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Rats made immobile and cataleptic by haloperidol, a dopamine receptor blocker, maintain their static stable equilibrium by employing a variety of allied postural support reflexes. Under some test conditions, competition between such reflexes occurs, and in haloperidol-treated rats, unlike undrugged controls, proprioceptive-tactile stimuli appear to be dominant over vestibular stimuli. We investigated this relationship in rats by testing their air-righting with and without simultaneous contact of the tail on a wooden platform. The rats were lightly held in a supine position by the shoulders and pelvis, with or without tail contact on a small wooden platform 47 cm above the ground. Undrugged rats showed the normal pattern of righting which involves axial rotation with cephalocaudal recruitment whether the tail is contacting the platform or not. Upon release, the haloperidol-treated rats (2.5 mg/kg) gripped the platform with their tail, which interfered with the air-righting reflex. This demonstrates that in haloperidol-treated rats, the dominance of tactile-proprioceptive postural support reflexes over those triggered vestibularly.
We compared vestibulo-ocular reflex, optokinetic reflex and postural function in subjects with insulin-dependent diabetes mellitus (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM), as well as non-diabetic controls. Both IDDM and NIDDM subjects exhibited significant deficits in gaze-holding in darkness (p < 0.05), small changes in vestibulo-ocular reflex (VOR) phase re velocity (p < 0.005) without a change in VOR gain, and a decrease in optokinetic reflex (OKR) slow phase velocity (p < 0.001). In addition, a smaller decrease was found in OKR quick phase amplitude (p < 0.02); postural sway was increased in both diabetic groups (p < 0.05), although this was not specific to the conditions of the Clinical Test of Sensory Interaction and Balance (CTSIB) that test vestibular contributions to postural stability. No differences were found in optokinetic afternystagmus or latency to circularvection. These results suggest that both IDDM and NIDDM are associated with deficits in gaze-holding, VOR and OKR function.
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In order to assess the influence of the cervical structures on the vestibulo-spinal reflex, and so on the posture, we have studied twenty subjects undergoing stimulation of the neck's proprioceptor structures. Each subject, by means of computerized stabilometric platform, has been submitted to the sensitized test with head retroflexed. In all cases significant variations of the posturographic parameters regarding in a particular way the total tracing length, the ellipse surface and the mean speed have been recorded. The results show that there are in man too stimulations of cervical origin which serve the purpose of maintaining posture; we can't exclude the possibility of the interference of an otolithic stimulation.
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The effects of neck vibration were studied in ten healthy subjects standing in unrestrained or restrained conditions. An involuntary slow and gradual displacement of the body axis associated with a subjective experience of forward tilt occurred during vibration of the dorsal surface of the neck. This effect was associated with a decreased tonic contration of the antigravitary lower limb musculature. The amplitude of the H-reflex from the soleus muscle, recorded at progressively increasing time interval after the onset of the vibratory stimulus, showed complex interactions: in particular, in unrestrained conditions an early inhibitory phase occurred at about 100 msec, followed by a short-lasting facilitatory phase between 150-300 msec and by a late long-lasting excitatory component which started 500-600 msec after the onset of vibratory stimulus. In restrained subjects, the late excitatory phase disappeared and was substituted by a delayed depression. In this instance, the short-lasting facilitatory phase appeared to be superimposed on a background of inhibition. The effects produced by the neck input on the H-reflex were attenuated during vibration of the dorsal muscolature at L4-L5. The mechanisms involved in the cervical control of posture and reflex movements following neck vibration are discussed.
In a reflexively standing acute decerebrate cat, the pontine tegmentum was electrically stimulated and the effects on postural muscle tone and locomotor movements evoked by stimulation of the mesencephalic locomotor region (MLR) were studied. A stimulating microelectrode was placed systematically at 1-mm increments throughout the pons (H - 2 to H - 10) at levels ranging from P0.0 to P6.5 dorsoventrally and mediolaterally from LR0 to L4 or R4. Another stimulating microelectrode was placed in the physiologically identified MLR. Stimuli delivered to the dorsomedial regions of the pontine tegmentum (P3 to P4, LR1.5 to 2.5, H - 4 to H - 6) resulted in simultaneous and bilateral suppression of tonic activities in the neck, lumbar back, forelimb and hindlimb muscles. The pontine inhibitory sites corresponded to the medial area of the central tegmental field (FTC) and the central area of the gigantocellular tegmental field (FTG), bilaterally. Effects of pontine induced suppression on those muscles were stimulus frequency and stimulus intensity-dependent and the effects persisted even after termination of the stimulation. With concomitant pontine stimulation, MLR-evoked locomotor movements were suppressed along with prolongation of the forelimb and hindlimb step cycles.
Vestibulospinal and vestibulocollic reflexes evoked by galvanic stimulation were studied in 20 normal volunteers. In an initial "baseline" study, subjects stood unsupported on a flat surface and a narrow base with their eyes shut and with their heads rotated to the left. The effects of vision, external support and increasing stance width were examined both individually and in combination. In a second series, the effects of the same factors were examined while subjects stood on a compliant surface. Short latency (SL) and medium latency (ML) vestibulospinal reflex responses were evoked using 4 mA/20 ms galvanic vestibular stimulation (GVS) and measured from the right soleus muscle. Vestibulocollic reflexes to short duration galvanic stimulation (4 mA/2 ms) were also measured under similar conditions. Both SL and ML vestibulospinal reflexes decreased significantly and to a similar degree with vision, external support and increasing stance width on a flat surface. On the compliant surface, stance width did not result in a decrease in the ML reflex. Reflex amplitudes further decreased in a non-linear fashion with each additional sensory modality ("factor") that was made available; the degree of attenuation due to the addition of a second and third factor closely approximated the product of the effect of each factor in isolation. Standing on a compliant surface resulted in enhancement of average SL and ML reflexes under all conditions. Vestibulocollic reflexes in contrast showed no significant modulation with vision, external support and stance width. The dissociation between vestibulocollic and vestibulospinal reflexes indicates that the modulation of vestibulospinal reflexes with task occurs proximal to the primary vestibular afferents. Vestibulospinal reflexes were largest when subjects stood on a narrow base, on a compliant surface, deprived of vision and external support, consistent with the importance of vestibular function under these conditions. Although attenuated, vestibulospinal reflexes were preserved in most subjects even when vision and external support were available and a wider stance width was adopted. The combination of different factors caused a multiplicative attenuation of the initial response.
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