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The semispinalis is a group of three muscles found in the human back: the semispinalis capitis, the semispinalis cervicis, and the semispinalis dorsi. Together extending from the base of the skull to the lower thoracic vertebrae, these muscles run parallel to the spine. They are counted among the transversospinales muscles, a group that is made up of the three semispinalis muscles, the multifidus, and the three rotatores muscles: the rotatores cervicis, rotatores thoracis, and rotatores lumborum. Like the other transversospinales muscles, those of the semispinalis group are responsible for extending and rotating the spine, although their individual actions are determined by their location.
By far the largest of the three, the semispinalis capitis is the uppermost muscle, found deep in the back of the neck. Rather than arising from a single tendon as do many muscles in the body, it originates via multiple tendons. These begin along the transverse processes, or sideways bony projections, of the upper six or seven thoracic vertebrae and lowest cervical vertebra, C7, as well as on the articular processes of C4-C6. From here the tendons form a single muscle that ascends along either side of the vertebral column and attaches to the occipital bone on the underside of the skull. As the capitis is situated in the neck and attaches to the head, it acts on the vertebrae that move the head, extending the head upward and slightly backward, and rotating the head on the neck.
The intermediate of the three is the semispinalis cervicis, which is shorter and narrower than the capitus. It is made up of fasciculi, bundled muscle fibers encased in a sheath of connective tissue called the perimysium. These fasciculi originate on the transverse processes of the topmost five or six thoracic vertebrae and run up the neck just deeper than the capitis, meaning that the cervical fibers are found underneath those of the capitus. They insert along the spinous processes, or backward bony projections, of C2-C5, attaching to the middle cervical vertebrae. This location dictates that the cervicis acts on the cervical and upper thoracic spine, extending the vertebrae upward.
Situated immediately beneath the fasciculi of the semispinalis cervicis are those of the semispinalis dorsi, the narrowest of the three muscles. Its tendons originate on the transverse processes of T6-T10 in the middle of the spine and inserts along six consecutive vertebrae: C6-C7 and T1-T4. These fibers act on the thoracic spine, extending the vertebrae upward as well as rotating the column to either side.
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