Stimulation Of Labyrinth Research Articles

Visual illusions induced by electrical stimulation of the labyrinth

In normal subjects, electrical stimulation of the labyrinth with surface electrodes located on the mastoid process induced illusions of shifting of a fixed point of light in darkness similar to the oculogyral illusion induced by rotatory vestibular stimulation. Monoaural anodal stimulation of the right labyrinth induced apparent shift of the target to the left; with cathodal stimulation, it shifted to the right; threshold current was 0.35–0.6 mA. When the current strength increased, the amplitude and rate of apparent movement of the target increased approximately linearly. With binaural, bipolar stimulation, the illusory movement of the target was toward the site of the cathode and the threshold decreased by 1.5–2.5 times. With binaural, monopolar stimulation, the target seemed to shift along the vertical and the threshold current was 1.4–3.0 mA. Eye movement appeared at substantially higher currents than those resulting in apparent movement of the target. It is suggested that visual illusions are linked not to vestibular eye-movement reactions, but to the effect vestibular signals have on the spatial perception system.

The ventrolateral medulla of the cat mediates vestibulosympathetic reflexes

Extracellular recordings were made from 94 neurons located in the ventrolateral medulla (VLM) whose firing rate was affected by vestibular nerve (VN) stimulation; 50 of these units were in the subretrofacial (SRF) nucleus, which contains cells that make direct excitatory connections with sympathetic preganglionic neurons. The sample included 12 SRF cells which were antidromically driven from the upper thoracic spinal cord and had conduction velocities of 10 m/s or less; the effect of VN stimulation on all but one of these units was inhibition. The onset latency of the response to VN stimulation was long [20.3 ± 3.7(S.E.M.) ms, n = 9, for the antidromically activated neurons and 12.1 ± 1.2ms, n = 73, for the others], suggesting that the effects were predominantly polysynaptic. In addition, most of the spontaneously active units tested (33/36) received convergent inputs from the carotid sinus nerve (CSN), as would be expected for neurons which influence sympathetic outflow. Vestibular-elicited inhibition of SRF neurons with projections to the intermediolateral cell column could account for late, long duration inhibition of sympathetic discharges produced by labyrinth stimulation.

Neurotransmitters Regulating Vestibular Commissural Inhibition in the Cat

The present study focuses on the transmitters which control the vestibular neural activity and, in particular, those which regulate commissural inhibition. Extracellular spike recordings were taken from single vestibular neurons of decerebrate cats. Seven-barreled electrodes were used. The barrels, except for the central one, were filled with several transmitter candidates and their specific inhibitors. After isolation of a type I neuron, the chemicals were iontophoretically applied to examine their effects on the activity of the neuron. The results were as follows: i) GABA and glycine markedly decreased spontaneous firing of the neurons, while serotonin did not change the activity. ii) Bicuculline abolished the inhibitory effects of GABA on the neurons. iii) Strychnine also abolished the effects of glycine, iv) Commissural inhibition caused by electrical stimulation of the contralateral labyrinth was not abolished by the application of strychnine but was abolished by bicuculline. We concluded that 1) vestibular type 1 neurons are controlled by GABAergic and glycinergic neurons, and that 2) the commissural inhibition is activated by the GABAA receptor, but not by the GABAB receptor.

Changes in Gain of Vestibulospinal Reflexes after Local Injection of β-adrenergic Substances in the Cerebellar Vermis of Decerebrate Cats

Experiments were performed in decerebrate cats to find out whether the noradrenergic afferent system to the cerebellar cortex modified the gain of the vestibulospinal (VS) reflexes and, if so, whether this effect was mediated through beta-adrenoceptors. Unilateral injection into the vermal cortex of the culmen of the beta-adrenergic agonist isoproterenol (0.25-0.50 microliter at 8-16 micrograms/microliters of saline) increased the gain of the ipsilateral and to a lesser extent also of the contralateral triceps brachii to roll tilt of the animal at 0.15 Hz, +/- 10 degrees. The opposite result was obtained after local injection of an equal dose of the beta-adrenergic antagonist, propranolol. Only slight changes in the phase angle of the responses were obtained. The effective sites were located within the parasagittal zone B which upon stimulation inhibited the activity of the ipsilateral triceps brachii. Since most of the Purkinje (P)-cells of this corticocerebellar area, which project to the lateral vestibular nucleus, fire out-of-phase with respect to the related VS neurons, we postulate that the beta-adrenergic system enhances the amplitude of the P-cell responses to labyrinth stimulation, thus increasing the gain of the VS reflex.

Chapter 27 Relationship of noradrenergic locus coeruleus neurones to vestibulospinal reflexes

The electrical activity of presumably noradrenergic locus coeruleus (LC) neurones was recorded in decerebrate cats during roll tilt of the animal at 0.15 Hz, +/- 10 degrees, leading to sinusoidal labyrinth stimulation. Among the tested units, some of which projected to the lumbosacral spinal cord, 56.7% responded to animal tilt. Most of these neurones were activated during side-up and depressed during side-down tilt of the animal, while a smaller proportion of units showed the opposite response pattern. This predominant response pattern of LC neurones and coeruleospinal (CS) neurones to animal tilt was opposite in activation polarity to that of vestibulospinal (VS) neurones projecting to the same segments of the spinal cord. Both the VS and the CS neurones exert a direct excitatory influence on ipsilateral limb extensor motoneurones. However, VS neurones excite corresponding Renshaw (R) cells, though due to activation of limb extensor motoneurones and their recurrent collaterals, the CS neurones may inhibit them. It appears, therefore, that during side-down animal tilt, the motoneurones innervating the ipsilateral limb extensors are excited by the increased discharge of VS neurones, while the corresponding R-cells are disinhibited due to the reduced discharge of CS neurones. The functional coupling between ipsilateral limb extensor motoneurones and the corresponding R-cells would then increase, just at the time in which these motoneurones are driven by the excitatory VS volleys, thus limiting the response gain of limb extensors to labyrinth stimulation. This hypothesis is supported by two facts: (1) R-cells linked with limb extensor motoneurones discharge during side-down tilt, thus firing in phase with the excitatory VS volleys, and (2) functional inactivation of the noradrenergic LC neurones increases the gain of the vestibulospinal reflexes acting on limb extensors.

Cholinergic brainstem sites for gain control of vestibulospinal reflexes in cats

Decerebrate cats were injected with carbachol into the locus coeruleus (LC) or with carbachol or bethanechol into the dorsal pontine reticular formation (pRF) of one side; recordings were made of the tonic contraction of forelimb extensor muscles of both sides and of their responses to sinusoidal roll tilt of the animal. Both drugs had similar effects when injected into the pRF: a decrease in the tonic contraction of limb extensors and a greatly enhanced amplitude and gain with slightly decreased phase lead in the responses to animal tilt of the forelimb extensor, triceps brachii, ipsilateral to the side of injection. Injected into the LC, carbachol produced a response opposite to the above: it increased the tonic contraction of limb extensors ipsilateral to the side of injection, but decreased the amplitude and gain of the EMG responses of limb extensor muscles to labyrinth stimulation induced by sinusoidal tilt. These findings did not depend on changes in posture since they were still observed when postural EMG activity was maintained constant by appropriate changes in static stretch of the muscle. Moreover, the magnitude of the effects increased in a dose-dependent manner. Results suggest that cholinergic activation of dorsal pRF neurons through muscarinic receptors increases the background discharge of medullary inhibitory reticulospinal (RS) system neurons, thus increasing their modulatory influence. Further, it is postulated that cholinergic activation of LC neurons would cause them to inhibit this tonic facilitatory drive by the pRF. Common to both sites of carbachol injection is the increase in phase lag of the EMG response of limb extensors to animal tilt. This is proposed to be due to the discharge of coerulospinal neurons that are directly or indirectly activated on the side of the injection.

Effect of movement and illusion of movement on human vestibulomotor response

Lateral stabilographic response to galvanic labyrinth stimulation was investigated in healthy subjects in the standing position. Vestibulomotor response increased during forwards volitional body tilt as well as involuntary tilt occurring in response to stimulating (by vibration) the proprioceptors of the anterior tibial muscles. An illusion of the forward body tilt induced by stimulating (vibrating) the proprioceptors of the triceps surae muscles with the trunk fastened in a fixed position was accompanied by practically the same intensification of vestibulomotor response as during actual body movement. It was concluded that reinforcement of vestibulomotor response during volitional movements is brought about by the spatial perception system.

Chapter 31 Noradrenergic and cholinergic mechanisms responsible for the gain regulation of vestibulospinal reflexes

Experiments performed in precollicular decerebrate cats have shown that dorsal pontine structures, including the LC and the related dorsal pRF, play an important role not only in the control of posture but also in the gain regulation of the VS reflexes. The LC neurons, which are not only noradrenergic but also NE-sensitive due to the existence of self-inhibitory synapses acting on a2-adrenoceptors, send inhibitory afferents to the dorsal pRF; on the other hand, these pontine reticular neurons, which are presumably cholinergic as well as cholinoceptive due to the existence of self-excitatory synapses, project excitatory afferents to the medullary inhibitory RS system. The increased discharge of these pontine reticular neurons and the related inhibitory RS neurons following local injection of cholinergic agonists into the dorsal pRF decreased the postural tonus in the ipsilateral limbs but greatly enhanced the amplitude of the EMG modulation and thus the response gain of ipsilateral limb extensors to labyrinth stimulation. This finding did not depend on the decreased postural activity, since it was still observed when the EMG activity was reflexly maintained by an increased static stretch of the muscle. Similar results were also obtained when the discharge of the pontine reticular neurons and the related inhibitory RS neurons was raised after local injection of an a2-adrenergic agonist into the LC, leading to functional inactivation of the noradrenergic LC neurons. On the other hand, an increased postural activity in the ipsilateral limbs as well as a reduced gain of the corresponding VS reflexes were obtained when the discharge of the pontine reticular neurons and the related inhibitory RS neurons was reduced, as shown after local injection of cholinergic agonists into the LC leading to activation of the noradrenergic neurons. There was also evidence that cholinergic excitatory afferents to the LC originated from the ipsilateral dorsal pRF. The effects described above were dose-dependent as well as site-specific, as shown by histological controls. In conclusion, the pontine structures described above operate as a variable-gain regulator acting at the motoneuronal level during the VS reflexes. Since the same structures are also responsible for the spontaneous fluctuations in posture related to the sleep waking cycle, they may well intervene as a control system in order to adapt to the animal state the response gain of limb extensors following labyrinth stimulation.

Chapter 7 The role of Renshaw cells in the dynamic control of posture during vestibulospinal reflexes

Contraction of limb extensors during side-down tilt of the animal can be attributed to an increased discharge of lateral VS neurons which exert an excitatory influence on ipsilateral extensor motoneurons (α response). However, in addition to these neurons there are other neuronal systems, such as the CS and the medullary inhibitory RS neurons which also respond to animal tilt, but with a predominant response pattern characterized by a reduced discharge during side-down tilt (β response). This finding is surprising since the CS projection, which originates from the NE-containing LC neurons, is inhibitory on R cells linked with limb extensor (and flexor) motoneurons, while the medullary RS projection, which is driven presumably by cholinergic and cholinoceptive neurons located in the dorsal pRF, is excitatory on them. Experiments were performed in precollicular decerebrate cats to investigate the relative influence that the CS, as well as the RS, neurons exert on R cells coupled with limb extensor motoneurons in the following experimental conditions: (1) during the state of postural activity in which the LC neurons showed a steady discharge while the pRF neurons, as well as the related medullary inhibitory RS neurons, fired at a very low rate; in this instance the gain of the VS reflexes was quite low; and (2) during episodes of reduced postural activity following systemic administration of an anticholinesterase (0.05–0.10 mg/ kg eserine sulphate), in which the discharge rate of LC neurons decreased, while that of the pRF neurons and the related medullary inhibitory RS neurons increased; in this instance the gain of the VS reflexes was greatly enhanced. In particular, the activity of R cells monosynaptically linked with gas-trocnemius-soleus motoneurons (GS R cells) was tested both in the animal at rest as well as during head rotation (at 0.026–0.15 Hz, ± 10°) performed after bilateral neck deafferentation, thus leading to sinusoidal stimulation of labyrinth receptors. In control animals at rest the GS R cells were silent or fired at a low rate; moreover, the same units were either unresponsive or displayed only small-amplitude α responses to labyrinth stimulation. These responses were attributed to the fact that during side-down head rotation the increased discharge of VS neurons activated the GS motoneurons and, through their recurrent collaterals, the related R cells. However, the decrease in firing rate of the CS neurons for the same direction of head rotation would reduce the inhibitory influence that these noradrenergic neurons exert on the GS R cells, thus enhancing the functional coupling of these units with their own extensor motoneurons. This would explain why the response gain of hindlimb extensors to labyrinth stimulation was negligible or absent in these preparations. After systemic injection of the anticholinesterase, leading to an increased discharge of cholinoceptive pRF neurons and the related medullary RS neurons, the GS R cells increased their firing rate in the animal at rest; however, all the R cells which prior to the injection were either unresponsive or showed an a response to head rotation, now showed a β response for the same parameters of labyrinth stimulation. In particular, a reduced discharge of the GS R cells occurred during side-down head rotation, as shown for the majority of the RS neurons. It appears, therefore, that the same R cells which in the control situation responded to the excitatory VS volleys acting through the GS motoneurons were now decoupled from their input motoneurons and underwent the most efficient control of the RS pathway. The reduced discharge of the GS R cells during side-down head rotation would lead to disinhibition of limb extensor motoneurons, thus enhancing the response gain of the corresponding muscle to labyrinth stimulation. The R cells could then act as a variable gain-regulator at moto-neuronal level during the VS reflexes.

Thermal stimulation of the vestibular labyrinth during orbital flight.

During the European Spacelab mission (SL1) in 1983, caloric testing was performed for the first time in long-term weightlessness. After 2 days into orbital flight an unequivocal caloric nystagmus was observed in both subjects tested which corresponded in both quality and intensity with that measured in one-g conditions on Earth. The subsequent D1 mission enabled the experiment to be repeated on further subjects and with improved measurement procedures. As with the SL1 findings, the observed caloric nystagmus response proved to be equivalent to that measured during baseline testing on Earth. Renewed consideration of peripheral and central mechanisms, which might be involved in the elicitation of the caloric response--both in one-g and zero-g environments--has led to the reopening of a number of associated issues. One important observation which has been addressed by various research groups concerns the influence of the labyrinth's orientation to the gravity vector on the caloric response. The present authors have examined a group of healthy subjects in various body positions in the sagittal plane. The interindividual variability in the response behavior was found to be high; indeed single cases were observed in which the nystagmus response did not invert from the supine to the prone positions. These findings are discussed together with earlier reports in the literature.

Modulation by eye position of neck muscle contraction evoked by electrical labyrinthine stimulation in the alert cat.

Modulation of vestibulo-spinal reflexes by gaze is a model system for studying interactions between voluntary and reflex motor activity. In the alert cat, the EMG of Splenius and Obliquus capitis muscles increases with ipsilateral gaze eccentricity during spontaneous eye movements. Labyrinth stimulation by current pulses evokes EMGs with latencies consistent with a three neuron vestibulocollic pathway. The amplitude of evoked activity increases with eye position. The directions in which eye movements increase EMG was usually the same for both spontaneous and induced EMG activity, namely, horizontal and ipsilateral. However, sometimes the increase in spontaneous EMG occurred with horizontal eye position, whereas the induced EMG changed with vertical eye position. Spontaneous and evoked EMG are then modulated by different eye position signals. Command signals reflecting eye position probably reach two different types of neurons in the vestibulo-collic pathway, most likely secondary vestibular neurons and neck muscle motoneurons.

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)

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)

The Caloric Stimulus: Temperature Generation Within the Temporal Bone

Temperature measurements were performed on isolated cadaveric temporal bones subjected to caloric irrigation of the external meatus. This was undertaken in an attempt to clarify the pathways of heat transfer involved in thermal stimulation of the temporal bone. On average, the maximum temperature change produced across the lateral semicircular canal was 0.47°C, occurring 74 s after the start of the 30-s irrigation. Temperature changes of approximately 1.5°C were found to occur in the air of the middle ear cleft, indicating significant heat transfer by the process of natural convection. By comparison, removal of the bony ridge joining the external meatus with the inner ear had little effect on the temperature generated across the lateral semicircular canal. It is therefore considered that natural convection within the middle ear cleft plays an important role in caloric stimulation of the vestibular labyrinth. This view is supported by the observation that there is a lack of a direct solid connection between the outer ear and lateral limb of the lateral semicircular canal.

Effects of microinjection of cholinergic agonists into the pontine reticular formation on the gain of vestibulospinal reflexes in decerebrate cats.

1. The question of which pontine neuronal groups and related receptors can mediate the cholinergic induction of the increased gain of vestibulospinal reflexes elicited by sinusoidal stimulation of labyrinth receptors was investigated by injecting in precollicular decerebrate cats either carbachol, which is a mixed muscarinic-nicotinic agonist, or bethanechol, which is a pure muscarinic agonist, via a cannula stereotaxically oriented in different pontine tegmental structures. 2. Injection of 0.1-0.2 microliter of carbachol solution (0.01-0.2 microgram/microliter of sterile saline) into the dorsal aspect of the pontine reticular formation (pRF), which slightly decreased the tonic contraction of limb extensors ipsilateral to the side of the injection, greatly increased the amplitude of the multiunit EMG response of the ipsilateral triceps brachii to roll tilt of the animal at 0.15 Hz, +/- 10 degrees, leading to selective stimulation of labyrinth receptors. Correspondingly, the response gain of the forelimb extensor to labyrinth stimulation increased. Moreover, a slight decrease in phase lead of the responses was observed. These findings were not attributable to decreased postural activity, since they were still observed when postural EMG activity was reflexly maintained by an increased static stretch of the muscle. No changes in the dynamic characteristics of the responses were observed in the contralateral triceps brachii. 3. The changes in posture as well as in response gain produced by the carbachol injection appeared suddenly, but partially declined to reach a plateau level which persisted for several hours before returning to the control level. Moreover, the magnitude of the effects increased in relation to the dose of the cholinergic agonist. 4. Histological controls indicated that the structure responsible for these postural and reflex changes was located in the dorsal aspect of the pontine tegmentum immediately ventral to the principal locus coeruleus (LC); this area corresponds to the peri-LC region and the surrounding pRF including the dorsal aspect of the central tegmental field. The effects were still obtained after chronic kainic acid lesioning of the gigantocellular area of the medulla. 5. An increase in gain of the vestibulospinal reflex which was as potent, dose-dependent, and site-specific as that previously observed with carbachol, appeared after injection of the pure muscarinic agonist bethanechol.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial perception and vestibulomotor responses in man

A study was made on normal human subjects, using a stabilograph to investigate changes in posture produced in response to transcutaneous galvanic stimulation of the right labyrinth. Results were obtained for different head positions and under the illusion of head and trunk rotation produced by stimulating (vibrating) the gulteus maximus muscle. In the absence of illusion of movement, the direction of the vestibulomotor response was determined by the position of the head in relation to the feed: with the normal head position, the body swayed on a frontal plane, and on a sagittal plane when the heat turned through 90°. Vestibulomotor responses were sagittally oriented, as with real head turning, when illusory head and trunk turning through 90° was produced by vibration. When the illusion of head rotation (in relation to the feet) was not produced by this stimulus, the direction of the postural response was not produced by this stimulus, the direction of the postural response was determined by the real orientation of the head. It is concluded that the spatial perception system plays a major part in controlling spatially oriented vestibulomotor responses.

The Caloric Stimulus: Temperature Generation Within the Temporal Bone

Temperature measurements were performed on isolated cadaveric temporal bones subjected to caloric irrigation of the external meatus. This was undertaken in an attempt to clarify the pathways of heat transfer involved in thermal stimulation of the temporal bone. On average, the maximum temperature change produced across the lateral semicircular canal was 0.47 degrees C, occurring 74 s after the start of the 30-s irrigation. Temperature changes of approximately 1.5 degrees C were found to occur in the air of the middle ear cleft, indicating significant heat transfer by the process of natural convection. By comparison, removal of the bony ridge joining the external meatus with the inner ear had little effect on the temperature generated across the lateral semicircular canal. It is therefore considered that natural convection within the middle ear cleft plays an important role in caloric stimulation of the vestibular labyrinth. This view is supported by the observation that there is a lack of a direct solid connection between the outer ear and lateral limb of the lateral semicircular canal.

Conditioning of crossed extensor reflex pathways by independent natural stimulation of labyrinth, neck and elbow joint afferents

The present experiments demonstrated that labyrinth, neck and elbow joint afferents interact to influence the excitability of crossed extensor reflex pathways projecting to triceps brachii in decerebrate cats. Reflex excitability was dependent on head, neck and elbow position such that it was increased when the elbow was held at full flexion and either the head maintained side down or the neck rotated side up. Such interaction between these afferents is likely to be important in postural control.

Synaptic Connections of Horizontal Canal Mediated Ascending Deiters Tract Axons on Medial Rectus Motoneurons in Cat

This study demonstrates the termination of ascending tract of Deiters' (ATD) axons on ipsilateral medial rectus (MR) motoneurons. Horseradish peroxidase (HRP) was iontophoretically injected into ATD axons which were recorded in the MR motoneuron pool of the oculomotor nucleus. MR motoneuron cell bodies were identified by retrograde transport of HRP injected into MR muscles in the orbit. ATD axons were identified by Type I responses to horizontal rotation, monosynaptic responses on stimulation of the ipsilateral labyrinth, and no response on contralateral labyrinth or contralateral abducens nucleus or on ipsilateral MR nerve stimulation. Light microscopic examination showed the main stem axons to be lateral to the medial longitudinal fasciculus, and terminal boutons were in contact with ipsilateral identified MR motoneurons (Furuya and Markham: Exp. Brain Res. 43: 289-303, 1981). Light microscopy and semi-thin sections showed boutons of ATD in contact with identified MR motoneuron cell bodies and proximal dendrites. The electron micrographs (EM) showed the HRP-injected ATD axons have synapses on MR motoneurons. ATD boutons made axosomatic and axodendritic synapses on MR motoneurons. The boutons contained numerous spheroidal synaptic vesicles. Several examples showed clear asymmetrical post-synaptic membrane specialization. This confirms the synaptic connection between horizontal canal activated elements in the ATD and MR motoneurons.

Influence of Renshaw cells on the response gain of hindlimb extensor muscles to sinusoidal labyrinth stimulation.

The contraction of limb extensor muscles during side-down roll tilt of the animal depends upon an increased discharge of excitatory vestibulospinal (VS) neurons (alpha-response) and a reduced discharge of inhibitory reticulospinal (RS) neurons of the medulla (beta-response), both acting on ipsilateral limb extensor motoneurons. In the decerebrate cat, a modulation of the multiunit EMG activity was clearly present in forelimb extensors, but was extremely weak or absent in hindlimb extensors. Experiments performed in decerebrate cats with the deefferented GS muscle fixed at a constant length have shown that Renshaw (R)-cells, monosynaptically coupled with gastrocnemius-soleus (GS) motoneurons, were either unresponsive or displayed only very weak, small amplitude alpha-responses to sinusoidal stimulation of labyrinth receptors elicited during slow head rotation after bilateral neck deafferentation. This effect was attributed to excitatory VS volleys acting on GS motoneurons and, through their recurrent collaterals, on the related R-cells. In these instances the recurrent inhibition of the GS motoneurons contributed to the very low gain of the EMG response of the corresponding muscles to labyrinth stimulation. Intravenous injection of an anticholinesterase (eserine sulphate, 0.05-0.1 mg/kg) at a dose that in previous experiments increased the firing rate of medullary RS neurons, while decreasing the decerebrate rigidity, slightly increased the discharge rate of R-cells linked with the GS motoneurons in the animal at rest; these findings suggest that the RS system inhibits the extensor motoneurons by exciting the related R-cells. All the R-cells, which prior to the injection were either unresponsive or showed an alpha-response to head rotation (at 0.026-0.15 Hz, +/- 10 degrees), after eserine sulphate showed a beta-response for the same parameters of labyrinth stimulation. In particular, a reduced discharge of the R-cells linked with the GS motoneurons occurred during side-down head rotation as shown for the majority of the RS neurons. It appears therefore that the same R-cells, which in the normal decerebrate cat responded to the excitatory VS volleys acting through the GS motoneurons, were now decoupled from their input motoneurons during head rotation, thus behaving as if they underwent the most efficient direct excitatory control of the RS system. The reduced discharge of the R-cells linked with the GS motoneurons during side-down head rotation would lead to disinhibition of these motoneurons, thus enhancing the response gain of the corresponding muscle to labyrinth stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)