Final answer:
Fibers entering and exiting the sympathetic trunks without synapsing are called white rami communicantes, which contain myelinated preganglionic sympathetic fibers.
Step-by-step explanation:
The fibers that enter and leave the sympathetic trunks without synapsing are called white rami communicantes. These fibers are myelinated axons that provide a short connection from a sympathetic chain ganglion to the spinal nerve containing the preganglionic sympathetic fiber. This direct route allows the sympathetic nervous system to regulate target effectors such as organs and glands without the need to synapse in a chain ganglion. In some cases, these fibers may bypass the chain ganglia altogether and continue as splanchnic nerves to synapse within prevertebral ganglia.