After the spinal nerves leave the intervertebral foramina, they divide into ventral and dorsal rami. Both rami contain mixed fibers that provide motor and sensory innervation. The dorsal rami are typically smaller than their ventral counterparts. They are responsible for innervation of the deep back muscles, suboccipital muscles, and the skin of the head, neck, and dorsal aspects of the trunk.

All dorsal rami of the spinal nerves (except for the first cervical (C1), fourth and fifth sacral (S4 - S5), and coccygeal (Co) spinal nerves) divide into two branches - the medial and lateral branches.

Both branches of the dorsal rami of the lumbar nerves separate at the junction between the facet joints and the upper edges of the transverse processes. Usually, the medial branches are smaller than the lateral branches, and they also have a more constant rather than variable course. Both branches contain sensory and motor fibers and are accompanied by blood vessels.

The medial branches arise immediately after the dorsal rami pierce the intertransverse ligaments and the intertransversarii muscles. These branches pass medially through the spaces between the accessory and mammillary processes in lumbar vertebrae, traveling toward the laminae of the vertebral arches. Here, they lie deep within the tissue of the multifidus muscles. At this point, the medial branches further divide into articular branches, muscular branches, and cutaneous branches. Overall, the medial branches supply the area from the midline to the facet joint line level in the lumbar vertebrae and to the intermediate sacral crest in the sacrum. They innervate the facet joints, multifidus muscles, medial intertransversarii lumborum muscles, and a small part of the skin medial to the facet joints.
The lateral branches also arise immediately after the dorsal rami pierce the intertransverse ligaments and the intertransversarii muscles. They innervate structures lateral to the facet joint line in the lumbar vertebrae and the intermediate sacral crest in the sacrum. Compared with the medial branches, these branches have a longer and more curved course. From the transverse processes of the lumbar vertebrae, they travel caudally, dorsally, and laterally deep to the iliocostalis lumborum and the lumbar part of the longissimus thoracis, innervating both muscles. These branches also innervate the latissimus dorsi muscle. The upper three lateral branches of the lumbar and sacral nerves (L1 - L3 and S1 - S3) are better developed, and they also give off cutaneous branches that form the superior cluneal nerves (Read more!) and the middle cluneal nerves (Read more!), respectively. The lateral branches often form anastomoses at the L1 - L3 levels. The lateral branch of the L4 nerve is purely muscular. The course of the lateral branches of the L4 and L5 segments is more variable, and in some cases, there is no lateral branch of the L5 nerve. Instead, the dorsal ramus of the L5 nerve connects with the dorsal ramus of the S1 nerve.

Some authors describe an additional third branch - the intermediate branch - between the T11 and L4 spinal nerves. If present, it usually arises at the L3 - L4 level and occasionally at the L1 - L2 level. It may also arise from a common trunk with the lateral branch. The intermediate and lateral branches often anastomose. This branch supplies the lumbar part of the longissimus thoracis muscle. Some authors also state that this branch extends to the skin.

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Dorsal rami of spinal nerves