Kruger Lawrence, Kavookjian Anahid M, Kumazawa Takao, Light Alan R, Mizumura Kazue
Department of Neurobiology, Geffen School of Medicine, University of California Medical Center, Los Angeles, California 90095, USA.
J Comp Neurol. 2003 Aug 18;463(2):197-211. doi: 10.1002/cne.10754.
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.
犬睾丸多模式伤害感受器单位的感受野由分支轴突的小束组成，这些轴突在覆盖生精小管的薄血管层内聚集成簇终止。这种有利的排列方式使得能够对一个扁平的、点状区域的电子显微照片进行连续重建，该区域是在记录单个具有生理特征的伤害感受器“单位”的冲动发放时确定的。感受野显示有多条轴突，有些有分支并有连续的含囊泡肿胀，类似于在其他组织中被称为终末并被认为构成冲动产生部位的结构。与此解释一致的是局部存在：（1）小线粒体簇，（2）糖原颗粒，以及（3）轴膜区域，其被施万细胞突起剥脱，因此是直接接触上皮基膜的“游离”末梢。最远端的区域显示出额外的形态学特征：（1）嵌入广泛颗粒基质中的囊泡区域，（2）源自滑面内质网的“轴突网”的终末前排列，取代了轴突细胞骨架成分，（3）透明球形囊泡以及一些颗粒状或致密核心囊泡的各种大小和电子透明度，以及（4）束状结构中最后一个施万细胞的特化。这些后者的特征可能反映了伤害感受器特化受体机制的差异，若无广泛的连续电子显微镜分析则难以检测到。或者，这些特征可能构成睾丸的区域特化。“游离神经末梢”这个术语可能不足以准确描述伤害感受器的形态。这些发现是在它们与伤害感受器特有的敏化香草酸受体机制可能存在的关系的背景下进行考虑的。
