The autochthonous back musculature is the part of the back musculature that attaches directly to the spine and provides straightening, rotation, and lateral tilt of the spine, as well as upright posture of the head. The term autochthonous was chosen because the musculature was created directly in place during the embryonic stage and did not “migrate” from other regions as most skeletal muscles do. The autochthonous back muscles are innervated by the dorsal branches of the spinal nerves.
What is autochthonous back musculature?
Autochthonous back muscles are laid down in place during the embryonic stage, directly adjacent to the spine, and are therefore referred to as autochthonous. In contrast, many other skeletal muscles are first laid out in other locations before emigrating to their destination while still in the embryonic stage of development. The structure and function of autochthonous back muscles is no different from the rest of the skeletal muscles. A distinguishing feature from the rest of the back musculature lies primarily in its innervation. The autochthonous back muscles are innervated by the dorsal branches of the spinal nerves, whereas the other back muscles are supplied by the ventral branches of the spinal nerves. Because of the main function of the autochthonous back muscles, the individual muscles are also grouped under the term musculus erector spinae, which could be translated as “spinal erector.” Overall, the autochthonous back musculature represents a very complex system of individual muscles belonging to either the lateral or medial muscle cord (tractus).
Anatomy and structure
The autochthonous back muscles do not differ in structure from the rest of the striated skeletal muscles that are subject to our will. The erectus spinae muscle is enveloped at the level of the thoracic and lumbar vertebrae by the superficial and deep sheets of the fascia thoracolumbalis, and at the level of the cervical vertebrae by the sheets of the fascia nuchae. The autochthonous dorsal muscles run in a canal formed partly by bones and partly by fibers, which is formed by the bony processes of the vertebrae or by the ribs and by the enveloping fascia. The individual muscles are composed of muscle fibrils, several hundred of which form each muscle fiber. The muscle fibers bundle together to form fiber bundles, which, when bundled together again, form the individual muscle. The actual motor of the muscles is formed by the myofibrils, which are composed of contractile proteins and do the actual work of contraction. The medial muscle cord is divided into the interspinous and transversospinous systems. Interspinous muscles connect spinous processes to each other, while muscles of the transversospinal system connect transverse processes to spinous processes above them, and may also skip one or more vertebrae. The lateral cord of the autochthonous spinal musculature is divided into the intertransverse, spinotransverse, and sacrospinal systems. It usually involves the complex muscular connection of transverse processes to each other or of spinous processes to transverse processes of different vertebral bodies.
Function and tasks
One of the main functions of the autochthonous back muscles is to straighten the spine and head. The fanning out of the lateral and medial muscle cords into numerous individual muscles that can be involuntarily controlled individually allows for very complex and delicate movement sequences and patterns. Through controlled unilateral muscle contractions of individual muscle groups, the spine can not only be bent forward and backward or laterally to the right or left and then straightened again, but rotations of the spine to the right and to the left are also possible to a certain degree. For example, spinotransverse muscles, which connect spinous processes to transverse processes of higher vertebrae, allow the spine to twist in the direction of muscle contraction when contracted unilaterally. Intertransverse muscles, which connect transverse processes with the transverse processes of vertebrae above, allow the spine to tilt in the direction of the activated muscle when contracted unilaterally. Bilateral contraction of the muscles results in extension of the spine.The deep neck muscles (musculi suboccipitales) are of particular importance. They enable fine motor movements of the head, which can be interconnected with fast messages of the sense of balance (vestibular system). Fine motor movements of the head were originally important for humans to better fixate moving objects such as enemy or prey while simultaneously moving themselves. The interaction of the various autochthonous back muscles is so complex that voluntary control is subject to a certain movement of the spine, but not to the decision as to which muscle parts must be brought into play for this purpose by contraction or by relaxation.
Diseases
Functional limitations of the autochthonous spinal musculature, as with other skeletal muscle parts, are due either to direct muscle diseases or to neurologic problems. Diseases that exclusively affect specific muscles of the back musculature occur relatively rarely. The most common complaints are caused by muscle tension and muscle hardening, which lead to one-sided loads on the spine and in serious cases can even trigger a herniated disc. Muscle tension in the back muscles is very common and usually triggers non-specific back pain. The tensions can be triggered by unusual and persistent one-sided static loads, which are aggravated by permanent stress. Continuous stress or too frequent acute stress phases lead to increased muscle tone due to an increased release of stress hormones, which promotes muscle tension and stiffness. In rare cases, the autochthonous back muscles may also be affected by genetically caused muscular dystrophies, leading to decreasing muscle performance. In very rare cases, the back muscles are affected by neuromuscular disease, in which signal transmission from the nerve to the muscle or sensory feedback from the muscle to the nerve is impaired, leading to weakening and degradation of the affected muscle.
