Three-dimensional microanatomy of mechanoreceptors and their possible mechanism of sensory transduction.

The fine structure of sensory nerve endings and their topographical relationships with surrounding tissues were examined by a combination of scanning and transmission electron microscopy in order to analyze the mechanism of mechanoreception. Observations were reported on Ruffini endings in periodontal ligaments of rat incisors, and on longitudinal lanceolate endings surrounding rat sinus hairs. Both types of receptors exhibited the triplet structure known as the axon-Schwann cell complex; a flattened axon terminal was sandwiched between two Schwann cell lamellae. The two receptor types additionally revealed their specific modifications at each distal end, where fine tuft-like processes of Schwann cells projected into surrounding tissues with finger-like projections of an axon terminal attached to their bases. In the Ruffini endings of the periodontal ligament, the terminal tufts coiled about collagen bundles in favor of continuous transmission of tissue distortions to their accompanying axon fingers. In the lanceolate endings of sinus hair follicles, the Schwann cell tufts were suspended in an amorphous matrix with only their end feet anchored to rigid connective tissue elements. Terminal axon fingers associated with these Schwann cell processes were supposed to transiently deflect during acceleration and deceleration phases of a given hair movement because of inertia. The present study proposes the terminal tuft of Schwann cell processes and their accompanying axon fingers as a structural complex which potentially contributes to mechano-electric transduction.