Information concerning events in the viscera reach the brain via two pathways; the vagus (the tenth cranial nerve), and the spinal nerves. The former deals mainly with signals used in physiological regulation, whereas the latter is concerned mainly with sensation. However afferents in the sacral parasympathetic nerves are also involved in sensory and reflex events related to the physiology of pelvic viscera.
Visceral signals that enter consciousness do so by utilising pathways used by somatic afferents entering the same spinal segments. Because of this arrangement, damaging events in the viscera give rise to sensations felt in the somatic domain, i.e. in skin or muscle. This is the basis of referred pain.
Many functions of internal organs are monitored by afferent neurones in the vagus and in spinal nerves.
The activity of some nerves that monitor events inside the body such as the levels of blood gases and pressures within the heart and blood vessels do not impinge on consciousness. Others, arising from the upper gastrointestinal tract, play a part in conscious behaviour, such as feeding and satiety. These types of afferent concerned with physiologcal regulation are found mainly in the vagal and glossopharyngeal nerves.
Another group of afferent neurones travel to the spinal cord within sympathetic and sacral parasympathetic nerves, and mediate pain and sensations relating to normal events in the bladder and other internal organs in the pelvis.
Key Words: Visceral afferents, vagal and spinal pathways; reflex and sensory functions
Afferent Pathways in Sympathetic and Parasympathetic Nerves
Image source: www.med.umich.edu (modified)
The afferent innervation of viscera is shown in green, and follow the paths of the sympathetic and parasymathetic nerves.
Vagal and Spinal Afferent Innervation of Viscera
Sensory nerve fibres arising in the viscera run in sympathetic and parasympathetic nerve trunks to reach the spinal cord or brainstem.
Vagal afferents have cell bodies in the nodose ganglia, and the spinal afferents have cell bodies in the dorsal root ganglia.
As a general rule, the vagal afferents are concerned with physiological regulation of the viscera, and the spinal pathway from viscera is concerned with visceral nociception.
However general rules have exceptions that include the irritant receptors of the trachea which respond to potential injury to the airway and cause coughing, and the 'J' receptors that may mediate or contribute to the unpleasant sensation of dyspnoea in some circumstances.
Another exception involves the spinal afferents innervating pelvic viscera that mediate normal sensations relating to continence and sexual activity as well as visceral nociception.
Some afferent in the urogenital and intestinal systems contribute to reflex control of these systems.
Role of Vagal Reflexes in Gastric Physiology
Image source: studydroid.com
Gastric afferents that respond to normal contractions and distension of the stomach participate in a series of reflex activities, many of which take place without conscious knowledge of the events.
Afferents to the upper GI Tract
The vagus nerve innervates the thoracic viscera and the upper gastro-intestinal tract and its sensory fibres monitor physiological events in these organs, such as the tension in the walls of the viscera and the chemical composition of the contents of viscera
Most vagal afferents to the GI tract are C-fibres..
The mesenteries receive a substantial innervation from the spinal cord, with sensory endings within the neuro-vascular bundles as they travel towards the viscera.
Vagal Afferents: Interactions with the Enteroendocrine System
Afferents innervating the intestinal mucosa include mechanoreceptors and chemoreceptors, that are important in the overall functioning of the gastro-intestinal tract as a whole.
There is an interplay between enteroendocrine (EEC) cells and visceral (primarily vagal) afferent neurons. Recent evidence suggests that the EEC cells are of importance in the secretion of incretin peptides and the activity of vagal afferents, and that both contribute to the regulation of glucose metabolism and insulin secretion by the pancreas (insulin secretion is promoted by vagal efferent pathways).
So, while insulin release remains the primary controller of glucose homeostasis, the incretin peptides and the vagus nerve both modulate the regulation of blood glucose.
EEC cells appear to monitor luminal carbohydrates, triglycerides and protein and may activate of intrinsic and extrinsic neurones.
Some vagal mucosal receptors in the stomach respond to the presence of acid in the gastric lumen and appear to play a part in the regulaation of gastic acid secretion.
Referred Pain and Viscero-somatic Convergence
Image source: medicguide
Diagram of the convergence of somatic and visceral afferents on the neurones of the anterolateral system.
Image source: higheredbcs.wiley
The coloured areas indicate the areas of the body surface where pain is experienced as a result of visceral overdistension or inflammation.
Visceral pain is one of the strongest painful sensations known to man. It arises from internal organs as a result of occlusion of arteries (as in a heart attack), friction between layers of pleura or peritoneum (as in pleurisy or rebound tenderness in the abdomen), stretch of mesenteries, overdistension of viscera, or inflammation.
The spinal afferents that accompany the autonomic nerves are responsible for initiating visceral pain, but the second order neurones that project rostrally also carry afferent traffic from the skin. This is because of viscero-somatic convergence - both pathways (visceral and somatic) converge on the same neurones in the dorsal horn.
A consequence of this is that activity in visceral afferents can be confused with somatic inputs within the same segments, and percieved as sensations within those somatic areas
afferents from the heart can induce sensations referred to the chest and arm (and sometimes in the neck)
afferents from the gall bladder can induce sensations in the abdominal skin, but also in the shoulder (because inflammation in that region can activate afferents of the phrenic nerve, whose origin is C4-5, which also innervates the shoulder)
afferents from the kidney and ureter can give rise to pain radiating from the small of the back to the groin.