Evidence for unmyelinated sensory fibres in the posterior columns in man.

The human posterior columns are generally described as a myelinated fibre pathway. The present study demonstrates that more than 25% of the component axons are unmyelinated. Many of these unmyelinated axons are labelled by antibodies to calcitonin gene-related peptide (CGRP), a substance found in sensory cells, axons and terminals. On this basis we suggest that there are significant numbers of unmyelinated primary afferent axons in the human posterior columns. These results indicate that unmyelinated sensory axons are more widespread than previously thought, and that they should be taken into account when considering stimulation of the posterior columns to relieve pain.