Saccadic eye movements evoked by microstimulation of the fastigial nucleus of macaque monkeys

Afferent and efferent connections of the oculomotor region of the fastigial nucleus in the macaque monkey

Afferent and efferent connections of the fastigial oculomotor region (FOR) were studied in macaque monkeys by using axonal transport of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). When injected HRP is confined to the FOR, retrogradely labeled cells appear in lobules VIc and VII of the ipsilateral vermis and in group b of the contralateral medial accessory olive (MAO). In reference to the maps of topographical organization, the extent of the effective site in the fastigial nucleus (FN) could be assessed from the distributions of labeled Purkinje cells (P cells) in the vermis and labeled olivary neurons in the MAO. In contrast to the unilateral nature of the P-cell and climbing-fiber projections, those from the other brainstem regions to the FOR were bilateral. Following the injection of HRP into the FOR, the largest number of retrogradely labeled cells appeared in the pontine nuclei. Although the number of labeled cells was greater on the contralateral side in both the peduncular and dorsomedial pontine nuclei (DMPN), the number of each side was virtually identical in the dorsolateral pontine nucleus (DLPN). In the nucleus reticularis tegmenti pontis (NRTP), labeled cells were located only in its medial and dorsolateral portions bilaterally. In the vestibular complex, labeled cells appeared in the superior (SVN), medial (MVN), and inferior vestibular nuclei (IVN) bilaterally. The lateral vestibular nucleus (LVN), including y group and the ventrolateral vestibular nucleus, were free of labeled cells. Labeled cells appeared also in the perihypoglossal nucleus (PHN) bilaterally. In the pontine raphe (PR) and paramedian pontine reticular formation (PPRF), labeled cells appeared bilaterally in the caudal third of the area between the oculomotor and abducens nuclei. Labeled cells appeared also in the mesencephalic and medullary reticular formation. Tracing of anterogradely labeled axons demonstrated that most fibers from the FOR decussated within the cerebellum and entered the brainstem via the contralateral uncinate fasciculus. Some crossed fibers ascended with the contralateral brachium conjunctivum and terminated in the midbrain tegmentum. A small contingent of fibers advanced further to the thalamus. In the mesodiencephalic junction, labeled terminals were found contralaterally in the rostral interstitial nucleus of medial longitudinal fasciculus (riMLF) and a medial portion of FOrel's H Field. They appeared also in the central mesencephalic reticular formation (cMRF), the periaqueductal gray (PAG), the posterior commissure nucleus, and the superior colliculus. The oculomotor and trochlear nuclei, the red nucleus, and the interstitial nucleus of Cajal were free of labeled terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

Connections of the caudal cerebellar interpositus complex in a new world monkey (Cebus apella)

The afferent and efferent connections of the cerebellar interpositus complex were studied in a capuchin monkey (Cebus apella) that had received a transcannular horseradish peroxidase implant into the caudal portion of the anterior interpositus nucleus and posterior interpositus nucleus. While the heaviest anterogradely labeled ascending projections were observed to the contralateral ventral posterolateral nucleus of the thalamus, pars oralis (VPLo), efferent projections were also observed to the contralateral ventrolateral thalamic nucleus (VLc) and central lateral (CL) nucleus of the thalamic intralaminar complex, magnocellular (and to a lesser extent parvicellular) red nucleus, nucleus of Darkschewitsch, zona incerta, nucleus of the posterior commissure, lateral intermediate layer and deep layer of the superior colliculus, dorsolateral periaqueductal gray, contralateral nucleus reticularis tegmenti pontis and basilar pontine nuclei (especially dorsal and peduncular), and dorsal (DAO) and medial (MAO) accessory olivary nuclei, ipsilateral lateral (external) cuneate nucleus (LCN) and lateral reticular nucleus (LRN), and to a lesser extent the caudal medial vestibular nucleus (MVN) and caudal nucleus prepositus hypoglossi (NPH), and dorsal medullary raphe. The heaviest retrograde labeling was corticonuclear Purkinje cells in the paramedian cerebellar cortex lateral to the vermis of lobules IV-VIII. Otherwise, retrogradely labeled sources of afferents were predominantly contralateral in the dorsal, dorsomedial, paramedian, and peduncular sectors of the basilar pons, NRTP, and dorsal accessory (DAO) and medial accessory (MAO) of olivary nuclei, but were predominantly ipsilateral in the LCN, LRN, and in the medullary reticular formation along the roots of the hypoglossal (XII) cranial nerve. It appeared that the connections with the contralateral dorsal basilar pons, NRTP, DAO and MAO, and ipsilateral LCN and LRN are reciprocal.(ABSTRACT TRUNCATED AT 250 WORDS)

Direction-selective saccadic-burst neurons in the fastigial oculomotor region of the macaque

Discharges of 68 neurons, recorded from the oculomotor region of the fastigial nucleus (FN) of the macaque, were characterized by a burst which started approximately 20 msec prior to the onset of a saccade in a direction contralateral to the recording site. These fastigial neurons also paused before saccades in their nonpreferred direction and then discharged with a subsequent burst. All units were spontaneously active but the tonic level of activity did not reflect eye position. They aggregated within the oculomotor region and the firing of 57 units (83.8%) was suppressed by stimulation of vermal lobule VII. Tonic discharges of six other units were closely related to eye position and located rostral to the burst units. Discharges of 16 units increased during the ipsilateral phase of sinusoidal head rotation and those of 50 units increased during contralateral head rotation. These neurons did not show saccade-related activity. They were widely scattered in the rostral and central portions of the FN. None of these units was inhibited by stimulation of vermal lobule VII.

Projections of the medial cerebellar nucleus to oculomotor-related midbrain areas in the rat: an anterograde and retrograde HRP study

The mesencephalic projections of the medial cerebellar nucleus (MCN) were studied in the rat by using the method of anterograde transport of wheat germ agglutinin/horseradish peroxidase to establish connections of the nucleus with oculomotor-related nuclei as a basis for its proposed role in eye movement. The principal targets of projections were the supraoculomotor ventral periaqueductal gray (PAG) and lateral PAG, and paraoculomotor cell groups (nucleus of Darkschewitsch and medial accessory nucleus of Bechterew). Lesser projections were observed to the intermediate layer of the superior colliculus, nucleus of the posterior commissure, and prerubral field. Following transcannular HRP gel implants into the oculomotor complex that included adjacent paraoculomotor nuclei, the largest number of retrogradely labeled cells was found in the caudal MCN. The findings suggest that the caudal MCN in the rat, like the primate fastigial nucleus, is involved in the control of eye movement.

Cerebellotectal pathways in the macaque: implications for collicular generation of saccades

The cerebellum is thought to modulate saccadic activity in the primate in order to maintain targeting accuracy, and the cerebellotectal pathway has been posited to play a role in this modulation. However, anatomical descriptions of this pathway in primates are sketchy and conflicting. To determine whether the organization of the cerebellotectal projection in primates is similar to that found in other species, neuroanatomical tracer transport techniques were utilized in two species of macaque monkey to label cerebellotectal somata and fiber terminations. Two pathways were found. One, the fastigiotectal pathway, is derived from cells in the caudal fastigial nucleus and projects bilaterally to the rostral end of the intermediate gray layer. The other pathway is derived from cells in the posterior interposed nucleus and the adjacent posterior wing of the dentate nucleus, and it terminates contralaterally throughout the ventral half of the intermediate gray and the deep gray layers. Both of these pathways terminate within the layers of the superior colliculus containing premotor, saccade-related neurons, but the differences in the distribution of their terminals and cells of origin suggest that these two pathways have different functions. Furthermore, the pattern of connections of these two pathways indicates that they do not function as a traditional feedback circuit. We suggest that the cerebellotectal pathways may instead modulate collicular activity in a more complex manner. For example, it may provide signals necessary for corrective saccades or for maintaining spatial registry between the different sensory representations supplied to the superior colliculus and its presaccadic output, which is organized into a motor map.

Inferior frontal eye field projections to the pursuit-related dorsolateral pontine nucleus and middle temporal area (MT) in the monkey

Inferior frontal eye field (FEF) projections to the dorsolateral pontine nucleus (DLPN), and corticocortical connections with the superior temporal sulcal (STS) cortex, were studied in five macaque monkeys which had received horseradish peroxidase (HRP) gel implants into the inferior prearcuate cortex (including area 45 of Walker, 1940). These connections were contrasted with those from the dorsal FEF (area 8a) in another macaque monkey. Findings of heavy inferior FEF projections to the ipsilateral DLPN (light to the contralateral DLPN) and reciprocal connections with the deep caudal bank and fundus of the superior temporal sulcus (STS), presumed to be the middle temporal (MT) visual area (Maunsell & Van Essen, 1983a), appeared to go hand in hand with more pronounced projections to the stratum superficialis of the superior colliculus (SC). In contrast, the HRP gel implant in the dorsal prearcuate cortex (area 8a of Walker, 1940) resulted in only very light projections to the ipsilateral DLPN, more pronounced projections to the dorsomedial pontine nucleus (DMPN), almost no projection to the stratum superficialis (SS), and more pronounced reciprocal connections with the upper bank of the STS, presumed to be the medial superior temporal (MST) area (Maunsell & Van Essen, 1983a). Both the inferior and dorsal FEF also had extensive reciprocal connections with the ventral intraparietal area (VIP; Maunsell & Van Essen, 1983a) in the caudal bank of the intraparietal sulcus. The correlated projections of the inferior FEF to the DLPN, MT area, and SS may explain its reported role in smooth pursuit (Lynch, 1987), in addition to its well-established role in the production of voluntary purposeful saccadic eye movements (Bruce et al., 1985).

Olivocerebellar and cerebelloolivary connections of the oculomotor region of the fastigial nucleus in the macaque monkey

Anatomical connections of the caudal portion of the fastigial nucleus (FN) with the inferior olive (IO) were studied in macaque monkeys with wheat-germ-agglutinin-conjugated horseradish peroxidase (WGA/HRP) and HRP. When injected HRP was confined to a caudal portion of the FN, retrogradely labeled Purkinje cells (P cells) appeared in the oculomotor vermis. We defined the area that receives the projection from vermal lobule VII as the fastigial oculomotor region. The same HRP injection resulted in retrograde labeling of IO neurons in an area of group b (of Bowman and Sladek: J. Comp. Neurol. 152:299-316, '73) of the contralateral medial accessory olive (MAO). This area was designated as the Z-portion because in the coronal section it appears like the letter "Z." Retrogradely labeled IO neurons were also found in the Z-portion when HRP was injected into the oculomotor vermis, indicating that neurons in this portion project to both the fastigial and vermal oculomotor regions. Anterogradely labeled axons from the contralateral fastigial oculomotor region also terminated in the Z-portion. When the effective site included a region anterior to the fastigial oculomotor region, labeled P cells appeared in lobule V and labeled IO neurons appeared in group a. Labeled terminals of fastigial fibers were also found in group a. When the effective site included a region ventral to the oculomotor region, labeled P cells appeared in vermal lobules VIII and IX and labeled IO neurons appeared in caudal parts of a and b, in addition to group c. HRP injection into the posterior interposed nucleus (PIN) resulted in labeling of P cells in the paravermal zone and of IO neurons in the rostral two-thirds of the MAO and the dorsal accessory olive (DAO). The location of the labeled terminals coincided with the region where the densest labeling of IO neurons was found. Thus, the olivary projections to both the cerebellar cortex and deep cerebellar nuclei and the nucleoolivary projection exhibited a closely related topographical organization.