Nociceptor structural specialization in canine and rodent testicular “free” nerve endings

Abstract

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.

Fine structure of Eimer's organ in the coast mole (Scapanus orarius)

“…It should be noted that the morphological qualities of both the central column free nerve endings and peripheral free nerve endings correspond well to those reported for the varicose (beaded) and fine (plain) free nerve endings, respectively, in human digital skin (Cauna, 1980). However, the peripheral terminals in this system appear to lack a distinct axonal reticulum and dense aggregations of microvesicles and appear to differ somewhat from the canine and rodent testicular free nerve endings described by Kruger et al (2003a), though without serial sectioning the length of the terminals, the presence of these structures cannot be ruled out.…”

Identification of Medium/High-Threshold Extrinsic Mechanosensitive Afferent Nerves to the Gastrointestinal Tract

“…We speculate that branch points are exposed to distortion during strong contractions or distension of the intestine. Branching nociceptor axons with regular swellings in extensive terminal fields are found on vascular structures in testis26, where ultrastructural specialisations have been detailed, comparable to nociceptor terminals in the knee joint, pleura and dura mater27. Whether the mesenteric mechanosensitive axonal structures identified here actually constitute “nerve endings” and share similar ultrastructural characteristics remains to be determined.…”

Phosphoinositide signaling in somatosensory neurons

“…These measurements were performed in isolated cell bodies of DRG neurons, where the ER is clearly present. It has also been shown that the nerve terminals of free nerve endings in the skin contain axonal reticulum, the equivalent of the ER, making it possible that store operated Ca 2+ channels function not only in the cell bodies, but also at the nerve terminals (Kruger et al, 2003). Interestingly, heat over 35 C° has been shown to induce clustering of STIM1 and activation of store operated Ca 2+ entry (Xiao et al, 2011).…”