The purpose of this study was to investigate neurophysiologically and anatomically the soft tissues of the dorsal compartment of the lumbar spine in order to understand better their possible role in low back pain. The focus was primarily on the lumbar facet joint and supraspinous ligament of the Sprague–Dawley rat. Microdissection of the dorsal ramus and electrophysiological and neuroanatomical studies of the dorsal ramus and its terminations in paravertebral tissue revealed that (a) there are mechanosensitive, slowly adapting fibers in the rat lumbar facet joint capsule;​ (b) there are slowly adapting, mechanosensitive units in the rat supraspinous ligaments that respond to tensile loading;​ both types of mechanosensitive units have high threshold;​ (c) mechanical stimulation of these tissues sometimes elicits afterdischarges lasting several minutes;​ (d) many extracellular recordings from the medial branch of the dorsal ramus appear to be reflex activity to mechanical stimulation;​ (e) silver impregnation of the rat joint capsule reveals individual axons, very few of which were encapsulated, suggesting that they terminate in free nerve endings;​ and (f) the nerves of the rat facet joint capsule contain 68, 160, and 200 kdalton polypeptide subunits of neurofilament protein (NFP). These results indicate that neurons of lumbar facet joint capsules and ligaments in the back are sensitive to mechanical strain and that the higher threshold neurons may serve a nociceptive (pain) function.