Afferents to the flocculus of the cerebellum in the rhesus macaque as revealed by retrograde transport of horseradish peroxidase

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.

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.

Topographical organization of olivocerebellar and corticonuclear connections in the rat--an WGA-HRP study: I. Lobules IX, X, and the flocculus

The organization of the olivocerebellar and corticonuclear relations for vermal lobules IX and X and the flocculus has been studied in the rat by using microinjections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). This axonal tracer allowed us to study simultaneously the olivocortical connections (revealed by retrograde transport) and corticonuclear connections (revealed by anterograde transport) from a single injection in the cerebellar cortex. The results indicate that four modules can be distinguished, each of which consists of a region of cerebellar cortex receiving afferents from a single small region of the inferior olive (IO) and sending efferents to one or several portions of the cerebellar nuclei and/or vestibular nuclei. The first module includes a medial part of lobule X as well as all the flocculus. It receives afferents from the dorsal cap (dc) and sends efferents to the small cell (s) zone of the dentate nucleus as well as to the medial vestibular (VM) nucleus and subnucleus y. The second module includes a medial parasagittal region located in lobules IX and X. It receives afferents from the ventrolateral outgrowth (vlo) and/or beta nucleus (vlo + beta nucleus) and sends efferents principally to the ventrolateral part of fastigial nucleus and to the superior vestibular (VS), inferior vestibular (VI), and VM nuclei. The third module includes a lateral parasagittal region in lobules IX and X. It receives afferents from the dorsomedial cell column (dmcc) of IO and sends efferents principally to the interpositus nucleus and subnucleus y. The fourth module includes the most lateral part of lobules IX and Xa. It receives afferents from the principal olive (PO) and sends efferents to the s zone of the dentate nucleus. These results are comparable to those obtained in the cat although a few differences are discussed.

Vestibular nuclear complex in the guinea pig: a cytoarchitectonic study and map in three planes

Cytoarchitectonic and fiberarchitectonic criteria were used in the preparation of a detailed map illustrating the vestibular nuclear complex of the guinea pig. The brainstems used for this study were serially cut at 16 micron in the transverse, the sagittal, or the horizontal plane. The sections were studied after being stained alternately with a combined cell and fiber staining method and a Nissl stain. The basic cytoarchitectonic features of the four main vestibular nuclei, their extent, as well as their relationship to the surrounding structures are described. Additionally, the location, topographical features, and the cytoarchitecture of the small groups (f,g,l,x,y,z) associated with the vestibular nuclei are reported. Group f is especially well developed and easily distinguishable in the guinea pig. Furthermore, a hitherto undescribed cell cluster found dorsal to the dorsal border of the superior vestibular nucleus is presented. The results and especially the differences from the descriptions of other species are discussed.

Response properties and visual receptive fields of climbing and mossy fibers terminating in the flocculus of the monkey

Response properties and visual receptive fields of climbing fibers and mossy fibers terminating in the cerebellar flocculus were studied in monkeys trained to fixate a stationary visual target. Among a total of 429 climbing fiber-related units (climbing fibers and complex spikes of Purkinje cells), 20 (4.9%) showed cyclic modulations in firing in response to sinusoidal retinal-slip velocities. Their receptive fields always included the fovea. Among 485 mossy fibers, 64 (13%) responded to the visual stimulation. Of the 64 visually responsive mossy fiber units, 39 (61%) responded exclusively to the retinal-slip velocity (visual mossy fibers and the remaining 25 mossy fibers (39%) responded also to the eye and head velocities (visuomotor mossy fibers). Receptive fields for 17 visual mossy fibers (17/39 or 44%) were within 10 degrees of fixation and those for 22 others (56%) were in the periphery. Receptive fields for all 25 visuomotor mossy fibers were in the periphery. Each mossy fiber unit had a unique velocity-tuning curve and, therefore, the response patterns of individual mossy fibers were different depending on the range of their velocity sensitivity and on the retinal-slip velocity applied.

Afferent and efferent connections of the dorsolateral precentral gyrus (area 4, hand/arm region) in the macaque monkey, with comparisons to area 8

The afferent and efferent connections of the dorsolateral precentral gyrus, the primary motor cortex for control of the upper extremity, were studied by using the retrograde and anterograde capabilities of the horseradish peroxidase (HRP) technique in three adult macaque monkeys that had received HRP gel implants in this cortical region. Reciprocal corticocortical connections were observed primarily with the supplementary motor area (SMA) in medial premotor area 6 and dorsal bank of the cingulate sulcus, postarcuate area 6 cortex, dorsal cingulate cortex (area 24), superior parietal lobule (area 5, PE/PEa), and inferior parietal lobule (area 7b, PF/PFop, including the secondary somatosensory SII region). In these heavily labeled regions, the associational intrahemispheric afferents originated primarily from small and medium sized pyramidal cells in layer III, but also from layer V. The SMA projections were columnar in organization. Intrahemispheric afferents from contralateral homologous and nonhomologous frontal and cingulate cortices also originated predominantly from layer III, but the connections from contralateral area 4 were almost exclusively from layer III. The bilateral connections with premotor frontal area 6 and cingulate cortices were not observed with parietal regions; i.e., only ipsilateral intrahemispheric parietal corticocortical connections were observed. There were no significant connections with prearcuate area 8 or the granular frontal (prefrontal) cortex. Subcortical afferents originated primarily from the nucleus basalis of Meynert, dorsal claustrum, ventral lateral (VLo and VLc), area X, ventral posterolateral pars oralis (VPLo), central lateral and centromedian thalamic nuclei, lateral hypothalamus, pedunculopontine nucleus, locus ceruleus and subceruleus, and superior central and dorsal raphe nuclei. Lesser numbers of retrogradely labeled neurons were observed in the nucleus of the diagonal band, mediodorsal (MD), paracentral, and central superior lateral thalamic nuclei, nucleus limitans, nucleus annularis, and the mesencephalic and pontine reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Mossy fibres sending retinal-slip, eye, and head velocity signals to the flocculus of the monkey

Discharges of mossy fibres were recorded from the cerebellar flocculus of monkeys trained to fixate a small visual target and to track the target when it moved slowly. The experimental paradigms used were designed to study neural responses to retinal-slip velocity, eye velocity, or head velocity, individually or in combination. Among 485 mossy-fibre units recorded from the flocculus, sixty-four units (or 13%) responded to movement of the visual stimulus in the horizontal plane. Two distinct groups of visual mossy fibres were found: they were designated 'visual units' (thirty-nine/sixty-four units or 61%) and 'visuomotor units' (twenty-five/sixty-four units or 39%). The visual units responded exclusively to the retinal-slip velocity. Stationary fixation was necessary for clear cyclic modulation of activity. Their responses declined when the retinal-slip velocity was reduced by eye movements in the same direction. The responses of the visual units were directionally selective and lagged behind the occurrence of 'turnabouts' (changes in direction of stimulus movement) and their peak discharges also lagged the occurrence of peak velocity. Each visual unit had a limited range of velocity sensitivity; in some units the range covered the velocity range of smooth-pursuit eye movements. The visuomotor units had visual receptive fields in the peripheral retina (outside of the central 10 deg); they received also oculomotor and vestibular signals. When the head was stationary, the visuomotor units responded to the target velocity (or visual stimulus velocity) which is the algebraic sum of the retinal-slip velocity and the eye velocity. Their responses reflected the retinal-slip velocity during stationary fixation and the eye velocity during smooth-pursuit eye movements. The responses to stimulus movements were, therefore, almost identical regardless of whether the eyes remained stationary or moved with the stimulus. In response to sinusoidal stimulus movements, the responses of the visuomotor units frequently preceded the stimulus velocity, and the phase lead relative to the velocity curve increased when the frequency of sinusoidal movements was increased. This reflected a relatively constant lead of neural discharges (circa 125 ms) during various frequencies. When the head was moved, the responses of the visuomotor units were dominated by the head velocity, and discharges in response either to the retinal-slip velocity or to the eye velocity (both in the direction opposite to the head velocity) were occluded.(ABSTRACT TRUNCATED AT 400 WORDS)

Brainstem terminations of extraocular muscle primary afferent neurons in the monkey

The central terminations of afferent nerve fibers from the extraocular muscles of the monkey were investigated by means of transganglionic transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA/HRP). Following injections of selected extraocular muscles with WGA/HRP, terminal labeling was apparent in the ipsilateral trigeminal sensory and cuneate nuclei. The density of trigeminal projections varied markedly from one rostrocaudal level to the next, being heaviest within the ventrolateral portion of pars interpolaris of the spinal trigeminal nucleus. A second extraocular muscle afferent representation was noted in ventrolateral portions of the cuneate nucleus. This projection was restricted to rostral portions of pars triangularis of the cuneate nucleus, partially overlapping the afferent termination from dorsal neck muscles. It is likely that some of the problems encountered in formulating conclusions regarding the functional role of extraocular muscle proprioception are due to a lack of detailed information of the central termination pattern of muscle afferents. Taken together, the present findings should provide a basis for further anatomical and physiological studies designed to elucidate the role played by extraocular muscle proprioceptors in vision and oculomotor control.

Afferents to the abducens nucleus in the monkey and cat

The abducens nucleus is a central coordinating element in the generation of conjugate horizontal eye movements. As such, it should receive and combine information relevant to visual fixation, saccadic eye movements, and smooth eye movements evoked by vestibular and visual stimuli. To reveal possible sources of these signals, we retrogradely labeled the afferents to the abducens nucleus by electrophoretically injecting horseradish peroxidase into an abducens nucleus in four monkeys and two cats. The histologic material was processed by the tetramethyl benzidine (TMB) method of Mesulam. In both species the largest source of afferents to the abducens nucleus was bilateral projections from the ventrolateral vestibular nucleus and the rostral pole of the medial vestibular nucleus. Scattered neurons were also labeled in the middle and caudal levels of the medial vestibular nucleus. Large numbers of neurons were labeled in the ventral margin of the nucleus prepositus hypoglossi in the cat and in the common margin of the nucleus prepositus and the medial vestibular nucleus in the monkey, a region we call the marginal zone. Substantial numbers of retrogradely labeled neurons were found in the dorsomedial pontine reticular formation both caudal and rostral to the abducens nuclei. In the monkey, large numbers of labeled neurons were present in the contralateral medial rectus subdivision of the oculomotor complex, while smaller numbers occurred in the ipsilateral medial rectus subdivision and elsewhere in the oculomotor complex. In the cat, large numbers of retrogradely labeled cells were present in a small periaqueductal gray nucleus immediately dorsal to the caudal pole of the oculomotor complex, and a few labeled neurons were also dispersed through the caudal part of the oculomotor complex. Occasional labeled neurons were present in the contralateral superior colliculus in both species. The size and distribution of the labeled neurons within the intermediate gray differed dramatically in the two species. In the cat, the retrogradely labeled neurons were very large and occurred predominantly in the central region of the colliculus, while in the monkey, they were small to intermediate in size and were distributed more uniformly within the middle gray. Among the afferent populations present in the monkey, but not in the cat, was a group of scattered neurons in the ipsilateral rostral interstitial nucleus of the medial longitudinal fasciculus and a denser, bilateral population in the interstitial nucleus of Cajal.(ABSTRACT TRUNCATED AT 400 WORDS)

Basal interstitial nucleus of the cerebellum: cerebellar nucleus related to the flocculus

We have shown that the monkey flocculus is not connected with any of the major, well-demarcated cerebellar nuclei. There is, however, a broadly distributed interstitial population of neurons in the white matter ventral to the cerebellar nuclei and extending into the peduncle of the flocculus; this population, previously undescribed in the monkey, has reciprocal connections with the flocculus (Langer et al., '85a,b). Several lines of evidence indicate that this collection of neurons, called the basal interstitial nucleus of the cerebellum (BIN/Cb), can justifiably be considered a nucleus. (1) Injection of horseradish peroxidase (HRP) into the flocculus always labels a group of neurons that lie immediately ventral to the well-demarcated cerebellar nuclei and extend posteromedially into the lateral margin of the nodulus and rostrolaterally around the caudal surface of the y-group, infiltrating the peduncle of the flocculus. (2) In Nissl-stained material there is a readily seen collection of neurons that are clearly distinct from the overlying cerebellar nuclei, with precisely the same distribution. These neurons have a characteristic morphology: they are intermediate-sized, chromatophilic, multipolar, and fusiform, and have rapidly tapering proximal dendrites. The cell nucleus is generally placed eccentrically in the cell body, against the plasma membrane or in one pole of the cell. The Nissl substance is usually finely granular in the center of the cell body and forms dense clumps adjacent to the cell membrane. (3) Anterograde label from injections of HRP or tritiated amino acids into the flocculus extends over the same group of neurons. In one brain with an HRP injection involving a part of the BIN/Cb there was a patchy, clustered distribution of labeled Purkinje cells extending throughout the flocculus and into the adjacent lateral parts of the simple lobule. The clusters were confined to the medial half of many of the floccular folia.

Floccular efferents in the rhesus macaque as revealed by autoradiography and horseradish peroxidase

To fulfill its putative role in short- and long-term modification of the vestibulo-ocular reflex, the flocculus of the cerebellum must send efferents to brainstem nuclei involved in the control of eye movements. In order to reveal the sites of these interactions, we determined the projections of the flocculus by autoradiography and orthograde transport of horseradish peroxidase in five rhesus macaques. Anterogradely labeled axons collected at the base of the injected folia and coursed caudally and medially between the middle cerebellar peduncle and the flocculus. They swept medially over the caudal surface of the middle cerebellar peduncle, over the dorsal surface of the cochlear nuclei, and then caudally along the lateral surface of the inferior cerebellar peduncle to pass over its dorsal surface in the cerebellopontine angle and terminate exclusively in the ipsilateral vestibular nuclei. Three contingents of axons could be differentiated. The axons of one group flowed caudally and medially into the y-group, which clearly received the densest floccular projection. Other, notably thicker, axons of this group continued rostrally and medially to terminate chiefly in the large-cell core of the superior vestibular nucleus. A second large contingent of thin axons streamed caudal and ventral to the y-group to form a compact tract adjacent to the lateral angle of the fourth ventricle and dorsal to the medial vestibular nucleus. Fibers from this tract (the angular bundle of Löwy) supplied a sizable projection to the rostral part of the medial vestibular nucleus and modest projection to the ventrolateral vestibular nucleus. A final group of fibers extended caudally and medially from the y-group in a plexus ventral to the dentate and interposed nuclei to terminate in the basal interstitial nucleus of the cerebellum (Langer, '85), a broadly distributed cerebellar nucleus on the roof of the fourth ventricle. The flocculus can affect vestibulo-ocular behavior only through these efferents to the vestibular nuclei and the basal interstitial nucleus of the cerebellum.