An in vitro model was developed to investigate the responses of afferent units in the lumbar spine to controlled loading as measured by a load-cell. The neuronal discharge was recorded simultaneously with loading. Three types of neuronal responses were observed. The first type of response involved phasic-type mechanoreceptors, which responded to movement, regardless of direction or initial position. The response did not outlast the movement phase of loading. These units may serve as velocity detectors. The second type of response was seen in slowly adapting low-threshold mechanoreceptors, which tended to respond to loading in the 0.3 to 0.5-kilogram range with an immediate and sustained increase in the rate of firing. This type of response appears to be associated with the activation of low-threshold group-II and group-III fibers, which were located in muscles and tendons inserting into the facet joint. The third type of response involved slowly adapting high-threshold mechanoreceptors, which could not be activated until a threshold of three to five kilograms had been exceeded. It appears that this type of response is at least partially due to the activation of high-threshold group-III and group-IV capsular afferent units, which may signal noxious mechanical stimulation.

CLINICAL RELEVANCE:​ This study demonstrated that mechanical loading of the lumbar spine, which resulted in posterolateral bending, activated low and high-threshold sensory fibers. It is hypothesized that capsular stretching in humans, even in the absence of other pathological changes, may activate capsular mechanoreceptors and nociceptors that may play a role in the initiation of facet-joint syndrome.