Mechanisms of spinal cord electrical stimulation action on autonomic functions

The method of transcutaneous electrical stimulation of the spinal cord (ESSC) has recently begun to be actively used for both experimental studies of human motor functions and the rehabilitation of motor function in patients with spinal cord pathology. The spinal cord is the most important center of the regulation of vital functions, and ESSC affects as spinal locomotor networks as the visceral system too, which should be taken into account for the development of an improved method of rehabilitation and its use in experiments on healthy volunteers. We present a review of studies on the possible mechanisms of ESSC effects on the peripheral and cerebral circulation, cardiovascular, respiratory, excretory, and digestive systems of mammals.     

Influence of artificial vestibular feedback on posture instability caused by asymmetric proprioceptive stimulation

Subjects kept a vertical posture, standing on a rigid support. Stability of a posture was estimated by the sizes of standard deviations (sigma) from average amplitudes of the subject's head fluctuation in respect to zero coordinates. To create a feedback on the vestibular input, transmastoidal bipolar galvanic stimulation was used. Changes of current in contour of feedback looked as linear function considering amplitude and velocity of the subject's head displacements. Varying the factors of feedback function, it was possible to reduce sigma for lateral sways increased (in comparison with their values at the quiet stance in the darkness) as a result of unilateral vibrating stimulation of m. gluteus medialis. The results specify inequality of "velocity" and "position" information for maintenance of vertical posture in different subjects. The results specify also the ability of the central nervous system (CNS) to revalue weights of various kinds of information entering via the same channel. The data confirm the hypothesis according to which galvanic vestibular input is capable to deliver in CNS and adequate information on the current orientation of the body. This information can be used for stabilization of a posture.     

The effect of lateral inclinations upon optokinetic and postoptokinetic nystagmus in pigeons

Optokinetic stimulation with the angle velocity 10 degrees/s was carried out in pigeons in differently bent postures. No difference in effects of dynamic and static inclinations upon optokinetic and postoptokinetic nystagmus was found. Neither any rigid connection was found between the inclination direction and the sign of the response change. The data obtained are at variance with the hypothesis of gravity-dependent changes of the "velocity accumulator" time constant as the only cause of changes in the optokinetic and postoptokinetic responses under conditions of dynamic or static inclinations.     

Artificial vestibular feedback in conditions of modified body scheme

Subjects standing in darkness on the rigid support, kept a vertical posture which was destabilized by vibration of the Achilles tendons. To create a feedback on the vestibular input, transmastoidal bipolar galvanic stimulation was used. Changes of current in the feedback contour looked as linear function considering amplitude and velocity of the subject's head displacements in reference to the vertical. To change the body scheme we used some posture configurations: turning of the head in relation to the trunk; turning of the trunk with the head fixed; joint turning of the head and trunk. As a result of these configurations, the head could be turned approximately at right angle in relation to the feet. In addition turning of one foot at right angle in relation to the other foot was used. Artificial feedback reduces body fluctuations caused by vibration only in the vertical plane which passes through interaural axis of the head. The authors assume that directional changes of vestibulo-motor responses and results of application of artificial feedback during changes of orientation of the head in relation to the feet can be connected to change of ensembles of vestibular hair cells, which signals dominate in responses of vestibulo-spinal neurones.     

Habituation of horizontal eye nystagmus in pigeon during alternation of central and eccentric rotations

Intact pigeons were rotated in the horizontal plane in the dark in different positions relatively to the rotation axis. At central rotations, the pigeon's head was in the rotation axis whereas at eccentric rotations it was displaced from the axis. Series of central and eccentric rotations were alternated. Each series consisted of 2-5 rotations using angular velocity trapezoids. All stimuli producing habituation were used at most 14 times each. Eccentric rotations did not prevent a gradual decrease of peak velocities of the slow component of primary nystagmus on transition from one series of central rotations to another in 17 pigeons (group 1). The increase of peak velocities was observed in 2 pigeons (group 2). In group 1, a direct dependence among alterations of this parameter of primary nystagmus, modifications of its duration, and variations of peak velocities of secondary nystagmus, were observed. If two identical stimuli did not follow in sequence directly, the effect of the second one produced same nystagmus changes as were observed in present pigeon by comparison of the first and last responses in the series of the central rotations. If they follow one by one, in many cases the second stimulus in the pair produced an increase of peak velocity of primary and secondary nystagmus and rise of delay of the point of primary nustagmus peak velocity. These variations were not random (probability, > 95%).     

Mechanisms of the otolith signal processing

A series of current hypotheses of mechanisms of the otolith signals processing and organization of adaptive responses of the oculomotor system to gravito-sensitive (otolith) inputs are reviewed: the "frequency segregation of the otolith signals", "gravito-inertial force resolution", "spatio-temporal transformation of the angular vestibulo-ocular reflex", "inertial processing of the vestibular signals", "asymmetry of the otolith signals", and their experimental corroboration.     

The effect of otolith entrances upon horizontal nystagmus induced by stimulation of semicircular ducts and retina in pigeons

The semicircular ducts in pigeons were stimulated either in an isolated way (the angular acceleration being 10 degrees/c2) or together with the otolith organs (the peak value of the interaural acceleration being 0.5 g). In one and the same situation, both the inhibitory and the activating otolith effects on nystagmus were found. In one and the same animal, changes of the postoptokinetic and canal nystagmuses could be qualitatively different. The results obtained contradict the hypothesis of changes in the "velocity accumulator" time constants as the only cause of changes in the postoptokinetic and rotatory nystagmuses under conditions of low-frequency otolith stimulation.     

Fragmental regulation of vestibuloocular responses

Intact pigeons were exposes to whole body centric and eccentric horizontal rotations in darkness during angular velocity trapezoids. The overall nystagmus alteration patterns were analysed. In 10 pigeons, all nystagmus alterations may be explained on the basis of the dynamics of peripheral otolith activity and central effects that are the same for all combinations of interacting inputs (type 1 patterns), whereas in other pigeons part of the nystagmus alterations were connected with some central effects that were individually specific and touch upon the responses to separate combinations of interacting input (type 2 patterns). It was observed the transformation of type 2 to type 1-patterns during the reiterated rotational trails and light nembutal anesthesia. A fragmentary control of the vestibulo-ocular responses seems to exist. This implies that the CNS is able to discern numerous kinds of bilaterally organized interacting inputs arising during different otolith membrane shifts.     

Interaction of the angular and linear components of the horizontal vestibulo-ocular reflex in the pigeon

Pigeons were exposed to centric and eccentric horizontal rotations in darkness by velocity trapezoid. Different in sign the duration alterations of the opposite directed horizontal eye nystagmus occurred during otolith membrane shifts in sagittal as well as frontal planes. A direct dependence was found between the duration alterations of the primary nystagmus phase and the peak value alterations of its slow phase velocity under increased (but not decreased) centrifugal force. In the both cases, if duration of the primary nystagmus phase was enlarged, duration of its secondary phase was diminished and vice versa. It suggests the otolith component does not decay up to zero by constant velocity and at once after rotation; by deceleration it is biphasic. In affirms the own hypothesis that the linear component is asymmetric central neuronal activity that modifies the canal component even if this activity by itself is not enough for eye movement initiation.     

Vestibular effects on posture instability evoked by moving visual environment and footing

Subjects held the vertical posture standing up on hard footing, having small degree of the freedom in the frontal plane. The stability of the vertical posture has been assessed by the standard deviations (sigma) from average amplitudes of the fluctuations of the subject's head (in frontal and sagittal planes) from conditional zero. Sinusoidal rotations of optokinetic cylinder, sinusoidal rotations of the footing, and combinations of these rotations, under phase shifts between the optokinetic cylinder and the footing, caused increase of sigma. The amplitude and velocity signal of the head deviations was transformed into low galvanic current applied to the mastoids and used as the artifical vestibular biofeedback. It was possible to reduce the value of the sigma for lateral tilts (raised in comparison with their values during stance in the dark as a result of destabilizing influence), varying coefficients of the biofeedback. At the same time, appropriate fluctuations in sagittal plane were not systematic.