Neck proprioceptors contribute to the modulation of muscle sympathetic nerve activity to the lower limbs of humans.

Several different strategies have now been used to demonstrate that the vestibular system can modulate muscle sympathetic nerve activity (MSNA) in humans and thereby contribute to the regulation of blood pressure during changes in posture. However, it remains to be determined how the brain differentiates between head-only movements that do not require changes in vasomotor tone in the lower limbs from body movements that do require vasomotor changes. We tested the hypothesis that neck movements modulate MSNA in the lower limbs of humans. MSNA was recorded in 10 supine young adult subjects, at rest, during sinusoidal stretching of neck muscles (100 cycles, 35° peak to peak at 0.37 ± 0.02 Hz) and during a ramp-and-hold (17.5° for 54 ± 9 s) static neck muscle stretch, while their heads were held fixed in space. Cross-correlation analysis revealed cyclical modulation of MSNA during sinusoidal neck muscle stretch (modulation index 45.4 ± 5.3 %), which was significantly less than the cardiac modulation of MSNA at rest (78.7 ± 4.2 %). Interestingly, cardiac modulation decreased significantly during sinusoidal neck displacement (63.0 ± 9.3 %). By contrast, there was no significant difference in MSNA activity during static ramp-and-hold displacements of the neck to the right or left compared with that with the head and neck aligned. These data suggest that dynamic, but not static, neck movements can modulate MSNA, presumably via projections of muscle spindle afferents to the vestibular nuclei, and may thus contribute to the regulation of blood pressure during orthostatic challenges.

The vestibular system does not modulate fusimotor drive to muscle spindles in contracting leg muscles of seated subjects.

We previously showed that sinusoidal galvanic vestibular stimulation (GVS) does not modulate the firing of spontaneously active muscle spindles in relaxed human leg muscles. However, given that there is little, if any, fusimotor drive to relaxed human muscles, we tested the hypothesis that vestibular modulation of muscle spindles becomes apparent during volitional contractions at levels that engage the fusimotor system. Unitary recordings were made from 28 muscle spindle afferents via tungsten microelectrodes inserted percutaneously into the common peroneal nerve of seated awake human subjects. Twenty-one of the spindle afferents were spontaneously active at rest and each increased its firing rate during a weak static contraction; seven were silent at rest and were recruited during the contraction. Sinusoidal bipolar binaural galvanic vestibular stimulation (±2 mA, 100 cycles) was applied to the mastoid processes at 0.8 Hz. This continuous stimulation produced a sustained illusion of "rocking in a boat" or "swinging in a hammock" but no entrainment of EMG. Despite these robust vestibular illusions, none of the fusimotor-driven muscle spindles exhibited phase-locked modulation of firing during sinusoidal GVS. We conclude that this dynamic vestibular input was not sufficient to modulate the firing of fusimotor neurones recruited during a voluntary steady-state contraction, arguing against a significant role of the vestibular system in adjusting the sensitivity of muscle spindles via fusimotor neurones.

Effects of deep and superficial experimentally induced acute pain on muscle sympathetic nerve activity in human subjects.

Human studies conducted more than half a century ago have suggested that superficial pain induces excitatory effects on the sympathetic nervous system, resulting in increases in blood pressure (BP) and heart rate (HR), whereas deep pain is believed to cause vasodepression. To date, no studies have addressed whether deep or superficial pain produces such differential effects on muscle sympathetic nerve activity (MSNA). Using microneurography we recorded spontaneous MSNA from the common peroneal nerve in 13 awake subjects. Continuous blood pressure was recorded by radial arterial tonometry. Deep pain was induced by intramuscular injection of 0.5 ml hypertonic saline (5%) into the tibialis anterior muscle, superficial pain by subcutaneous injection of 0.2 ml hypertonic saline into the overlying skin. Muscle pain, with a mean rating of 4.9 +/- 0.8 (S.E.M.) on a 0-10 visual analog scale (VAS) and lasting on average 358 +/- 32 s, caused significant increases in MSNA (43.9 +/- 10.0%), BP (5.4 +/- 1.1%) and HR (7.0 +/- 2.0%) - not the expected decreases. Skin pain, rated at 4.9 +/- 0.6 and lasting 464 +/- 54 s, also caused significant increases in MSNA (38.2 +/- 12.8%), BP (5.1 +/- 2.1%) and HR (5.6 +/- 2.0%). The high-frequency (HF) to low-frequency (LF) ratio of heart rate variability (HRV) increased from 1.54 +/- 0.25 to 2.90 +/- 0.45 for muscle pain and 2.80 +/- 0.52 for skin pain. Despite the different qualities of deep (dull and diffuse) and superficial (burning and well-localized) pain, we conclude that pain originating in muscle and skin does not exert a differential effect on muscle sympathetic nerve activity, both causing an increase in MSNA and an increase in the LF:HF ratio of HRV. Whether this holds true for longer lasting experimental pain remains to be seen.

Head and cervical spine posture in behaving rats: implications for modeling human conditions involving the head and cervical spine.

The aim of this study was to define the temporal and spatial (postural) characteristics of the head and cervical vertebral column (spine) of behaving rats in order to better understand their suitability as a model to study human conditions involving the head and neck. Time spent in each of four behavioral postures was determined from video tape recordings of rats (n = 10) in the absence and presence of an intruder rat. Plain film radiographic examination of a subset of these rats (n = 5) in each of these postures allowed measurement of head and cervical vertebral column positions adopted by the rats. When single they were quadruped or crouched most (∼80%) of the time and bipedal either supported or free standing for only ∼10% of the time. The introduction of an intruder significantly (P < 0.0001) reduced the proportion of time rats spent quadruped (median, from 71% to 47%) and bipedal free standing (median, from 2.9% to 0.4%). The cervical spine was orientated (median, 25-75 percentile) near vertical (18.8°, 4.2°-30.9°) when quadruped, crouched (15.4°, 7.6°-69.3°) and bipedal supported (10.5°, 4.8°-22.6°) but tended to be less vertical oriented when bipedal free standing (25.9°, 7.7°-39.3°). The range of head positions relative to the cervical spine was largest when crouched (73.4°) and smallest when erect free standing (17.7°). This study indicates that, like humans, rats have near vertical orientated cervical vertebral columns but, in contrast to humans, they displace their head in space by movements at both the cervico-thoracic junction and the cranio-cervical regions.

Commissural neurons in the cat upper cervical spinal cord.

We have begun a study of the intrinsic circuitry of the cat's upper cervical cord, in part to elucidate the role of spinal interneurons in vestibulocollic reflexes. Using retrograde labelling with Fluoro-Gold and intraspinal microstimulation, we have identified commissural neurons projecting to the contralateral ventral horn. Neurons tended to be in the medial half of lamina VIII. Approximately half of the neurons were propriospinal neurons that could be activated antidromically from the rostral border of the cervical enlargement. Most of the tested, spontaneously active neurons were driven by stimulation of the ipsi- and/or contralateral vestibular nerve, in some cases disynaptically.

An in vivo method for studying afferent fibre activity from cervical paravertebral tissue during vertebral motion in anaesthetised cats.

We describe a method for characterizing and studying afferent input to the central nervous system arising from deep axial structures of the neck during defined cervical vertebral movement. Multiple or single unit recordings of afferent activity arising from identified receptive fields in these tissues can now be studied in situ while simultaneously inducing 'natural' stimulation of mechanoreceptors during well defined movements of the intact vertebral column. When combined with existing strategies for extracellular and intracellular recordings of neurones, the methods described here will allow in vivo investigation of the central effects of functionally identified afferents innervating identified receptive fields located in deep paravertebral tissues during a variety of discrete movements of individual vertebra. This has particular importance in determining the relative role that afferents innervating specific axial tissues have on identified neurones in the central nervous system. It will allow determination of the 'bias' of input to projection cells, such as 'hyperconvergent' neurones, during natural movement. Furthermore, it will allow investigation of their role in the control of somatic and autonomic reflex behaviour.

Neurons in the dorsal column nuclei of the rat respond to stimulation of neck mechanoreceptors and project to the thalamus.

Electrophysiological recordings were made from neurons in the dorsal column nuclei which were activated by stimulation of muscle and cutaneous receptors in the neck of the rat. 222 units were studied, 158 (71%) of which responded to activation of cutaneous mechanoreceptors while 64 (29%) were activated by muscle receptors. The response patterns of 12 neurons with input from receptors in neck muscles were tested more fully. Their response patterns strongly suggested that 6 were activated by muscle spindle afferents while the other 6 were activated by Golgi tendon organ afferents. 18 (8%) of the neck-responsive neurons in the medulla were shown to project rostrally to the thalamus.

The ventrolateral medulla of the cat mediates vestibulosympathetic reflexes.

Extracellular recordings were made from 94 neurons located in the ventrolateral medulla (VLM) whose firing rate was affected by vestibular nerve (VN) stimulation; 50 of these units were in the subretrofacial (SRF) nucleus, which contains cells that make direct excitatory connections with sympathetic preganglionic neurons. The sample included 12 SRF cells which were antidromically driven from the upper thoracic spinal cord and had conduction velocities of 10 m/s or less; the effect of VN stimulation on all but one of these units was inhibition. The onset latency of the response to VN stimulation was long [20.3 +/- 3.7 (S.E.M.) ms, n = 9, for the antidromically activated neurons and 12.1 +/- 1.2 ms, n = 73, for the others], suggesting that the effects were predominantly polysynaptic. In addition, most of the spontaneously active units tested (33/36) received convergent inputs from the carotid sinus nerve (CSN), as would be expected for neurons which influence sympathetic outflow. Vestibular-elicited inhibition of SRF neurons with projections to the intermediolateral cell column could account for late, long duration inhibition of sympathetic discharges produced by labyrinth stimulation.

Responses of neurons in the rostral ventrolateral medulla of the cat to natural vestibular stimulation.

To investigate the neural substrate of vestibulo-sympathetic reflexes, we studied the responses of neurons in the rostral ventrolateral medulla (RVLM) of decerebrate cats to natural stimulation of the labyrinth in vertical and horizontal planes. The RVLM is a major source of excitatory inputs to sympathetic preganglionic neurons. The animals used in these studies were baroreceptor-denervated and vagotomized and had a cervical spinal transection so that inputs from tilt-sensitive receptors outside of the labyrinth did not influence the units we recorded. Of the 38 neurons whose type of vertical vestibular inputs could be classified, the majority (27) received signals mainly from otolith organs. Only 4 of the neurons received inputs predominantly from vertical semicircular canals, and 7 were classified as having convergent inputs from otoliths and canals that were spatially aligned (2 cells) or misaligned (5 cells). In addition, only 2 of 68 neurons tested responded to sinusoidal horizontal rotations in a manner typical of brainstem neurons that receive inputs from the horizontal semicircular canals. Thus, the vestibular inputs to the RVLM appear to come mainly from otolith receptors. In labyrinthectomized cats, we were unable to locate neurons with responses to tilt similar to those of cells recorded in labyrinth-intact cats, confirming that the responses attributed to vertical vestibular inputs were produced by signals from the labyrinth. In animals whose semicircular canals had been rendered dysfunctional by plugging, we only recorded responses similar to those of neurons classified as having mainly otolith inputs in canal-intact animals, indicating that the dynamic behavior of these cells does not depend upon canal inputs. The presence of otolith inputs to the RVLM is consistent with the hypothesis that this region mediates vestibulo-sympathetic reflexes involved in correcting posturally-related changes in blood pressure.

Vestibular influences on cat lumbar paravertebral muscles.

Recordings were made from nerves innervating ventral lumbar paravertebral muscles (quadratus lumborum) during natural vestibular stimulation in vertical planes. The best direction of vestibular stimulation for producing an increase in nerve activity was near nose-up pitch in two-thirds of the animals, but was near nose-down pitch in the others. The response gain (re. position) was flat across the frequency range of 0.05-1 Hz, and the responses persisted throughout 11-s static tilts. These data suggest that otolith organs activated by pitch influence activity in the lumbar paravertebral muscles. These responses may serve to stabilize the vertebral column during movements involving body pitch, such as vertical climbing.

Neck afferent involvement in cardiovascular control during movement.

It is well established that labyrinth and neck afferent information contributes to the regulation of somatomotor function during movement and changes in posture. There is also convincing evidence that the vestibular system participates in the modulation of sympathetic outflow and cardiovascular function during changes in posture, presumably to prevent orthostatic hypotension. However, the labyrinth organs do not provide any signals concerning body movements with respect to the head. In contrast, the neck receptors, particularly muscle spindles, are well located and suited to provide information about changes in body position with respect to the head and vestibular signals. Studies in the cat suggest that neck afferent information may modulate the vestibulosympathetic reflex responses to head-neck movements. There is some evidence in the cat to suggest involvement of low threshold mechanoreceptors. However, human studies do not indicate that low threshold mechanoreceptors in the neck modulate cardiovascular responses. The human studies are consistent with the studies in the cat in that they demonstrate the importance of otolith activation in mediating cardiovascular and sympathetic responses to changes in posture. This paper briefly reviews the current experimental evidence concerning the involvement of neck afferent information in the modulation of cardiovascular control during movement and changes in posture.

Influences of neck afferents on sympathetic and respiratory nerve activity.

It is well established that the vestibular system influences the sympathetic nervous system and the respiratory system; presumably, vestibulosympathetic and vestibulorespiratory responses participate in maintaining stable blood pressure and blood oxygenation during movement and changes in posture. Many brainstem neurons that generate vestibulospinal reflexes integrate signals from the labyrinth and neck muscles to distinguish between head movements on a stable body and whole body movements. In the present study, responses were recorded from the splanchnic (sympathetic), hypoglossal (inspiratory) and abdominal (expiratory) nerves during stimulation of the C2 dorsal root ganglion or C2 or C3 nerve branches innervating dorsal neck muscles. Stimulation of neck afferents using low current intensities, in many cases less than twice the threshold for producing an afferent volley recordable from the cord dorsum, elicited changes in sympathetic and respiratory nerve activity. These data suggest that head rotation on a stable body would elicit both cervical and vestibular inputs to respiratory motoneurons and sympathetic preganglionic neurons. The effects of cervical afferent stimulation on abdominal, splanchnic and hypoglossal nerve activity were not abolished by transection of the brainstem caudal to the vestibular nuclei; thus, pathways in addition to those involving the vestibular nuclei are involved in relaying cervical inputs to sympathetic preganglionic neurons and respiratory motoneurons. Transection of the C1-3 dorsal roots enhanced responses of the splanchnic and abdominal nerves to pitch head rotations on a fixed body but diminished responses of the hypoglossal nerve. Thus, neck and vestibular afferent influences on activity of respiratory pump muscles and sympathetic outflow appear to be antagonistic, so that responses will occur during whole body movements but not head movements on a stationary trunk. In contrast, neck and vestibular influences on tongue musculature are complementary, presumably to produce tongue protrusion either during movements of the head alone or of the whole body.

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.

A descriptive study of the force and displacement profiles of the toggle-recoil spinal manipulative procedure (adjustment) as performed by chiropractors.

The aim of this study was to determine the variability of the thrust parameters produced by practitioners performing a high velocity spinal manipulative therapy technique (toggle-recoil) normally applied to the neck. Fourteen participants performed three thrust trials, separated by >30minutes, on a patient simulation device. Force and displacement generated during the thrusts were simultaneously recorded and analysed off line. Peak thrust force ranged from 18.2 to 246N with a mean of 111.2N (SD 48.8). Time to peak thrust force ranged from 20 to 100ms, mean 67.5 ms (SD 13.1). Peak thrust displacement ranged from 6.1 to 28.9mm, mean 24.1mm (SD 4.9) and time to peak thrust displacement ranged from 22.5 to 105ms, mean 59.4ms (SD 13.8). This study demonstrates that the force and displacement induced by any individual practitioner on a simulator can vary by up to 50% during a toggle-recoil thrust. Furthermore, different practitioners may vary in their force by as much 100% and in displacement by 50% when the toggle-recoil spinal manipulative procedure is performed.

Vestibular inputs do not influence the fusimotor system in relaxed muscles of the human leg.

Descending vestibular pathways have been shown to influence recruitment thresholds of alpha motoneurones in both human and cat. However, whereas parallel connections to the fusimotor system have been shown in the cat, such connections have not yet been demonstrated in humans. In the present study we investigated whether vestibular inputs can influence the firing of spontaneously active muscle spindles in the leg via activation of gamma motoneurones. Unitary recordings were made from 30 muscle spindle afferents via tungsten microelectrodes inserted percutaneously into the common peroneal nerve of seated awake human subjects. Sinusoidal bipolar binaural galvanic vestibular stimulation (GVS; frequency 0.2, 0.5, 0.8 Hz, amplitude +/-2 mA, 100 cycles) was applied to the mastoid processes. This continuous stimulation produced a sustained frequency-dependent illusion of "rocking in a boat" or "swinging in a hammock". Despite these robust illusions none of the spontaneously active muscle spindles exhibited phase-locked modulation of firing during sinusoidal GVS. We conclude that this dynamic vestibular input was not sufficient to recruit gamma motoneurones, which are known to have little spontaneous activity in relaxed human muscles.

Reflex effects of vertebral subluxations: the peripheral nervous system. An update.

BACKGROUND: The traditional chiropractic vertebral subluxation hypothesis proposes that vertebral misalignment cause illness, disease, or both. This hypothesis remains controversial. OBJECTIVE: To briefly review and update experimental evidence concerning reflex effects of vertebral subluxations, particularly concerning peripheral nervous system responses to vertebral subluxations. DATA SOURCE: Information was obtained from chiropractic or scientific peer-reviewed literature concerning human or animal studies of neural responses to vertebral subluxation, vertebral displacement or movement, or both. CONCLUSION: Animal models suggest that vertebral displacements and putative vertebral subluxations may modulate activity in group I to IV afferent nerves. However, it is not clear whether these afferent nerves are modulated during normal day-to-day activities of living and, if so, what segmental or whole-body reflex effects they may have.

The somatosensory system of the neck and its effects on the central nervous system.

BACKGROUND: There is some evidence to suggest that dysfunction in the sensory system of the neck may result in a gamut of signs and symptoms. However, a sound understanding of the somatosensory system in the neck and its normal influence on the central nervous system is essential before signs and symptoms can be identified as representations of ill health or disease arising from the neck. OBJECTIVE: To briefly review current knowledge of the somatosensory system of the neck and to consider its connections and influence on the central nervous system. DATA SOURCES: Information was obtained from peer-reviewed scientific journals and proceedings of scientific meetings that have investigated or considered anatomical and physiological aspects of the sensory system in the necks of human and nonhuman vertebrates. CONCLUSION: Studies involving human and nonhuman vertebrates have provided considerable information about the anatomy of the sensory receptors located in the neck and about where information from these receptors is relayed in the spinal cord and brain. Physiological experiments involving electrical and natural stimulation of the head and neck regions have identified a role for some of these receptors in neck-evoked reflexes. It is clear that in addition to signaling nociception, the somatosensory system of the neck may influence the motor control of the neck, eyes, limbs, respiratory muscles and possibly the activity of some preganglionic sympathetic nerves.

Torticollis: a review of etiology, pathology, diagnosis and treatment.

A literature review of the etiology and pathology of torticollis has been undertaken from which six classifications are suggested and discussed. Evidence is presented that the neuromuscular control mechanisms pertinent to the cervical spine are vulnerable at various levels and that a specific etiology of torticollis is yet to be identified. A brief discussion of the diagnosis and treatment of torticollis is presented.