The viscera receive dual sensory innervation. The majority of visceral sensory fibres terminate in the spinal cord but sensory fibres contained in the vagus and pelvic nerves, which terminate in the brain stem and lumbosacral spinal cord, respectively, also innervate the same visceral organs. Figure 1 illustrates this unique pattern of innervation for the gastrointestinal tract, classically referred to as sympathetic and parasympathetic, but more appropriately designated by nerve name (for example, hypogastric nerve, pelvic nerve).

Visceral sensory axons are almost exclusively thinly myelinated Aδ fibres and unmyelinated C fibres. Activity in most visceral sensory neurones, whether in vagal afferent fibres or spinal afferent fibres, does not reach consciousness. For example, there is regular input into the central nervous system from gastric and hepatic chemoreceptors, aortic baroreceptors, etc., that is not perceived.

Sensory innervation of the gastrointestinal tract involves all layers of a viscus (mucosa, muscle, and serosa) and visceral receptors exhibit chemosensitivity, thermosensitivity, and mechanosensitivity. Visceral receptors, in fact, are generally polymodal in character (that is, respond …