branch of the inferior thyroid artery. In addition, it receives some blood from the superficial branch of the transverse cervical artery, a branch of the thyrocervical trunk.

Function

When the costal attachment of the scalenus posterior muscle is fixed, the unilateral contraction of the muscle results in the ipsilateral lateral flexion of the neck.

When the vertebral attachment is fixed, the muscle helps stabilize or elevate the 2nd rib during respiration.

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