Neurology and neuropathology of the pancreatic innervation.

The innervation of the digestive tract, which is known to control a wide array of functions including smooth muscle contractility, splanchnic microcirculation, epithelial cell absorption/secretion and endocrine secretion, is composed of an intrinsic component consisting of neuronal cell bodies and nerve processes of the enteric neurons (i.e., the enteric nervous system) and an extrinsic component (i.e., nerve fibers and cellular bodies localized outside the digestive tract) [1, 2]. The extrinsic nerve fibers can be anatomically and functionally classified in afferents, conveying sensory information from the digestive system to the central nervous system, and efferent nerves, through which the central nervous system-mediated response (excitatory or inhibitory) occurs. The extrinsic fibers belong to the sympathetic and parasympathetic systems [1, 2, 3, 4]. The mammalian pancreas, like the gastrointestinal tract, embryologically develops from an outgrowth of the primitive foregut and is richly innervated being composed of a variety of myelinated or unmyelinated nerve fibers, thick nerve bundles and aggregates of neural cell bodies known as intrapancreatic ganglia. These ganglionic structures are randomly scattered throughout the pancreatic parenchyma and represent the intrinsic neural component of the pancreatic nerve supply [5, 6]. The two main extrinsic components are anatomically identified in the vagus nerves (anterior and posterior branches) and the splanchnic nerve trunks. The vagus nerves reach the pancreas directly or, alternatively, they pass across the preaortic chain of the sympathetic ganglia. Post-ganglionic sympathetic fibers, whose neural cell bodies are located in the superior mesenteric and celiac ganglia, run with the splanchnic nerves. The afferent system, primarily involved in sensory/pain transmission to the central nervous system, is composed of thin unmyelinated fibers running with either the parasympathetic pathways (vagi) or the sympathetic inputs (splanchnic nerves). The cell bodies of these nerve processes can be located either in the dorsal root ganglia (the so-called spinal afferents) or in the nodose ganglia (vagal afferents) [4, 5, 6].