Srinivasu Kallakuri

Pain Generation in Lumbar and Cervical Facet Joints

Facet joints are implicated as a major source of neck and low-back pain. Both cervical and lumbar facet syndromes have been described in the medical literature. Biomechanical studies have shown that lumbar and cervical facet-joint capsules can undergo high strains during spine-loading. Neuroanatomic studies have demonstrated free and encapsulated nerve endings in facet joints as well as nerves containing substance P and calcitonin gene-related peptide. Neurophysiologic studies have shown that facet-joint capsules contain low-threshold mechanoreceptors, mechanically sensitive nociceptors, and silent nociceptors. Inflammation leads to decreased thresholds of nerve endings in facet capsules as well as elevated baseline discharge rates. Recent biomechanical studies suggest that rear-end motor-vehicle impacts give rise to excessive deformation of the capsules of lower cervical facet joints. Still unresolved is whether this stretch is sufficient to activate nociceptors in the joint capsule. To answer this question, recent studies indicate that low stretch levels activate proprioceptors in the facet-joint capsule. Excessive capsule stretch activates nociceptors, leads to prolonged neural afterdischarges, and can cause damage to the capsule and to axons in the capsule. In instances in which a whiplash event is severe enough to injure the joint capsule, facet capsule overstretch is a possible cause of persistent neck pain.

Effect of Nucleus Pulposus on the Neural Activity of Dorsal Root Ganglion

Study Design. This study was designed to investigate, using neurophysiologic techniques in an in vivo rat model, the effect of application of nucleus pulposus to the nerve root on the neural activity of the dorsal root ganglion and the corresponding receptive fields. Objectives. To assess a further role of the dorsal root ganglion in mechanisms of radicular pain in lumbar disc herniation. Summary of Background Data. It has been suggested that the epidural application of autologous nucleus pulposus without mechanical compression causes nerve root inflammation and related radicular pain in lumbar disc herniation. Concerning the dorsal root ganglion, its mechanical hypersensitivity and potential for generating ectopic discharges have been reported. However, the effect of autologous nucleus pulposus on the dorsal root ganglion is uncertain. Methods. In adult Sprague-Dawley rats spontaneous neural activity was recorded from the surgically exposed L5 dorsal root using electrophysiologic techniques, and the mechanosensitivity of L5 dorsal root ganglia and corresponding receptive fields on the hind paw were measured using calibrated nylon filaments. Autologous nucleus pulposus from the tail or fat was implanted at the L5 nerve root. Neural activity was monitored for 6 hours. Results. Spontaneous neural activity in the nucleus pulposus group gradually increased and showed significant differences compared with the fat group from 2.5 to 6 hours after exposure. The mechanosensitivity of the dorsal root ganglia showed significant increases compared with the fat group. Conclusions. After application of nucleus pulposus to the nerve root, the dorsal root ganglion demonstrated increased excitability and mechanical hypersensitivity. These results suggest that nucleus pulposus causes excitatory changes in the dorsal root ganglion.

Demonstration of Substance P, Calcitonin Gene-Related Peptide, and Protein Gene Product 9.5 Containing Nerve Fibers in Human Cervical Facet Joint Capsules

Study Design. Human cervical facet joint capsules were evaluated by immunohistochemistry. Objectives. To study the neuropeptide innervation of the cadaveric cervical facet joint capsules. Summary of Background Data. Various clinical and biomechanical studies indicate a role for cervical facet joint capsules in the etiology of neck pain. However, studies on innervation of these capsules are very limited. There is also a dearth of studies on the neuropeptide nature of this innervation. Methods. Facet joint capsules harvested from unembalmed cadavers were studied by the avidin biotin peroxidase method for the presence of nerve fibers. Neuropeptide innervation was investigated by using antisera to substance P and calcitonin gene-related peptide. Antisera to protein gene product 9.5 (PGP 9.5), a general neuronal marker, were also used. Results. In a study of 12 human cervical facet joint capsules, short segments of substance P were observed in 6 capsules, while fibers reactive to calcitonin gene-related peptide were demonstrated in 7 capsules. Nerve fibers immunoreactive to protein gene product 9.5 were also observed in 9 of the 14 capsules studied. Protein gene product 9.5 reactive fibers were the most extensively distributed fibers, observed as bundles and also as single fibers. Conclusions. An abundance of protein gene product 9.5 reactive nerve fibers indicates an extensive innervation of the cervical facet joint capsules. The presence of substance P and calcitonin gene-related peptide reactive nerve fibers in a population of these lends credence to cervical facet joint capsules as a key source of neck pain.

Effects of phospholipase A2 on lumbar nerve root structure and function.

Study Design. To investigate the effects of phospholipase A2 on the neurophysiology and histology of rat lumbar spinal nerves and the corresponding behavioral changes. Objectives. To study possible mechanisms of sciatica. Summary of Background Data. The pathophysiology of sciatica is uncertain, although mechanical, chemical, and ischemic factors have been proposed. Methods. Phospholipase A2 was injected into the rat L4‐L5 epidural space, and the rats were observed for 3 or 21 days. Behavioral studies were conducted daily during the survival period. On the 3rd or 21st day, extracellular nerve recordings were made from dorsal roots, to determine discharge properties and mechanical sensitivity. The nerve roots were then sectioned for a light‐microscopic examination. Results. Motor weakness of hind limbs and altered sensation were observed. In the 3‐day phospholipase A2 groups, squeezing the dorsal roots at the L4‐L5 disc level (force = 0.8 g) evoked sustained ectopic discharge that lasted approximately 8 minutes. Squeezing the roots distal to the L4‐L5 area did not result in sustained discharges. In sham, control, and 21‐day phospholipase A2 groups, squeezing the dorsal roots elicited only a transient firing that lasted approximately 0.1 second. Loss of myelin was seen in the nerve root cross sections in the 3‐day group, and remyelination was observed in the 21‐day group. No abnormality was found in the control groups. Conclusions. Based on these studies, it is hypothesized that phospholipase A2 causes demyelination that results in hypersensitive regions where ectopic discharge may be elicited by mechanical stimulation. These ectopic discharges may be a source of sciatica. We believe that, as long as these irritating factors are present, the hypersensitive nerve root nerve will continue to fire, and sciatic pain will persist.

Distribution of A-δ and C-fiber receptors in the cervical facet joint capsule and their response to stretch

BACKGROUND It has been proposed that cervical facet joint capsules are a major source of whiplash pain. However, there is a paucity of neurophysiologic data to support this hypothesis. The purposes of this study were to determine the distribution of A-delta and C-fiber sensory receptors in the facet joint capsule and to test their patterns of response to stretch and related sensory function. METHODS Laminectomy from C4 to C7 was performed in seventeen goats, while they were under general anesthesia, to expose the C6 nerve roots. Customized dual bipolar electrodes were used to record neural activity from one of the C6 branches. An 8 or 15-V electrical stimulus was used to provoke receptor activity in nine designated areas on the dorsal part of the C5-C6 facet joint capsule. Receptors were classified on the basis of conduction velocities. The waveform of an identified receptor was set up as a template to determine its neural activity in response to capsular stretch. The characteristics of each single receptors response to capsular stretch were analyzed to determine its sensory function as a mechanoreceptor or nociceptor. RESULTS Two hundred and forty-eight receptors on the dorsal part of the C5-C6 facet joint capsule were evoked by electrical stimulation in the seventeen goats. More C-fiber receptors were found on the dorsolateral aspect of the facet joint capsule, where tendons and muscles were attached. The response to stretch of 120 receptors, from twelve goats, were analyzed to classify them into one of four categories (high-threshold mechanoreceptors, non-saturated low-threshold mechanoreceptors, saturated low-threshold mechanoreceptors, and silent receptors) or as unclassified receptors. CONCLUSIONS The existence of receptors in the facet joint capsule indicates that the capsule has pain and proprioceptive sensory functions.

Effects of Nucleus Pulposus on Nerve Root Neural Activity, Mechanosensitivity, Axonal Morphology, and Sodium Channel Expression

Study Design. This study analyzed the effects of autografted nucleus pulposus on nerve root axon morphology, neurophysiologic function, and sodium channel expression. Objectives. To investigate the chronic effects of the epidural implantation of nucleus pulposus on nerve root morphology, neural activity, ectopic discharge, mechanosensitivity, and sodium channel expression. Summary of Background Data. It has been reported that ectopic discharges were recorded antidromically from sural nerve on compressing nucleus pulposus exposed spinal nerves. However, it is not clear what the effects of nucleus pulposus are on ectopic discharges recorded directly from the spinal nerve roots. It is also not clear what the effects of nucleus pulposus are on the threshold pressure to provoke ectopic discharges in the spinal nerves. Sodium channel content increases in remodeling axons after nerve injury, but it is not clear what the effects of nucleus pulposus are on sodium channel expression in spinal nerve. Methods. Forty-six male Sprague-Dawley rats were used, 20 in a nucleus pulposus-implanted group, 18 in a fat-implanted group, and 8 in a normal group. Fresh autografted nucleus pulposus or fat tissue was implanted into the dorsal epidural space at the L4–L5 disc level. On the 7th, 21st, or 42nd day, neurophysiologic recordings were made to determine nerve root response to compression. Nerve roots were then harvested to determine sodium channel protein concentration and histologic changes in the nerve root. The correlations between sodium channel density and neural activity and mechanosensitivity of dorsal root were analyzed statistically. Results. Ectopic discharge rate was higher in nucleus pulposus 7-day group. Threshold pressure to evoke ectopic discharges was lower in the nucleus pulposus 7-day group, and higher in the nucleus pulposus 42-day group compared to the normal group. Sodium channel protein density increased in the nucleus pulposus 7-day and nucleus pulposus 21-day group compared to normal nerve. Sodium channel density changes were not correlated to threshold pressure. Ectopic discharge rate increased with increase of sodium channel density in the nerve roots. The number of axons with neuropathy increased in the nucleus pulposus 7-day and 21-day groups. Conclusions. Acute exposure of nerve root to nucleus pulposus resulted in increased number of axons with neuropathy, higher intensity of ectopic discharges on compression, and nerve mechanosensitization. Chronic exposure resulted in mechanical desensitization. Changes of sodium channel density were correlated to ectopic discharge rate.

An immunohistochemical study of innervation of lumbar spinal dura and longitudinal ligaments.

Study Design. An immunocytochemical study of nerve fibers in lumbar spinal dura and longitudinal ligaments was conducted in New Zealand white rabbits. Objectives. To demonstrate the presence of nerve fibers and to establish the presence of nociceptive and sympathetic nerve fibers in lumbar dura and longitudinal ligaments. Summary of Background Data. The role of dura as a source of low back pain is still unclear, and the data present a somewhat conflicting picture of the nature of nociceptive innervation in this tissue. Methods. An immunocytochemical method was used to study dura and longitudinal ligaments from New Zealand White rabbits. Results. Numerous fine nerve fibers and some small bundles were demonstrated in both the dura and the longitudinal ligaments. In dorsal dura, the fibers were seen at lateral margins running toward midline. In ventral dura and longitudinal ligaments, the fibers were seen throughout the substance of these tissues. A population of substance P, calcitonin gene‐related peptide, and tyrosine hydroxylase‐reactive nerve fibers were observed in all the tissues. In addition, fibers exhibiting nicotinamide adenine dinucleotide phosphate diaphorase activity were also observed, indicating the presence of nitric oxide in dura. Conclusions. The results clearly demonstrate an extensive distribution of nerve fibers in dura and longitudinal ligaments. The presence of a significant number of putative nociceptive fibers supports a possible role for these structures as a source of low back pain and radicular pain.

Recording of Neural Activity From Goat Cervical Facet Joint Capsule Using Custom-designed Miniature Electrodes

Study Design. To establish a methodology for the neurophysiologic study of mechanoreceptors in the cervical facet joint capsule. Objectives. To test a custom designed miniature dual bipolar electrode for recording the neural activity in cervical dorsal roots. To determine if the neural activity from different receptors in the capsule can be differentiated using this methodology. Summary of Background Data. Injury to cervical facet joint capsules has been regarded as an important source of whiplash pain, but no neurophysiologic study has been performed to demonstrate or characterize sensory nerve function in the capsule. Methods. Nineteen goats weighing 34 to 55 kg were used under general anesthesia. A C4–C6 laminectomy was performed to expose the C6 nerve root. Custom designed miniature dual bipolar electrodes were used to record neural activity in the left C6 branches. Electrical and mechanical stimuli were used to evoke receptor activity in the dorsal aspect of the C5/6 capsule. Conduction velocities (CVs) of evoked units were determined by electrical stimulation and dual-bipolar-electrode recording methods. The units were classified based on their CVs. The waveform of each classified unit was saved as a template for later single unit discharge search among multiunit discharges during the stretch of the capsule. The C5/6 facet joint with capsule was pulled by a computer-controlled actuator instrumented with a load cell at a rate of 0.5 mm per second. The evoked neural activity and load were recorded, digitized, and analyzed to determine CV, discharge rate, and response to the stretch. Results. Miniature bipolar electrodes recorded the neural activity in both channels, with single unit CVs being measured. There was no discernible motion between the electrode and dorsal root when the capsule was pulled. Both local compression and stretch on capsule evoked multiunit discharges. A-&bgr;, A-&dgr;, and C-fiber units were found among these multiunit discharges. The rate of single unit and multiunit discharges increased during capsule stretch in the physiologic range and afterdischarges occurred beyond the physiologic range. Conclusions. The novel miniature electrodes not requiring a micromanipulator made it feasible and reliable to record neural activity from short cervical spinal roots. Waveforms of different units could be identified, making it possible to study sensory functions of the facet joint capsule. A-&bgr;, A-&dgr;, and C-fiber units were found responding to mechanical stimuli, indicating that facet joint capsule has functional proprioceptors and nociceptors.

Quantitative Relationship between Axonal Injury and Mechanical Response in a Rodent Head Impact Acceleration Model

A modified Marmarou impact acceleration model was developed to study the mechanical responses induced by this model and their correlation to traumatic axonal injury (TAI). Traumatic brain injury (TBI) was induced in 31 anesthetized male Sprague-Dawley rats (392±13 g) by a custom-made 450-g impactor from heights of 1.25 m or 2.25 m. An accelerometer and angular rate sensor measured the linear and angular responses of the head, while the impact event was captured by a high-speed video camera. TAI distribution along the rostro-caudal direction, as well as across the left and right hemispheres, was determined using β-amyloid precursor protein (β-APP) immunocytochemistry, and detailed TAI injury maps were constructed for the entire corpus callosum. Peak linear acceleration 1.25 m and 2.25 m impacts were 666±165 g and 907±501 g, respectively. Peak angular velocities were 95±24 rad/sec and 124±48 rad/sec, respectively. Compared to the 2.25-m group, the observed TAI counts in the 1.25-m impact group were significantly lower. Average linear acceleration, peak angular velocity, average angular acceleration, and surface righting time were also significantly different between the two groups. A positive correlation was observed between normalized total TAI counts and average linear acceleration (R(2)=0.612, p<0.05), and time to surface right (R(2)=0.545, p<0.05). Our study suggested that a 2.25-m drop in the Marmarou model may not always result in a severe injury, and TAI level is related to the linear and angular acceleration response of the rat head during impact, not necessarily the drop height.

Structural and Functional Changes in Nerve Roots Due to Tension at Various Strains and Strain Rates: An In-Vivo Study

This study investigates the functional and structural responses of spinal nerve roots in vivo to various strains and strain rates. Seventy-two L5 dorsal nerve roots from male Sprague-Dawley rats were each subjected to a predetermined strain (<10%, 10-20%, and >20%; n = 8) and rate (0.01 mm/sec, 1 mm/sec, or 15 mm/sec; n = 24). Neurophysiologic recordings were performed before and after stretch to determine changes in conduction velocity (CV), amplitude, and area of the compound action potential (CAP). Morphological injury as evident by primary and secondary axotomy as well as impaired axoplasmic transport (IAT) was determined using the palmgren silver impregnation technique and betaAPP immunostaining, respectively. The results from neurophysiologic recordings indicate that as strain and rate increased, there was a decrease in CV, amplitude, and area of the CAP. Further, high strains led to a complete conduction block that appeared to be rate dependent. Strains of 16%, 10%, and 9%, at 0.01 mm/sec, 1 mm/sec, and 15 mm/sec, respectively, led to 50% probability of complete conduction block in the nerve roots. Results from histological assessment indicate an increase in periaxonal spacing (secondary axotomy) and torn fibers (primary axotomy), as well as impaired IAT, with increasing strain and rate. Overall, the results from the current study indicate that (1) functional nerve root injuries as evident by changes in the CV, amplitude, and area of the CAP are strain- and rate-dependent; (2) high strains at low rates cause complete conduction block in the roots, while a similar block was observed at lower strains at the high rate; (3) the extent of IAT and primary and secondary axotomy occurred concomitant with functional injury and were strain- and rate-dependent.