An in vivo preparation for investigating neural responses to controlled loading of a lumbar vertebra in the anesthetized cat

Abstract

This paper describes a method for applying controlled loads to a lumbar vertebra while recording in vivo from primary afferents innervating the lumbar paraspinal tissues. Unlike the appendicular skeleton, the vertebral column poses a unique challenge for neurophysiological investigations. Distances between paraspinal tissues and the spinal cord are short. In addition, substantial removal of the paraspinal tissues to access the spinal roots or spinal cord appears necessary. The preparation described in this report takes advantage of the anatomical fact that the L6 dorsal root enters the spinal cord 2-2.5 vertebral segments rostral to its passage through the intervertebral foramina. This effectively lengthens the distance between the lumbar paraspinal tissues and central recording sites. The preparation has five unique features: (1) the L6 and L7 vertebrae remain intact; (2) lumbar paraspinal tissues and their attachments to the L6 and L7 vertebrae remain intact on one side of the vertebral column; (3) the intact L6 vertebra can be loaded at its spinous process; (4) the magnitude of the load applied at the L6 spinous process can be controlled with a feedback motor; (5) the direction of load relative to the long axis of the spine can be controlled. Using this preparation, single unit recordings were obtained from the L6 dorsal root during controlled loading of the L6 lumbar vertebra at its spinous process. The responses of two paraspinal muscle proprioceptors to vertebral loading are characterized in this report. With existing electrophysiological techniques this preparation can be used to study central processing of paraspinal inputs. By combining mechanical loading of the lumbar spine in the presence of inflammatory mediators this preparation can contribute to the understanding of the mechanisms by which interactions between mechanical and chemical stimuli likely produce low back pain.