Dynamic relations between natural vestibular inputs and activity of forelimb extensor muscles in the decerebrate cat. I. Motor output during sinusoidal linear accelerations

Differences in coding provided by proprioceptive and vestibular sensory signals may contribute to lateral instability in vestibular loss subjects

One of the signatures of balance deficits observed in vestibular loss subjects is the greater instability in the roll compared to pitch planes. Directional differences in the timing and strengths of vestibular and proprioceptive sensory signals between roll and pitch may lead to a greater miscalculation of roll than pitch motion of the body in space when vestibular input is absent. For this reason, we compared the timing and amplitude of vestibular information, (observable in stimulus-induced head accelerations when subjects are tilted in different directions), with that of proprioceptive information caused by stimulus induced rotations of ankle and hip joints [observable as short latency (SL) stretch responses in leg and trunk muscle EMG activity]. We attempted to link the possible mode of sensory interaction with the deficits in balance control. Six subjects with bilaterally absent vestibular function and 12 age-matched controls were perturbed, while standing, in 8 directions of pitch and roll support surface rotation in random order. Body segment movements were recorded with a motion analysis system, head accelerations with accelerometers, and muscle activity with surface EMG. Information on stimulus pitch motion was available sequentially. Pitch movements of the support surface were best coded in amplitude by ankle rotation velocity, and by head vertical linear acceleration, which started at 13 ms after the onset of ankle rotation. EMG SL reflex responses in soleus with onsets at 46 ms provided a distal proprioceptive correlate to the pitch motion. Roll information on the stimulus was available simultaneously. Hip adduction and lumbo-sacral angular velocity were represented neurally as directionally specific short latency stretch and unloading reflexes in the bilateral gluteus medius muscles and paraspinal muscles with onsets at 28 ms. Roll angular accelerations of the head coded roll amplitude and direction at the same time (31 ms). Significant differences in amplitude coding between vestibular loss subjects and controls were only observed as a weaker coding between stimulus motion and head roll and head lateral linear accelerations. The absence of vestibular inputs in vestibular loss subjects led to characteristic larger trunk in motion in roll in the direction of tilt compared to pitch with respect to controls. This was preceded by less uphill flexion and no downhill extension of the legs in vestibular loss subjects. Downhill arm abduction responses were also greater. These results suggest that in man vestibular inputs provide critical information necessary for the appropriate modulation of roll balance-correcting responses in the form of stabilising knee and arm movements. The simultaneous arrival of roll sensory information in controls may indicate that proprioceptive and vestibular signals can only be interpreted correctly when both are present. Thus, roll proprioceptive information may be interpreted inaccurately in vestibular loss subjects, leading to an incorrect perception of body tilt and insufficient uphill knee flexion, especially as cervico-colic signals appear less reliable in these subjects as an alternative sensory input.

Functional coupling of the stabilizing gaze reflexes during vertical linear motion in the alert cat

Eye-head coordination is mainly achieved by means of stabilizing reflexes (VOR, VCR, OKR) and orienting movements (eye-neck surgery) underlying the close cooperation of the visual and vestibular systems in gaze stabilization. The functional coupling of these different sensorimotor subsystems has been principally analysed using rotatory stimulation of the whole body and/or of the visual surround. The aim of the present study was to investigate the dynamic properties of these stabilizing gaze reflexes and their coupling during linear motion in the vertical plane. These investigations were performed in the alert cat under open-loop conditions (head fixed). Otolith stimulation consisted of vertically translating the cat in total darkness using sinusoidal linear motion (0.025 Hz-1.39 Hz; 290 mm peak-to-peak amplitude). Optokinetic stimulation was provided by sinusoidally moving a pseudo-random visual pattern in front of the cat and in the vertical plane, with identical kinematic parameters. Normal visual-otolith interaction was performed by translating the cat in front of the stationary visual surround while conflicting interaction was provided by moving the animal and the visual pattern in phase and at the same velocity (visual stabilization). Results showed that the vertical otolith-neck reflex is very poorly developed or absent in the low frequency range of motion (0.025 Hz-0.25 Hz) while consistent EMG activity is found during pure optokinetic stimulation. EMG responses are in phase with the visual surround velocity in the upward direction and with the upward OKR velocity. A close correlation is observed between the EMG gain and the OKR gain, which both decrease in this low frequency range, indicating that gaze stabilization would be mainly ensured by the OKR and a functional oculo-collic coupling or eye-neck surgery in the vertical plane. On the contrary, gaze stabilization is principally achieved by way of the otolith-neck reflex in the higher frequency range of motion (above 0.25 Hz). EMG responses recorded during otolith stimulation exhibit a relatively constant gain and a phase lead with respect to motion velocity which progressively reduces as the stimulus frequency increases up to 1.39 Hz. When present, EMG responses evoked during the optokinetic stimulation show strong gain attenuation and phase lag. Normal visual-otolith interaction induces neck muscle activity which parallels the optokinetic and the otolith responses in the low and high frequency ranges, respectively. The motor responses are however improved in terms of gain and phase values in the whole frequency range when both sensory inputs are combined.(ABSTRACT TRUNCATED AT 400 WORDS)

The influence of adequate vestibular stimulation on evoked locomotor muscle activity in the decerebrated guinea-pig

The influence of adequate vestibular stimulation on locomotor muscle activity has been investigated in the decerebrated guinea-pig. Locomotor activity was evoked by electrical stimulation of the mesencephalic locomotor region, whose location has been ascertained in this animal. Vestibular stimulation was performed by cyclic tiltings about the longitudinal and transverse axes and swinging along the vertical axis. The translation frequency was in the range of 0.02-0.8 Hz with an amplitude of +/-20% for tilting and 40 mm for swinging. Vestibular stimulation was accompanied by distinct changes in locomotor electromyographic activity of fore- and hindlimb antagonist muscles. During stimulation the intensity of discharges in extensor and flexor muscles corresponding to the stance and swing phases of the locomotor cycle was modulated; the alternation of antagonist muscle activity was not as a rule disturbed. The changes in muscle activity had the same pattern and similar phase-frequency properties to those observed under analogous vestibular stimulation during the maintenance of steady posture. It is suggested that the vestibular system is of considerable importance for the regulation of locomotor muscle activity. During locomotion the vestibular system influences mainly spinal motor output but does not act on the locomotor generator.

Convergent effects from vestibulospinal tract and primary cutaneous afferent fibers on motoneurons to ankle extensor and flexor muscles in humans

The latency and magnitude of cutaneomuscular reflexes, evoked by stimulation of the sural nerve in the contralateral and ipsilateral tibialis anterior and soleus, were investigated in normal human subjects during static tilts in the pitch axis. For all subjects the test reflex consisted of oscillating sequences of excitation and inhibition, each muscle exhibiting a characteristic pattern defined by its latency and sign of the initial phase. The latency of the various components and the sign of the initial phase did not vary with angle of tilt. However, the results of an ANOVA demonstrate a highly significant tilt-dependent modulation of the amplitude of the test reflex for the initial inhibitory phase of the contralateral tibialis anterior. We propose that this tilt-dependent effect on the crossed cutaneomuscular reflex originates from activity in the otolith organ receptors.

Response of central vestibular neurons to horizontal linear acceleration in the rat

Responses of central vestibular neurons to horizontal sinusoidal translation (F:0.25Hz) were recorded in albino rat. 57.5% of vestibular neurons were responding to this stimulation by a modulation of their firing rate, the mean phase angle of the response, averaged from the whole population being 22 +/- 79 deg. lag, relative to the peak of contralateral acceleration. Dynamic characteristics of phase and gain were studied and appeared to be different from previous reports on primary afferents: the gain decreased or was flat with increasing acceleration at one frequency, and the phase lag which was flat in the same conditions increased with increasing frequency. A phase lead of some units has been observed at low frequency (0.1 Hz). Regarding the convergence between otolith and canal inputs on nuclear vestibular neurons, it was shown that the major pattern of convergence is between canal and otolith inputs of same polarity.

Reciprocal responses to tilt of neurones within both fore- and hindlimb regions of Deiters' nucleus

In decerebrate cats with cerebellum intact the frequency response of 102 neurons located within the lateral vestibular nucleus (LVN) to sinusoidal stimulation of vestibular receptors was analyzed. Positional sensitive units, showing a reciprocal pattern of response to lateral tilting, characterized by an excitation during ipsilateral and a depression during contralateral tilt, were equally found in the rostroventral (forelimb) and dorsocaudal (hindlimb) divisions of the LVN. No unit was found to be excited during both ipsilateral and contralateral tilts. A comparison between these findings and those reported in cerebellectomized preparations indicates that the reciprocal pattern of response to tilt of neurons, particularly located in the hindlimb region of the LVN, depends upon the anatomical integrity of the cerebellum.

Properties of central vestibular neurons fired by stimulation of the saccular nerve

In 4 cats all vestibular afferents in one labyrinth except those innervating the saccular macula were transected and allowed to degenerate. 23--53 days after the initial surgery the central connections of the remaining saccular nerve were studied under chloralose anesthesia. Stimulation of the saccular nerve evoked N1 field potentials in the ipsilateral lateral and descending vestibular nuclei; little or no field potential activity was seen in the superior nucleus. The distribution of field potentials overlapped with that of neurons of origin of the vestibulospinal tracts. Forty-two neurons in the ipsilateral vestibular nuclei, many in the lateral nucleus, responded, often monosynaptically, to stimulation of the saccular nerve with single or double shocks; some of the neurons projected to the spinal cord. All saccular-fired neurons were tested for commissural actions by stimulation of the contralateral vestibular nerve. Many were facilitated, almost none were inhibited. In agreement with earlier work, we conclude that commissural inhibition may be a property of the canal system only.

Dynamic relations between natural vestibular inputs and activity of forelimb extensor muscles in the decerebrate cat. II. motor output during rotations in the horizontal plane

Decerebrate cats with the spinal cord sectioned at low thoracic levels were submitted to rotations in the horizontal plane. The position of the animal with respect to the axis of rotation was such that horizontal canal afferents were activated either alone or in combination with macular afferents. The EMG activity from the triceps brachii muscles of both forelimbs was recorded. The main findings were as follows. (1) The motor output to each forelimb extensor is increased by an increase in the activity of the horizontal canal afferents from the contralateral labyrinth. The phase of the motor output with respect to that of the vestibular afferents shows a lag which increases with frequency, reaching about 85 degrees at 1.0 Hz. (2) The macular and horizontal canal inputs are independently processed in the central nervous system and the motor output in response to both inputs applied simultaneously is a linear summation of the outputs expected for each of the inputs.

Dynamic relations between natural vestibular inputs and activity of forelimb extensor muscles in the decerebrate cat. III. Motor output during rotations in the vertical plane

Decerebrate cats with the spinal cord sectioned at low thoracic levels were subjected to sinusoidal rotations in the vertical plane, while recording the EMG activity of both triceps brachii muscles. The applied stimulus activated utricular and saccular receptors as well as the vertical semicircular canals. Using results presented in preceding papers 2,3, namely the dynamic response characteristics of the motor output to macular inputs and the fact that the responses to otolith and canal stimulation summate linearly 3, it is deduced that the dynamic characteristics of the motor output in response to stimulation of the vertical canals are the same as those to horizontal canal stimulation. The implication of these findings vis-à-vis the problem of central processing of vestibular inputs and the problem of postural stability is discussed.