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Epaxial muscles of the trunk

Other Terms: Back, Back muscles, Muscles of the back


The epaxial muscles, or vertebral extensors, develop on the dorsal side of the vertebral column and skull. These muscles develop from the myotomal epimere of all the trunk somites and are found along the entire length of the vertebral column and back of the occipital bone. They comprise the intrinsic muscles of the vertebral column, which are often referred to as the “true back muscles.” The vertebral extensors form four distinct muscle groups. These groups are, from superficial to deep, the spinotransversales, the erector spinae, the transversospinales, and the intersegmental muscles. However, each of the four groups does not extend the entire length of the vertebral column, so in some regions not all four layers are represented. It is also important to recognize that muscles from the upper limb and caudal branchial arch migrate superficial to the epaxial muscles and completely cover them. In general, the epaxial muscles vary in length and attach between different vertebrae along the length of the vertebral column. Some of the shorter, deep muscles originate (attach) on one vertebra and insert (attach) onto an adjacent vertebra or the skull, thus spanning only one intervertebral joint. On the other hand, the longer superficial muscles might span numerous vertebrae and produce movements at multiple intervertebral joints. While the majority of the muscles attach from one vertebra to another vertebra, some also attach to the occipital bone at the base of the skull (a bone with the same developmental origins as the vertebrae) or to the vertebral ends of the ribs. All of the muscles in this group share a common nerve supply; that is, they are all innervated by the posterior (dorsal) rami of the spinal nerves.


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