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S M Highstein

Publications and source records attributed to S M Highstein.

At least 73 records · Page 4Linked to original sources
PubMed1988

Structure-function relationships in the primate superior colliculus. I. Morphological classification of efferent neurons.

1. Neurons in the superior colliculus (SC) of anesthetized paralyzed squirrel monkeys were injected intracellularly with horseradish peroxidase (HRP) to establish a morphological classification of tectal efferent neurons in this species. These neurons were physiologically identified by their antidromic responses following stimulation of the contralateral predorsal bundle or SC. These cells also responded with postsynaptic potentials to stimulation of the ipsilateral substantia nigra and cerebral peduncle and the contralateral tectum. 2. Quantitative light microscopic analysis of the somatodendritic profiles and axonal trajectories of 27 recovered cells revealed the existence of three major groups of tectal efferent neurons: L (n = 7), X (n = 8), and T (n = 12). 3. L neurons are small or medium size cells with relatively elaborate dendritic trees and are located mainly in the superficial layers of the SC. They participate in the ipsilateral descending and dorsal ascending tectofugal bundles. Intrinsic collaterals of L axons deploy a large number of boutons both near the parent cell body and more ventrally within the deeper tectal layers. 4. X neurons are mostly large in size and multipolar in shape with relatively complex dendritic trees. Their cell bodies are situated mainly in the stratum griseum intermedium and occasionally in the stratum opticum. Axons of X neurons participate in the crossed descending and ipsilateral ventral ascending projections of the SC. In addition, the axonal system of about half of the X neurons includes recurrent collaterals. 5. T neurons are located mainly in the ventral stratum opticum and the dorsal stratum griseum intermedium. They have small or medium-sized, trapezoid or ovoid cell bodies and relatively simple radiating or vertical dendritic trees. Their axons usually participate in two of the major tectofugal bundles besides providing a commissural component and recurrent collaterals. 6. Morphological details revealed in the present study support the notion that distinct tectofugal axonal systems originate from efferent neurons of the primate SC that differ both as to their location in the tectum as well as the appearance of their somata and dendritic trees. The resulting morphological classification of tectal efferent cells provides a framework for the analysis of tectal function in terms of populations of identified neurons.

Animals↗
PubMed1988

Structure-function relationships in the primate superior colliculus. II. Morphological identity of presaccadic neurons.

1. Microelectrodes filled with horseradish peroxidase (HRP) were inserted in the superior colliculus (SC) of alert squirrel monkeys. Spontaneous eye movements were monitored in the dark during recording and intraaxonal injection of fibers carrying presaccadic signals. 2. Analysis of the relationship between neuronal activity and saccadic parameters indicates that saccade-related neurons can be functionally classified into: 1) vectorial long-lead burst neurons (n = 31), and 2) directional long-lead burst neurons. 3. Vectorial long-lead burst neurons have little if any spontaneous activity and burst intensely before spontaneous saccades within their movement fields with a latency of approximately 20 ms. Their cell bodies were recovered mostly (4/5) in the stratum opticum of the SC. The mediolateral and anteroposterior location of these tectal long-lead burst neurons (TLLBs) together with their movement fields are consistent with existing descriptions of the motor map of the deeper tectal layers. Due to their somatodendritic morphology and pattern of axonal trajectories, TLLBs belong to the T group of tectal efferent neurons that was described in our companion report. Through its branched axonal system each TLLB can relay a signal coding intended eye displacement to reticular targets of the predorsal bundle (PDB), contralateral tectum, ipsilateral mesencephalic reticular formation (MRF), and rostrally located ipsilateral targets of the SC, besides participating in intratectal information processing. 4. Recovered tectal neurons (n = 4) with activity not related to spontaneous saccades participate in the predorsal and ventral ascending tectofugal bundles as well as the projection to the ipsilateral mesencephalic reticular formation. They do not participate in the commissural projection of the SC and need not have recurrent collaterals. Due to their somatodendritic morphology and pattern of axonal trajectories, these cells belong to the X group of tectal efferent neurons that was described in the preceding paper. 5. Recovered cells of origin of directional long-lead burst fibers recorded in the SC (n = 5) are located in the tectorecipient portion of the MRF and their axonal terminals are entirely contained within the SC. The high-frequency portion of the discharge of these reticulotectal long-lead burst neurons (RTLLBs) precedes most contraversive saccades by approximately 19 ms.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1988

Sensory-to-motor transformations in the vestibular system.

The vestibulo-ocular reflex is a compensatory reflex that results in eye movements that are 180 degrees out of phase with movements of the head but that match head velocity. Because of these reflex eye movements that are equal, but opposite to head movement, the viewed object remains on the fovea of the retina during head movement, thus resulting in visual acuity that is not degraded by visual image slip on the retina. This reflex is compensatory over a large spectrum of head movements in any plane of space. This is accomplished by a spatial and temporal transformation of the input from the vestibular semicircular canals to the motoneurons that innervate the extraocular muscles. The reflex is a three-neuron arc. The middle leg of the reflex is accomplished by secondary vestibular neurons whose axons branch to innervate more than one extraocular muscle. These secondary neurons thus program an eye movement rather than the contraction of a single extraocular muscle. These programmed eye movements that match the plane of the particular semicircular canal that is the input to the reflex constitute the spatial transformation. Primary vestibular afferents innervating the semicircular canals have a broad range of response dynamics that either lead, lag or are in phase with head velocity. The predominant vestibular primary afferent input to the middle leg of the reflex, the same secondary neurons as mentioned above, is parcellated so that afferents more in phase with head velocity predominate.(ABSTRACT TRUNCATED AT 250 WORDS)

Head↗
PubMed1987

Anatomical and physiological characteristics of vestibular neurons mediating the horizontal vestibulo-ocular reflex of the squirrel monkey.

The anatomical characteristics of vestibular neurons, which are involved in controlling the horizontal vestibulo-ocular reflex, were studied by injecting horseradish peroxidase (HRP) into neurons whose response during spontaneous eye movements had been characterized in alert squirrel monkeys. Most of the vestibular neurons injected with HRP that had axons projecting to the abducens nucleus or the medial rectus subdivision of the oculomotor nucleus had discharge rates related to eye position and eye velocity. Three morphological types of cells were injected whose firing rates were related to horizontal eye movements. Two of the cell types were located in the ventral lateral vestibular nucleus and the ventral part of the medial vestibular nucleus (MV). These vestibular neurons could be activated at monosynaptic latencies following electrical stimulation of the vestibular nerve; increased their firing rate when the eye moved in the direction contralateral to the soma; had tonic firing rates that increased when the eye was held in contralateral positions; and had a pause in their firing rate during saccadic eye movements in the ipsilateral or vertical directions. Eleven of the above cells had axons that arborized exclusively on the contralateral side of the brainstem, terminating in the contralateral abducens nucleus, the dorsal paramedian pontine reticular formation, the prepositus nucleus, medial vestibular nucleus, dorsal medullary reticular formation, caudal interstitial nucleus of the medial longitudinal fasciculus, and raphé obscurus. Eight of the cells had axons that projected rostrally in the ascending tract of Deiters and arborized exclusively on the ipsilateral side of the brainstem, terminating in the ipsilateral medial rectus subdivision of the oculomotor nucleus and, in some cases, the dorsal paramedian pontine reticular formation or the caudal interstitial nucleus of the medial longitudinal fasciculus. Two MV neurons were injected that had discharge rates related to ipsilateral eye position, generated bursts of spikes during saccades in the ipsilateral direction, and paused during saccades in the contralateral direction. The axons of those cells arborized ipsilaterally, and terminated in the ipsilateral abducens nucleus, MV, prepositus nucleus, and the dorsal medullary reticular formation. The morphology of vestibular neurons that projected to the abducens nucleus whose discharge rate was not related to eye movements, or was related primarily to vertical eye movements, is also briefly presented.

Abducens Nerve↗
PubMed1987

Anatomical and physiological characteristics of vestibular neurons mediating the vertical vestibulo-ocular reflexes of the squirrel monkey.

The morphology of 35 vestibular neurons whose firing rate was related to vertical eye movements was studied by injection of horseradish peroxidase intracellularly into physiologically identified vestibular axons in alert squirrel monkeys. The intracellularly injected cells were readily classified into four main groups. One group of cells, down position-vestibular-pause neurons (down PVPs; N = 12), increased their firing rate during downward eye positions, paused during saccades, and were located in the medial vestibular nucleus (MV) and the adjacent ventrolateral vestibular nucleus (VLV). They had axons that crossed the midline and ascended in the medial longitudinal fasciculus (MLF) to terminate in the trochlear nucleus, the lateral aspect of the caudal oculomotor nucleus, and the dorsal aspect of the rostral oculomotor nucleus. A second group of cells (N = 15) were also located in the MV and VLV, but increased their firing rate during upward eye positions, and paused during saccades. These cells had axons that crossed the midline and ascended in the contralateral MLF to terminate in the medial aspect of the oculomotor nucleus. A third group of cells (N = 4) were located in the superior vestibular nucleus, generated bursts of spikes during upward saccades, and increased their tonic firing rate during upward eye positions. These cells had axons that ascended laterally to the ipsilateral MLF to terminate in regions of the trochlear and oculomotor nuclei similar to those in which down PVPs terminated. A fourth group of cells (N = 4), located in the VLV, had axons that projected to the spinal cord, although they had firing rates that were significantly correlated with vertical eye position. Electrical stimulation of the vestibular nerve evoked spikes at monosynaptic latencies in each of the above classes of cells, six of which were injected with horseradish peroxidase. Each group of cells had collateral projections to other areas of the brainstem. Some of the neurons that projected to the contralateral trochlear and oculomotor nuclei had collaterals that crossed the midline to terminate in the oculomotor nucleus ipsilateral to the soma, and some gave rise to small collaterals that terminated in the abducens nucleus. Other areas of the brainstem that received collateral inputs from neurons projecting to oculomotor and trochlear nuclei included the interstitial nucleus of Cajal, the caudal part of the dorsal raphe nucleus, the nucleus raphe obscurus, Roller's nucleus, the intermediate and caudal interstitial nuclei of the MLF, and the nucleus prepositus.

Animals↗
PubMed1987

Inputs from regularly and irregularly discharging vestibular nerve afferents to secondary neurons in the vestibular nuclei of the squirrel monkey. I. An electrophysiological analysis.

1. The electrical excitability of vestibular nerve afferents is related to their discharge regularity (23). Irregular (I) afferents are more excitable than regular (R) afferents. We explored the possibility that the differences in electrical excitability could be used to determine the profile of monosynaptic inputs from the ipsilateral vestibular nerve (Vi) to secondary neurons of the vestibular nuclei. The growth of monosynaptic Vi excitatory postsynaptic potentials (EPSPs) as shock strength is increased should reflect the kinds of afferent input that a secondary neuron receives. We were particularly interested in seeing if cells in the vestibular nuclei could be distinguished as R or I neurons depending on whether they received predominantly regular or irregular inputs. Barbiturate-anesthetized squirrel monkeys were used. 2. Recordings were made from vestibular nerve afferents. Shock strength was expressed as multiples of T, the value needed to recruit 10% of the afferents or, as determined empirically, to evoke a detectable field potential in the vestibular nuclei. Most I afferents (85/87 = 98%) were recruited below 4 X T, whereas most R afferents (197/212 = 93%) were first activated above 4 X T. The relation between latent period and electrical excitability was flat for units with thresholds in the range 1-4 X T. Latent periods increased for units with higher thresholds, especially those first activated above 8 x T. The threshold differences between I and R afferents are maximal if the shock falls at approximately half the mean interval after a naturally occurring action potential. The same results were obtained by having each unit fire to a maximal (16-32 X T) conditioning shock and then determining the threshold to a test shock presented 4 ms later. The latter stimulus configuration was used to study the Vi monosynaptic inputs to secondary neurons. The test shock was raised by successive doublings from 1 X T to the strength of the conditioning shock (16-32 X T). 3. Intracellular recordings were made from neurons located in the superior vestibular nucleus or the rostral parts of the medical or lateral vestibular nuclei. Amplitudes and latent periods of Vi EPSPs were measured from averages of several repetitions of each stimulus pair. Each EPSP was calculated by subtracting the extracellular from the intracellular averaged response. Of the 122 neurons sampled, 115 were judged to be monosynaptically related to the ipsilateral vestibular nerve because their Vi EPSPs had latent periods in the range of 0.7-1.4 ms.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1987

Inputs from regularly and irregularly discharging vestibular nerve afferents to secondary neurons in the vestibular nuclei of the squirrel monkey. II. Correlation with output pathways of secondary neurons.

1. Intracellular recordings were made from secondary neurons in the vestibular nuclei of barbiturate-anesthetized squirrel monkeys. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulation of the ipsilateral vestibular nerve (Vi) were measured. An electrophysiological paradigm, described in the preceding paper (26), was used to determine the proportion of irregularly (I) and regularly (R) discharging Vi afferents making direct connections with individual secondary neurons. The results were expressed as a % I index, an estimate for each neuron of the percentage of the total Vi monosynaptic input that was derived from I afferents. The secondary neurons were also classified as I, R, or M cells, depending on whether they received their direct Vi inputs predominantly from I or R afferents or else from a mixture (M) of both kinds of Vi fibers. The neurons were located in the superior vestibular nucleus (SVN) or in the rostral parts of the medical or lateral (LVN) vestibular nuclei. 2. Antidromic activation or reconstruction of axonal trajectories after intrasomatic injection of horseradish peroxidase (HRP) was used to identify three classes of secondary neurons in terms of their output pathways: 1) cerebellar-projecting (Fl) cells innervating the flocculus (n = 26); 2) rostrally projecting (Oc) cells whose axons ascended toward the oculomotor (IIIrd) nucleus (n = 27); and 3) caudally projecting (Sp) cells with axons descending toward the spinal cord (n = 13). Two additional neurons, out of 21 tested, could be antidromically activated both from the level of the IIIrd nucleus and from the spinal cord. 3. The Vi inputs to the various classes of relay neurons differed. As a class, Oc neurons received the most regular inputs. Sp neurons had more irregular inputs. Fl neurons were heterogeneous with similar numbers of R, M, and I neurons. The mean values (+/- SD) of the % I index for the Oc, Fl, and Sp neurons were 34.7 +/- 24.7, 51.9 +/- 30.4, and 61.8 +/- 18.0%, respectively. Only the Oc neurons had a % I index that was similar to the proportion of I afferents (34%) in the vestibular nerve (cf. Ref. 26). 4. The commissural inputs from the contralateral vestibular nerve (Vc) also differed for the three projection classes. Commissural inhibition was most common in Fl cells: 22/25 (88%) of the neurons had Vc inhibitory postsynaptic potentials (IPSPs) and 1/25 (4%) had a Vc EPSP. In contrast, Vc inputs were only observed in approximately half the Oc and Sp neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1986

Anatomy and physiology of saccadic burst neurons in the alert squirrel monkey. I. Excitatory burst neurons.

Saccadic burst neurons in the pontine reticular formation have been implicated in the generation of saccades in the horizontal plane on the basis of lesion and extracellular recording studies in the cat and monkey. In the present study, saccadic burst neurons were anatomically and physiologically characterized with intraaxonal recording and injection of horseradish peroxidase in the alert squirrel monkey. A population of burst neurons were found that appear analogous to the excitatory burst neurons (EBNs) described previously in the cat. All neurons are located in the caudal pontine reticular formation and have a major axonal projection to the ipsilateral abducens nucleus. Additional projections were found to the medial vestibular nucleus, the nucleus prepositus, and regions of the pontine and medullary reticular formation rostral, ventral, and caudal to the abducens. All neurons fire exclusively during saccades and have a discharge pattern similar to that of medium-lead burst neurons described previously in the cat and monkey. In most neurons the saccadic burst begins 5-15 msec before saccade onset. Linear relationships exist between burst duration and saccade duration, number of spikes in the burst and saccade amplitude, and firing frequency and instantaneous velocity. Physiological activity of each neuron shows the closest relationship with the amplitude of the saccade component in a particular direction. For all neurons, this on-direction is in the ipsilateral hemifield and is predominantly horizontal, but may have either an upward or downward vertical component. These results support a major role for the EBNs in the monkey in generating the saccadic burst in abducens motoneurons, as well as in contributing to the oculomotor activity in other classes of premotor neurons.

Abducens Nerve↗
PubMed1986

Anatomy and physiology of saccadic burst neurons in the alert squirrel monkey. II. Inhibitory burst neurons.

Electrophysiological and intracellular labelling studies in the cat have identified a population of saccadic burst neurons in the medullary reticular formation that have an inhibitory, monosynaptic projection to the contralateral abducens nucleus. In the present study, intraaxonal recording and injection of horseradish peroxidase were used to identify and characterize the corresponding population of inhibitory burst neurons (IBNs) in the alert squirrel monkey. Squirrel monkey IBNs are located in the reticular formation ventral and caudal to the abducens nucleus and project contralaterally to the abducens. Additional contralateral projections are present to the vestibular nuclei, the nucleus prepositus, and the pontine and medullary reticular formation rostral and caudal to the abducens. All neurons fire a burst of spikes during saccades and are silent during fixation. In most neurons the burst begins 5-15 msec before saccade onset. The number of spikes in the saccadic burst is linearly related to the amplitude of the component of the saccade in the neuron's on-direction. Linear relationships also exist between burst duration and saccade duration and between firing frequency and instantaneous eye velocity. For all neurons, the on-direction is in the ipsilateral hemifield, with a vertical component that may be either upward or downward. Neurons with projections to the vertically related descending and superior vestibular nuclei tend to have on-directions with larger vertical components than neurons that lack these projections. These results, together with those on excitatory burst neurons reported in the preceding paper, demonstrate a reciprocal organization of burst neuron input to the abducens in the monkey similar to that found in the cat and indicate a major role for these neurons in generating the oculomotor activity in motoneurons as well as in other classes of premotor neurons.

Animals↗
PubMed1986

Morphology and physiology of abducens motoneurons and internuclear neurons intracellularly injected with horseradish peroxidase in alert squirrel monkeys.

Axons of abducens motoneurons and internuclear neurons were penetrated with HRP-filled glass microelectrodes in alert squirrel monkeys. The firing rate of these axons and spontaneous eye movements were recorded and the axons were then injected with HRP for subsequent visualization of the recorded cells. Soma-dendritic and axon and axonal terminal morphology were studied for possible correlation with firing frequency. The physiology of squirrel monkey abducens neurons is qualitatively similar to their counterparts in the rhesus monkey and the cat, being primarily correlated with the position and velocity of the eyes. The locations of moto- and internuclear neurons are similar in the squirrel monkey and cat as are the axonal projections and terminals. However, squirrel monkey abducens cells are smaller than their feline counterparts and have dendrites that are confined to the cellular borders of the abducens nucleus. The size of the soma and proximal dendrites of moto- and internuclear neurons are poorly correlated with either their threshold for recruitment or their tonic eye position sensitivity. However, cells with smaller dendritic trees tended to have higher saccadic eye velocity sensitivity than those with larger trees. Three types of internuclear neurons were distinguishable upon the basis of their axon collaterals. All cells terminated within the medial rectus subdivision of the oculomotor nucleus. One class of cells did not give rise to collaterals before projecting to the oculomotor nucleus and the other classes gave rise to collaterals that terminated in the intermediate and/or caudal interstitial nuclei of the median longitudinal fasciculus. Within the IIIrd nucleus internuclear terminations were usually confined to a single subgroup of medial rectus motoneurons.

Abducens Nerve↗
PubMed1986

Organization of the efferent vestibular nuclei and nerves of the toadfish, Opsanus tau.

The efferent vestibular nuclei and nerves were studied in the toadfish, Opsanus tau, with morphological and electrophysiological techniques. The origin and course of the efferent vestibular nerves was extensively documented. One major morphological observation was that the efferent nerves comprise a peripheral network that is anatomically distinct, and separable by dissection from the primary afferents innervated by each end organ. These anatomically distinct nerves are likely to be a major asset in physiological studies of efferent vestibular function. The retrograde transport of horseradish peroxidase (HRP) from each of the nerves innervating the vestibular and lateral line organs was used to delineate the subgroups of efferent neurons projecting to these end organs. The efferent vestibular nuclei are located in the posterior medulla in and around the median longitudinal fasciculi (MLF). We divided the nuclei cytoarchitecturally into lateral, medial, and dorsal subdivisions. The lateral cells had bilateral dendritic trees while the dorsal cells had ipsilateral, unilateral dendritic trees. There was a higher proportion of lateral cells that innervated the canal organs and the utricle while the dorsal cells tended to innervate the other organs. The total number of cells obtained by summing those from separate nerve label was twice the total cell count present in the nuclei. Indirectly, this indicates that some cells project to more than one end organ. Efferent neurons were penetrated with glass microelectrodes, and their end organs and patterns of connectivity with other end organs were investigated by stimulating various vestibular nerves. Posterior semicircular canal efferent cells are electrically coupled to each other and could be activated electrically or chemically by stimulating other ipsilateral or contralateral vestibular nerves. It is suggested that electrical coupling might be responsible for the uniform behavior of these cells under certain conditions. Morphological and physiological experiments suggested that the semicircular canals are innervated by their own, exclusive populations of efferent neurons while other end organs may share efferent innervation. Single cells were injected intracellularly with HRP and their morphology was studied and characterized by light microscopy. Intracellular label confirmed the morphological features demonstrated by retrograde transport of HRP and also revealed that some cells had central axon collaterals that terminated within the MLF. These morphological and physiological results provide a basis for understanding the behavior of efferent vestibular neurons in the alert animal.

Acoustic Maculae↗
PubMed1985

Action of the efferent vestibular system on primary afferents in the toadfish, Opsanus tau.

Spinalized toadfish were held in a lucite chamber and perfused through the mouth with running seawater. Primary vestibular afferents and vestibular efferent axons and somas were studied with glass microelectrodes. Vestibular semicircular canal afferent and efferent axons were visually identified and penetrated with glass microelectrodes. Afferents responded to pulses of injected current with trains of action potentials, whereas efferents responded with only a single spike. This differential response to injected current served to further distinguish these two classes of nerve fibers that share the same canal nerve for part of their course. When current pulses were injected into efferent somadendritic recording sites, cells responded with trains of action potentials similar to those seen in other central nervous system neurons. Semicircular canal afferents were spontaneously active and occupied the same spectrum of regularity as vestibular afferents recorded in other species. Behavioral arousal evoked by lightly touching the fish on the snout or over the eye resembled spontaneous arousal observed in the field and consisted of eye withdrawal, fin erection, and attempted swimming. Efferent vestibular neurons were spontaneously active and increased their frequency of discharge when the fish was behaviorally aroused. Most efferents were briskly activated by behavioral arousal, but the time constant of the decay of their responses was variable ranging from 100 to 600 ms. Not only touch, but multimodal stimuli were capable of increasing the level of spontaneous activity of efferent vestibular neurons. The shortest latency to behavioral activation was 160 ms. Vestibular primary afferents also manifested increase in neuronal activity with behavioral activation. Irregularly discharging afferents were much more responsive than regularly discharging afferents. One rare case of transient inhibition in a regularly discharging afferent is illustrated. Severing the efferent vestibular nerve blocked behavioral activation in vestibular primary afferents. Electrical stimulation of the efferent vestibular nerve produced excitatory postsynaptic potentials (EPSPs) at latencies within the monosynaptic range in vestibular primary afferents. These monosynaptic EPSPs could produce action potentials in primary afferents or could sum with subthreshold depolarizations produced by current passed through the microelectrode to initiate impulses.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1984

New ultrastructural features in the vestibular labyrinth of the toadfish, Opsanus tau.

Electron microscopy of the toadfish vestibular labyrinth demonstrated the usual cytoarchitectural efferent-hair cell organization with type II hair cells having efferent synaptic terminals directly onto the hair cell bodies. Additional, unexpected findings were: direct efferent-afferent synapses; and two types of synaptic vesicles either found both in the same efferent terminals or in two separate classes of efferent terminals.

Afferent Pathways↗
PubMed1983

The superior vestibular nucleus: an intracellular HRP study in the cat. I. Vestibulo-ocular neurons.

Superior vestibular neurons were penetrated with horseradish peroxidase (HRP)-loaded glass microelectrodes in anesthetized cats. Responses to electrical stimulation of the oculomotor complex and the vestibular nerves were characterized and selected neurons were injected with HRP. Neurons antidromically activated by oculomotor complex stimulation were generally monosynaptically excited by the ipsilateral vestibular nerve. Notable was the absence of strong commissural inhibition by stimulation of the contralateral vestibular nerve. Light microscopy of antidromically identified injected cells demonstrated that these cells are predominantly located at the central levels of the superior vestibular nucleus along the incoming vestibular nerve fibers but a few are found at more caudal levels. Cell bodies, elongated or pyramidal, are mainly medium-sized to large (30-50 micrometers). Dendritic trees extend in a plane at an acute to the collaterals of the vestibular nerve fibers. Dendrites remain within the nuclear territory and generally display an isodendritic branching pattern. Dendritic spines and appendages are mainly distributed on secondary and distal dendrites. A few terminal enlargements similar to growth cones are observed in these neurons. Axons of these neurons project rostrally via the medial longitudinal fasciculus, while a minor projection via the brachium conjunctivum is also found. Axon collaterals, when present, originate in the nucleus itself and in the pontine reticular formation.

Animals↗
PubMed1983

The superior vestibular nucleus: an intracellular HRP study in the cat. II. Non-vestibulo-ocular neurons.

Superior vestibular neurons were penetrated with horseradish peroxidase (HRP)-loaded glass microelectrodes in anesthetized cats and identified electrophysiologically following electrical stimulation of the vestibular nerves and oculomotor complex. Neurons that were not antidromically activated from the oculomotor complex were stained by intracellular injection of horseradish peroxidase. Three types of neurons are identified according to their initial axonal trajectories into the cerebellum, the dorsal pontine reticular formation, or the brachium conjunctivum. Ipsilateral vestibular nerve input to all neurons is primarily monosynaptic and excitatory, whereas the contralateral is inhibitory. The neurons are located in the periphery of the superior vestibular nucleus. Soma diameters range from 20.5 micrometers to 44 micrometers. Most neurons exhibit globular and ovoid cell bodies. The dendritic arbors are intermediate between iso- and allodendritic branching patterns. The few spines and dendritic appendages present are distributed mainly distally on the dendrites. Soma size does not correlate with axon diameter, number of dendrites, or dendritic territories.

Animals↗
PubMed1982

The vestibulothalamic pathway: contribution of the ascending tract of Deiters.

Axoplasmic transport techniques were used to determine the contribution of the ascending tract of Deiters (ATD) to the vestibulothalamic projection in cats. Large injections of HRP into the thalamus centered on the border region between the ventrobasal complex and the caudal ventrolateral nucleus resulted in bilateral retrograde labeling of cells in the vestibular nuclear complex and the nucleus prepositus hypoglossi (PH). Similar thalamic injections were also made in animals with extensive bilateral lesions of the medial longitudinal fasciculus (MLF) and the brachium conjunctivum (BC). HRP-positive neurons in these cases were localized principally to the ventral lateral vestibular nucleus and adjacent superior vestibular nucleus ipsilateral to the thalamic injection, evidence that vestibulothalamic neurons in these nuclei may project to the thalamus over the unlesioned ATD. Injections of [35S]methionine into the rostral vestibular nuclear complex in animals with MLF and BC lesions confirmed these findings, demonstrating orthograde transport of radiolabel in the ATD with termination in thalamus. These experiments document a contribution of the ATD to the ipsilateral vestibulothalamic projection; other sources of the vestibulothalamic pathway (PH, Y group) likely travel through projection systems destroyed in the lesions made in the present study.

Afferent Pathways↗