Cutaneous corpuscles of the cephalic integument of the Texas rat snake: morphological and electrophysiological studies

Abstract

The structure and function of corpuscle-like structures which were found in the cephalic integument of the Texas rat snake, Elaphe ohsoleta lindheimeri,_were_investigated. These structures, henceforth called touch cotpuscles, were identif~ed by .the ~tereomicroscope as surface elevations. Their number and distribution were determined in cephalic scales of.intact skin and shed exuviae. The num.ber of.touch . corpus·cles per unit area of skin was great.er in. anterior cephalic scales than posterior scales. The· density of touch corpuscles decreased as the . . size of the snakes increa.sed • . The surface morphology of the ·touch corpuscles was studied with the scanning electron microscope. Each .elevation consisted of a crater~ like structure containing a central peg which ext.ended out of th!= circular depressi':ln• Histologically, the touch corpuscle appeared af> an organized group of cells and nerve. fiber:s which evaginated into the epidermis and became encapsulated by epidermal cells. Studies using Sudan black B. ·and silver impregnation stains r~vealed .. that 3 or 4 myelinated nerve fibers (5 p diameter) approached the base of a corpuscle, became unmyelinated~ and branched into 2 or 3 smaller fibers (1 p diameter). which entered the corpuscle. Fibers were traced near their terminatiOns in the apex of the corpuscle with ·Bodian protargol nerve stain. Ultrastructurally, the touch corpuscle possessed axially positioned central cells whose attentuated processes extended towards the periphery. Unmyelinated nerve fibers were grouped between processes, and each fiber retained its Sch,;vann cell investment. An enveloping basal lamina and adjacent circumferential layer of unorganized collagen fibers surrounded the inner components of the corpuscle. Out~r surrounding·epidermal cells ( po!:jsessed fewer bundles of keratin tonofilaments and increC).sed numbers of ·mitochondria·than regular adjacent epidermis. The structure of each cell layer of the epidermis was described and the fine structure of lipids and cholest.erol ester clefts within the lacunar layer was studi·ed. The re~ponse ptoperties of 146 sitigle nerve fibers teased from the superficial branch of the maxillary nerves of .11 rat· snakes were s~udied. It was determined that . the touch corpuscle. was a rapidly. adapting me.chano- . receptor which was optimally excited by the movement of a mechanical stimulus across the surface peg. Receptive fields, stirimlus threshplds, adaptation rates, and adequate stimuli were determined for other noncorpuscle nerve fibers •. It was conclud~d.that the touch :corpuscle of the Texas rat snake is a structurally and functionally unique mechanoreceptor.