Visceral nociceptors

The scientific interpretation of the mechanisms of visceral sensation and visceral pain has included from the beginning two different and opposing ideas: that the viscera are innervated by separate classes of sensory receptors, some concerned with autonomie regulation and some concerned with sensation, including pain; and that internal organs are innervated by a single and homogeneous class of sensory receptors that at low frequencies of activation send normal regulatory signals and at high frequencies, induced by intense stimuli, signal pain. This chapter addresses three ways to approach this central point of disagreement: looking at the evidence in favour of the existence of separate classes of sensory receptor in the viscera, particularly the results that show the presence of high-threshold sensory receptors in internal organs; looking at the evidence in favour of the existence of a single and homogeneous class of sensory receptor in the viscera and the results that show that these receptors can encode a wide range of stimulus intensities; and a discussion of the properties of the so-called ‘silent’ nociceptors in internal organs and their possible functional roles