Posterior (Dorsal) Primary Rami
Spinal nerve exits through the intervertebral foramen and branches into the ventral ramus, dorsal ramus and rami communicantes. What does ventral rami do?
Ventral rami (also called anterior primary rami) of the cervical and lumbar region supply the muscles and skin of the upper and lower limbs through brachial and lumbar plexuses respectively. The ventral rami of the thoracic region supply the abdominal muscles and skin on the anterior and lateral aspect of the trunk.
What does dorsal rami do?
The dorsal rami (also called posterior primary rami) are as a rule smaller than the anterior rami. After the division they are directed backward. Dorsal rami divide into medial and lateral branches. The medial branch of the dorsal primary ramus also supplies articular branches to the zygopophyseal joints and the periosteum of the vertebral arch. In the neck and upper back, the medial branch continues through the deep and superficial back muscles to supply overlying skin. In the lower back, the lateral branch does this. Generally they supply the muscles and skin of the posterior part of the trunk with the exceptions of those of C1, S4 and S5. Dorsal rami remain distinct from each other, and each innervates a narrow strip of skin and muscle along the back, more or less at the level from which the ramus leaves the spinal nerve.