Neurobiology of visceral afferent neurons: neuroanatomy, functions, organ regulations and sensations.

Visceral organs are innervated by vagal and spinal visceral afferent neurons which serve as interface between visceral domain and brain. They have multiple functions, one of which is the encoding of mechanical and chemical events and the relay of these messages to the CNS. Vagal afferent neurons project viscerotopically to the nucleus of the solitary tract in the medulla oblongata. Spinal visceral afferent neurons project segmentally to the laminae I and V and deeper of the spinal dorsal horn. Visceral pain and discomfort are associated with spinal visceral afferents. Functionally there exist general classes of visceral afferents, the compositions of which are distributed according to the type and function of visceral organ: low-threshold mechanosensitive afferents responding to distension and contraction and other stimuli; specific chemosensitive afferents (probably only vagal); and high-threshold mechanosensitive afferents. Normally mechano-insensitive spinal visceral afferents which are chemosensitive may be recruited in pathophysiological conditions. Visceral events which lead to the generation of distinct organ regulations, reflexes and sensations may be encoded by functionally specific sets of afferents or by the intensity-coding in afferents or by both. Pain elicited from some visceral organs may not be associated with the activation of specific sets of 'visceral nociceptors' but with the intensity of discharge in spinal visceral afferents.