Sensory innervation in the inner conical body of the vibrissal follicle-sinus complex of the rat.

The innervation of the inner conical body of the vibrissal follicle-sinus complex of the rat was examined by high-voltage and conventional transmission electron microscopy of serial and semi-serial sections. The inner conical body is innervated by axons supplied almost exclusively by several superficial vibrissal nerves that arise from the infraorbital branch of the trigeminal nerve and converge upon the neck of the follicle-sinus complex. Each superficial vibrissal nerve contains a few A delta myelinated axons and several bundles of 20-30 unmyelinated axons. These axons enter the inner conical body and distribute circumferentially within 7-10 ring-like arrays that encircle the vibrissal follicle and are stacked through the superficial-to-deep extent of the inner conical body. Each ring consists of 1 or 2 myelinated axons and several small bundles of 2-15 unmyelinated axons enclosed in sheaves of parallel collagen fibrils. Myelinated axons provide exclusively lanceolate endings that may arise at the termination of the axon or at nodes of Ranvier. Within the small bundles, unmyelinated axons individually terminate in succession as abrupt cytoplasmic swellings referred to as cytoplasmic blebs, which contain mitochondria or clusters of clear or dense-core vesicles. Because of their affiliation with collagen fibrils and the proximity of myelinated axons, the blebbed endings may have been misinterpreted as Ruffini endings in previous studies. Their structure, distribution, and origin from unmyelinated axons suggest that the blebbed endings may constitute a unique array of low-threshold C-mechanoreceptors.