OBJECTIVE: To determine the nature and distribution of afferent fibers to the interspinous tissues and facet joints of the lumbar spine in the rat.
DESIGN: Dissection and photography of medial branch of the primary dorsal ramus; histological and electron microscopic examination of the medial branch; measurement of conduction velocities of fibers within the medial branch; recording of compound nerve activity in medial branch in response to mechanical and chemical stimulation of interspinous tissues and facet joints.
RESULTS: In the rat, the medial branch of the primary dorsal ramus of lumbar spinal nerves is normally distributed to the facet joints and interspinous tissues one and two segmental levels caudad to its origin. This nerve contains unmyelinated and myelinated afferents with conduction velocities within the ranges of C fibers, and A-delta and A-beta fibers. The tissues served by this nerve are sensitive to mechanical and noxious chemical stimulation.
CONCLUSIONS: There are many structural and functional similarities in the innervation of the lumbar spine in rats and humans. However, there are anatomical variants and, in rats, the medial branch of the primary dorsal ramus, which serves the interspinous tissues and facet joints, is distributed more caudally than in humans. This information should be taken into account in extrapolating experimental results from rats to the human situation.
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