Internuclear Neurons Research Articles

Recruitment order of cat abducens motoneurons and internuclear neurons.

Abducens neurons undergo a dose-dependent synaptic blockade (either disinhibition or complete blockade) when tetanus neurotoxin (TeNT) is injected into the lateral rectus muscle at either a low (0.5) or a high dose (5 ng/kg). We studied the firing pattern and recruitment order in abducens neurons both in control and after TeNT injection. The eye position threshold for recruitment of control abducens neurons was exponentially related to the eye position and velocity sensitivities. We also found a constancy of recruitment threshold for different eye movement modalities (spontaneous, optokinetic, and vestibular). Exponential relationships were found, as well, for eye velocity sensitivity during saccades and for position and velocity sensitivities during the vestibulo-ocular reflex. Likewise, inverse relationships were found between recruitment threshold or position sensitivity with the antidromic latency in control abducens neurons. These relationships, however, did not apply following TeNT treatment. Neuronal firing after TeNT appeared either disinhibited (low dose) or depressed (high dose), but the relationships between neuronal sensitivities and recruitment still applied. However, the pattern of recruitment shifted toward the treated side as more inputs were blocked by the low- and high-dose treatments, respectively. Nonetheless, although the recruitment-to-sensitivity relationships persisted under the TeNT synaptic blockade, we conclude that synaptic inputs are determinant for establishing the recruitment threshold and recruitment spacing of abducens motoneurons and internuclear neurons.

Grafting of a new target prevents synapse loss in abducens internuclear neurons induced by axotomy

The loss of afferent synaptic boutons is a prominent alteration induced by axotomy on adult central neurons. In this work we attempted to prove whether synapse loss could be reverted by reconnection with a new target. We severed the medial longitudinal fascicle of adult cats and then transplanted embryonic cerebellar primordia at the lesion site immediately after lesion. As previously shown, the transected axons from abducens internuclear neurons penetrate and reinnervate the graft [J Comp Neurol 444 (2002) 324]. By immunocytochemisty and electron microscopy we studied the synaptology of abducens internuclear neurons under three conditions: control, axotomy and transplant (2 months of survival time). Semithin sections of the abducens nucleus were immunostained against calretinin, to identify abducens internuclear neurons, and either synaptophysin (SF), to label synaptic terminals, or glial fibrillary acidic protein (GFAP) to detect the astrocytic reaction. Optical and linear density of SF and GFAP immunostaining were measured. Data revealed a significant decrease in the density of SF-labeled terminals with a parallel increase in GFAP-immunoreactive elements after axotomy. On the contrary, in the transplant group, the density of SF-labeled terminals was found similar to control, and the astrocytic reaction induced by lesion was significantly reduced. At the ultrastructural level, synaptic coverage and linear density of boutons were measured around the somata of abducens internuclear neurons. Whereas a significant reduction in both parameters was found after axotomy, cells of the transplant group received a normal density of synaptic endings. The ratio between F- and S-type boutons was found similar in the three groups. Therefore, these findings indicate that the grafting of a new target can prevent the loss of afferent synaptic boutons produced by the axotomy.

Dynamics of Abducens Nucleus Neuron Discharges During Disjunctive Saccades

In this report, we provide the first characterization of abducens nucleus neuron (ABN) discharge dynamics during horizontal disjunctive saccades. These movements function to rapidly transfer the visual axes between targets located at different eccentricities and depths. Our primary objective was to determine whether the signals carried by ABNs during these movements are appropriate to drive the motion of the eye to which they project. We also asked whether ABNs encode eye movements similarly during disjunctive saccades and disjunctive fixation. To address the first objective we 1) assessed whether we could predict the discharge dynamics of individual neurons during disjunctive saccades based on their discharge properties during conjugate saccades and 2) directly estimated the sensitivity of individual neurons to either the ipsilateral/contralateral eye or the conjugate/vergence position and velocity using bootstrap statistics. Our main finding was that during disjunctive saccades in the direction ipsilateral to the recording site (ON-direction), the majority of ABNs preferentially encoded the velocity and the position of the ipsilateral eye. The remaining neurons predominantly encoded the conjugate motion of the eyes (i.e., were equally sensitive to the motion of both eyes). Generally, ipsilateral/contralateral eye based models better described neuronal discharges than conjugate/vergence based models, yet both model structures yielded similar conclusions. Moreover, the preferred eye of individual neurons based on their position and velocity sensitivities were generally well matched. We also found that for saccades in the OFF-direction, the pausing behavior of ABNs was similar during conjugate and disjunctive saccades, with the exception that for movements of small amplitudes, more ABNs paused during conjugate saccades. Finally, we found that putative motoneurons and internuclear neurons encoded ON- and OFF-direction disjunctive saccades in a similar manner. To address our second objective, we compared the discharge properties of individual ABNs during disjunctive saccades and disjunctive fixation. Good coherence was observed between the preferred eye of individual ABNs during the two behaviors. Taken together, our results indicate that although individual ABNs can encode the motion of both eyes to various degrees, the population drive of ABNs accounts for most of the movement of the ipsilateral eye during disjunctive saccades and disjunctive fixation.

Intrinsic determinants of synaptic phenotype: an experimental study of abducens internuclear neurons connecting with anomalous targets

The present experiments investigate the role of postsynaptic neurons in the morphological differentiation of presynaptic terminals that are formed de novo in the adult CNS. Abducens internuclear neurons in the adult cat were chosen as the experimental model. These neurons project onto the contralateral medial rectus motoneurons of the oculomotor nucleus. Abducens internuclear axon terminals were identified by their anterograde labeling with biocytin and analyzed at the electron microscopic level. To promote the formation of new synapses, two different experimental approaches were used. First, after the selective ablation of medial rectus motoneurons with ricin, abducens internuclear neurons reinnervated the neighboring oculomotor internuclear neurons. Second, after axotomy followed by embryonic cerebellar grafting, abducens internuclear axons invaded the implanted tissue and established synaptic connections in both the molecular and granule cell layer. Boutons contacting the oculomotor internuclear neurons developed ultrastructural characteristics that resembled the control synapses on medial rectus motoneurons. In the grafted cerebellar tissue, abducens internuclear axons and terminals did not resemble climbing or mossy fibers but showed similarities with control boutons. However, labeled boutons analyzed in the granule cell layer established a higher number of synaptic contacts than controls. This could reflect a trend towards the mossy fiber phenotype, although labeled boutons significantly differed in every measured parameter with the mossy fiber rosettes found in the graft. We conclude that at least for the abducens internuclear neurons, the ultrastructural differentiation of axon terminals reinnervating novel targets in the adult brain seems to be mainly under intrinsic control, with little influence by postsynaptic cells.

Firing properties of axotomized central nervous system neurons recover after graft reinnervation.

Axotomy produces changes in the electrical properties of neurons and in their synaptic inputs, leading to alterations in firing pattern. We have considered the possibility that these changes occur as a result of the target deprivation induced by the lesion. Thus, we have provided a novel target to axotomized central neurons by grafting embryonic tissue at the lesion site to study the target dependence of discharge characteristics. The extracellular single-unit electrical activity of abducens internuclear neurons was recorded in the alert behaving cat in control, after axotomy, and after axotomy plus the implantation of cerebellar primordium. As recently characterized (de la Cruz et al. [2000] J. Comp. Neurol. 427:391-404), firing alterations induced by axotomy included an overall decrease in firing rate and a loss of eye-related signals, i.e., eye position and velocity neuronal sensitivities, that do not resume to normality with time. The grafting of a novel target to the injured abducens internuclear neurons restored the normal firing and sensitivities as recorded in the majority of units. To study the reinnervation of the implant, we performed anterograde labeling with biocytin combined with electron microscopy visualization. Axons of abducens internuclear neurons grew into the transplant sprouting into granule cell and molecular layers, as characterized by the immunostaining for gamma-aminobutyric acid and calbindin D-28k. Ultrastructural examination of labeled axons and boutons revealed the establishment of synaptic contacts, mainly axodendritic, with different cell types of the grafted cerebellar cortex. Therefore, these data indicate that axotomized central neurons resume to normal firing after the reinnervation of a novel target.

Interocular timing differences in the horizontal components of human saccades

The occurrence of systematic interocular differences in the time of initiation of saccades in various directions was investigated in normal human subjects ( n=4). Saccades were recorded binocularly with scleral sensor coils on each of the eyes with a temporal resolution of 0.1 ms (sampling frequency 10 000 Hz). Analysis was done in the velocity domain after digital differentiation. It was found that, in the initial phase of horizontal saccades, the nasalward moving eye lagged the temporalward moving eye consistently by slightly less than 1 ms. No such difference was found in vertical (upward or downward) saccades. In oblique saccades, the systematic initial lag of the nasalward moving eye was similarly present in the horizontal component of the saccade, but absent in the vertical component. It is postulated that this interocular timing difference in horizontal saccades is due to the additional synaptic delay caused by the abducens internuclear neuron in the pathway to the medial rectus muscle of the eye.

Reversible deafferentation of abducens motoneurons and internuclear neurons with tetanus neurotoxin.

Tetanus neurotoxin (TeNT) is a blocker of synaptic vesicle exocytosis in central synapses with preferential affinity for inhibitory neurotransmission. Following its intramuscular injection, TeNT is retrogradely and trans-synaptically transported towards the premotor terminals. Therefore, we have used TeNT as a tool to study the consequences of functional deafferentation on motoneurons following its peripheral administration. For this, we injected the toxin into the lateral rectus muscle at doses of 5 or 0.5 ng/kg and recorded the discharge activity of abducens motoneurons and internuclear neurons in the alert cat. Our results showed that: (i) TeNT blocked selectively the afferent inhibitory signals on abducens neurons only when used at a low dose, whereas both excitatory and inhibitory synaptic drive was lost after the high dose treatment; (ii) all effects were reversible within one month; and (iii) strikingly, the internuclear neurons of the abducens nucleus showed similar discharge alterations to the motoneurons, suggesting a TeNT action on shared common afferences.

Limits to the capacity of transplants of olfactory glia to promote axonal regrowth in the CNS.

Olfactory bulb ensheathing cell (OBEC) transplants promoted axonal regeneration in the spinal cord dorsal root entry zone and in the corticospinal tract. However, OBECs failed to promote abducens internuclear neuron axon regeneration when transplanted at the site of nerve fibre transection. In experiments performed in both cats and rats, OBECs survived for up to 2 months, lining themselves up along the portion of the regrowing axons proximal to the interneuron cell body. However, OBECs migrated preferentially towards abducens somata, in the direction opposite to the oculomotor nucleus target. OBECs seem to promote nerve fibre regeneration only where preferred direction of glial migration coincides with the direction of axonal growth towards its target.

GABA-immunoreactive internuclear neurons in the ocular motor system of lampreys

The presence of internuclear neurons in the abducens and oculomotor nuclei of lampreys [González, M.J., Pombal, M.A., Rodicio, M.C. and Anadón, R., Internuclear neurons of the ocular motor system of the larval sea lamprey, J. Comp. Neurol. 401 (1998) 1–15] indicates that coordination of eye movements by internuclear neurons appeared early during the evolution of vertebrates. In order to investigate the possible involvement of inhibitory neurotransmitters in internuclear circuits, the distribution of γ-aminobutyric acid (GABA) in the extraocular motor nuclei of the lamprey was studied using immunocytochemical techniques. Small GABA-immunoreactive (GABAir) neurons were observed in the three ocular motor nuclei. Numerous GABAir neurons were observed in the group of internuclear neurons of the dorsal rectus oculomotor subnucleus. A second group of GABAir neurons was observed among and below the trochlear motoneurons. Two further groups of GABAir interneurons, periventricular and lateral, were located in the abducens nucleus among the cells of the caudal rectus and the ventral rectus motor subnuclei, respectively. In addition to the presence of GABAir neurons, in all the ocular motor nuclei the motoneurons were contacted by numerous GABAir boutons. Taken together, these results suggest that GABA is involved as a neurotransmitter in internuclear pathways of the ocular motor system of lampreys.

Early components of the human vestibulo-ocular response to head rotation: latency and gain.

To characterize vestibulo-ocular reflex (VOR) properties in the time window in which contributions by other systems are minimal, eye movements during the first 50-100 ms after the start of transient angular head accelerations ( approximately 1000 degrees /s(2)) imposed by a torque helmet were analyzed in normal human subjects. Orientations of the head and both eyes were recorded with magnetic search coils (resolution, approximately 1 min arc; 1000 samples/s). Typically, the first response to a head perturbation was an anti-compensatory eye movement with zero latency, peak-velocity of several degrees per second, and peak excursion of several tenths of a degree. This was interpreted as a passive mechanical response to linear acceleration of the orbital tissues caused by eccentric rotation of the eye. The response was modeled as a damped oscillation (approximately 13 Hz) of the orbital contents, approaching a constant eye deviation for a sustained linear acceleration. The subsequent compensatory eye movements showed (like the head movements) a linear increase in velocity, which allowed estimates of latency and gain with linear regressions. After appropriate accounting for the preceding passive eye movements, average VOR latency (for pooled eyes, directions, and subjects) was calculated as 8.6 ms. Paired comparisons between the two eyes revealed that the latency for the eye contralateral to the direction of head rotation was, on average, 1.3 ms shorter than for the ipsilateral eye. This highly significant average inter-ocular difference was attributed to the additional internuclear abducens neuron in the pathway to the ipsilateral eye. Average acceleration gain (ratio between slopes of eye and head velocities) over the first 40-50 ms was approximately 1.1. Instantaneous velocity gain, calculated as Veye(t)/Vhead(t-latency), showed a gradual build-up converging toward unity (often after a slight overshoot). Instantaneous acceleration gain also converged toward unity but showed a much steeper build-up and larger oscillations. This behavior of acceleration and velocity gain could be accounted for by modeling the eye movements as the sum of the passive response to the linear acceleration and the active rotational VOR. Due to the latency and the anticompensatory component, gaze stabilization was never complete. The influence of visual targets was limited. The initial VOR was identical with a distant target (continuously visible or interrupted) and in complete darkness. A near visual target caused VOR gain to rise to a higher level, but the time after which the difference between far and near targets emerged varied between individuals.

Discharge characteristics of axotomized abducens internuclear neurons in the adult cat.

The aim of the present work was to characterize the axotomy-induced changes in the discharge properties of central nervous system neurons recorded in the alert behaving animal. The abducens internuclear neurons of the adult cat were the chosen model. The axons of these neurons course through the contralateral medial longitudinal fascicle and contact the medial rectus motoneurons of the oculomotor nucleus. Axotomy was carried out by the unilateral transection of this fascicle (right side) and produced immediate oculomotor deficits, mainly the incapacity of the right eye to adduct across the midline. Extracellular single-unit recording of abducens neurons was carried out simultaneously with eye movements. The main alteration observed in the firing of these axotomized neurons was the overall decrease in firing rate. During eye fixations, the tonic signal was reduced, and, on occasion, a progressive decay in firing rate was observed. On-directed saccades were not accompanied by the high-frequency spike burst typical of controls; instead, there was a moderate increase in firing. Similarly, during the vestibular nystagmus, neurons hardly modulated during both the slow and the fast phases. Linear regression analysis between firing rate and eye movement parameters showed a significant reduction in eye position and velocity sensitivities with respect to controls, during both spontaneous and vestibularly induced eye movements. These firing alterations were observed during the 3 month period of study after lesion, with no sign of recovery. Conversely, abducens motoneurons showed no significant alteration in their firing pattern. Therefore, axotomy produced long-lasting changes in the discharge characteristics of abducens internuclear neurons that presumably reflected the loss of afferent oculomotor signals. These alterations might be due to the absence of trophic influences derived from the target.

Response of abducens internuclear neurons to axotomy in the adult cat.

The highly specific projection of abducens internuclear neurons on the medial rectus motoneurons of the oculomotor nucleus constitutes an optimal model for investigating the effects of axotomy in the central nervous system. We have analyzed the morphological changes induced by this lesion on both the cell bodies and the transected axons of abducens internuclear neurons in the adult cat. Axotomy was performed by the transection of the medial longitudinal fascicle. Cell counts of Nissl-stained material and calretinin-immunostained abducens internuclear neurons revealed no cell death by 3 months postaxotomy. Ultrastructural examination of these cells at 6, 14, 24, and 90 days postaxotomy showed normal cytological features. However, the surface membrane of axotomized neurons appeared contacted by very few synaptic boutons compared to controls. This change was quantified by measuring the percentage of synaptic coverage of the cell bodies and the linear density of boutons. Both parameters decreased significantly after axotomy, with the lowest values at 90 days postlesion ( approximately 70% reduction). We also explored axonal regrowth and the possibility of reinnervation of a new target by means of anterograde labeling with biocytin. At all time intervals analyzed, labeled axons were observed to be interrupted at the caudal limit of the lesion; in no case did they cross the scar tissue to reach the distal part of the tract. Nonetheless, a conspicuous axonal sprouting was present at the caudal aspect of the lesion site. Structures suggestive of axonal growth were found, such as large terminal clubs, from which short filopodium-like branches frequently emerged. Similar findings were obtained after parvalbumin and calretinin immunostaining. At the electron microscopy level, biocytin-labeled boutons originating from the sprouts appeared surrounded by either extracellular space, which was extremely dilated at the lesion site, or by glial processes. The great majority of labeled boutons examined were, thus, devoid of neuronal contact, indicating absence of reinnervation of a new target. Altogether, these data indicate that abducens internuclear neurons survive axotomy in the adult cat and show some form of axonal regrowth, even in the absence of target connection.

Neural organization from the superior colliculus to motoneurons in the horizontal oculomotor system of the cat.

The neural organization of the superior colliculus (SC) projection to horizontal ocular motoneurons was analyzed in anesthetized cats using intracellular recording and transneuronal labeling. Intracellular responses to SC stimulation were analyzed in lateral rectus (LR) and medial rectus (MR) motoneurons and internuclear neurons in the abducens nucleus (AINs). LR motoneurons and AINs received excitation from the contralateral SC and inhibition from the ipsilateral SC. The shortest excitation (0.9-1.9 ms) and inhibition (1.4-2.4 ms) were mainly disynaptic from the SC and were followed by tri- and polysynaptic responses evoked with increasing stimuli or intensity. All MR motoneurons received excitation from the ipsilateral SC, whereas none of them received any short-latency inhibition from the contralateral SC, but some received excitation. The latency of the ipsilateral excitation in MR motoneurons (1.7-2.8 ms) suggested that this excitation was trisynaptic via contralateral AINs, because conditioning SC stimulation spatially facilitated trisynaptic excitation from the ipsilateral vestibular nerve. To locate interneurons mediating the disynaptic SC inputs to LR motoneurons, last-order premotor neurons were labeled transneuronally after injecting wheat germ agglutinin-conjugated horseradish peroxidase into the abducens nerve, and tectoreticular axon terminals were labeled after injecting dextran-biotin into the ipsilateral or contralateral SC in the same preparations. Transneuronally labeled neurons were mainly distributed ipsilaterally in the paramedian pontine reticular formation (PPRF) rostral to retrogradely labeled LR motoneurons and the vestibular nuclei, and contralaterally in the paramedian pontomedullary reticular formation (PPMRF) caudomedial to the abducens nucleus and the vestibular nuclei. Among the last-order premotor neuron areas, orthogradely labeled tectoreticular axon terminals were observed only in the PPRF and the PPMRF contralateral to the injected SC and seemed to make direct contacts with many of the labeled last-order premotor neurons in the PPRF and the PPMRF. These morphological results confirmed that the main excitatory and inhibitory connections from the SC to LR motoneurons are disynaptic and that the PPRF neurons that receive tectoreticular axon terminals from the contralateral SC terminate on ipsilateral LR motoneurons, whereas the PPMRF neurons that receive tectoreticular axon terminals from the contralateral SC terminate on contralateral LR motoneurons.

Internuclear neurons of the ocular motor system of the larval sea lamprey.

The internuclear neurons of the ocular motor system of lampreys are characterized here for the first time. Horseradish peroxidase (HRP), fluorescein-, or Texas red-(TRDA) coupled dextran-amine applied into the oculomotor nucleus of larval lamprey (Petromyzon marinus) retrogradely labeled two populations of contralateral abducens interneurons, one lateral and the other periventricular. Tracer application to the abducens nucleus anterogradely labeled thick contralateral fibers that specifically contact the medial rectus motor subnucleus by means of large boutons. Local application of TRDA to this subnucleus allowed identification of the lateral abducens interneurons as the origin of this projection. Electron microscopy of the medial rectus motor subnucleus showed large boutons bearing round synaptic vesicles that contact on the perikarya, as well as small boutons with pleomorphic vesicles. This lateral rectus (abducens) -- medial rectus (oculomotor) internuclear projection of lampreys appears to be similar to those involved in the coordination of horizontal eye movements in mammals. The periventricular abducens interneurons projected bilaterally to other oculomotor subnuclei. Tracer application to the abducens nucleus labeled a group of small interneurons in the ipsilateral dorsal rectus motor subnucleus. Anterograde labeling indicates that oculomotor interneurons project ipsilaterally to the ventral rectus abducens subnucleus, thus, corresponding to oculomotor interneurons found in mammals and frogs. The interneurons of the dorsal rectus and ventral rectus motor subnuclei are probably involved in the control of conjugate vertical eye movements. The present results strongly suggest that the internuclear coordination of conjugate eye movements appeared in the earliest vertebrates. The homologies of extraocular muscles of lampreys and gnathostomes were reexamined.

Localization of parvalbumin, calretinin, and calbindin D-28k in identified extraocular motoneurons and internuclear neurons of the cat.

Calcium-binding proteins have been shown to be excellent markers of specific neuronal populations. We aimed to characterize the expression of calcium-binding proteins in identified populations of the cat extraocular motor nuclei by means of immunohistochemistry against parvalbumin, calretinin, and calbindin D-28k. Abducens, medial rectus, and trochlear motoneurons were retrogradely labeled with horseradish peroxidase from their corresponding muscles. Oculomotor and abducens internuclear neurons were retrogradely labeled after horseradish peroxidase injection into either the abducens or the oculomotor nucleus, respectively. Parvalbumin staining produced the highest density of immunoreactive terminals in all extraocular motor nuclei and was distributed uniformly. Around 15-20% of the motoneurons were moderately stained with antibody against parvalbumin, but their axons were heavily stained, indicating an intracellular segregation of parvalbumin. Colchicine administration increased the number of parvalbumin-immunoreactive motoneurons to approximately 85%. Except for a few calbindin-immunoreactive trochlear motoneurons (1%), parvalbumin was the only marker of extraocular motoneurons. Oculomotor internuclear neurons identified from the abducens nucleus constituted a nonuniform population, because low percentages of the three types of immunostaining were observed, calbindin being the most abundant (28.5%). Other interneurons located within the boundaries of the oculomotor nucleus were mainly calbindin-immunoreactive. The medial longitudinal fascicle contained numerous parvalbumin- and calretinin-immunoreactive but few calbindin-immunoreactive axons. The majority of abducens internuclear neurons projecting to the oculomotor nucleus (80.7%) contained calretinin. Moreover, the distribution of calretinin-immunoreactive terminals in the oculomotor nucleus overlapped that of the medial rectus motoneurons and matched the anterogradely labeled terminal field of the abducens internuclear neurons. Parvalbumin immunostained 42% of the abducens internuclear neurons. Colocalization of parvalbumin and calretinin was demonstrated in adjacent semithin sections, although single-labeled neurons were also observed. Therefore, calretinin is proven to be a good marker of abducens internuclear neurons. From all of these data, it is concluded that parvalbumin, calretinin, and calbindin D-28k selectively delineate certain neuronal populations in the oculomotor system and constitute valuable tools for further analysis of oculomotor function under normal and experimental conditions.

Simulated recruitment of medial rectus motoneurons by abducens internuclear neurons: synaptic specificity vs. intrinsic motoneuron properties.

Ocular motoneuron firing rate is linearly related to conjugate eye position with slope K above recruitment threshold theta. Within the population of ocular motoneurons K increases as theta increases. These differences in firing rate between motoneurons might be determined either by the intrinsic properties of the motoneurons, or by differences in synaptic input to them, or by a combination of the two. This question was investigated by simulating the input signal to medial rectus motoneurons (MR-MNs) from internuclear neurons of the abducens nucleus (INNs). INNs were represented as input nodes in a two-layer neural net, each with weighted connections to every output node representing an MR-MN. Individual simulated MR-MNs were assigned parameters corresponding to an intrinsic current threshold I(R) and an intrinsic frequency-current (f-I) slope gamma. Their firing rates were calculated from these parameters, together with the effective synaptic current produced by their synaptically weighted INN inputs, with the use of assumptions employed in computer simulations of spinal motoneuron pools. The experimentally observed firing rates of MR-MNs served as training data for the net. The following two training conditions were used: 1) synaptic weights were fixed and the intrinsic parameters of the MR-MNs were allowed to vary, corresponding to the situation in which each MR-MN receives a common synaptic drive and 2) intrinsic MR-MN properties were fixed and synaptic weights were allowed to vary. In each case, the varying quantities were trained with a form of gradient descent error reduction. The simulations revealed the following three problems with the common-drive model: 1) the recruitment of INNs produced nonlinear responses in MR-MNs with low thetas; 2) the range of I(R)s required to reproduce the observed range of theta were generally larger than those measured experimentally for cat ocular motoneurons; and 3) the intrinsic f-I slope gamma increased with I(R). Experimental data from cat indicate that gamma decreases with I(R). When synaptic weights were allowed to vary, all three problems with the common-drive model were overcome. This required MR-MNs receiving selective input from INNs with similar firing rate thresholds. These results suggest that the differences in firing rate properties among MR-MNs in relation to steady-state eye position cannot be derived from their intrinsic properties alone but result at least partly from differences in their synaptic inputs. An MR-MN's individual set of synaptic inputs constitutes, in effect, a premotor receptive field.

Excitatory and Inhibitory Vestibular Pathways to the Extraocular Motor Nuclei in Goldfish

Electrophysiological, ultrastructural, and immunohistochemical techniques were utilized to describe the excitatory and inhibitory vestibular innervation of extraocular motor nuclei in the goldfish. In antidromically activated oculomotor motoneurons, electrical stimulation of the intact contralateral vestibular nerve produced short-latency, variable amplitude electrotonic excitatory postsynaptic potentials (EPSPs) at 0.5-0.7 ms followed by chemical EPSPs at 1.0-1.3 ms. Stimulation of the ipsilateral vestibular nerve produced small amplitude membrane hyperpolarizations at a latency of 1.3-1.7 ms in which equilibrium potentials were slightly more negative than resting potentials. The inhibitory postsynaptic potentials (IPSPs) reversed with large amplitudes after the injection of chloride ions suggesting a proximal soma-dendritic location of terminals exhibiting high efficacy inhibitory synaptic conductances. In antidromically identified abducens motoneurons and putative internuclear neurons, electrical stimulation of the contralateral vestibular nerve produced large-amplitude, short-latency electrotonic EPSPs at 0.5 ms followed by chemical depolarizations at 1.2-1.3 ms. Stimulation of the ipsilateral vestibular nerve evoked IPSPs at 1.4 ms that were reversed after injection of current and/or chloride ions. gamma-Aminobutyric acid (GABA) antibodies labeled inhibitory neurons in vestibular subdivisions with axons projecting into the ipsilateral medial longitudinal fasciculus (MLF). Putative GABAergic terminals surrounded oculomotor, but not abducens, motoneurons retrogradely labeled with horseradish peroxidase. Hence the spatial distribution of GABAergic neurons and terminals appears highly similar in the vestibuloocular system of goldfish and mammals. Electron microscopy of motoneurons in the oculomotor and abducens nucleus showed axosomatic and axodendritic synaptic endings containing spheroidal synaptic vesicles establishing chemical, presumed excitatory, synaptic contacts with asymmetric pre- and/or postsynaptic membrane specializations. The majority of contacts with spheroidal vesicles displayed gap junctions in which the chemical and electrotonic synapses were either en face to dissimilar or adjacent to one another on the same soma/dendritic profiles. Another separate set of axosomatic synaptic endings, presumed to be inhibitory, contained pleiomorphic synaptic vesicles with symmetric pre- and/or postsynaptic membrane specializations that never included gap junctions. Excitatory and inhibitory synaptic contacts appeared equal in number but were more sparsely distributed along the soma-dendritic profiles of oculomotor as compared with abducens motoneurons. Collectively these data provide evidence for both disynaptic vestibular inhibition and excitation in all subdivisions of the extraocular motor nuclei suggesting the basic vestibulooculomotor blueprint to be conserved among vertebrates. We propose that unique vestibular neurons, transmitters, pathways, and synaptic arborizations are homologous structural traits that have been essentially preserved throughout vertebrate phylogeny by a shared developmental plan.

Influence of the postsynaptic target on the functional properties of neurons in the adult mammalian central nervous system.

In this review we have attempted to summarize present knowledge concerning the regulatory role of target cells on the expression and maintenance of the neuronal phenotype during adulthood. It is well known that in early developmental stages the survival of neurons is maintained by specific neurotrophic factors derived from their target tissues. Neuronal survival is not the only phenotype that is regulated by target-derived neurotrophic factors since the expression of electrophysiological and cytochemical properties of neurons is also affected. However, a good deal of evidence indicates that the survival of neurons becomes less dependent on their targets in the adult stage. The question is to what extent are target cells still required for the maintenance of the pre-existing or programmed state of the neuron; i.e., what is the functional significance of target-derived factors during maturity? Studies addressing this question comprise a variety of neuronal systems and technical approaches and they indicate that trophic interactions, although less apparent, persist in maturity and are most easily revealed by experimental manipulation. In this respect, research has been directed to analyzing the consequences of disconnecting a group of neurons from their target-by either axotomy or selective target removal using different neurotoxins-and followed (or not) by the implant of a novel target, usually a piece of embryonic tissue. Numerous alterations have been described as taking place in neurons following axotomy, affecting their morphology, physiology and metabolism. All these neuronal properties return to normal values when regeneration is successful and reinnervation of the target is achieved. Nevertheless, most of the changes persist if reinnervation is prevented by any procedure. Although axotomy may represent, besides target disconnection, a cellular lesion, alternative approaches (e.g., blockade of either the axoplasmic transport or the conduction of action potentials) have been used yielding similar results. Moreover, in the adult mammalian central nervous system, neurotoxins have been used to eliminate a particular target selectively and to study the consequences on the intact but target-deprived presynaptic neurons. Target depletion performed by excitotoxic lesions is not followed by retrograde cell death, but targetless neurons exhibit several modifications such as reduction in soma size and in the staining intensity for neurotransmitter-synthesizing enzymes. Recently, the oculomotor system has been used as an experimental model for evaluating the functional effects of target removal on the premotor abducens internuclear neurons whose motoneuronal target is destroyed following the injection of toxic ricin into the extraocular medial rectus muscle. The functional characteristics of these abducens neurons recorded under alert conditions simultaneously with eye movements show noticeable changes after target loss, such as a general reduction in firing frequency and a loss of the discharge signals related to eye position and velocity. Nevertheless, the firing pattern of these targetless abducens internuclear neurons recovers in parallel with the establishment of synaptic contacts on a presumptive new target: the small oculomotor internuclear neurons located in proximity to the disappeared target motoneurons. The possibility that a new target may restore neuronal properties towards a normal state has been observed in other systems after axotomy and is also evident from experiments of transplantation of immature neurons into the lesioned central nervous system of adult mammals. It can be concluded that although target-derived factors may not control neuronal survival in the adult nervous system, they are required for the maintenance of the functional state of neurons, regulating numerous aspects of neuronal structure, chemistry and electro-physiology.(ABSTRUCT TRUNCATED)

A bilateral model integrating vergence and the vestibulo-ocular reflex.

The majority of previous modelling studies of vergence and the vestibulo-ocular reflex (VOR) have postulated arbitrary structures mainly on the basis of input-output behavioural relationships. Such models were developed following traditional schemes of oculomotor organization, based upon the notion of independence between different oculomotor subsystems. This impedes the simulation of complex binocular interactions and associated central activities. In contrast to preceding studies, the mathematical model for binocular control presented here was developed fully on physiological and anatomical grounds which reflect the organization and functional properties of known vergence and VOR premotor centres. Computer simulations show the model properly simulates the main observed characteristics in the discharge of several premotor and motor nuclei during slow vergence and the VOR in the dark. In particular, the model reproduces the activity profiles of abducens internuclear neurons, secondary vestibular cells, tonic prepositus hypoglossi neurons and ocular motoneurons during vergence and the VOR. It also simulates the activity of mesencephalic neurons whose discharge is modulated by vergence parameters alone. It is shown that given recent neurophysiological and behavioural findings, ocular reflexes cannot be properly modelled as separate independent subsystems whereas a single, unified modelling approach can produce results consistent with observed data. This study also shows how changes in the functional activity of shared pathways in a single two-sided structure produce vergence and conjugate integrators whose function relies on coupled loops across the brainstem: separate, dedicated operators are not necessary to replicate data. This provides evidence that challenges previous studies supporting the existence of separate vergence and conjugate integrators to transform velocity to position signals in the brainstem. A major implication of this study is that it questions the validity of testing conjugate and vergence systems independently, neglecting potential interactions.

Are laws that govern behavior embedded in the structure of the CNS? The case of Hering's law

The pattern of axonal terminations of individual premotoneuronal medium lead burst neurons (MLBs) has been elucidated with the help of the intraaxonal recording and horseradish peroxidase injection technique in alert behaving monkeys. These findings indicate that individual MLBs do not influence individual muscles; instead they influence groups of muscles. Horizontal medium lead burst neurons (EBNs) project to ipsilateral lateral rectus motoneurons directly, and could contact contralateral medial rectus motoneurons indirectly, through the internuclear neurons of the ipsilateral abducens nucleus. Also, upward MLBs project to motoneurons innervating both the superior rectus and the inferior oblique muscles of both eyes. Finally, downward MLBs project to ipsilateral motoneurons innervating the ipsilateral inferior rectus muscle and the contralateral superior oblique muscle. All in all, the pattern of oculomotor terminations of MLBs provides experimental support for Hering's law of equal innervation. EBNs also project to the nucleus prepositus hypoglossi while vertical MLBs also project to the interstitial nucleus of Cajal (NIC). These structures are thought to participate in the process of “velocity to position integration”, in the horizontal and vertical planes respectively. Intracellular recording from individual neurons of the NIC in alert behaving monkeys followed by biocytin or horseradish peroxidase injections demonstrates that their axons pass through the posterior commissure on their way to vertical extraocular motoneurons. It also demonstrates that these fibers carry a phasic signal related to saccades and a tonic signal related to eye position in a precise quantitative manner. Work is in progress to determine whether their pattern of oculomotor terminations is also appropriate for Hering's law.