Ultrastructural features of functionally identified primary afferent neurons with C (unmyelinated) fibers of the guinea pig: classification of dorsal root ganglion cell type with reference to sensory modality.

Intracellular labeling of neurons permitted a direct correlation of neuronal profiles with sensory modality of cutaneous receptors. In the guinea pig, 47 neurons in the dorsal root ganglion (displaying C-fiber conduction velocities) were labeled with horseradish peroxidase (HRP) by iontophoresis after determining the sensory modality. Receptive field were explored with systematic "natural" stimuli. Cell areas of all C-fiber units were measured by tracing the cellular contour in light microscopy. The mean cellular diameter calculated from cell areas was 21.8 microns in the second cervical ganglion of the guinea pig. Mean cell diameter for high-threshold mechanoreceptors was 20.9 microns, 24.6 microns for polymodal nociceptors, and 25.7 microns for mechanical-cold nociceptors. Electron microscopic observations showed that all labeled neurons of C-fiber units had profiles of small, dark type-B neurons. Neurons representative of each sensory modality exhibited different cell features, each belonging to a distinct subtype of small B neurons. High-threshold mechanoreceptor and mechanical-cold nociceptor displayed a peripheral lamellar arrangement of cisternae of endoplasmic reticulum (ER), corresponding to the B1 subtype. Polymodal nociceptor units were characteristically of the B2 subtype, in which stacks of long and short cisternae of ER were distributed randomly throughout the cytoplasm, and the arrangement of Golgi bodies varied among these cells. Cooling receptors displayed poorly developed, flattened cisternae of ER and numerous vesicles, typical of the B3 subtype. These results imply that all C-fiber cells belong to the small B-type cell category and that the ultrastructural features of the neuron in the dorsal root ganglion may reflect the sensory modality of the receptive field.