The anatomy of human lower lumbar zygapophyseal joint postero-medial fibrous capsules, and their synovial folds, was investigated by high-power light microscopy and transmission electron microscopy. A description of nerves in the postero-medial joint capsule and the synovial folds is presented. The diameter of paravascular myelinated nerve fibres in the synovial folds was 0.2 micron (micron). Small myelinated nerves (0.6 to 12 microns) course through the synovial folds, not in association with blood-vessels, suggesting that these nerves are nociceptive and, therefore, may have clinical significance in spinal pain.