Library

Notes: Summary The differential contributions of static versus dynamic visual cues to postural control were studied in human subjects. Lateral body oscillations were measured with accelerometers located at head, hips and ankle levels, while subjects righted their balance under various mechanical conditions: i) on either a soft (foam rubber) support or a hard one, and ii) in either the classical or the sharpened Romberg stance. The visual pattern (horizontal or vertical rectangular grating) was illuminated with either a stroboscopic bulb or a normal one, and control measurements were also taken in darkness for each mechanical condition. Acceleration signals were processed into their frequency power spectra, the mean area and shape of which were taken to characterize the postural skills involved and the effects of either the visual suppressions or the mechanical destabilizations. Although dynamic visual cues have already been found to play a major role in the control of lateral body sway (Amblard and Crémieux 1976), we demonstrate here that static visual cues, the only ones available under stroboscopic illumination, also make a clear though minor contribution. Hence we suggest the existence of two modes of visual control of lateral balance in man, which are well separated in terms of the frequency range of body sway: the first mechanism, which operates below 2 Hz and is strobe-resistant, seems to control the orientation of the upper part of the body; the second mechanism, which operates above 4 Hz, centers on about 7 Hz and is strobe-vulnerable, seems to immobilize the body working upwards from the feet. Thus static visual cues may slowly control re-orientation or displacement, whereas dynamic visual cues may contribute to fast stabilization of the body. In between the frequency ranges at which these two visuomotor mechanisms come into play, at about 3 Hz, there is what we call a “blind frequency”, a visually neutral sway frequency which may arise from the incompatibility of visual reorientation with visual stabilization, and where vision appears unable to reduce postural sway to any marked extent. Transmission of the destabilization produced by suppression of visual cues or by mechanical methods from one anatomical level to another is also briefly discussed in terms of bio-mechanical constraints, and the correlations between various pairs of levels are considered.

Notes: Summary Four cats labyrinthectomized shortly after birth (DELAB) exhibited the classical vestibular syndrome and recovery, while their motor development was otherwise unimpaired. As adults, they were tested for visual vestibular substitution in a locomotor task with either orientation requirements (tilted platforms) or balance requirements (narrow platforms). Visual motion cues or static visual cues were controlled using normal or stroboscopic lighting, or darkness. Measurements of the average speed of locomotion showed that: Although all cats increase their speed when more visual cues become available, a marked deficit occurs in darkness only in the DELAB cats. With either vestibular cues alone or static visual cues alone, cats are able to reach the same level of performance in the tilted platform test, which suggests a total visual-vestibular interchangeability in orientation. DELAB cats perform very poorly in the narrow rail test. When continuous vision is allowed in the narrow rail test the DELABs' performance rises but does not match that of the control group. A specific deficit in balance for the DELAB group is thus reduced by normal continuous vision as compared to stroboscopic vision, suggesting a significant, though imperfect, substitution of motion visual cues for the missing dynamic vestibular cues. Dynamic visual cues play only a minor role in most situations, when locomotory speed is high. This results support the view that both the vestibular and the visual system can subserve two distinct functions: dynamic information may stabilize the stance in narrow unstable situations, during slow locomotion. and static orientation cues may mainly control the direction for displacement. Possible interactions between head positioning and body orientation in the DELAB cats are discussed.

Notes: In crustaceans, glutamatergic excitation at the neuromuscular synapse has been extensively studied. Fewer reports exist of the central and possibly inhibitory actions of glutamate on neurons. The present study analyses the response of intracellularly identified motoneurons, which innervate the proximal leg muscles, to local glutamate pressure applications in the neuropil, in an in vitro thoracic preparation of the crayfish Procambarus clarkii. L-Glutamate application always inhibited motoneuron activity, with a decrease in input resistance. The resulting depolarization or hyperpolarization could usually be reversed within 10 mV of the resting potential. The response persisted in neurons pharmacologically isolated with Cd2+ or tetrodotoxin. The reversal potential of the response to glutamate was displaced in a low-chloride solution. Similar responses were obtained with GABA. Application of GABA blocked the glutamate response in a competitive manner. Both responses were suppressed by β-guanidino-propionic acid, a competitive antagonist for GABA receptors. This indicates that glutamate activates a chloride-GABA receptor-channel. Micromolar concentrations of picrotoxin reduced both the L-glutamate and the GABA inhibitory responses, thereby unmasking a smaller, picrotoxin-resistant effect of glutamate (but not of GABA), which was excitatory and sensitive to 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results suggest dual and opposite roles for motoneuron glutamatergic connections–a peripheral (well known) net excitatory one and a central net inhibitory one. Direct inhibition of motoneurons by L-glutamatergic neurons is to be expected.

Notes: Reflex leg levation habituates during repeated electrical stimulation of mechanosensory afferents in the dactyl of the fifth walking leg of the crayfish, Procambarus clarkii. This was investigated in decerebrate crayfish, and reproduced in an isolated thoracic ganglion preparation. In vivo, trains of stimuli delivered every 2.5 s produced a gradual decrease in the amplitude of the mechanical response, and a concomitant decrease in the number of impulses per burst in the levator muscle myogram. Near complete recovery occurred after 10 min rest, and transient dishabituation was observed after electrical stimulation of the telson. Less frequent or stronger stimuli led to less rapid habituation. In vitro, the same parametric characteristics of habituation were observed in the levator nerve responses, while the intrinsic variability of the reflex was reduced. The response decrement was shown to be unrelated to changes in the afferent excitation. Evoked polysynaptic excitatory postsynaptic potentials (EPSPs) in levator motorneurons decreased in parallel with the levator neurogram. This decrease was unrelated to any change in the resting membrane potential of the levator motorneurons. Intemeurons with habituating EPSPs, antagonistic depressor motorneurons with habituating inhibitory postsynaptic potentials and non-habituating responses in other motorneuronal groups were also found. These findings point to a central locus of habituation upstream from the motorneurons, and offer prospects for a detailed investigation of the mechanisms of habituation in a polysynaptic system.

Notes: Abstract Cuticular Stress Detector afferents (CSD1 and CSD2) in the 5th walking legs of crayfish (Pacifastacus leniusculus, Procambarus clarkii) have been studied in an in vitro preparation allowing intracellular recordings to be made from the central terminals of primary afferent fibres during mechanical stimulation of the sense organs. Biocytin anterograde fills and transverse sections of the sensory nerves showed CSD1 to comprise fewer and more heterogeneous fibres than CSD2. Lucifer yellow filling of single fibres showed branching patterns compatible with monosynaptic projections to some motorneuronal groups. Whole nerve recordings during sinusoidal or ramp stimuli showed an important contribution from units with phasic properties. The intracellular recordings identified three features unique to CSD1: 1. Many ‘on-off’ units have a phasic response to both increases and decreases of force. 2. Many ‘high threshold’ units respond only to high amplitude vibratory stimuli. 3. A few sensory fibres have a main branch projecting rostrally within the interganglionic connectives, possibly as far as the brain. In vivo recordings of CSD1 activity during forward locomotion on a treadmill showed a discharge occurring in advance of, as well as during, the power stroke. It is therefore suggested that at least some CSD1 fibres encode active as well as passive force during locomotion.

Notes: Abstract The reflex connections made by Cuticular Stress Detector afferents (CSD1 and CSD2) with motorneurones of the four proximal muscle groups in the 5th walking legs of crayfish (Procambarus clarkii, Pacifastacus leniusculus) have been studied in an in vitro preparation. Reflex responses to mechanical stimulation of the CSDs were studied in single neurones by means of intracellular techniques. Within each motorneurone pool, both excitatory and inhibitory reflex responses occurred, although sometimes no reflex connections were found. When present, they could be classified into ‘levation’ and ‘depression’ reflexes, corresponding to negative and positive feedback effects respectively. Each motorneurone receives input from a number of different CSD afferents (mean values between 3.0 and 5.8). Using electrophysiological and pharmacological tests, it was demonstrated that at least 32% of all connections were monosynaptic. In preparations showing fictive locomotion, phasic CSD stimulation was shown to be able to entrain anterior levator and depressor motorneurone activity in 95% of cases. The results thus demonstrate the importance of sensory feedback from the CSDs in shaping the final motor output.

Notes: Abstract In decerebrate cats, rotation about the longitudinal axis of the animal, leading to sinusoidal stimulation of labyrinth receptors, produces a tonic contraction of limb extensors during side-down tilt (α responses) and of dorsal neck extensors during side-up tilt (β responses). These changes in posture are mediated, at least in part, by lateral vestibular nucleus (LVN) neurons, with response characteristics to stimulation of macular and/or canal receptors that have so far been evaluated at the level of either unidentified vestibulospinal (VS) neurons or vestibulo-collic neurons projecting to the upper cervical cord. In the present study we investigated the dynamics of the responses of VS neurons projecting to the lumbosacral segments of the spinal cord to increasing frequencies of tilt (from 0.026 to 0.32 Hz, ±10°). All the recorded units showed an average phase lead with respect to position of +25.6±5.5° (SE) at the tilt frequency of 0.026 Hz. Most of these neurons (n=32) were particularly activated during side-down tilt (α responses) and showed either a stable phase or an increase in phase lead of the responses with increasing frequency of tilt. At the tilt frequency of 0.026 Hz, the smaller the phase lead of the responses, the larger was the response gain. Moreover, the smaller the phase lead of the responses at that frequency of tilt, the smaller the increase in gain but the larger was the increase in lead of the responses obtained by increasing the stimulation frequency up to 0.32 Hz. Through this set of finely organized changes in unit response characteristics, the overall output of this population of neurons increased, while the phase angle of the responses reached the mean value of +64.9±2.6° (SE), thus becoming more related to the velocity than to the positional signal. The remaining units (n=7), which were mainly activated during side-up tilt (β responses), displayed an increase in phase lag of the responses to increasing frequency of stimulation, which reached the mean value of-118.9±14.5° (SE) at 0.32 Hz. The differences in the dynamic properties of these VS neurons projecting to the lumbosacral cord, with respect to those of previously recorded populations of VS neurons, are discussed.

Notes: Abstract 1. 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 (lVS 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). 2. Among the lVS 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°. The gain and the sensitivity of the first harmonic responses corresponded on the average to 0.47±0.44, SD, impulses·s−1·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〈0.05). 3. Among the lVS 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·s−1·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. 4. Histological controls indicated that 102 of 129 (i.e. 79.0%) lVS neurons tested for labyrinth stimulation and 86 of 109 (i.e. 78.9%) lVS neurons tested for neck stimulation were located in the dorsocaudal part of LVN, the remaining lVS neurons being located in the rostroventral part of LVN. 5. The observation that the predominant response pattern of the lVS neurons to roll tilt was just opposite to that of lVS neurons to neck rotation indicates that the motoneurons innervating ipsilateral hindlimb extensors were excited by an increased discharge of vestibulospinal neurons during side-down tilt but they were disfacilitated by the reduced discharge of vestibulospinal neurons during side-down neck rotation; the opposite would occur during side-up animal tilt or neck rotation. These findings were compared with those of previously recorded LVN neurons, whose descending axons were not identified as projecting to upper or lower segments of the spinal cord. It was then possible to evaluate the role that the LVN exerts not only in the control of the limb but also of the neck extensor musculature.

Notes: Abstract 1. In decerebrate cats with the cerebellum intact we recorded the activity of lateral vestibulospinal neurons projecting to lumbosacral segments of the spinal cord (lVS neurons) and related the resting discharge, as well as the response characteristics of these neurons to roll tilt of the animal and neck rotation, with the cell size inferred from the conduction velocity of the corresponding axons. 2. A slight negative correlation was found between resting discharge rate and conduction velocity of the whole population of lVS neurons responsive and unresponsive to animal tilt and neck rotation, so that the faster the conduction velocity, the lower was the unit discharge at rest. This correlation, however, was found only for the dorsocaudal LVN neurons, which contributed to the majority of lVS units, but not for the rostroventral LVN neurons. Moreover, it affected the units unresponsive but not those responsive to vestibular stimulation; the opposite, however, occurred for the units tested to neck stimulation. These findings indicate that the static properties of the lVS neurons can only in part be related to cell size. 3. If we consider the lVS neurons responsive to roll tilt of the animal (76 neurons) and neck rotation (75 neurons) at the standard parameters of 0.026 Hz, ±10°, no significant correlation was found between gain (impulses·s−1·deg−1) of the labyrinth or neck responses and conduction velocity of the axons. Thus, due to the presence of slight negative relation between resting discharge and conduction velocity of the axons, larger neurons exhibited a greater percentage modulation (sensitivity) to the labyrinth and neck inputs than smaller neurons; this correlation involved particularly the dcLVN neurons. These findings suggest that the afferent pathways driven during dynamic stimulation of labyrinth and neck receptors produce an higher number or density of synaptic contacts on lVS neurons of increasing size. 4. No significant differences in the means of resting discharge, conduction velocity, gain and sensitivity were found between all the lVS units responding to labyrinth and neck inputs. These findings indicate that the effectiveness of the two inputs was almost comparable and did not vary in different units as a function of cell size. 5. The lVS neurons were mainly excited during side-down animal tilt and side-up neck rotation. Although these neurons showed the same spectrum of conduction velocity as those displaying the opposite response patterns, the response gains of the predominant populations of units were on the average higher than those obtained from the remaining populations of units. The peak of the unit responses occurred with an average phase lead of +21.0±27.2°, SD with respect to the extreme animal displacements, and an average phase lead of +52.0±18.3, SD with respect to the extreme neck displacements; yet no difference was found in the average phase angle of responses of the two populations of slow and fast units to animal tilt or neck rotation. These findings indicate that the quantitative and qualitative organization of the synaptic inputs represent the critical factor controlling response characteristics of comparable size lVS neurons to vestibular and neck stimulations. 6. A comparison between the present results and those obtained in previous experiments performed after bilateral ablation of the paleocerebellum permitted to evaluate the role that the inhibitory area of the cerebellum exerts on the resting discharge, as well as on the dynamic characteristics of response of different size lVS neurons to animal tilt and neck rotation.