Unmyelinated fibers in human spinal ventral roots: C4 to S2

Study design: Histological examination of human spinal ventral roots. Objectives: To determine the proportion of unmyelinated fibers in human ventral roots from the 4th cervical (C4) to 2nd sacral (S2) segment, and to evaluate differences in the proportions of unmyelinated fibers between the cervical, thoracic, lumbar and sacral segments, and between autonomic and other segments. Setting: University Teaching Hospital, Busan, Korea. Methods: Eight embalmed adult human cadavers (six males and two females; mean age 56.3 years) were collected. The ventral root samples were obtained by transverse cuts of the ventral roots within 1 cm proximal to the medial portion of the dorsal root ganglion from the C4 to S2 segment. The number of unmyelinated and myelinated fibers was counted in four fields, and the mean number of unmyelinated fibers was calculated. The percentage of unmyelinated fibers was calculated from the ratio of unmyelinated fibers to total fibers (myelinated fibers + unmyelinated fibers). Results: The mean percentages of unmyelinated axons in cervical (C4-C8), thoracic (T1-T12), lumbar (L1-L5) and sacral (S1-S2) ventral roots were 16.3, 21.4, 17.8 and 20.7%, respectively. The percentage of unmyelinated fibers in thoracic ventral roots was higher than that for other segments (P<0.001). There was no significant difference in proportions of unmyelinated fibers between the sympathetic segments (T11-L2), parasympathetic segments (S2) and the other segments (C4-T10 and L3-S1) (P = 0.1784). Conclusions: Approximately 20% of human spinal ventral root fibers were unmyelinated. The proportion of unmyelinated fibers was highest in the thoracic segments.