Deep Back Muscles: Anatomy and Function

• Deep muscles of the back are also known as intrinsic back muscles or erector spinae.
• There are three groups of back muscles: extrinsic, intermediate, and deep.
• Extrinsic back muscles are superficial and include the trapezius, latissimus dorsi, levator scapula, and rhomboids.
• Intermediate back muscles include the serratus posterior superior and inferior.
• Deep muscles are intrinsic back muscles.
• Intrinsic back muscles maintain posture and facilitate vertebral column movements.
• These muscles are innervated by the dorsal rami of spinal nerves and enclosed by deep fascia.
• Intrinsic back muscles are organized into superficial, intermediate, and deep layers.

Superficial Layer

• Includes the splenius capitis and splenius cervicis muscles.
• Acting alone, these muscles laterally bend and rotate the head to the same side.
• Working together, these muscles extend the neck.

Intermediate Layer

• Arranged in three groups from lateral to medial:
• Iliocostalis muscles (lumborum, thoracis, and cervicis).
• Longissimus muscles (thoracis, cervicis, and capitis).
• Spinalis muscles (thoracis, cervicis, and capitis).
• Acting alone, these muscles laterally bend the vertebral column.
• Acting together, these muscles extend the vertebral column and head.

Deep Layer

• Arranged in three groups from superficial to deep includes:
• Semispinalis muscles (capitis, cervicis, and thoracis).
• Multifidius muscles.
• Rotatores muscles.
• Extend the head and vertebral column in addition to rotating the vertebral column to the opposite side.
• Minor deep layer muscles include interspinal, intertransverse, and levatores costarum muscles.

Deep Muscles of the Neck

• Two muscle groups at the root of the neck is known as prevertebral muscles.
• These muscles lie posterior to the prevertebral fascia.
• Include anterior and lateral vertebral muscles.

Anterior Vertebral Muscles

• Includes muscles such as longus capitis, longus colli, rectus capitis anterior, and rectus capitis lateralis.

Lateral Vertebral Muscles

• Includes the Splenius capitis (considered in the back)
• Includes the Levator scapulae (considered in the upper limb)
• Includes the Anterior, middle and posterior scalene muscles

Longus Capitis Muscle

• Innervated by the ventral rami of C1-C3 spinal nerves.
• Flexes the head with anterior bending relative to atlanto-occipital joints.

Longus Colli Muscle

• Innervated by the ventral rami of C2-C6 spinal nerves.
• Flexes the neck with anterior bending of the cervical vertebrae C2-C7.
• Acting unilaterally, rotates the head to the opposite side at the atlanto-axial joint.

Rectus Capitis Anterior

• Innervated by branches from C1 and C2 spinal nerves.
• Flexes the head.

Rectus Capitis Lateralis

• Innervated by branches from C1 and C2 spinal nerves.
• Flexes the head and helps stabilize the atlanto-occipital joint.

Anterior Scalene Muscle

• Innervated by C4-C6 spinal nerves.
• Elevates the first rib, flexes, and laterally rotates the neck.

Middle Scalene Muscle

• Innervated by the ventral rami of the C3-C8 spinal nerves.
• Elevates the first rib during forced inspiration and bends the neck laterally.

Posterior Scalene Muscle

• Innervated by the ventral rami of the C7 and C8 spinal nerves.
• Elevates the second rib during forced inspiration and bends the neck laterally.
• During inspiration, scalene muscles elevate the first and second ribs increasing the thorax's volume.
• Scalene muscles are classified as accessory muscles of respiration.
• Two things that pass between the anterior and middle scalene muscles are the subclavian artery and the trunks of the brachial plexus.
• The phrenic nerve may also pass between these muscles.

Suboccipital Region

• The suboccipital region lies on the upper part of the posterior aspect of the neck.
• It extends between the occipital bone and the first two cervical vertebrae.

Muscles of the Suboccipital Region

• The rectus capitis posterior major muscle extends from the spinous process of the axis to the lateral part of the inferior nuchal line.
• The rectus capitis posterior minor muscle extends from the posterior tubercle of the atlas to the lateral part of the inferior nuchal line.
• The inferior oblique muscle of the head (obliquus capitis inferior muscle) extends from the spinous process of the axis to the transverse process of the atlas.
• The superior oblique muscle of the head (obliquus capitis superior muscle) extends from the transverse process of the atlas to the upper part of the inferior nuchal line.
• The above muscles are innervated by the suboccipital nerve (dorsal ramus of the C1 spinal nerve).
• These muscles function in extending or rotating the head.

Suboccipital Triangle

• Its borders are:
  • Superomedial: rectus capitis posterior major
  • Superolateral: obliquus capitis superior muscle
  • Inferolateral: obliquus capitis inferior muscle
  • Floor: posterior atlanto-occipital membrane
  • Roof: semispinalis capitis muscle
• Contains the vertebral artery and the suboccipital nerve.