Responses of Mechanosensitive Afferents to Manipulation of the Lumbar Facet in the Cat

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Abstract
The response of mechanosensitive afferent nerve endings in the lumbar spine to manipulation of a lumbar facet isolated using a unique surgical approach was studied in anesthetized adult cats. To characterize sensory nerve endings in the lumbar spine with respect to their receptive field and conduction velocity and to assess their response to facet joint motion. Previous studies have identified the presence of encapsulated endings in normal human facet capsules and have documented the presence of mechanosensitive units responsive to spinal loading. Previous neurophysiologic studies have used preparations that stripped all paraspinous musculature away from the field to expose the facets and lamina. A unique hemilaminectomy approach was developed that permitted physiologic loading of the lumbar facet without disturbing its overlying musculature. Recordings of single unit afferent activity were made from fine filaments teased from the L6 dorsal root. Response to L5-L6 facet motion was studied by applying cranial, craniomedial, and medial distractive forces and lateral compressive forces to the facet joint. Single unit recordings were obtained from 16 afferents with receptive fields in the lumbar spine. Seven of 16 afferents had receptive fields in or near the facet, and the remaining nine afferents had receptive fields in paraspinal tissues some distance from the facet joint. There were nine Group II afferents, three Group IV, and four unclassified afferents. The majority of endings responded in a graded fashion relative to the direction of force applied. Mechanosensitive endings in the lumbar spine show graded sensitivity to the direction of facet manipulation. These Group III and IV afferents can reside some distance from the facet joint and remain sensitive to facet motion.

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