Optic nystagmus and vestibular nuclei: Unitary activity of vestibular neurons during nystagmus

The behavior of 102 vestibular neurons during optic nystagmus was investigated in 25 guinea pigs with extracellular microelectrodes. The recorded vestibular neurons were electrophysiologically identified by their orthodromic response to ipsilateral labyrinthine stimulation and by antidromic activation from the medial longitudinal fascicle. Of the 102 recorded units, 92 were modulated by the electrical stimulation of at least one optic nerve. The presence of vestibular neurons sensitive to the direction of nystagmus induced by labyrinthine or optic stimulation was also analyzed.     

Postsynaptic potentials in neurons of the pigeon''s optic tectum in response to afferent stimulation from the retina and other visual structures: an intracellular study

The responses of the cells in the pigeon's optic tectum to electrical stimulation of the contralateral optic nerve, the ipsilateral visual Wulst and the opposite optic tectum were intracellularly recorded. Optic nerve or visual Wulst stimulation elicited 3 types of responses: (1) a pure EPSP which gave rise to one or two action potentials; (2) an EPSP which sometimes gave rise to a spike, followed by an IPSP; and (3) a pure IPSP. Opposite tectum stimulation evoked in the tectal cells either a pure IPSP or a pure EPSP. The mono- or polysynaptic nature of the pathways involved in the excitatory and inhibitory responses of the tectal cells was assessed by increasing the frequency of the optic nerve stimulation. At low stimulus rates (2-6 Hz), all the excitatory events showing latencies longer than 5 ms were blocked suggesting that they were polysynaptic. Excitatory events having latencies shorter than 5 ms were generally able to follow high rate frequencies of optic nerve stimulation (40, 50 or 90 Hz) and we considered them to be monosynaptic. All but 3 IPSPs evoked by optic nerve stimulation, were blocked by stimulus rates beyond 5 Hz. Thus, although most IPSPs are generated through polysynaptic paths, direct retino-tectal inhibitory paths may also exist. The latency of the responses of individual cells to optic nerve, visual Wulst and opposite tectum stimulation show that the polysynaptic IPSPs to optic nerve stimulation did not involve relays in the visual Wulst or the opposite tectum.     

Localization of optic tectal input to the ventral dendrite of the goldfish Mauthner cell

Although visually evoked Mauthner cell (M-cell) startle responses occur in the goldfish, the afferent projections underlying these reactions have not been previously studied. We have recorded from the M-cell while stimulating the left optic nerve and/or right optic tectum and have traced projections of the optic nerve and restricted areas of the optic tectum using HRP histochemistry and autoradiography. Tectal stimulation elicits similar postsynaptic potentials (PSPs) in both M-cells. The responses recorded in the right (ipsilateral) cell were localized to its ventral dendrite. The existence of uncrossed tectal projections to the ventral dendrite was confirmed morphologically following application of horseradish peroxidase (HRP) to the optic tectum. The PSPs contained both inhibitory and excitatory components, but with adequate stimulus strength, excitation of either M-cell dominated. Thus, this pathway is probably sufficient to trigger visually evoked startle responses mediated by the M-cell. Stimulation of the left optic nerve also evoked PSPs capable of bringing both M-cells to threshold. The blockage of this response by conditioning stimulation of the right tectum suggests that the visual information is relayed to the M-cells through this structure. In support of these findings, no label was found near any portion of the M-cell after either intraocular injection of tritiated proline or application of HRP to the cut end of the optic nerve. In summary, visual input to the M-cell is mediated via projections from the tectum, is segregated onto the ventral dendrite, and is capable of bringing this neuron to threshold. This pathway presumably accounts for the demonstrated behavioral efficacy of visual stimuli in evoking a startle response.     

Physiological Characterization of Pretectal Neurons Projecting to the Láteral Geniculate Nucleus in the Cat

Single neurons in the pretectal nucleus of the optic tract and posterior pretectal nucleus were extracellularly recorded in anaesthetized cats and tested for antidromic activation after electrical stimulation of the ipsilateral dorsal lateral geniculate nucleus. Cells were further characterized by their response latencies to electrical stimulation of the optic nerve head and the optic chiasm, and by responses to various visual stimuli. 46 out of 188 neurons (24%) were antidromically activated from the lateral geniculate nucleus at response latencies of 0.6 - 2.6 ms. They had low spontaneous activities and preferred fast-moving visual stimuli. 29 of the antidromically activated neurons (63%) could be activated from the optic chiasm with response latencies of 4–10 ms. Together with the mean conduction time of 0.8 ms between the optic nerve head and the optic chiasm, this indicates that they receive an indirect retinal input via fast-conducting Y-fibres. Sometimes antidromically activated neurons spontaneously showed irregular burst activity. During unidirectional stimulation with a large moving visual stimulus, burst activity became more regular, and interburst intervals and the duration of single bursts decreased. After the stimulus was stopped, interburst intervals slowly increased until prestimulation activity was restored. The response properties of these neurons could reflect the transfer of saccade-related visual as well as oculomotor signals through the pretectum to the visual thalamus.     

Galvanic stimulation evokes short-latency EMG responses in sternocleidomastoid which are abolished by selective vestibular nerve section

OBJECTIVE: To describe vestibulocollic responses in sternocleidomastoid (SCM) evoked by transmastoid galvanic (DC) stimulation. METHODS: We studied the averaged responses in the unrectified EMG of SCM to transmastoid galvanic stimulation (5 mA/2 ms) and also to 100 dB clicks. Two patients with Meniere's disease were studied both before and after unilateral selective vestibular nerve section. RESULTS: Transmastoid galvanic stimulation produced a positive-negative biphasic EMG response at short latency in the SCM ipsilateral to the side of cathode placement, which resembled that which followed vestibular activation by loud clicks (p13/n23). Selective unilateral vestibular nerve section abolished this galvanic-evoked response. CONCLUSIONS: Galvanic-evoked vestibulocollic responses can be recorded in SCM. This is a new method of studying vestibular reflex function which may have application in the clinical assessment of vestibular disorders.     

Projections of the dorsal and lateral terminal accessory optic nuclei and of the interstitial nucleus of the superior fasciculus (posterior fibers) in the rabbit and rat

The projections of the dorsal and lateral terminal accessory optic nuclei (DTN and LTN) and of the dorsal and ventral components of the interstitial nucleus of the superior fasciculus (posterior fibers; inSFp have been studied in the rabbit and rat by the method of retrograde axonal transport following injections of horseradish peroxidase into oculomotor-related brainstem nuclei. The projections of the ventral division of the inSFp have been further investigated in rabbits with the anterograde axonal transport of 3H-leucine. The data show that the projections of the DTN, LTN, and inSFp are remarkably similar in rabbit and rat. The DTN projects heavily to the ipsilateral medial terminal accessory optic nucleus (MTN), nucleus of the optic tract, and dorsal cap of the inferior olive. The DTN projects sparsely to the ipsilateral visual tegmental relay zone and to the contralateral superior and lateral vestibular nuclei. The LTN and dorsal component of the inSFp are found to share the same basic connections; both project heavily to the ipsilateral nucleus of the optic tract and visual tegmental relay zone and send a moderately sized projection to the ipsilateral MTN. However, while the dorsal component of the inSFp sends significant ipsilateral projections to both rostral and caudal portions of the dorsal cap, only a few LTN neurons appear to follow this example and only by projecting to the rostral part of the dorsal cap. In addition, both the LTN and dorsal component of the inSFp send sparse contralateral projections to the MTN, nucleus of the optic tract, and visual tegmental relay zone; and the dorsal component of the inSFp also provides a sparse contralateral projection to both rostral and caudal portions of the dorsal cap. The ventral component of the inSFp projects heavily to the ipsilateral visual tegmental relay zone and moderately to the ipsilateral MTN and nucleus of the optic tract. The ventral inSFp projects sparsely to the contralateral MTN, the nucleus of the optic tract, and the visual tegmental relay zone. A few of its neurons target the ipsilateral dorsal cap of the inferior olive. Unlike the DTN (present study) and the MTN (Giolli et al.: J. Comp. Neurol. 227:228-251, '84; J. Comp. Neurol. 232:99-116, '85a), the LTN and the inSFp of the rabbit and rat lack projections to the superior and lateral vestibular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)     

Axonal guidance of developing optic nerves in the frog. II. electrophysiological studies of the projection from transplanted eye primordia.

When a primordial eye was transplanted to the ear position in Rana pipiens embryos, the optic nerve from the ectopic eye penetrated the medulla and invariably established a tract in the dorsolateral white matter of the ipsilateral spinal cord. In response to visual stimulation of the transplanted eye, extracellular recordings with metal microelectrodes were conducted with the spinal cords of post-metamorphic animals. Visual activity in the spinal cord could only be recorded in those experimental animals in which the transplanted optic nerve succeeded in penetrating the medulla. This activity was frequently encountered in the gray matter of the cord well below the dorsolateral position of the transplanted optic tract. The discharge characteristics and adaptation properties of the visual activity were often similar to that of optic nerve fibers from normal eyes suggesting that axons or their collaterals branch off from the transplanted optic tract and arborize within the spinal cord. However, occassionally stimulation of the transplanted eye evoked activity with adaptation and/or response characteristics unlike that of normal optic nerve fibers. Visual activity in the spincal cords of our experimental animals could be driven by moving small dark objects within circumscribed regions in the visual field of the transplanted eye. However we were unable to find any evidene of a systematic mapping of the transplanted retina within these abnormally penetrated spinal cords.     

The contralateral input to the dorsal horn of the spinal cord in the decerebrate spinal rat

Input from the contralateral limb and tail was examined in the lumbar dorsal horn of decerebrate spinal rats. Fifty-three cells were recorded from laminae 4, 5 and 6 and classified according to their ipsilateral response to natural and electrical stimulation. Twenty-nine (54%) of these cells were found to have inhibitory contralateral fields. This inhibition was evoked by noxious pinching or heating of the skin. In most cases the inhibitory field was a mirror image of the excitatory ipsilateral field although it also often included the tail. Activity evoked by natural and electrical stimulation as well as spontaneous activity was inhibited by contralateral skin stimulation. Noxious specific and wide dynamic range cells displayed these fields but low threshold mechanoreceptive cells did not. Twenty-six cells (49%) received direct short-latency excitatory input from the contralateral sciatic nerve; this correlated well with the presence of contralateral fields. Trains of stimuli applied to the contralateral sciatic nerve at Aδ- and C-fibre strength resulted in inhibition of the cell whereas trains of Aβ strength had no effect. The results demonstrate the existence of segmental contralateral control over dorsal horn cell activity, not involving supraspinal pathways.     

Vestibular input to the caudate nucleus

Unilateral stimulation of peripheral vestibular nerve branches in encephale isolé cats evoked potentials of 3- to 7-msec latency in the banks of the contralateral anterior suprasylvian sulcus and 4- to 15-msec latency in the head of the caudate nucleus bilaterally. The responses were distributed in the dorsomedial region of the ipsilateral caudate and dorsolateral region of the contralateral caudate. Stimulation of peripheral cochlear nerve branches also evoked responses in areas of caudate partially overlapping those responsive to vestibular stimulation. Stimulation of the vestibular projection area of cortex evoked a response in caudate but did not alter the response to vestibular nerve stimulation. Ablation of the vestibular projection area of cortex did not alter the vestibular responses in caudate. However, caudate and cortical responses to vestibular nerve stimulation were severely decreased by lesions of the magnocellular portion of medial geniculate body. We conclude that there are vestibular projections to the caudate nuclei which require passage through the medial geniculate body and probably other thalamic nuclei, but not through the vestibular projection area of cortex.     

Morphology, axonal projection pattern, and responses to optic nerve stimulation of thalamic neurons in the salamander Plethodon jordani

In the salamander Plethodon jordani, the morphology and axonal projections of thalamic (TH) neurons and their responses to electrical optic nerve stimulation were determined by intracellular recording and biocytin labeling under in vitro, whole-brain conditions. Based on their axonal projections, labeled neurons (n = 76) were divided into the following groups: TH1 neurons, with mostly ipsilateral projections to the striatum; TH2 neurons, with ipsilateral or bilateral projections to the medial amygdala and nucleus accumbens; TH3 neurons, with bilateral projections to the medial and dorsal pallium; TH4 neurons, with mostly ipsilateral projections to the striatum and ipsilateral projections to the tectum opticum, tegmentum, and rostral medulla oblongata; and TH5 neurons, with ipsilateral projections to the tegmentum, medulla oblongata, and rostral spinal cord without (TH5.1) or with (TH5.2) additional projections to the optic tectum. TH1-TH4 neurons are found in the dorsal thalamus and around the sulcus medialis, and TH5 neurons are found in the ventral thalamus. Labeled neurons with ascending projections, i.e., the more dorsally situated TH1-TH4 neurons, are mostly inhibited by electrical stimulation of the optic nerve and have significantly longer latencies (mean +/- S.D., 42.1 +/- 11.6 msec) than neurons with exclusively descending projections, i.e., the ventrally located TH5 neurons (8.5 +/- 6.1 msec), which receive the bulk of retinal afferents and show excitation at electrical optic nerve stimulation. Neurons recorded without labeling in the dorsal thalamus likewise exhibit mostly inhibition and have significantly longer latencies (35.7 +/- 18.9 msec) than those recorded in the ventral thalamus (10.9 +/- 7.7 msec), which mostly show excitation. None of the neurons recorded in the dorsal thalamus followed repetitive stimulation of the optic nerve. Thus, neurons situated in the dorsal thalamus and projecting to pallial or subpallial telencephalic targets are unlikely to receive monosynaptic or oligosynaptic, excitatory retinal input. Accordingly, no retino-thalamo-telencephalic pathway homologous to that found in amniotes appears to exist in salamanders.     

Responses of afferent and efferent neurons to visual inputs in the vestibular nerve of the frog

In the frog immobilized by intralymphatic injection of D-tubocurarine, stimulation of the visual apparatus with either electrical shocks applied to the optic chiasma or light pulses elicited, in many cases, an increase and a decrease of firing of efferent and afferent vestibular neurons, respectively, recorded from the horizontal semicircular canal nerve. Optokinetic stimulation was completely inefficient in modulating the efferent and afferent discharge. These results show that stimulation of the visual system can modify vestibular apparatus fuctioning at the most peripheral level. However, it is likely that the effects observed were due to an arousal phenomenon or/and to a motor corollary discharge.     

Responses of the optic tectum to telencephalic stimulation in catfish

In order to test physiologically for cerebrotectal connections in a fish, averaged evoked potentials and unit responses were recorded from the optic tectum following electrical stimulation applied to the telencephalon in the siluroid teleost Ictalurus nebulosus. A single shock applied to the area dorsalis centralis (Dc) of the telencephalon, and only to this area, elicits a sequence of deflections in the ipsilateral optic tectum: an initial negative peak at about 8 ms, (= N8), a larger N25 and a slow P50-N95. The configurations, depth profiles, latencies and susceptibility to repetitive stimulation, together with the known tectal anatomy, suggest that the first wave is due to the afferent fibers from the telencephalon and that N25 is due to deep tectal neurons. Telencephalic input exerts a conditioning effect on the field potentials and unit responses evoked by direct optic nerve shock. Such a shock elicits, in the contralateral tectum, small negative, optic tract axon peaks followed by a large N6, believed to be postsynaptic, and a still later P12. As a first approximation it is argued that the telencephalic input and the retinal input are activating different sets of neuronal elements in the optic tectum, since the configuration and depth profile of the telencephalic and optic nerve shock-elicited potentials are different. A conditioning Dc stimulus has a long-lasting effect on the form of the optic nerve field potential, maximally when the pallial shock precedes the optic by about 90 ms. The effect, observed by subtracting the conditioned from the unconditioned tectal response to optic nerve shock, is a difference wave with N11 and P20. The unit activity from deep tectal laminae is either activated or accelerated following Dc stimulation, while superficially located neurons are not affected. In another group of tectal units, the optic nerve shock-induced response is depressed by a preceding pallial dorsalis centralis stimulus. The evidence is compatible with the assumption of direct projections from Dc to the deep layers of the tectum, but the timing could also permit indirect pathways. In any case, the influence is not simple or identical for different tectal cell classes.     

Monocular and binocular optic inputs to salamander pretectal neurons: intracellular recording and HRP labelling study

Intracellular recordings and horseradish peroxidase injections were performed in the pretectum and adjacent tegmentum of Salamandra salamandra, while both optic nerves were electrically stimulated. In approximately half of the recorded units no spikes could be evoked but rather graded postsynaptic potentials. The latter type morphologically showed features of interneurons. From a total of 48 recorded units, nearly 60% were excited only by the contralateral optic nerve, whereas approximately 40% were binocular. For the most part (10/19) the binocular cells were excited by the contralateral and inhibited by the ipsilateral optic nerve. Fewer neurons (7/19) received excitatory inputs from both optic nerves. The latency distribution of the monocular cells shows a maximum of 20-30 ms. The same maximum exists for the contralateral inputs to the binocular cells, whereas the ipsilateral inputs to these units were nearly as frequent with latencies of 20-30 and 40-50 ms. Since neurons with the short ipsilateral latencies always had parts of their dendrites within the ipsilateral ocular projection field, a feature which was lacking in the cells with long ipsilateral latencies, it is possible that the longer latencies are due to indirect ipsilateral inputs. Efferents of labelled dorsal pretectal cells reach the contralateral pretectum via the posterior commissure, the basal optic neuropil of the accessory optic system and the tegmental white substance. More ventrally located cells often reach the pretectal and the basal optic neuropil with their dendrites. Axons of this type descend to the medulla oblongata via the medial longitudinal fasciculus.     

Optical detection of neural function in the chick visual pathway in the early stages of embryogenesis

We investigated the developmental pattern of functional synaptogenesis in the chick visual pathway using a multiple-site optical recording method. Responses to optic nerve stimulation were recorded from the diencephalon and mesencephalon of the chick embryo. The first excitatory postsynaptic responses to optic nerve stimulation appeared in the contralateral diencephalon at Hamburger-Hamilton stage 27, which corresponds to an incubation day 5.5 (E5.5). At more developed stages, the optical signals evoked by optic nerve stimulation spread to several different regions, including the tectum and extra-tectal visual nuclei. We constructed maps of neural activity in the diencephalon and mesencephalon at different stages to investigate the spatio-temporal patterns of functional development in the chick visual system. The maps revealed that distinct postsynaptic response areas in the extra-tectal regions showed different onsets of activity, suggesting that the corresponding visual nuclei exhibit different time courses of functional synaptogenesis. We also identified the onset and location of the first functional synaptic connection in the optic tectum, which had been a point of controversy in earlier studies. In the tectal region, the action potential and the excitatory postsynaptic potential first appeared at E8, although these signals were recognized in the tecto/tegmental region at E7. The response area expanded with retinotectal fibre elongation, and reached the area centralis at E9. These results show that the onset of synaptic function in the tectum occurs 2-3 days earlier than was previously reported.     

Spinal reflex pattern to foot nociceptive stimulation in standing humans

Ipsi- and contralateral patterns of lower limb nociceptive reflex responses were studied in 6 normal subjects in free standing position. Once the position was stabilized, only ankle extensor muscles showed consistent tonic activity while ankle flexors and knee extensors and flexors were virtually silent. Reflex responses, elicited by painful electrical stimuli to the skin of the plantar and dorsal aspect of the foot, were recorded from ipsi- and contralateral quadriceps (Q), biceps femoris (Bic), tibialis anterior (TA) and soleus (Sol) muscles. Plantar foot stimulation evoked a large excitatory response in the ipsilateral TA at about 80 ms and a smaller responses in Bic and Q at 70 ms and 110 ms, respectively. Ipsilateral excitatory effects after dorsal foot stimulation consisted of a Bic response at about 75 ms. In addition to excitatory effects, both plantar and dorsal foot stimulation evoked long-lasting suppression of ipsilateral Sol background activity starting at about 60 ms. Contralaterally, the only nociceptive effects after plantar or dorsal foot stimulation were a small excitatory response of Sol at about 85 ms. Evidence is provided that only excitatory responses were contingent upon nociceptive volley. The main mechanical effects seen after plantar stimulation were dorsiflexion of the foot without loss of heel contact with the floor; no withdrawal response of the foot followed nociceptive dorsal stimulation. Our main conclusion is that only reflex nociceptive responses serving to avoid the stimulus without conflicting with limb support function are expressed. The mechanisms reconciling nociceptive action and postural function of the lower limbs are discussed.     

Inhibitory postsynaptic potentials evoked in hypoglossal motoneurons by lingual nerve stimulation

The percent magnitude of a short- and a long-lasting inhibitory postsynaptic potential (IPSP) produced in tongue retractor and protruder motoneurons by lingual nerve stimulation was studied in cats. In the retractor motoneurons, stimulation of the ipsilateral lingual nerve produced primarily the short-lasting IPSP, and the neurons had received synaptic input primarily from the afferent fibers in the contralateral lingual nerve generating the long-lasting IPSP. In the protruder motoneurons, in contrast, there was no pronounced difference in the IPSP patterns evoked by stimulation of the ipsilateral and contralateral lingual nerve.     

Involvement of the inferior olivary neurons in vestibular compensation

Vestibular decompensation induced by spinal cord transection in left labyrinthectomized guinea pigs provoked asymmetrical excitability of the inferior olivary nuclei. In the right nucleus, spinal deafferentation induced a significantly increased response to electrical stimulation of the contralateral radial nerve and decreased response to ipsilateral radial nerve stimulation. In the left nucleus, opposite results were obtained. Increased responses were recorded in the I.O. neurons during electrical stimulation of the radial nerve ipsilateral to a previous hemilabyrinthectomy, and reduced responses during the electrical stimulation of the radial nerve of the opposite side. Since the inferior olive impinges on the vestibular nuclei both directly and indirectly through the cerebellar loop, it is possible that the inferior olive is involved in the spinal compensation of the vestibular deficits resulting from the hemilabyrinthectomy.     

Lack of visual suppression in the rabbit lateral geniculate nucleus during blink reflex

Stimulation of the supraorbital branch of the trigeminal nerve (SO) elicited eye blinks in the rabbit, but did not decrease the amplitude of visual cortical evoked potential from stimulation of the optic chiasm (OX). In addition, the SO stimulation neither induced an inhibitory postsynaptic potential (IPSP) in LGN cells, nor activated inhibitory interneurons in the thalamic reticular nucleus (TRN), which proved to mediate both recurrent inhibition and saccadic suppression in the dorsal lateral geniculate nucleus (LGN). All these indicate that there is no visual suppression in the rabbit LGN during blink reflex.     

The Isolated and Perfused Brain of the Guinea-pig In Vitro

We describe here an isolated and perfused in vitro adult guinea-pig whole brain preparation which is an extension of the previously described in vitro brainstem–cerebellum preparation, Viability was tested by the analysis of trans-synaptic responses along the visual pathways following the electrical stimulation of the optic nerve or the optic radiations. The evoked field potentials were recorded in the dorsal lateral geniculate, the superior colliculus and the visual cortex. The distribution of extracellular currents was studied using current source density analysis, in order to determine the amplitude, time course and spatial organization of the synaptic activity at these sites. The study indicates that field potentials were very similar to those described in vivo. These data demonstrate the survival of a complex adult sensory system in vitro and suggest that this preparation can be used for the analysis of multisynaptic circuits in the mammalian brain.     

An anatomical and electrophysiological study of the centrifugal visual system in the lamprey (Lampetra fluviatilis)

The centrifugal visual system of the lamprey Lampetra fluviatilis was investigated using various neurohistophysiological methods: intraocular injections of [³H]adenosine, fluorescent tracer (Evans Blue) and the iontophoretic deposit of HRP on the optic nerve. Retrogradely labeled neurons were identified bilaterally within the nucleus M5 of Schober and contralaterally in the reticular mesencephalic area (RMA). Comparison of the various orthograde and retrograde labeling results indicated that the neurons of M5 and RMA were labeled via retrograde axonal transport of the different tracers in the retinopetal system and not by orthograde transneuronal processes or from extraretinal pathways. Part of the anatomical data regarding RMA as a site of origin of the centrifugal visual system was confirmed using electrophysiological techniques involving evoked potential and unit cell recordings in RMA following electrical stimulation of the optic nerve. The experiments were performed in the curarized animal under conditions of either normal blood circulation perfusions of adapted physiological saline, or with a solution known to block chemical synaptic transmission. Various electrophysiological criteria, including the results obtained during the conditions of reversible chemical synaptic blockade, indicated that the responses in RMA reflect an antidromic process. The anatomical organization of the centrifugal visual system in the lamprey is compared to that found in different gnathostome vertebrate species. Several hypotheses concerning the marked interspecies differences related either to the number and the topographical location of the centrifugal neurons as well as the evolution of this system are advanced.