Animal Tilt Research Articles

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)

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)

Tonic inhibitory influences of locus coeruleus on the response gain of limb extensors to sinusoidal labyrinth and neck stimulations.

Previous experiments had shown that in decerebrate cats activation of limb extensor motoneurons during side-down roll tilt of the animal or side-up neck rotation depends on both an increased discharge of excitatory vestibulospinal (VS) neurons and a reduced discharge of inhibitory reticulospinal (RS) neurons of the medulla, thus leading to disinhibition of limb extensor motoneurons. The present experiments were performed to find out whether the locus coeruleus (LC) complex keeps under its tonic inhibitory control the medullary inhibitory RS neurons and, if so, whether this structure intervenes in the gain regulation of the vestibular and neck reflexes acting on the limb extensor musculature. In precollicular decerebrate cats with good postural rigidity of the four limbs, the amplitude of modulation and thus the response gain of the first harmonic component of multiunit EMG responses of limb extensors to sinusoidal stimulation of labyrinth and neck receptors (at 0.15 Hz, +/- 10 degrees) were quite small in forelimb muscles (triceps brachii) and almost negligible or absent in hindlimb muscles (triceps surae). Electrolytic lesion limited to the LC complex decreased the tonic contraction of limb extensors, but greatly increased in the forelimbs (and brought to the light in the hindlimbs) the response modulation of extensor muscles to the same parameters of labyrinth or neck stimulation. Correspondingly, the response gain increased, but no change in the phase angle of the responses was observed. Both changes in posture, as well as in response gain of the limb extensors to labyrinth and neck stimulation, fully developed some time after the LC lesion. This increase in response gain of the vestibular and neck reflexes acting on the limb extensor muscles did not depend on the decrease in postural activity following the LC lesion, since it was still obtained when an increased static stretch of the extensor muscle following passive flexion of the limb compensated for the reduced EMG activity. Moreover, the slope of the regression line relating the gain of the multiunit EMG response of the triceps brachii to animal tilt with the base frequency greatly increased following lesioning of the LC, thus indicating that for the same background discharge of the muscle the amplitude of modulation, and thus the response gain, increased significantly. The effects described above involved mainly, but not exclusively, the limbs ipsilateral to the side of the lesion.(ABSTRACT TRUNCATED AT 400 WORDS)

Convergence and interaction of neck and macular vestibular inputs on reticulospinal neurons

Extracellular recordings were obtained in decerebrate eats from neurons located in the inhibitory area of the medullary reticular formation, namely in the medial aspects of the nucleus reticularis gigantocellularis, magnocellularis and ventralis. Of 127 medullary reticular units examined, 77 were reticulospinal neurons antidromically identified following stimulation of the spinal cord at T 12 L 1; the remaining 50 neurons were not activated antidromically. Unit firing rate was analyzed under separate stimulation of macular vestibular, neck, or combined receptors by using sinusoidal rotations about the longitudinal axis at 0.026 Hz, 10 peak amplitude. Among the 127 reticular units, 84 (66.1%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 93 (73.2%) responded to neck rotation. Convergence of macular and neck inputs was found in 71–127 (55.9%) reticular neurons; in these units, the gain as well as the sensitivity of the first harmonic of response corresponded on the average to 0.49 ±0.41. SD imp s deg and 5.10 ± 4.27, SD % deg for the neck responses and to 0.40 ± 0.39, SD imp s deg and 3.90 ± 3.80, SD % deg for the macular responses, respectively. Most of the convergent reticular units were maximally excited by the direction of stimulus orientation. the first hormonic or responses showing an average phase lead of about +42.7 with respect to neck position and +24.9 with respect to animal position. Two populations of convergent neurons were observed. The first group of units (58/71, i.e. 81.7%) showed reciprocal (“out ol phase”) responses to the two inputs in that they were mainly excited during side-down neck rotation, but inhibited during side-down animal tilt. The remaining group of units (13 71, i.e. 18.3%) showed parallel (“in phase”) responses to the two inputs and they were mainly excited by side-down neck rotation and animal tilt. The response characteristics of medullary reticular neurons to the combined neck and macular inputs. elicited during head rotation, closely corresponded to those predicted by a vectorial summation of the individual neck and macular responses. In particular, “out of phase” units displayed small amplitudes and large phase leads of the responses with respect to head position, when both types of receptors were costimulated. In contrast, “in phase” units displayed large amplitude and small phase leads during head rotation. The findings are discussed in relation to the possibility that presumably inhibitory reticulospinal neurons contribute with excitatory vestibulospinal neurons to the postural adjustments of the limb musculature during the labyrinth and neck reflexes.

Cholinergic mechanism controlling the response gain of forelimb extensor muscles to sinusoidal stimulation of macular labyrinth and neck receptors.

The multiunit EMG activity of the triceps brachii was recorded in precollicular decerebrate cats during roll tilt of the animal or neck rotation at the frequencies of 0.026-0.15 Hz and at the peak amplitude of 10 degrees, leading to selective stimulation of labyrinth or neck receptors. The first harmonic component of the EMG responses to labyrinth stimulation was characterized by an increased activity during side-down tilt of the animal and a decreased activity during side-up tilt; however, just the opposite changes were elicited for the same directions of neck rotation. The peak of the responses was closely related to the extreme animal or neck displacement, thus being attributed to stimulation of position-sensitive macular labyrinth and receptors. Moreover, the modulation as well as the gain of the EMG responses were small in amplitude. Intravenous injections of an anticholinesterase at a dose which in some instances slightly decreased the extensor tonus as well as the background activity of the triceps brachii (eserine sulphate, 0.05-0.075 mg/kg), greatly enhanced the response gain of this extensor muscle to animal tilt or neck rotation at the parameters reported above. This finding was also observed in the absence of any decrease in spontaneous EMG activity of the extensor muscle after injection of the anticholinesterase. In no instance did the phase angle of the response change following these injections. The increased gain of the EMG response of the forelimb extensor muscle to sinusoidal stimulation of labyrinth and neck receptors was first observed 5-10 min after the injection and reached the highest value in about one hour. This effect, was not only time-dependent, but also state-dependent. In fact, the increase in response gain described above either did not occur or was negligible during the sudden recovery of the extensor rigidity which occurred either spontaneously or after somatosensory stimulations. The effects elicited by eserine sulphate were reversed within seconds by a 0.1-0.5 mg/kg dose of atropine sulphate, an anticholinergic drug. It is postulated that for the same labyrinthine or neck signal giving rise to excitatory vestibulospinal volleys acting on extensor motoneurons, the amplitude of the EMG modulation of limb extensor muscles depends on the activity of a cholinergic system.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of medullary reticulospinal neurons to sinusoidal stimulation of labyrinth receptors in decerebrate cat.

The electrical activity of 168 individual neurons located in the medullary reticular formation, namely, in the medial aspects of the nucleus reticularis gigantocellularis, magnocellularis, and ventralis, has been recorded in precollicular decerebrate cats during sinusoidal tilt about the longitudinal axis of the whole animal, leading to stimulation of labyrinth receptors. In particular, 93 neurons were activated antidromically by stimulation of the spinal cord at T12 and L1 (1RS neurons); the remaining 75 neurons were not activated antidromically (RF neurons). Among these medial reticular neurons tested, 64 of 93 (i.e., 69%) 1RS neurons and 49 of 75 (i.e., 65%) RF neurons responded to slow rotation of the animal at the standard frequency of 0.026 Hz and at the peak amplitude of displacement of 10 degrees. A periodic modulation of firing rate of the units was observed during the sinusoidal stimulus. In particular, 71 of 113 units (i.e., 63%) were excited during side-up and depressed during side-down tilt, whereas 24 of 113 units (i.e., 21%) showed the opposite behavior. In both instances, the peak of the responses occurred with an average phase lead of about +25 degrees with respect to the extreme side-up or side-down position of the animal. The remaining 18 units (i.e., 16%) showed a prominent phase shift of the peak of their response with respect to animal position. Within the explored region of the medulla, the proportion of units excited during side-up tilt was higher at caudal levels, whereas that of the units excited during side-down tilt was higher at rostral levels. Units displaying intermediate phase angle of the responses predominated at intermediate levels of the medulla. Responses to animal tilt were detectable at 1 degree of peak displacement. The gain (impulses x s-1 x deg-1) of the responses of reticulospinal neurons did not change by increasing the peak amplitude of tilt from 5 to 20 degrees at the fixed frequency of 0.026 Hz. This finding indicates that the system was relatively linear with respect to the amplitude of displacement. By varying the frequency of stimulation from 0.008 to 0.32 Hz at the fixed amplitude of 10 degrees, two populations of reticulospinal neurons were observed. In the first, the gain and the phase angle of response remained relatively unmodified against changes in frequencies: these positional responses were attributed to stimulation of macular receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Direction-specific optokinetic modulation of monosynaptic hind limb reflexes in cats

The excitability of hindlimb extensor and flexor motoneurons is tonically modulated by animal tilt and a large visual stimulus rotating about the animal's line of sight. This direction-specific modulation is opposite for extensor and flexor motoneurons and opposite for optokinetic and vestibular stimuli, thus combining to a functionally significant pattern of postural reflexes.