Frequency analysis of vestibular influence on extensor motoneurons. II. Relationship between neck and forelimb extensors

Vestibular Contributions to Primate Neck Postural Muscle Activity during Natural Motion

To maintain stable posture of the head and body during our everyday activities, the brain integrates information across multiple sensory systems. Here, we examined how the primate vestibular system, independently and in combination with visual sensory input, contributes to the sensorimotor control of head posture across the range of dynamic motion experienced during daily life. We recorded activity of single motor units in the splenius capitis and sternocleidomastoid muscles in rhesus monkeys during yaw rotations spanning the physiological range of self-motion (up to 20 Hz) in darkness. Splenius capitis motor unit responses continued to increase with frequency up to 16 Hz in normal animals, and were strikingly absent following bilateral peripheral vestibular loss. To determine whether visual information modulated these vestibular-driven neck muscle responses, we experimentally controlled the correspondence between visual and vestibular cues of self-motion. Surprisingly, visual information did not influence motor unit responses in normal animals, nor did it substitute for absent vestibular feedback following bilateral peripheral vestibular loss. A comparison of muscle activity evoked by broadband versus sinusoidal head motion further revealed that low-frequency responses were attenuated when low- and high-frequency self-motion were experienced concurrently. Finally, we found that vestibular-evoked responses were enhanced by increased autonomic arousal, quantified via pupil size. Together, our findings directly establish the vestibular system's contribution to the sensorimotor control of head posture across the dynamic motion range experienced during everyday activities, as well as how vestibular, visual, and autonomic inputs are integrated for postural control.SIGNIFICANCE STATEMENT Our sensory systems enable us to maintain control of our posture and balance as we move through the world. Notably, the vestibular system senses motion of the head and sends motor commands, via vestibulospinal pathways, to axial and limb muscles to stabilize posture. By recording the activity of single motor units, here we show, for the first time, that the vestibular system contributes to the sensorimotor control of head posture across the dynamic motion range experienced during everyday activities. Our results further establish how vestibular, autonomic, and visual inputs are integrated for postural control. This information is essential for understanding both the mechanisms underlying the control of posture and balance, and the impact of the loss of sensory function.

Task, muscle and frequency dependent vestibular control of posture

The vestibular system is crucial for postural control; however there are considerable differences in the task dependence and frequency response of vestibular reflexes in appendicular and axial muscles. For example, vestibular reflexes are only evoked in appendicular muscles when vestibular information is relevant to postural control, while in neck muscles they are maintained regardless of the requirement to maintain head on trunk balance. Recent investigations have also shown that the bandwidth of vestibular input on neck muscles is much broader than appendicular muscles (up to a factor of 3). This result challenges the notion that vestibular reflexes only contribute to postural control across the behavioral and physiological frequency range of the vestibular organ (i.e., 0-20 Hz). In this review, we explore and integrate these task-, muscle- and frequency-related differences in the vestibular system's contribution to posture, and propose that the human nervous system has adapted vestibular signals to match the mechanical properties of the system that each group of muscles controls.

Vestibular control of the head: possible functions of the vestibulocollic reflex

Here, we review the angular vestibulocollic reflex (VCR) focusing on its function during unexpected and voluntary head movements. Theoretically, the VCR could (1) stabilize the head in space during body movements and/or (2) dampen head oscillations that could occur as a result of the head's underdamped mechanics. The reflex appears unaffected when the simplest, trisynaptic VCR pathways are severed. The VCR's efficacy varies across species; in humans and monkeys, head stabilization is ineffective during low-frequency body movements in the yaw plan. While the appearance of head oscillations after the attenuation of semicircular canal function suggests a role in damping, this interpretation is complicated by defects in the vestibular input to other descending motor pathways such as gaze premotor circuits. Since the VCR should oppose head movements, it has been proposed that the reflex is suppressed during voluntary head motion. Consistent with this idea, vestibular-only (VO) neurons, which are possible vestibulocollic neurons, respond vigorously to passive, but not active, head rotations. Although VO neurons project to the spinal cord, their contribution to the VCR remains to be established. VCR cancelation during active head movements could be accomplished by an efference copy signal negating afferent activity related to active motion. Oscillations occurring during active motion could be eliminated by some combination of reflex actions and voluntary motor commands that take into account the head's biomechanics. A direct demonstration of the status of the VCR during active head movements is required to clarify the function of the reflex.

Electromyographic activity of dorsal neck muscles in squirrel monkeys during rotations in an upright or upside down posture

Electromyographic (EMG) activity was recorded from occipitoscapularis, semispinalis, and splenius neck muscles in five alert squirrel monkeys during 0.25-Hz rotations about horizontal axes oriented at 22.5 degrees intervals, including pitch, roll, and intermediate axes. The animals were oriented in either upright or upside down posture. In the upright posture, all monkeys exhibited compensatory EMG activity with maximal activation during rotations about axes between pitch in the pitch forward direction and contralaterally directed roll. Response timing varied across animals with EMG peaks ranging from near pitch forward head velocity to near pitch forward head position. When the head was upside down, response dynamics and directionality were altered to varying degrees in different monkeys. The greatest change in response to head inversion was seen in the monkey that had response phases closest to head position, the least in the animal with phases closest to head velocity. The monkey with EMG response peaks closest to position phase showed nearly 180 degrees inversion of responses when the head was upside down, suggesting that in this monkey a righting reflex mediated by utricular signals was activated in the upside down posture. The monkey with EMG response peaks closest to velocity phase may have lacked a righting response and exhibited only a canal-mediated compensatory vestibulocervical reflex in both upright and upside down postures. The results suggest that reflex contraction of neck muscles in response to passive head rotation includes an interplay of compensatory and righting responses that varies from animal to animal.

Predicting vestibular, proprioceptive, and biomechanical control strategies in normal and pathological head movements

Little is known of the functionality of the vestibulocollic reflex (VCR) and cervico-collic reflex (CCR) during head and neck movements caused by perturbations of the trunk. Previously, we formulated mathematical expressions for these neck reflexes and incorporated them into a model of horizontal plane head movements. The formalism of this neuromechanical model allowed us to examine separately the main components of head movement control. In the present study, we examine selected parameters within the main components of the model, and associate variations of these parameters with disease processes affecting head and neck movements, such as loss of sensory input or modification in central or motor function. Our simulations led us to several conclusions. First, the probable use of the VCR and CCR in yaw plane head movements is to tune the head response. In the time domain, they diminish natural head oscillations (head wobble) related to head mechanics. Equivalently, in the frequency domain, they reduce the amplitude of head wobble (resonances) around 2 Hz. Second, our simulations suggest that the VCR is about ten times stronger than the CCR in normal humans. Moreover, this disproportion is associated with only very minor contributions from the CCR in yaw. Third, head oscillations (or instability) can be generated by mechanical or neural changes in the head and neck system. Finally, readjustments of central nervous system dynamic operations could provide mechanisms to compensate for sensory and motor dysfunction caused by disease.

Orienting-related eye-neck neurons of the medial ponto-bulbar reticular formation do not participate in horizontal canal-dependent vestibular reflexes of alert cats

Ponto-bulbar reticular formation neurons, including identified reticulospinal neurons, were studied in alert, head-fixed cats. Orienting-related neurons of "eye-neck" type (ENNs) were selected on the basis of qualitative correlations of their discharges with visually triggered eye saccades and electromyographic activity (EMG) of dorsal neck muscles. It was tested whether ENNs participate both in visually triggered gaze shifts requiring eye-head coordination and in gaze-stabilizing movements, such as vestibulo-ocular and vestibulo-collic reflexes (VOR, VCR). Firing patterns were studied during passive sinusoidal rotation (0.2-1.0 Hz; 2.0-21.5 deg peak-to-peak) in the horizontal plane. Responses to electrical stimulation of the superior colliculus and the vestibular nerve were recorded to assess the convergence of tectal and vestibular synaptic inputs. The same methods were applied to a control sample of neurons with discharges apparently "unrelated" to orienting movements. ENNs did not show any modulation of firing rate correlated to compensatory VOR or VCR during passive sinusoidal rotations. Among "unrelated" cells, the fraction of modulated units was close to that reported for reticular neurons projecting in the medial reticulospinal tract. Phasic and sustained components of ENN bursts were associated with anticompensatory movements induced by rotation, such as quick phases, ocular beating field shift, and the increase of EMG activity in neck muscles acting in the direction of passive rotation. Monosynaptic excitation from the contralateral superior colliculus was observed in 92.3% of ENNs, but only 2 out of 17 tested showed an excitatory response to vestibular nerve stimulation. In the control group of "unrelated" neurons the proportions of monosynaptic tectal and excitatory vestibular nerve inputs were, respectively, 75.6 and 71.4%. It is concluded that ENNs are specifically related to active gaze shifts, derived from either visual or from head velocity inputs. Rhombencephalic connections of vestibular nuclei to these neurons appear to be quite weak. Parallel inputs from the mid- or forebrain must be assumed to explain their firing patterns during rotation-induced anticompensatory gaze shifts. Within the studied range of frequencies and amplitudes of passive rotation, ENNs did not participate in the vestibulo-collic reflex. It is therefore unlikely that reticular neurons controlling orienting eye-neck synergies act also as a premotor pathway for gaze-stabilizing movements.

Comparison of neck muscle activation patterns during head stabilization and voluntary movements

The motor system that controls the neck musculature serves two major functions: stabilization of the head in the face of external perturbations or body movements, and generation of voluntary or orientating head movements. Typically the latter are thought to be mediated by complex pathways involving cerebral cortex and superior colliculus while stabilization is thought to be mediated by simple short-loop pathways that generate vestibulocollic and cervicocollic reflexes (VCR and CCR). Our work has been directed towards evaluating the extent to which the VCR and CCR are in fact responsible for head stabilization, and to determining how the motor patterns produced by these reflexes compare with those produced by the voluntary head movement system. To address these questions we have analysed the dynamic and spatial (kinematic) properties of the head movement system in cats and humans.

Effects of stimulation of vestibular and neck receptors on Deiters neurons projecting to the lumbosacral cord

The activity of lateral vestibular nucleus (LVN) neurons, antidromically identified by stimulation of the spinal cord at T12 and L1, thus projecting to the lumbosacral segments of the spinal cord (IVS neurons), was recorded in precollicular decerebrate cats during rotation about the longitudinal axis either of the whole animal (labyrinth input) or of the body only while the head was kept stationary (neck input). Among the IVS neurons tested for vestibular stimulation, 76 of 129 units (i.e. 58.9%) responded to roll tilt of the animal at the standard parameters of 0.026 Hz, +/- 10 degrees. The gain and the sensitivity of the first harmonic responses corresponded on the average to 0.47 +/- 0.44, SD, impulses X s-1 X deg-1 and 3.24 +/- 3.15, SD, %/deg, respectively. As to the response patterns, 51 of 76 units (i.e. 67.1%) were excited during side-down and depressed during side-up tilt, whereas 15 (i.e. 19.7%) showed the opposite behavior. In both instances the peak of the responses occurred with an average phase lead of about +21.0 +/- 27.2, SD, deg with respect to the extreme side-down or side-up position of the animal. Moreover, the former group of units showed almost a twofold larger gain with respect to the latter group (t-test, p less than 0.05). Among the IVS neurons tested for neck stimulation, 75 of 109 units (68.8%) responded to neck rotation at the standard parameters. The gain and the sensitivity of the first harmonic responses corresponded on the average to 0.49 +/- 0.40, SD, impulses X s-1 X deg-1 and 3.30 +/- 3.42, SD, %/deg, respectively, thus being similar to the values obtained for the labyrinth responses. However, 59 of 75 units (i.e. 78.6%) were excited during side-up neck rotation and depressed during side-down neck rotation, while 8 of 75 units (i.e. 10.7%) showed the opposite pattern. In both instances the peak of the responses occurred with an average phase lead of +52.0 +/- 18.3, SD, deg for the extreme side-up or side-down neck displacements. Further, the former group of units showed a larger gain than the latter group. Histological controls indicated that 102 of 129 (i.e. 79.0%) IVS neurons tested for labyrinth stimulation and 86 of 109 (i.e. 78.9%) IVS neurons tested for neck stimulation were located in the dorsocaudal part of LVN, the remaining IVS neurons being located in the rostroventral part of LVN.(ABSTRACT TRUNCATED AT 400 WORDS)

The sagittal vestibulocollic reflex and its interaction with neck proprioceptive afferents in the decerebrate cat

The sagittal vestibulocollic reflex (v.c.r.) evoked by nose-up, nose-down movements of the head, has been studied in the neck extensor muscle biventer cervicis in the decerebrate cat. Nose-down movements of the head increased, and nose-up movements decreased, electromyographic (e.m.g.) activity in the biventer cervicis muscles of the left and right sides. At low frequencies of sinusoidal head movement (0.1-0.5 Hz), the gain of the sagittal v.c.r. was approximately constant, and e.m.g. modulation showed a phase lead of about 40 deg with respect to head position. At higher frequencies (2-5 Hz), v.c.r. gain increased at a rate close to 40 dB/decade, and phase lead increased to approach 150 deg. The relation between head movement and v.c.r. activity may be described by a transfer function containing two lead terms, with time constants of 0.07 and 0.23 s, and two lag terms, with time constants of 5.3 and 9.9 s. When movements of the head were accompanied by stretching of the biventer cervicis muscles, the gain of the sagittal v.c.r. was increased threefold, at all frequencies between 0.1 and 5 Hz, with no substantial change in phase. Sinusoidal stretching of the biventer cervicis muscles, with the head stationary, evoked a stretch reflex (cervicocollic reflex, c.c.r.) the behaviour of which was similar to that of a second-order system described by a transfer function containing two lead terms with time constants of 0.07 and 0.16 s. This difference in dynamics between the v.c.r. and the c.c.r. indicates that the lag terms in the v.c.r. transfer function reflect the frequency-response properties of the vestibular pathway to the biventer cervicis muscles, as they do not appear when the same muscles participate in the c.c.r. The vectorial differences between the frequency-response of the sagittal v.c.r. with and without concomitant stretching of the biventer cervicis muscles is quantitatively similar to the frequency-response of the c.c.r. evoked by sinusoidal stretching. The inputs from the vestibular and stretch receptors thus appear to sum linearly to produce the increase in v.c.r. gain, at least over the frequency range 0.1-1 Hz. Since most head movements, and all voluntary head movements, involve a rotation of the head in relation to the neck, the potentiation of the gain of the v.c.r. by afferents from stretch receptors (presumably muscle spindles) in the neck muscles is an important factor in the normal reflex stabilization of head position.

Discharge activity of spindle afferents from the gastrocnemius-soleus muscle during head rotation in the decerebrate cat

The activity of spindle afferents originating from both primary and secondary endings of the isometrically extended (6-8 mm) gastrocnemius-soleus (GS) muscle was recorded in precollicular decerebrate cats during sinusoidal head rotation about the longitudinal axis above a stationary body. In the first group of experiments to test the influence of vestibular volleys on fusimotor neurons, an acute bilateral neck deafferentation at C1-C3 was performed to eliminate possible influences arising from neck receptors; head rotation (0.026 Hz, +/- 15 degrees) induced a weak periodic rate modulation in 6/38 (15.8%) of the tested spindle afferents; the average response gain was 0.18 +/- 0.12, SD imp./s/deg (mean firing rate, 18.9 +/- 2.8 imp./s), and the average phase angle was -43.2 +/- 47.0 degrees, SD lag with respect to ipsilateral side-down displacement of the head (alpha-response pattern). In a second group of experiments head rotation studied after acute bilateral section of VIII cranial nerve, thereby stimulating only neck receptors, failed to influence in a reliable manner the firing rate of 38 additional spindle afferents. In a third group of experiments in which both VIII nerves and cervical dorsal roots were left intact, head rotation induced a response in 7/45 (15.6%) of the tested spindle afferents similar to that observed after cervical deafferentiation and thus depended on stimulation of labyrinth receptors alone. Over the examined frequency range of head rotation from 0.015 to 0.325 Hz (+/- 15 degrees), the response gain of spindle afferents was relatively stable during sinusoidal labyrinth stimulation. For most of the spindle afferents the phase angle of the response elicited at the lower frequencies was related to the direction of head orientation towards the ipsilateral sidedown, thus being attributed to labyrinth volleys originating from macular receptors; at 0325 Hz the stimulus was less effective and some units showed a phase advance relative to head position which was attributed to costimulation of canal receptors. Displacement of the muscle under study obtained by either rotation of the whole animal or body alone beneath a stationary head elicited a periodic modulation of spindle afferent discharge, independent of head orientation or type of preparation, in 51/73 (70%) of the muscle spindles tested; the average response gain was 0.20 +/- 0.19, SD imp./s/deg, and an average phase lead of +14.1 +/- 20.5 degrees, SD with respect to the peak of the ipsilateral side-down displacement of the body or of the animal was observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Roll tilt reflexes after vestibulospinal tract lesions

The effects of lesions of the vestibulospinal tracts on vestibular reflexes evoked by roll tilt in forelimb and neck extensors were examined in decerebrate cats. Sectioning the medial longitudinal fasciculus, which contains the medial vestibulospinal tract, had no major effect on the phase of the reflex, although some gain was usually lost at high stimulus frequencies. Spinal lesions at C2-C3, both cord hemisections and more restrictive lesions which cut the lateral vestibulospinal tract, produced two major effects on the forelimb. Background EMG activity was usually abolished in the triceps ipsilateral to the lesion, with partial loss of activity in the opposite limb. The tilt reflex response in the ipsilateral limb appeared normal, although it was usually necessary to raise the background excitability of the preparation by administering L-Dopa in order to observe the reflex. In contrast, the response in the contralateral limb showed a phase reversal of 180 deg at low stimulus frequencies, implying that the reflex in intact cats receives a crossed otolith-spinal input. Responses in the neck extensors splenius and biventer, recorded from compartments caudal to the spinal lesion, were relatively unaffected.

Eye and head movements during vestibular stimulation in the alert rabbit

Rabbits passively oscillated in the horizontal plane with a free hand tended to stabilize their head in space (re: earth-fixed surroundings) by moving the head on the trunk (neck angular deviation, NAD) opposite the passively imposed body rotation. The gain (NAD/body rotation) of head stabilization varied from 0.0 to 0.95 (nearly perfect stability) and was most commonly above 0.5. Horizontal eye movement (HEM) was inversely proportional to head-in-space stability, i.e. the gaze (sum of HEM, NAD, and body rotation) was stable in space (regardless of the gain of head stabilization). When the head was fixed to the rotating platform, attempted head movements (head torque) mimicked eye movements in both the slow and fast phases of vestibular nystagmus; tonic eye position was also accompanied by conjugate shifts in tonic head torque. Thus, while eye and head movements may at times be linked, that the slow eye and head movements vary inversely during vestibular stimulation with a free head indicates that the linkage is not rigid. Absence of a textured stationary visual field consistently produced a response termed 'visual inattentiveness,' which was characterized by, among other things, a reduction of head and gaze stability in space. This behavioral response could also be reproduced in a subject allowed vision during prolonged vestibular stimulation in the absence of other environmental stimuli. It is suggested that rabbits optimize gaze stability (re: stationary surroundings), with the head contributing variably, as long as the animal is attending to its surroundings.

Compensation of postural reactions to free-fall in the vestibular neurectomized monkey. Role of the visual motions cues

In previous studies a contribution of vision to vestibular-dependent muscle responses during free-fall was found in the intact monkey, and the role of remaining labyrinthine afferents in compensation of these postural reactions was studied in vestibular neurectomized monkeys. In the present investigation we have compared the role of visual motion cues in the recovery of muscle responses to fall in unilateral (U.N.) and bilateral vestibular neurectomized (B.N.) baboons. During free-fall, electromyographic (EMG) responses were recorded from splenius capitis, soleus and tibialis anterior muscles. EMG activities were recorded in two randomly presented conditions: with normal motion of the visual world (NV) and with the visual world stabilized with respect to the baboon's head (SV) until 6 weeks after surgery. In B.N. baboons, results showed that condition SV was accompanied by a very strong motor depression during the entire test period. A greater decrease was observed in the splenius and soleus muscles. In U.N. baboons, significantly depressed EMG responses were recorded in the SV condition during the first two stages of compensation only (0--2 weeks), in all tested muscles except the tibialis anterior muscle. On the other hand, these motor depressions appeared to depend upon the level of neuronal resting activity in the vestibular nuclei. It is inferred that the partial recovery of muscle responses to fall observed in B.N. baboons in the NV condition is mainly due to visual information concerning motion, which replaces to the labyrinthine afferents. In U.N. baboons, the visual motion cues would fulfil only a transitory substitution function by supplying the decrease of neuronal activity in the vestibular nuclei. Later on, full compensation would be carried out by means of the remaining labyrinth.

The response of horizontal semicircular canal afferents to sinusoidal rotation in the cat

Dynamic characteristics of primary vestibular afferents innervating the horizontal semicircular canal were studied in decerebrate, unanesthetized cats. Activities of individual afferent fibers were recorded intracranially by glass micropipettes. Frequency of sinusoidal rotation was varied from 0.014 Hz to 0.42 Hz, and phase and gain properties were examined. All of the fibers recorded fired spontaneously, and their firing rate ranged from 7 to 128 spikes/sec. Regularity of firing, phase lags, and gains were calculated in individual fibers. There was a tendency that the units with high spontaneous firing rates showed regular firing, larger phase lags, and lower gains that the units with low spontaneous firing rates. The transfer function of the system (firing rate of the primary afferent per angular acceleration of the head) was (formula: see text). A high frequency phase lead component was needed to account for the data obtained, indicating a slight deviation from the relationship predicted by the torsion pendulum model. The present phase properties were compared with those of vestibular nucleus neurons reported previously. It was suggested that a group of vestibular nucleus neurons transmits fairly faithfully the phase properties of primary afferents, and that another group of vestibular nucleus neurons receive additional influences from central structures, exhibiting larger phase lags than primary afferents.

Forelimb reflexes modulated by tonic neck positions in cats

The modulation of monosynaptic forelimb reflexes by tonic neck positions was investigated in cats with the head fixed. Lateral flexion of the body in a horizontal plane markedly facilitates reflexes of the deep radial nerve (DR) in the ipsilateral forelimb, while the antagonistic ulnar nerve (ULN) reflexes are strongly inhibited. Opposite effects are seen after contralaternal body movement. Dorsiflexion of the body clearly increases DR-reflexes and exerts a reciprocal although more pronounced inhibition on ULN reflexes. Opposite effects appear after ventriflexion. The reflex modulation starts with head-body displacements of approximately 5 degrees and increases with increasing angles. Furthermore reflex modulation does not depend on the intact cerebrum and cerebellum. The comparison of forelimb and hindlimb reflexes shows a decrease of the neck influences along the spinal cord.

Modulation of hindlimb reflexes by tonic neck positions in cats

In cats with the head fixed in a stereotactic frame a slight facilitation of the gastrocnemiussoleus (GS) monosynaptic reflexes in the ipsilateral hindlimb, accompanied by an inhibition of the monosynaptic reflex of the antagnostic deep peroneal nerve (DP), occurs following lateral flexion and rotation around the body axis at an angle of 25 degrees. Following dorsiflexion of the body a moderate inhibition of the extensor reflexes (GS) up to 10% and a reciprocal slight excitation in the same range of the monosynaptic reflexes of flexor muscles (DP) is recorded. On the other hand after ventriflexion of the body there is a marked inhibition in the range of 50% of all the monosynaptic extensor and flexor reflexes for all the stimulus intensities used. This inhibition can already be demonstrated in ventriflexion of 10 degrees-15 degrees. All these patterns of reflex modulation are similar in cortically intact, ischemically decorticated and precollicularly decerebrated cats. Furthermore these patterns are not essentially influenced by cerebellectomy, although all reflex amplitudes are reduced. However, the spinal reflexes can not be modulated by these body movements after acute spinalization.