The intrinsic back muscles are part of the trunk musculature. It comprises a system of deep back muscles at the vertebral column ranging from the pelvis up to the head. They are located in an osteofibrous canal, bounded by the vertebrae, the ribs and the thoracolumbar fascia. All intrinsic back muscles are innervated by the posterior branches of the spinal nerves. Embryologically they derive from the dorsal part of the myotomes (epimere) which reside locally during the entire development for which reason they are also referred to as true or autochthonous back muscles. In contrast the back muscles deriving from the ventral part of the myotomes (hypomere), e.g. latissimus dorsi muscle and trapezius muscle, migrate into the trunk musculature subsequently (secondary back muscles).
The intrinsic back musculature is subdivided into a medial and lateral tract and the deep neck muscles.
The medial tract extends between the spinal and transverse processes of the vertebrae. It is innervated by the medial branches of the posterior branches of the spinal nerves. One differentiates between the spinal and transversospinal system.
- The spinalis muscle and the interspinales muscles belong to the spinal system. They connect the spinal processes of neighboring vertebrae. The spinalis muscle originates slightly laterally from the interspinales muscles.
- The transversospinal system comprises the short and long rotatores muscles, multifidus muscle and the semispinalis muscle. They all run diagonally from the transverse to the spinal processes. The short rotatores muscles insert at the next higher vertebrae, the long rotatores muscles at the second higher vertebrae. The multifidus muscle skips 2-4 vertebrae, the semispinalis muscle even 4-7 vertebrae. Furthermore the semispinalis muscle has a robust insertion at the occipital bone.
The lateral tract is a system of long longitudinal muscles. It lies more superficially than the medial tract and inserts more laterally. It is innervated by the lateral branches of the posterior branches of the spinal nerves. One differentiates between the sacrospinal, intertransverse and spinotransverse system.
- The sacrospinal system consists of the longissimus muscle and iliocostalis muscle. Both share an origin tendon at the thoracolumbar fascia. They have numerous origins and insertions at the ribs and transverse processes. In addition the longissimus capitis muscle inserts at the mastoid process of the temporal bone.
- The splenius muscle builds the spinotransverse system. It originates at the spinal processes of the upper thoracic and cervical vertebrae and runs laterocranially.
- The intertransverse system comprises the intertransversarii muscles and levatores costarum muscles. They extend between two neighboring transverse processes. The levatores costarum muscles are innervated by both the anterior and posterior branches of the spinal nerves (double innervation). Part of the intertransversarii muscles are even innervated only by anterior branches of the spinal nerves for which reason they do not count as intrinsic back muscles.
Deep neck muscles
The deep neck muscles (suboccipital muscles) are innervated by the posterior branch of the first spinal nerve (C1), the suboccipital nerve, and are therefore considered as intrinsic back muscles. They consist of four paired muscles:
- Rectus capitis posterior major muscle (spinal process of axis → inferior nuchal line)
- Rectus capitis posterior minor muscle (posterior tubercle of atlas → inferior nuchal line)
- Obliquus capitis superior muscle (transverse process of atlas → inferior nuchal line)
- Obliquus capitis inferior muscle (spinal process of axis → transverse process of atlas)
The intrinsic back musculature is essential for the stabilization and movement of the vertebral column. Bilateral contraction stretches the body (dorsal extension), unilateral contraction bends the body to one side (lateral flexion). The rotation is effected with the assistance of the oblique abdominal muscles. The deep neck muscles support the movement of the atlanto-occipital joint. They have particularly many muscle spindles contributing to the proprioception of the head.