Saccadic eye movements were evoked with weak currents applied to a circumscribed vermal area. The area was confined to lobule VII in the majority of the monkeys and coincided with the distribution of saccade-related neural activity. We defined this area as the oculomotor vermis and studied its anatomical connections with wheat germ-agglutinin conjugated horseradish peroxidase (WGA/HRP) and HRP. When injected HRP was confined to the oculomotor vermis, most labeled Purkinje axons terminated ipsilaterally in an ellipsoidal region in the mediocaudal aspect of the fastigial nucleus. Retrogradely labeled cells were found in two relatively circumscribed regions in the fastigial nucleus: one group was in the lateral half of the ellipsoidal terminal region and the other group was in a spherical region near the lateral margin of the nucleus. Following the injection of HRP into the oculomotor vermis, the largest population of retrogradely labeled neurons was found in the nucleus reticularis tegmenti pontis. Labeled cells were located only in the medial and dorsolateral portions of the nucleus. The cell aggregates in the dorsolateral portion merged with densely labeled cells of the processus tegmentosus lateralis. The second largest population of labeled cells was found in the pontine nuclei. Approximately 28% of the labeled pontine cells aggregated in the paramedian pontine nucleus, whereas the other labeled pontine cells were widely distributed in the dorsal part of the pontine peduncular nucleus and the dorsolateral pontine nucleus. Labeled cells were scattered also in the pontine raphe, the paramedian pontine reticular formation, and the interfascicular nucleus at the rostral level of the hypoglossal nucleus. Fewer labeled cells were discovered in the vestibular nuclear complex and the prepositus hypoglossi. In the inferior olivary nucleus, labeled cells were located in the subnucleus b of the medial accessory nucleus.

The internal lateral (IL) subnucleus of the parabrachial nucleus (PB), which is one of the seven lateral subnuclei of the PB, receives information from the spinal cord. The IL subnucleus perhaps relays nociceptive signals to the intralaminar nuclei of the thalamus, apparently being implicated in the motivational-affective component of pain reaction. However, cells of origin of spinal fibers to the IL subnucleus have not been investigated sufficiently. We intended to clarify these cells by injection of fast blue or wheat germ agglutinin-conjugated horseradish peroxidase into the IL subnucleus and/or other lateral subnuclei in the rat. When the tracer was injected into the IL subnucleus, many cells were labeled bilaterally in laminae I, V, and VII, and in the dorsolateral and dorsomedial parts of the lateral funiculus throughout the entire length of the spinal cord. A small number of labeled cells appeared ipsilaterally in laminae II-IV and VI in the upper cervical segments and contralaterally in laminae VIII and X throughout the spinal cord. Labeled cells in lamina I were more numerous ipsilaterally than contralaterally in the first two cervical segments but were more numerous contralaterally than ipsilaterally in the remaining spinal segments. Labeled cells were seen with a contralateral predominance in lamina VII, but with an ipsilateral predominance in lamina V and in the dorsolateral and dorsomedial parts of the lateral funiculus. With tracer injected into the lateral subnuclei of the PB, excluding the IL subnucleus, labeled cells were found primarily in lamina I throughout the entire length of the spinal cord. These results show that cells giving rise to spinoparabrachial fibers were more numerous and more widely distributed than previously reported.

1. Systematic exploration throughout the deep cerebellar nuclei and white matter disclosed that the region from which saccadic eye movements (saccades) were evoked with weak currents (less than 10 microA) was confined to the fastigial nucleus and the adjacent white matter. 2. When an electrode for stimulation was advanced in the cerebellum, saccades were evoked in the direction of the stimulated side (ipsilateral saccades) as it entered the low-threshold region. In some tracks, particularly when the electrode was advanced in the medial portion of the fastigial nucleus, the direction of the evoked saccades changed from the ipsilateral to the contralateral. 3. The mappings with microstimulation disclosed that the ipsilateral saccades were elicited from a relatively wide region that included almost the full extent of the fastigial nucleus. The low-threshold region continued in the white matter caudally into vermal lobule VII and rostrally into the dorsal aspect of the brachium conjunctivum. On the other hand, the contralateral saccades were evoked from a relatively circumscribed region in the ventromedial portion of the fastigial nucleus. 4. The reversal in the direction of the horizontal component occurred always in a narrow zone in the core of the fastigial nucleus. The caudal part of this zone coincided with an ellipsoidal region where anterogradely labeled axons of the Purkinje cells terminated when HRP was injected into vermal lobule VII. 5. When bicuculline (0.2-1 microgram) was injected in the ellipsoidal region, the ipsilateral saccades evoked from the dorsocaudal aspect of the region were suppressed for several hours. On the other hand, the contralateral saccades evoked from the ventromedial portion of the fastigial nucleus were either unchanged or enhanced. 6. Because the ipsilateral saccades were suppressed by bicuculline, they were most probably evoked by stimulation of the presynaptic component of gamma-amino-butyric acid-(GABA) mediated synapses, namely the axons of Purkinje cells. 7. Because stimulation of the presynaptic component of the inhibitory synapses evoked ipsilateral saccades, activation of the postsynaptic component would evoke contralateral saccades. In fact, the distribution of the fastigial sites yielding contralateral saccades coincided with the course of axons of fastigial neurons arising in the ellipsoidal region. It is most likely, therefore, that the contralateral saccades were evoked by stimulation of fastigial neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

The course of spinocerebellar fibers in the rat spinal cord was investigated by injecting horseradish peroxidase into the cerebellar anterior vermis after complete transection of the left inferior and right superior cerebellar peduncles. By this procedure, fibers passing via the inferior cerebellar peduncles (icp-fibers) were labeled retrogradely on the right side of the spinal cord, whereas fibers passing via the superior cerebellar peduncles (scp-fibers) were labeled on the left side. Crossed icp-fibers were located diffusely in the anterior and lateral funiculi in the sacral to lower lumbar segments. They gradually migrated laterally and dorsally in these funiculi and received many uncrossed icp-fibers moving laterally in the lateral funiculus from the gray substance in the upper lumbar to lower thoracic segments. These mixed fibers shifted more dorsally and laterally in the anterior and lateral funiculi to aggregate in the narrow peripheral zone of the lateral funiculus in the upper thoracic and lower cervical segments, and received many crossed fibers in the upper cervical segments. There were more icp-fibers than scp-fibers through the spinal cord. However, the extent of scp-fibers in the anterior and lateral funiculi was essentially the same as that for icp-fibers, except that a few scp-fibers were found in the dorsolateral marginal zone of the lateral funiculus. It has been generally accepted that the dorsal spinocerebellar tract ascends in the dorsal half of the lateral funiculus and enters the cerebellum via the inferior cerebellar peduncle, whereas the ventral spinocerebellar tract ascends in the ventral half of it and takes the superior cerebellar peduncle route. The results of this study suggest that it is necessary to revise this concept.