Joel G. Pickar

Mechanoreceptor endings in human thoracic and lumbar facet joints

Study Design. Histologic analysis of normal human facet capsules to determine the density and distribution of encapsulated nerve endings in the thoracic and lumbar spine. Objectives. To quantify the extent of mechanoreceptor innervation in normal facet tissues and determine the relative distribution of three specific receptor types with respect to thoracic and lumbar segments. Summary of Background Data. Ongoing studies of spinal innervation have shown that human facet tissues contain mechanoreceptive endings capable of detecting motion and tissue distortion. The hypothesis has been advanced that spinal proprioception may play a role in modulating protective muscular reflexes that prevent injury or facilitate healing. Methods. Whole facet capsules harvested from seven healthy adult patients were processed using a gold chloride staining method and cut into 35‐micron sections for histologic analysis. No sampling was performed; all sections were analyzed. Receptor endings were classified by the method of Freeman and Wyke if they met the following three criteria: 1) encapsulation, 2) identifiable morphometry, and 3) consistent morphometry on serial sections. Results. One Type 1 and four Type 2 endings were identified among 10 thoracic facet capsules. Five Type 1, six Type 2, and one Type 3 ending were identified among 13 lumbar facet capsules. Occasional atypical receptive endings were noted that did not fit the established classification. Unencapsulated free nerve endings were seen in every specimen, but were not quantified. Conclusions. Encapsulated nerve endings are believed to be primarily mechanosensitive and may provide proprioceptive and protective information to the central nervous system regarding joint function and position. A consistent, but small population of receptors has been found previously in cervical facets, but innervation of the thoracic and lumbar levels is less consistent. This suggests that proprioceptive function in the thoracic and lumbar spine is less refined and, perhaps, less critical than in the cervical spine.

Responses of mechanosensitive afferents to manipulation of the lumbar facet in the cat.

Study Design 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. Objectives 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. Summary of Background Data 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. Methods A unique hemilaminectomy approach was developed that permitted physiologic loading of the lumbar facet without disturbing its overlying musculature. Recordings of single unit affarent 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. Results 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 III afferents, three Group IV, and four unclassified afferents. The majority of endings responded in a graded fashion relative to the direction of force applied. Conclusions 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.

Electrophysiologic evidence for an intersegmental reflex pathway between lumbar paraspinal tissues.

Study Design. Electrophysiologic recordings were obtained from a lumbar paraspinal nerve or muscle in the anesthetized cat while electrically stimulating a paraspinal nerve or facet capsule in an adjacent lumbar segment. A variety of approaches were used to demonstrate the reflex nature of both the nerve and the muscle response. Objective. The primary purpose of this study was to seek electrophysiologic evidence for the presence of intersegmental reflexes between adjacent lumbar vertebral segments. A second purpose of this study was to confirm a previous procedure used to evoke paraspinal reflexes. This previous work had shown that electrical stimulation of the L1–L2 facet joint capsule elicits electromyographic activity from multifidus muscle one to two vertebral segments caudal to the stimulated facet in a porcine preparation. Summary of Background Data. Biomechanical approaches have stressed the need for spinal stability to avoid conditions that could give rise to low back dysfunction. It seems reasonable to believe that reflex interactions between vertebral segments contribute to the sensorimotor integration of lumbar paraspinal tissues. It also seems reasonable to believe that alterations or abnormal elicitation of these reflexes could contribute to biomechanical changes associated with low back pain and paraspinal muscle spasm. Methods. Experiments were performed on 23 &agr;-chloralose anesthetized adult cats. In eight cats the L3, L4, and L5 medial branch from each dorsal ramus was exposed and placed on a bipolar hook electrode. In six cats the L4 medial branch was stimulated and a compound action potential was recorded from the L3 medial branch. In three of the six cats the L5 medial branch was stimulated and a compound action potential was recorded from the L3 medial branch. In one cat the L4 medial branch was stimulated and a compound action potential was recorded from the L5 medial branch. In one cat the L3 medial branch was stimulated and a compound action potential was recorded from the L5 medial branch. At the end of each protocol the medial branch was cut just proximal to the stimulating electrode to confirm that the compound action potential was reflexive in nature and not initiated by volume conduction. In 15 cats three approaches were used to confirm that multifidus electromyographic activity evoked by electrical stimulation of a lumbar facet capsule was reflexive in nature: 1) by anesthetizing the site of the sensory endings, i.e., the facet capsule, 2) by injecting lidocaine intrathecally to block neural conduction centrally, i.e., within the spinal canal, or 3) by cutting the afferent pathway, i.e., the medial branch of the dorsal ramus. Results. Electrical stimulation of the medial branch of the dorsal ramus innervating the medial-most lumbar paraspinal tissues evoked a compound action potential in the medial branch innervating the medial-most paraspinal tissues one and two segments away. Stimulating voltages between 2 and 70 V were necessary to evoke the compound action potential. Each compound action potential was reflexive in nature because cutting the lumbar medial branch proximal to its contact with the stimulating electrode abolished each compound action potential. The conduction velocity of the reflex ranged from 3.5 to 6.1 m/sec. Electrical stimulation of a lumbar facet capsule evoked lumbar multifidus muscle electromyographic activity. However, injecting lidocaine intrathecally or transecting the medial branch of the dorsal ramus had no effect on electromyographic activity. Injecting lidocaine into the facet or into the multifidus muscle around the facet joint (near the stimulating electrode) significantly decreased the magnitude of the multifidus electromyography. Conclusion. These results indicate that afferent impulses conveyed by the medial branch of the dorsal ramus reflexly altered efferent activity to an adjacent lumbar segment. This intersegmental paraspinal reflex may span at least one or two vertebral segments. The data suggest that electrical stimulation of the facet joint capsule may not have reflexly elicited multifidus activity because neither chemical interruption (intrathecal lidocaine) nor physical interruption (nerve transection) of the presumed reflex pathway diminished or abolished the electromyographic response. Volume conduction of the stimulating currents likely elicited multifidus activity during electrical stimulation of the facet capsule. When using electrical stimulation of neural paraspinal tissues to evoke reflex muscle activity, appropriate control experiments must be performed to clearly demonstrate the reflexive nature of the response.

Spinal Manipulative Therapy and Somatosensory Activation

Manually-applied movement and mobilization of body parts as a healing activity has been used for centuries. A relatively high velocity, low amplitude force applied to the vertebral column with therapeutic intent, referred to as spinal manipulative therapy (SMT), is one such activity. It is most commonly used by chiropractors, but other healthcare practitioners including osteopaths and physiotherapists also perform SMT. The mechanisms responsible for the therapeutic effects of SMT remain unclear. Early theories proposed that the nervous system mediates the effects of SMT. The goal of this article is to briefly update our knowledge regarding several physical characteristics of an applied SMT, and review what is known about the signaling characteristics of sensory neurons innervating the vertebral column in response to spinal manipulation. Based upon the experimental literature, we propose that SMT may produce a sustained change in the synaptic efficacy of central neurons by evoking a high frequency, bursting discharge from several types of dynamically-sensitive, mechanosensitive paraspinal primary afferent neurons.

Effect of Spinal Manipulation Duration on Low Threshold Mechanoreceptors in Lumbar Paraspinal Muscles : A Preliminary Report

Study Design. Electrophysiologic recordings were obtained from low threshold primary afferent neurons innervating lumbar multifidus and longissimus muscles in the anesthetized cat. Objective. The purpose of this study was to classify sensory nerve endings in lumbar paraspinal muscles and characterize their responses to biomechanical loads applied over a range of durations that encompass those occurring during spinal manipulation. Summary of Background Data. Neural responses arising from the mechanical input during spinal manipulation are thought to contribute to this maneuver’s therapeutic effects. Because manual therapies are distinguished to a large extent on the basis of the speed with which they are applied, it is important to understand how their rate of application affects the signaling properties of primary afferent neurons innervating paraspinal tissues. If alterations in sensory input do contribute to the mechanism of spinal manipulation’s therapeutic effect, it seems reasonable to expect that these primary afferents would respond to spinal manipulation in some unique fashion. Methods. Experiments were performed on 6 adult cats. A L4–L5 laminectomy was performed and the L6 dorsal roots exposed. The L6–L7 vertebrae and associated paraspinal tissues remained intact bilaterally, including lumbodorsal fascia, multifidus, longissimus, iliocostalis muscles, and deeper tissues. Forceps were clamped tightly onto the lateral surfaces of the L6 spinous process through a thin narrow, slit in the lumbodorsal fascia. Single unit afferent activity was recorded from fine filaments teased from the L6 dorsal root. Instantaneous discharge frequency was calculated. Afferents were classified based on von Frey threshold, conduction velocity, and responses to direct muscle stimulation and to succinylcholine injection. Spinal manipulative-like loads were applied to the L6 vertebra (posterior to anterior) using a programmable electronic feedback control system. Force-time profiles were half-sine waves with durations of 25, 50, 100, 200, 400, and 800 milliseconds delivered at constant magnitudes of 33%, 66%, or 100% body weight. Results. The 6 afferents were classified as low threshold mechanoreceptors based on von Frey thresholds being less than 6 g. Five afferents were Group I or II muscle proprioceptors and one afferent was a Group III muscle mechanoreceptor. The receptive field for 2 of the 6 afferents was in the multifidus muscle and the receptive field of the remaining 4 afferents was in the longissimus muscle. In general, the mean instantaneous discharge frequency for all 6 afferents increased abruptly as the duration of the impulse approached 100 milliseconds. An increase in loading magnitude (33% vs. 66% vs. 100% body weight) did not appear to systematically affect the discharge from the 6 low threshold mechanoreceptors. Conclusions. This preliminary report suggests that abrupt changes in neural discharge (instantaneous frequency) of low threshold muscle mechanoreceptors of the lumbar spine occur as the duration of a biomechanical load approaches that typically used during spinal manipulation. These changes could comprise part of the mechanism contributing to this intervention’s physiologic effects. Further studies are warranted to better understand the signaling properties of a wider range of sensory receptors as well as determine the central effects of these high frequency discharges.

Vertebral position alters paraspinal muscle spindle responsiveness in the feline spine: effect of positioning duration

Proprioceptive information from paraspinal tissues including muscle contributes to neuromuscular control of the vertebral column. We investigated whether the history of a vertebras position can affect signalling from paraspinal muscle spindles. Single unit recordings were obtained from muscle spindle afferents in the L6 dorsal roots of 30 Nembutal‐anaesthetized cats. Each afferents receptive field was in the intact muscles of the low back. The L6 vertebra was controlled using a displacement‐controlled feedback motor and was held in each of three different conditioning positions for durations of 0, 2, 4, 6 and 8 s. Conditioning positions (1.0–2.2 mm dorsal and ventral relative to an intermediate position) were based upon the displacement that loaded the L6 vertebra to 50–60% of the cats body weight. Following conditioning positions that stretched (hold‐long) and shortened (hold‐short) the spindle, the vertebra was repositioned identically and muscle spindle discharge at rest and to movement was compared with conditioning at the intermediate position. Hold‐short conditioning augmented mean resting spindle discharge by +4.1 to +6.2 impulses s−1; however, the duration of hold‐short did not significantly affect this increase (F4,145= 0.49, P= 0.74). The increase was maintained at the beginning of vertebral movement but quickly returned to baseline. Conversely, hold‐long conditioning significantly diminished mean resting spindle discharge by −2.0 to −16.1 impulses s−1 (F4,145= 11.23, P < 0.001). The relationship between conditioning duration and the diminished resting discharge could be described by a quadratic (F1,145= 9.28, P= 0.003) revealing that the effects of positioning history were fully developed within 2 s of conditioning. In addition, 2 s or greater of hold‐long conditioning significantly diminished spindle discharge to vertebral movement by −5.7 to −10.0 impulses s−1 (F4,145= 11.0, P < 0.001). These effects of vertebral positioning history may be a mechanism whereby spinal biomechanics interacts with the spines proprioceptive system to produce acute effects on neuromuscular control of the vertebral column.

Relationship between Biomechanical Characteristics of Spinal Manipulation and Neural Responses in an Animal Model: Effect of Linear Control of Thrust Displacement versus Force, Thrust Amplitude, Thrust Duration, and Thrust Rate

High velocity low amplitude spinal manipulation (HVLA-SM) is used frequently to treat musculoskeletal complaints. Little is known about the interventions biomechanical characteristics that determine its clinical benefit. Using an animal preparation, we determined how neural activity from lumbar muscle spindles during a lumbar HVLA-SM is affected by the type of thrust control and by the thrusts amplitude, duration, and rate. A mechanical device was used to apply a linear increase in thrust displacement or force and to control thrust duration. Under displacement control, neural responses during the HVLA-SM increased in a fashion graded with thrust amplitude. Under force control neural responses were similar regardless of the thrust amplitude. Decreasing thrust durations at all thrust amplitudes except the smallest thrust displacement had an overall significant effect on increasing muscle spindle activity during the HVLA-SMs. Under force control, spindle responses specifically and significantly increased between thrust durations of 75 and 150 ms suggesting the presence of a threshold value. Thrust velocities greater than 20–30 mm/s and thrust rates greater than 300 N/s tended to maximize the spindle responses. This study provides a basis for considering biomechanical characteristics of an HVLA-SM that should be measured and reported in clinical efficacy studies to help define effective clinical dosages.

Effects of thrust amplitude and duration of high-velocity, low-amplitude spinal manipulation on lumbar muscle spindle responses to vertebral position and movement.

OBJECTIVE Mechanical characteristics of high-velocity, low-amplitude spinal manipulations (HVLA-SMs) can vary. Sustained changes in peripheral neuronal signaling due to altered load transmission to a sensory receptors local mechanical environment are often considered a mechanism contributing to the therapeutic effects of spinal manipulation. The purpose of this study was to determine whether variation in an HVLA-SMs thrust amplitude and duration alters the neural responsiveness of lumbar muscle spindles to either vertebral movement or position. METHODS Anesthetized cats (n = 112) received L6 HVLA-SMs delivered to the spinous process. Cats were divided into 6 cohorts depending upon the peak thrust force (25%, 55%, 85% body weight) or thrust displacement (1, 2, 3 mm) they received. Cats in each cohort received 8 thrust durations (0-250 milliseconds). Afferent discharge from 112 spindles was recorded in response to ramp and hold vertebral movement before and after the manipulation. Changes in mean instantaneous frequency (∆MIF) during the baseline period preceding the ramps (∆MIFresting), during ramp movement (∆MIFmovement), and with the vertebra held in the new position (∆MIFposition) were compared. RESULTS Thrust duration had a small but statistically significant effect on ∆MIFresting at all 6 thrust amplitudes compared with control (0-millisecond thrust duration). The lowest amplitude thrust displacement (1 mm) increased ∆MIFresting at all thrust durations. For all the other thrust displacements and forces, the direction of change in ∆MIFresting was not consistent, and the pattern of change was not systematically related to thrust duration. Regardless of thrust force, displacement, or duration, ∆MIFmovement and ∆MIFposition were not significantly different from control. CONCLUSION Relatively low-amplitude thrust displacements applied during an HVLA-SM produced sustained increases in the resting discharge of paraspinal muscle spindles regardless of the duration over which the thrust was applied. However, regardless of the HVLA-SMs thrust amplitude or duration, the responsiveness of paraspinal muscle spindles to vertebral movement and to a new vertebral position was not affected.

Head repositioning errors in normal student volunteers: a possible tool to assess the neck's neuromuscular system

BackgroundA challenge for practitioners using spinal manipulation is identifying when an intervention is required. It has been recognized that joint pain can interfere with the ability to position body parts accurately and that the recent history of muscle contraction can play a part in that interference. In this study, we tested whether repositioning errors could be induced in a normal population by contraction or shortening of the neck muscles.MethodsIn the experimental protocol, volunteers free of neck problems first found a comfortable neutral head posture with eyes closed. They deconditioned their cervical muscles by moving their heads 5 times in either flexion/extension or lateral flexion and then attempted to return to the same starting position. Two conditioning sequences were interspersed within the task: hold the head in an extended or laterally flexed position for 10 seconds; or hold a 70% maximum voluntary contraction in the same position for 10 seconds. A computer-interfaced electrogoniometer was used to measure head position while a force transducer coupled to an auditory alarm signaled the force of isometric contraction. The difference between the initial and final head orientation was calculated in 3 orthogonal planes. Analysis of variance (1-way ANOVA) with a blocking factor (participants) was used to detect differences in proprioceptive error among the conditioning sequences while controlling for variation between participants.ResultsForty-eight chiropractic students participated: 36 males and 12 females, aged 28.2 ± 4.8 yrs. During the neck extension test, actively contracting the posterior neck muscles evoked an undershoot of the target position by 2.1° (p <0.001). No differences in repositioning were found during the lateral flexion test.ConclusionThe results suggest that the recent history of cervical paraspinal muscle contraction can influence head repositioning in flexion/extension. To our knowledge this is the first time that muscle mechanical history has been shown to influence proprioceptive accuracy in the necks of humans. This finding may be used to elucidate the mechanism behind repositioning errors seen in people with neck pain and could guide development of a clinical test for involvement of paraspinal muscles in cervical pain and dysfunction.

Neural responses to the mechanical parameters of a high-velocity, low-amplitude spinal manipulation: effect of preload parameters.

OBJECTIVE The purpose of this study was to determine how the preload that precedes a high-velocity, low-amplitude spinal manipulation (HVLA-SM) affects muscle spindle input from lumbar paraspinal muscles both during and after the HVLA-SM. METHODS Primary afferent activity from muscle spindles in lumbar paraspinal muscles were recorded from the L6 dorsal root in anesthetized cats. High-velocity, low-amplitude spinal manipulation of the L6 vertebra was preceded either by no preload or systematic changes in the preload magnitude, duration, and the presence or absence of a downward incisural point. Immediate effects of preload on muscle spindle responses to the HVLA-SM were determined by comparing mean instantaneous discharge frequencies (MIF) during the HVLA-SMs thrust phase with baseline. Longer lasting effects of preload on spindle responses to the HVLA-SM were determined by comparing MIF during slow ramp and hold movement of the L6 vertebra before and after the HVLA-SM. RESULTS The smaller compared with the larger preload magnitude and the longer compared with the shorter preload duration significantly increased (P = .02 and P = .04, respectively) muscle spindle responses during the HVLA-SM thrust. The absence of preload had the greatest effect on the change in MIF. Interactions between preload magnitude, duration, and downward incisural point often produced statistically significant but arguably physiologically modest changes in the passive signaling properties of the muscle spindle after the manipulation. CONCLUSION Because preload parameters in this animal model were shown to affect neural responses to an HVLA-SM, preload characteristics should be taken into consideration when judging this interventions therapeutic benefit in both clinical efficacy studies and in clinical practice.