Distribution of chemosensitive primary sensory afferents in the central nervous system of the rat.

Abstract

AbstractBy making use of the selective neurotoxic effect of capsaicin on chemosensitive primary sensory neurons, a new experimental approach has been made for the direct morphological demonstration of the distribution of unmyelinated primary sensory afferent fibers to the spinal cord and sensory nuclei of cranial nerves. Chemosensitive primary sensory neurons are known to take part in the transmission of a specific sensory modality, i.e., chemogenic pain.For the demonstration of axon terminal degeneration, the Fink‐Heimer technique I was used. It was shown that the chemosensitive primary afferents terminated in Rexed's laminae I and II of the spinal cord. In the brainstem degenerating fibers were found to be distributed in the spinal trigeminal nuclei and the solitary system. In the caudal part of the trigeminal nucleus caudalis degeneration argyrophilia was noted both in the subnucleus marginalis and subnucleus gelatinosus, while in the rostral part of the nucleus degeneration was almost exclusively confined to the subnucleus gelatinosus. In addition, a small number of degenerating terminals were found in the nucleus oralis. Heavy terminal degeneration was observed in the nucleus of the solitary tract and the adjoining nucleus commissuralis. Degenerating structures were also found in a small region in the dorsolateral part of the area postrema.It is concluded that the degenerating afferent fibers terminating in the sensory nuclei of cranial nerves and the spinal cord represent central projections of chemosensitive primary sensory neurons, supplying the skin, mucous membranes, and different visceral areas by the Vth, IXth, and Xth cranial and spinal nerves, respectively.The participation of chemosensitive neurons in somato‐ and viscerosensory functions, other than chemogenic pain, is also discussed. It is suggested that these chemosensitive primary sensory neurons may, at least in part, be identical with substance P‐containing sensory ganglion cells.