Final answer:
Splanchnic nerves originate from preganglionic fibers that exit the spinal cord and bypass the chain ganglia to synapse with collateral ganglia, like the celiac, superior mesenteric, and inferior mesenteric ganglia. These nerves are integral for autonomic control of the abdominal organs as part of the sympathetic nervous system.
Step-by-step explanation:
Splanchnic nerves are derived from preganglionic fibers that exit the spinal cord through the ventral nerve root. These preganglionic fibers do not terminate within the chain ganglia but continue as either the greater splanchnic nerve or lesser splanchnic nerve. For example, the greater splanchnic nerve, at the level of T5, travels beyond the sympathetic chain ganglia to synapse with a collateral ganglion outside the chain, such as the celiac ganglion, which then provides postganglionic innervation to abdominal organs such as the stomach.
The collateral ganglia, or prevertebral ganglia, which include the celiac, superior mesenteric, and inferior mesenteric ganglia, receive input from the splanchnic nerves. These ganglia are crucial for controlling organs within the abdominal cavity and form part of the enteric nervous system. It's important to note that within this system, the preganglionic fibers release acetylcholine (ACh) which activates ganglionic neurons through nicotinic receptors, while the postganglionic neurons typically release norepinephrine to excite the target organs as part of the sympathetic nervous response.