Final answer:
Spinal nerve trunks are formed by the union of dorsal and ventral nerve roots, leading to major nerve networks like cervical, brachial, lumbar, and sacral plexuses that give rise to crucial nerves like the phrenic and sciatic nerves.
Step-by-step explanation:
The spinal nerve trunks are formed by the combination of sensory and motor axons that separate into two nerve roots: the sensory axons enter the spinal cord as the dorsal nerve root while the motor fibers, including both somatic and autonomic nerves, emerge as the ventral nerve root. Each spinal nerve trunk can be associated with specific spinal cords and may develop into larger nerve networks known as plexuses, such as the cervical, brachial, lumbar, and sacral plexuses. For instance, the cervical plexus consists of axons from spinal nerves C1 through C5, and the brachial plexus reorganizes nerves from C4 through T1. Significant nerves such as the phrenic nerve, originating from the cervical plexus, and the sciatic nerve, arising from the sacral plexus, illustrate the intricate branching and vital functions of these spinal nerve trunks.