Final answer:
The most accurate statement about plexuses is that only ventral rami form plexuses, such as the major cervical, brachial, lumbar, and sacral plexuses.
Step-by-step explanation:
The statement about plexuses that is most correct is: 'Only ventral rami form plexuses'. Plexuses are networks of overlapping nerve fibers from the ventral rami of spinal nerves. The dorsal rami conduct signals for the posterior body, and they do not form plexuses. The major plexuses in the human body are the cervical, brachial, lumbar, and sacral plexuses. For example, the cervical plexus is formed by axons from spinal nerves C1 through C5, and the brachial plexus is composed of spinal nerves C4 through T1, which serve the arms. These plexuses carry both motor and sensory fibers and are crucial for bodily functions such as movement and sensation. The statement that the dorsal rami of all spinal nerves unite to form complex networks or that each branch of the plexus contains fibers from a single spinal nerve is incorrect. Additionally, there is no thoracic plexus formed by the ventral rami of thoracic spinal nerves, as thoracic nerves typically do not form a plexus but rather directly innervate the intercostal spaces.