How many kinds of visceral afferents?

Most afferent signals from the viscera do not give rise to conscious experience and yet they participate in the complex neural control of visceral functions. Studies of the extrinsic visceral afferent nerves supplying the digestive tract have been hampered because their nerve endings are intermingled with the extensive net of nerve fibres from the enteric nervous system, which also contains intrinsic primary afferent neurones (IPANs). The extrinsic afferent neurones (EPANs) to the digestive tract are unmyelinated and are located either in vagal or spinal
ganglia where they are mixed with somatic afferent neurones and reach the gut via vagal, splanchnic, pelvic, and pudendal nerves. Anterograde fillings in vivo and in vitro have been used to reveal the mode of termination within the gut wall. The nerve endings of vagal afferents in the stomach are intraganglionic
laminar endings (IGLEs) or intramuscular arrays (IMAs) and these supply almost all of the digestive tract. IGLEs are the site of mechanosensory transduction. Similar IGLEs supply the rectum (rectal IGLEs) running in the pelvic nerves. Spinal afferent neurones terminate as free nerve endings within the different gut
layers. Different neurochemicals are present in the visceral afferents to the digestive tract but little is known of their specific function. Mucosal visceral afferent nerve fibres are activated and modulated by substances such as
serotonin and cholecystokinin, released from mucosal enteroendocrine cells. Spinal visceral afferents provide collaterals to enteric and prevertebral ganglia
and blood vessels where they are likely to play an efferent function. Intestinofugal neurones provide another source of neural activity that leaves
the digestive tract to affect neurones in prevertebral ganglia and in the spinal cord. Identification of afferent nerve endings in the digestive tract and the correlation with their function is in its infancy.