Dorsal Horn Plasticity Relating to Low Back Pain



WJ Roberts, RG Gillete and RC Kramis

Dow Neurological Sciences Institute

Good Samaritan Hospital and Medical Care

Portland, Oregon, 97212, USA

Acute and chronic low back pain are major medical problems in the industralized world. However, the origins of these pains are poorly understood, and few studies of nociceptive systems have been directed specifically toward the lumbar region.

We have begun studies of somatosensory systems serving the low back in pentobarbital anesthetized cats and have found specific differences in afferent projections into the dorsal horn and in the characteristics of dorsal horn neurons subserving proximal regions (including the lumbar region, hip, and/or proximal hindlimb) compared to the analogous elements serving the distal hindlimb.

The most distinctive difference observed was in the complexity of afferent input onto dorsal horn neurons. Most (72% of 118) of those having proximal receptive fields received excitatory input from both dorsal and ventral compartments, including skin, and paraspinal or proximal hindlimb muscles, and other deep tissues such as zygapophysial joint capsules, tendons, periosteum, intervertebral discs, and dura. Fewer neurons had receptive fields restricted to just skin (23%) and very few to just deep tissues (5%). Most previous studies of neurons with distal receptive fields reported a predominance of cutaneous input.

The receptive fields of neurons having proximal receptive fields were much larger than those reported for neurons with distal fields, typically including part of the low back, and hip and proximal hindlimb. Many (82%) had receptive fields that crossed the midline, unlike the strictly ipsilateral receptive fields of neurons with distal fields.

The patterns of responding to cutaneous stimuli in neurons with proximal receptive fields were qualitatively similar to those of neurons with distal receptive fields. WDR (multireceptive units (69%) typically had an area of low mechanical threshold superimposed on a larger high-threshold area. However, responding to mechanical stimulation of muscles, joint capsules and other deep tissues appeared to be continuously graded over a wide range of stimulus intensities. Other neurons responded maximally to low intensity stimuli (8%) or were initially nociceptive specific (22%).

Noxious stimulation (bradykinin or 6% saline injections, or 60s skin pinch) were used as a conditioning stimulus in testing of 34 units. Most (65%) showed persistent increases in responding to non-noxious mechanical stimuli and expansions of both low-threshold and high-threshold receptive fields. In some, the high-threshold field expanded from the hip region to include the entire hindquarter and low back. Significantly, expansions of deep-tissue fields were also noted. The expanded receptive fields retracted slowly over several hours in some units. Five neurons that were initially nociceptive-specific became clearly responsive to non-noxious stimulation after noxious conditioning. Injection of algogens onto a zygapophysial joint was, for many units, the most effective conditioning stimulus.

In summary, most nociceptive neurons recorded from the lateral dorsal horn in this study had very complex receptive fields in both skin and deep tissues of the low back, hip and proximal hindlimb. These neurons showed marked increases in responding and expansion of receptive fields following brief noxious conditioning stimuli. Such neurons are well suited to subserve diffuse low back pain, which can occur after focal injury to any one of several lumbar tissues.

The Canadian Chiropractic Association.


By W.J. Roberts; R.G. Gillette and R.C. Kramis

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