Biomedical subjects

A R Marchand

Publications and source records attributed to A R Marchand.

23 records · Page 2Linked to original sources

PubMed1987

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)

Action Potentials↗

PubMed1987

Effects of roll tilt of the animal and neck rotation on different size vestibulospinal neurons in decerebrate cats with the cerebellum intact.

In decerebrate cats with the cerebellum intact we recorded the activity of lateral vestibulospinal neurons projecting to lumbosacral segments of the spinal cord (IVS 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. A slight negative correlation was found between resting discharge rate and conduction velocity of the whole population of IVS 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 IVS 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 IVS neurons can only in part be related to cell size. If we consider the IVS neurons responsive to roll tilt of the animal (76 neurons) and neck rotation (75 neurons) at the standard parameters of 0.026 Hz, +/- 10 degrees, no significant correlation was found between gain (impulses X s-1 X 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 IVS neurons of increasing size. No significant differences in the means of resting discharge, conduction velocity, gain and sensitivity were found between all the IVS 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. The IVS 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.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1987

Convergence of neck and macular vestibular inputs on vestibulospinal neurons projecting to the lumbosacral segments of the spinal cord.

The activity of LVN neurons was recorded in decerebrate cats and analyzed during separate stimulation of macular vestibular and neck receptors elicited by sinusoidal rotation about the longitudinal axis at 0.026 Hz, 10 degrees peak amplitude. Of 119 LVN units examined, the great majority, i.e. 106, were vestibulospinal neurons antidromically identified following stimulation of the spinal cord at T12-L1, thus projecting to the lumbosacral segments of the spinal cord (IVS neurons); the remaining 13 units were nonantidromically activated. Among the 119 LVN neurons, 77 (64.7%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 81 (68.1%) responded to neck rotation. Convergence of macular and neck inputs was found in 58/119 (48.7%) lateral vestibular neurons; in these units, the gain as well as the sensitivity of the first harmonic of responses corresponded on the average to 0.58 +/- 0.45, S.D. imp./sec/deg and 4.39 +/- 3.58, S.D.%/deg for the neck responses and 0.52 +/- 0.49, S.D. imp./sec/deg and 3.85 +/- 3.35, S.D.%/deg for the macular responses, respectively. In addition to these convergent units, 19/119 (16.0%) and 23/119 (19.3%) lateral vestibular units responded to selective stimulation either of macular receptors or of neck receptors only. These units, which showed on the average an higher firing rate and a lower conduction velocity of the corresponding vestibulospinal axons than the convergent units, displayed a significantly lower response gain and sensitivity to animal tilt and neck rotation with respect to those obtained from convergent units. Most of the convergent lateral vestibular units were maximally excited by the direction of stimulus orientation, the first harmonic of responses showing an average phase lead of +51.4 degrees with respect to neck position and +21.9 degrees with respect to animal position. Two populations of convergent neurons were observed. The first group of units (53/58, i.e. 91.4%) showed reciprocal ("out-of-phase") responses to the two inputs in that they were mainly excited during side-down animal tilt and side-up neck rotation. The remaining group of units (5/58, i.e. 8.6%) showed parallel ("in phase") responses to the two inputs and they were mainly excited by side-up neck rotation and animal tilt. Interestingly, the former group of units displayed an average gain and sensitivity to the labyrinth and neck inputs which were more than twice higher than the values obtained from the latter group of units.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1985

Lateral orientation and stabilization of human stance: static versus dynamic visual cues.

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: on either a soft (foam rubber) support or a hard one, and 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.

Adult↗

PubMed1984

Locomotion in adult cats with early vestibular deprivation: visual cue substitution.

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.

Animals↗