Nociceptor structural specialization in canine and rodent testicular "free" nerve endings

The receptive fields (RFs) of polymodal nociceptor units of canine testis consist of small fascicles of branching axons ending as clusters within the thin vascular layer overlying the seminiferous tubules. This propitious arrangement enabled serial reconstruction of electron micrographs of a flat, punctate zone, identified during recording of impulse discharge of a single physiologically characterized nociceptor "unit." The RFs showed multiple axons, with some branching and sequential vesicle-containing swellings, similar to what have been called terminals and assumed to constitute impulse generating sites in other tissues. Consistent with this interpretation is the local presence of: (1) clusters of small mitochondria, (2) glycogen granules, and (3) sectors of axolemma denuded of Schwann cell processes and thus "free" endings directly contacting the epithelial basal lamina. The most distal sectors showed additional morphologic properties: (1) zones of vesicles embedded in an extensive granular matrix, (2) preterminal arrays of "axonal reticulum" derived from the smooth endoplasmic reticulum displacing axonal cytoskeletal elements, (3) a variety of sizes and electron lucency in clear, spherical vesicles and a few granular or dense-core vesicles, and (4) specialization in the last Schwann cell of the fascicle. These latter features may reflect differences in the specialized receptor mechanisms of nociceptors that are difficult to detect without extensive serial electron microscopic analysis. Alternatively, these features may constitute a regional specialization of the testis. The term free nerve ending is perhaps an insufficient and inaccurate descriptor of the morphology of nociceptors. These findings are considered in the context of their possible relation to the sensitized vanilloid receptor mechanism unique to nociceptors.