Vestibular compensation: tonic spinal influence upon spontaneous descending vestibular nuclear activity

How Does the Central Nervous System for Posture and Locomotion Cope With Damage-Induced Neural Asymmetry?

In most vertebrates, posture and locomotion are achieved by a biomechanical apparatus whose effectors are symmetrically positioned around the main body axis. Logically, motor commands to these effectors are intrinsically adapted to such anatomical symmetry, and the underlying sensory-motor neural networks are correspondingly arranged during central nervous system (CNS) development. However, many developmental and/or life accidents may alter such neural organization and acutely generate asymmetries in motor operation that are often at least partially compensated for over time. First, we briefly present the basic sensory-motor organization of posturo-locomotor networks in vertebrates. Next, we review some aspects of neural plasticity that is implemented in response to unilateral central injury or asymmetrical sensory deprivation in order to substantially restore symmetry in the control of posturo-locomotor functions. Data are finally discussed in the context of CNS structure-function relationship.

Recovery of gaze stability during vestibular regeneration

Many motion related behaviors, such as gaze stabilization, balance, orientation, and navigation largely depend on a properly functioning vestibular system. After vestibular insult, many of these responses are compromised but can return during the regeneration of vestibular receptors and afferents as is known to occur in birds, reptiles, and amphibians. Here we characterize gaze stability in pigeons to rotational motion during regeneration after complete bilateral vestibular loss via an ototoxic antibiotic. Immediate postlesion effects included severe head oscillations, postural ataxia, and total lack of gaze control. We found that these abnormal behaviors gradually subsided, and gaze stability slowly returned to normal function according to a temporal sequence that lasted several months. We also found that the dynamic recovery of gaze function during regeneration was not homogeneous for all types of motion. Instead high-frequency motion stability was first achieved, followed much later by slow movement stability. In addition, we found that initial gaze stability was established using almost exclusive head-response components with little eye-movement contribution. However, that trend reversed as recovery progressed so that when gaze stability was complete, the eye component had increased and the head response had decreased to levels significantly different from that observed in normal birds. This was true even though the head-fixed VOR response recovered normally. Recovery of gaze stability coincided well with the three stage temporal sequence of morphologic regeneration previously described by our laboratory.

Bilateral vestibular loss leads to active destabilization of balance during voluntary head turns in the standing cat

The purpose of this study was to determine the source of postural instability in labyrinthectomized cats during lateral head turns. Cats were trained to maintain the head in a forward orientation and then perform a rapid, large-amplitude head turn to left or right in yaw, while standing freely on a force platform. Head turns were biomechanically complex with the primary movement in the yaw plane accompanied by an ipsilateral ear-down roll and nose-down pitch. Cats used a strategy of pushing off by activating extensors of the contralateral forelimb while using all four limbs to produce a rotational moment of force about the vertical axis. After bilateral labyrinthectomy, the initial components of the head turn and accompanying postural responses were hypermetric, but otherwise similar to those produced before the lesion. However, near the time of peak yaw velocity, the lesioned cats produced an unexpected burst in extensors of the contralateral limbs that thrust the body to the ipsilateral side, leading to falls. This postural error was in the frontal (roll) plane, even though the primary movement was a rotation in the horizontal (yaw) plane. The response error decreased in amplitude with compensation but did not disappear. We conclude that lack of vestibular input results in active destabilization of balance during voluntary head movement. We postulate that the postural imbalance arises from the misperception that the trunk was rolling contralaterally, based on signals from neck proprioceptors in the absence of vestibular inputs.

Plasticity mechanisms in vestibular compensation in the cat are improved by an extract of Ginkgo biloba (EGb 761)

The effects of administration of an extract of Ginkgo biloba (EGb 761) on vestibular compensation was studied in unilateral vestibular neurectomized cats. This experimental model of CNS plasticity was investigated by using behavioral tests (postural disorders compensation, locomotor balance recovery), electrophysiological (spontaneous and evoked neck muscle activity) and neurophysiological (spontaneous firing rate recovery of deafferented vestibular cells) recordings, and immunocytochemical methods (synaptic loss and synaptic reoccupation within the deafferented vestibular nuclei). In all experiments, EGb 761 was administered over 30 days at daily doses of 50 mg/kg IP. The results showed a faster recovery in the EGb-treated group of cats as compared to an untreated control group. EGb administration strongly accelerated postural and locomotor balance recovery. Concomitantly, spontaneous neck muscle activity, vestibulo-collic reflexes and spontaneous firing rate of vestibular units located on the lesioned side were restored earlier. Morphological correlates characterized by a more rapid synaptic reoccupation were found in the deafferented medial vestibular nucleus by means of immunoreactive labelling using an antibody against a synaptic vesicle-associated protein (synaptophysin), but they displayed a longer time-constant in comparison with the behavioral and neurophysiological data. These results clearly demonstrate that EGb 761 acts on recovery mechanisms considered as key processes in vestibular compensation. They suggest that this substance would possess neurotrophic and/or neuritogenic properties improving functional recovery after CNS injury.

Compensation of horizontal canal related activity in the medial vestibular nucleus following unilateral labyrinth ablation in the decerebrate gerbil. II. Type II neurons

The spontaneous activity and dynamic responses to sinusoidal horizontal head angular acceleration of type II horizontal semicircular canal related neurons in the medial vestibular nucleus (MVN) were recorded bilaterally in decerebrate Mongolian gerbils (Meriones unguiculatus) under three experimental conditions: normal labyrinths intact, acutely following unilateral labyrinthine lesion, and four to seven weeks following labyrinthine lesion. The number of type II neurons detected contralateral to the lesion was greatly reduced both in the acutely hemilabyrinthectomized animals and following compensation. The gain of the responses was depressed bilaterally acutely following the lesion. A greater reduction in response gain was noted in cells contralateral to the lesion. The gain of the contralateral type II responses increased with time such that in the compensated animal bilaterally symmetric gains were recorded. While the significant changes which occur in the gain of type II neurons with recovery from peripheral vestibular lesions can largely be attributed to type I neurons on the other side of the midline, changes in type I neurons were not entirely reflected in the type II population. The spontaneous activity of type II neurons did not undergo any significant changes following the labyrinthine lesion. We present a model utilizing the dynamic responses to estimate the functional recovery of commissural connections in compensated animals. The overall gain of the contralateral type I to ipsilateral type I commissural polysynaptic pathway appears to improve, while the efficacy in the reverse direction remains depressed, suggesting that modifications in commissural connections, particularly involving the type II to type I connections within the MVN on the injured side, mediate aspects of behavioral recovery.

Mechanisms of recovery following unilateral labyrinthectomy: a review

This paper reviews the literature on the mechanisms responsible for the behavioural recovery which occurs following unilateral labyrinthectomy (UL), UL causes a syndrome of ocular motor and postural disorders, which diminish over time in a process of behavioural recovery known as vestibular compensation. Electrophysiological studies show that the VIIIth nerve does not undergo a functional recovery, therefore vestibular compensation has been attributed to CNS plasticity. However, the nature of the plasticity responsible for vestibular compensation is not understood. Single-neuron studies have demonstrated that a significant recovery of resting activity has occurred in the vestibular nuclei (VN) ipsilateral to the UL by the time symptoms such as spontaneous nystagmus and roll head tilt (static symptoms) have largely disappeared. However, many of the deficits in the response of VN neurons to head acceleration persist and may be permanent. This lack of recovery in the response of neurons to head acceleration correlates with the incomplete and sometimes poor recovery of the vestibulo-ocular and vestibulo-spinal reflex responses to head movement (dynamic symptoms). The major neuronal change in the VN during vestibular compensation appears to be the recovery of resting activity in the VN ipsilateral to the UL, although this recovery is more pronounced in the medial VN than in the lateral VN. The mechanism responsible for the regeneration of resting activity in VN neurons is unknown. In frogs, there is evidence to suggest that transcommissural synaptic input to the VN, from the contralateral (intact) labyrinth, increases in efficacy.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of spinal cord transection on spontaneous activity recorded from type I neurons of the medial vestibular nucleus in compensated hemilabyrinthectomized gerbils

Spontaneous activity was recorded from type I neurons of the medial vestibular nuclei (MVN) in unanesthetized, decerebrate gerbils to determine the effect of spinal cord transection on compensation following labyrinthectomy. Immediately after labyrinthectomy there was an increase in activity of type I neurons on the intact side and an absence of activity on the injured side. Following compensation from labyrinthectomy, the distribution and activity rates approximated those of nonlabyrinthectomized animals. Spinal cord transection resulted in an increase in activity in type I MVN neurons contralateral to the labyrinthectomy in compensated animals and bilaterally in nonlabyrinthectomized animals. These results illustrate that type I neurons apparently are under an indirect inhibitory control from both the contralateral labyrinth and the spinal cord. In compensated animals spinal cord inhibition exists only on the intact side. This suggests that the symmetry in type I activity bilaterally in the compensated animal is in part the result of asymmetric spinal cord input.

Survival of function in the deafferentated vestibular nerve

After it has been deafferentated, spontaneous spike activity in the vestibular nerve is not abolished, but is drastically reduced 1-3 months post-op. If the nerve stump is cut again in the compensated animal, similarly directed postural asymmetries are re-induced, although of milder intensity. Electrical stimulation of the vestibular nerve stump evokes monosynaptic excitation in the vestibular nuclei and a vestibulo-ocular reflex at normal thresholds despite partial degeneration in the nerve. Therefore, the deafferentated nerve is capable of exerting compensatory influences upon the central nervous system.