Intrinsic back muscles
The intrinsic back muscles are part of the trunk musculature. They comprise a system of well-developed, deep muscles, which collectively extend along the length of the vertebral column. They are located in an osteofibrous canal, bounded by the vertebrae, ribs and thoracolumbar fascia,.
The intrinsic muscles of the back are extremely important in maintaining posture and movements of the vertebral column.
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 extrinsic back muscles which are derived 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 rami of the spinal nerves.
The medial tract can be further subdivided into the spinal, and transversospinal groups.
- The spinalis muscles (spinalis thoracicis, spinalis cervicis, and spinalis capitis) and the interspinales muscles belong to the spinal group of medial intrinsic muscles. They connect the spinal processes of neighboring vertebrae. The spinalis muscle originates slightly laterally from the interspinales muscles.
- The transversospinal groups comprises the short and long rotatores muscles, multifidus muscle and the semispinalis muscles. They all run diagonally from the transverse to 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.
Lateral tractThe lateral tract is a collection 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 rami of the spinal nerves. The lateral tract can be further subdivided into sacrospinal, intertransverse and spinotransverse muscular groups.
- The sacrospinal group 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 muscles build the spinotransverse system. They originate at the spinal processes of the upper thoracic and cervical vertebrae and runs laterocranially.
- The intertransverse group 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 rami 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). Rotation of the trunk 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.