In one breath
Viscera carry the same kind of nociceptors as somatic tissue, but far fewer, so visceral pain is diffuse and badly localized and comes with autonomic upset. Its usual trigger is distension of a hollow organ, plus ischemia and chemical irritation, and its afferents travel with autonomic nerves before joining the lateral spinothalamic tract. Because those visceral afferents share second-order neurons with somatic ones, the pain is often felt in a somatic structure supplied by the same embryonic segment — referred pain, as in a heart attack felt down the left arm. Four theories explain it: dermatomal, convergence, facilitation and experience.
Builds on: Pain: concepts, receptors and fibers · Pain pathways and perception · Leads to: Specific pain syndromes · Sensory lesions and sensory function tests
Visceral pain
- Its usual quality is aching. It can also be burning, as in peptic ulcer, or anginal, as in coronary ischemia. When intense it turns sharp or penetrating.
- It is diffuse and poorly localized, and it comes with autonomic symptoms.
- It often radiates, or is referred, to structures well away from the organ.
- The physiology of visceral and referred pain was opened up by Henry Head (1861–1940), who also worked on skin sensation and on recovery after a nerve is cut.
Receptors
- The receptors are the same nociceptors as in somatic structures, only far fewer in number.
- With so few nerve endings, the edges of the painful zone are blurred, and the pain is poorly localized.
Stimulus
| Stimulus | Note |
|---|---|
| Distension | the main one, for a hollow viscus |
| Ischemia | as in coronary or mesenteric occlusion |
| Chemical irritation | acid, inflammatory mediators |
- Worth knowing, though not in your pages: note what is not on that list. Cutting or burning a viscus is largely painless, which is why bowel can be handled with only the abdominal wall anesthetized. Distension, ischemia and spasm are what hurt.
Afferent fibers
- Visceral pain afferents reach the CNS along autonomic nerves. Their cell bodies sit in the DRG and in the matching cranial nerve ganglia.
- Three cranial nerves bring visceral pain fibers in: the vagus, the glossopharyngeal and the facial.
| Source | Route |
|---|---|
| Esophagus, stomach, small intestine, proximal colon | mostly vagus → nucleus of the solitary tract (NTS) → thalamus |
| Abdominal viscera | also along sympathetic fibers |
| Pelvic structures | along lumbosacral roots |
| Face and eye | trigeminal nerve (Trigeminal pathway and trigeminal neuralgia) |
- Because the fibers run with sympathetic nerves, visceral pain drags autonomic signs along with it: sweating, a change in blood pressure.
Ascending pathway
- From the cord the fibers run in the lateral spinothalamic tract — the very same route as somatic pain (Pain pathways and perception).
- Thalamus → cortex by the thalamocortical radiation.
- In the cortex the visceral representation is poorly organized, and the visceral areas are mixed in with the somatic ones. That is a second reason the pain cannot be placed.
Special characteristics
- Poorly localized, because the viscera hold few receptors.
- Distension is the key stimulus; inflammation of a viscus raises its sensitivity to pain.
- Unpleasant out of proportion: it carries a strong affective component, and its afferents stimulate the vomiting centre, so nausea comes with it.
- Autonomic symptoms accompany it — blood pressure changes, sweating — because the afferents run in sympathetic and parasympathetic pathways.
- Muscle spasm and rigidity, especially with abdominal pain, and especially once the inflammation reaches the peritoneum. The muscle guarding of an acute abdomen is protective: a rigid wall keeps the viscera from further injury.
- It is often referred elsewhere.
Somatic and visceral pain compared
| Feature | Somatic | Visceral |
|---|---|---|
| Receptors | plentiful | few |
| Stimulus | cut, crush, prick, burn | distension, ischemia, irritants |
| Quality | sharp (superficial), aching (deep) | aching, burning, anginal; diffuse |
| Localization | good | poor |
| Afferents | somatic nerves | autonomic nerves |
| Autonomic signs | uncommon | usual |
| Nausea, vomiting | no | often |
| Referral | rare | common |
| Muscle response | local reflex contraction | guarding, wall rigidity |
| Cortical map | discrete | vague, mixed with somatic |
Referred pain
- Definition: pain that is felt in a somatic structure when the real trouble is irritation or injury of a viscus.
- It is usually not referred to the skin lying over the organ, but to other skin supplied by the same embryonic spinal segment.
| Condition | Pain felt at |
|---|---|
| Acute myocardial infarction | inner (ulnar) aspect of the left arm |
| Acute cholecystitis | tip of the right shoulder |
Applied: why cholecystitis hurts in the shoulder
An inflamed gall bladder sits against the diaphragm and irritates it, and the diaphragm reports through the phrenic nerve. Diaphragm and shoulder tip grow from the same dermatomal segments, so the brain reads the signal as coming from the shoulder.
Worth knowing, though not in your pages: the same logic explains shoulder-tip pain from any irritation under the diaphragm, such as blood or pus there after a ruptured spleen.
Dermatomal theory
- Visceral pain is referred to a structure that develops from the same embryonic segment, that is, the same dermatome.
- Heart and the inner arm come from one embryonic segment, so infarct pain runs down the ulnar border of the left arm.
Convergence theory
- Visceral and somatic afferents converge on the same second-order neuron in the spinothalamic tract. They must: second-order neurons are fewer than the first-order fibers arriving.
- So the fiber that normally carries pain from the skin also carries pain from the viscus.
- The cortex cannot always say which end the signal came from, because the ascending fiber is the same one. It therefore refers the pain to the somatic area as well as to the organ.
Facilitation theory
- Collaterals from the visceral afferents end on the same spinothalamic neurons that receive somatic afferents.
- Visceral activity therefore facilitates — strengthens — the somatic pain signal, so that even a minor somatic input is now enough to be felt as pain. The somatic input itself does the signalling; the viscus only primes the neuron.
Convergence or facilitation?
They look alike, and examiners lean on the difference.
- Convergence: one second-order neuron, two inputs onto it; the visceral signal itself travels up a somatic line, and the cortex misreads its source.
- Facilitation: the visceral input is a collateral that only turns up the gain on the somatic neuron, so ordinary somatic traffic starts to hurt.
Experience theory
- Past experience shapes where pain is felt. Pain may be referred to a site where the patient has hurt before, rather than to its usual place.
- Example: pain from inflamed abdominal viscera normally goes to the midline, but in someone who has had abdominal surgery it is referred to the surgical scar, wherever that lies.
Draw it: convergence and facilitation
Draw the two side by side, each as a small piece of dorsal horn with one second-order neuron going up the spinothalamic tract.
- Convergence: two first-order fibers arrive, one labelled visceral afferent (from a heart symbol) and one labelled somatic afferent (from a patch of skin). Both synapse on the same second-order neuron. Draw the single axon up to the thalamus, and write “cortex cannot tell them apart” beside it.
- Facilitation: the somatic afferent synapses on the second-order neuron as before, and the visceral afferent sends only a collateral onto that same neuron. Mark the collateral with a plus sign and label it “facilitates”, then note that a weak somatic input now fires the cell.
- Under both, add the dermatome line: visceral organ and referred skin area from the same embryonic segment.
Exam-answer skeleton: "What is referred pain? Give examples and explain its theories" (short note)
- Define referred pain, and set it in context: visceral pain is diffuse, poorly localized and often referred.
- The rule: referred not to the skin over the organ but to skin of the same embryonic segment.
- The two standard examples: myocardial infarction to the ulnar side of the left arm; cholecystitis to the right shoulder tip, through diaphragm and phrenic nerve.
- Dermatomal theory, with the heart and arm example.
- Convergence theory, with the diagram: fewer second-order neurons than afferents.
- Facilitation theory, with the diagram: visceral collaterals raise the gain on somatic neurons.
- Experience theory, with the surgical-scar example.
- One line on why it matters clinically: the site of pain need not be the site of disease.
Asked in exams
- Referred pain and why it happens: Jan 2011, essay · Jan 2012, 4 marks · Aug 2017, 4 marks · Oct/Nov 2018, 4 marks · Sep/Oct 2019, 4 marks · Jan 2021, 3 marks · Aug 2024, MCQ · Oct 2024, MCQ
- Visceral pain: Jan/Feb 2022, MCQ