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

The fine structure of sensory nerve endings and their topographical relatio nships with surrounding tissues were examined by a combination of scanning and transmission electron microscopy in order to analyze the mechanism of m echanoreception. Observations were reported on Ruffini endings in periodont al ligaments of rat incisors, and on longitudinal lanceolate endings surrou nding rat sinus hairs, Both types of receptors exhibited the triplet struct ure known as the axon-Schwann cell complex; a flattened axon terminal was s andwiched between two Schwann cell lamellae. The two receptor types additio nally revealed their specific modifications at each distal end, where fine tuft-ne 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 abo ut collagen bundles in favor of continuous transmission of tissue distortio ns to their accompanying axon fingers. In the lanceolate endings of sinus h air follicles, the Schwann cell tufts were suspended in an amorphous matrix with only their end feet anchored to rigid connective tissue elements. Ter minal axon fingers associated with these Schwann cell processes were suppos ed to transiently deflect during acceleration and deceleration phases of a given hair movement because of inertia. The present study proposes the term inal tuft of Schwann cell processes and their accompanying axon fingers as a structural complex which potentially contributes to mechano-electric tran sduction.