Final answer:
Dorsal rami do not form plexuses. The nerve plexuses—cervical, brachial, lumbar, and sacral—are composed of ventral rami of spinal nerves and contain both sensory and motor fibers affecting different body regions.
Step-by-step explanation:
Dorsal rami do not unite to form plexuses; rather, it is the ventral rami of spinal nerves that contribute to the formation of the nerve plexuses. Specifically, nerve plexuses are networks of intersecting nerves supplying various body regions. In humans, there are four primary nerve plexuses that arise from the merging of the ventral rami of spinal nerves in certain areas:
The cervical plexus, formed by axons from spinal nerves C1 through C5, supplies the posterior neck and head, along with the phrenic nerve to the diaphragm.
The brachial plexus, originating from spinal nerves C4 through T1, gives rise to nerves serving the arms, including the radial, axillary, ulnar, and median nerves.
The lumbar plexus, arising from all the lumbar spinal nerves, enervates the pelvic area and the anterior leg, with the femoral nerve being a major nerve in this plexus.
The sacral plexus, formed by spinal nerves from L4 to S4, is known for the sciatic nerve that innervates the posterior leg.
These plexuses each contain both sensory and motor fibers that originate from sensory neuron axons in dorsal root ganglia (for sensory input) and motor neuron axons in the anterior horn of the spinal cord (for muscle contraction).