Visceral vs Somatic Pain

In contrast to the somatic system (a misleading term since the viscera are certainly of the body), relatively little is known about the mechanisms of visceral pain sensation. We do know, however, that whereas some characteristics are shared between the visceral and nonvisceral

(somatic) systems, there are also significant differences. Therefore, results from experiments on somatic tissue cannot automatically be assumed to correlate with the visceral organs. The major features (4) that differentiate visceral from nonvisceral pain are as follows:

1. Visceral pain is not evoked from all viscera: A large portion of the viscera is innervated by afferents and their receptive endings that convey purely regulatory, or at least non-noxious, information, and many forms of (presumed) noxious stimulation do not produce a conscious sensation of discomfort or pain in the viscera.

2. Visceral pain is not always linked to tissue injury: Acute colonic pain, for example, can be evoked by colonic distension without any associated tissue damage, whereas cutting or crushing of the colon does not reliably evoke pain.

3. Visceral pain is referred to other locations: This, alongside the following observations, is related to the convergence of visceral and nonvisceral pathways onto second-order neurons in the spinal cord. Angina, for example, is referred to the upper left shoulder and left arm.

4. Visceral pain is diffuse and poorly localized: The proportion of visceral to nonvisceral afferents is low; thus each unit must represent a larger receptive field in the viscera compared to an equivalent unit in skin, muscle, or joint. Viscerovisceral convergence (e.g., colon and bladder) onto the same second-order spinal cord neurons also contributes to the poor localization of visceral pain.

5. Visceral pain is accompanied by motor and autonomic reflexes: Primary afferent neurons innervating the viscera possess axon collaterals that synapse with autonomic system secretory and motor neurons in prevertebral ganglia. Moreover, the emotional component of undefined visceral pain, such as chest pain, is typically greater than that associated with skin, muscle, or joint insult.

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