The precise nuclear origins of projections from the brainstem reticular formation to the thalamus were identified in rats using two retrograde tracing substances: wheat germ agglutinin-horseradish peroxidase conjugate, and Fluoro-Gold. Injections of these tracers were made into a variety of thalamic nuclei, including the intralaminar nuclei (most of these also involved the lateral part of the mediodorsal nucleus), the central part of the mediodorsal nucleus, the ventrolateral/ ventromedial nuclei, and the ventral posterolateral/ventral posteromedial nuclei. Counts of retrogradely labeled cells were done on a large sample of select cases. The data generated by these cell counts indicate that brainstem reticular projections to the intralaminar/lateral mediodorsal complex are fairly strong, as are those to the ventrolateral/ventromedial nuclear complex. Ascending reticular projections to the mediodorsal nucleus per se are somewhat weaker, while those to the ventrobasal complex (or at least the ventral posterolateral nucleus) are weaker still. As a whole, reticular neurons projecting to the thalamus are by far most numerous in the midbrain, and then decline gradually at successively caudal levels through the pons and medulla. Midbrain reticular groups evincing very strong ascending projections include nucleus reticularis (n.r.) pedunculopontinus (particularly its pars compactus), n.r. cuneiformis and n.r. subcuneiformis (together known as the deep mesencephalic nucleus). Strong thalamic projections arise from the medial part of n.r. pontis oralis, the medial (beta) part of n.r. pontis caudalis, and the mid-pontine dorsomedial tegmental area. Within the medullary reticular formation, a ‘trans-nuclear field’ of neurons encompassing n.r. paragigantocellularis dorsalis and dorsal parts of n.r. gigantocellularis and n.r. parvocellularis was consistently labeled contralateral to the injection site. In general, ascending reticulothalamic projections are largely ipsilateral from midbrain reticular groups, bilateral from pontine reticular groups, and contralateral from medullary reticular groups. Within individual reticular nuclei, the morphology of labeled neurons is identical to that reported previously by this laboratory subsequent to spinal, cortical, or cerebellar tracer injections, thus strengthening our hypothesis that the various brainstem reticular nuclei can be distinguished on the basis of neuronal morphology. As a whole, thalamic-projecting reticular neurons are mostly small or medium-sized cells.

The precise nuclear origins of projections from the brainstem reticular formation to the thalamus were identified in rats using two retrograde tracing substances: wheat germ agglutinin-horseradish peroxidase conjugate, and Fluoro-Gold. Injections of these tracers were made into a variety of thalamic nuclei, including the intralaminar nuclei (most of these also involved the lateral part of the mediodorsal nucleus), the central part of the mediodorsal nucleus, the ventrolateral/ ventromedial nuclei, and the ventral posterolateral/ventral posteromedial nuclei. Counts of retrogradely labeled cells were done on a large sample of select cases. The data generated by these cell counts indicate that brainstem reticular projections to the intralaminar/lateral mediodorsal complex are fairly strong, as are those to the ventrolateral/ventromedial nuclear complex. Ascending reticular projections to the mediodorsal nucleus per se are somewhat weaker, while those to the ventrobasal complex (or at least the ventral posterolateral nucleus) are weaker still. As a whole, reticular neurons projecting to the thalamus are by far most numerous in the midbrain, and then decline gradually at successively caudal levels through the pons and medulla. Midbrain reticular groups evincing very strong ascending projections include nucleus reticularis (n.r.) pedunculopontinus (particularly its pars compactus), n.r. cuneiformis and n.r. subcuneiformis (together known as the deep mesencephalic nucleus). Strong thalamic projections arise from the medial part of n.r. pontis oralis, the medial (beta) part of n.r. pontis caudalis, and the mid-pontine dorsomedial tegmental area. Within the medullary reticular formation, a ‘trans-nuclear field’ of neurons encompassing n.r. paragigantocellularis dorsalis and dorsal parts of n.r. gigantocellularis and n.r. parvocellularis was consistently labeled contralateral to the injection site. In general, ascending reticulothalamic projections are largely ipsilateral from midbrain reticular groups, bilateral from pontine reticular groups, and contralateral from medullary reticular groups. Within individual reticular nuclei, the morphology of labeled neurons is identical to that reported previously by this laboratory subsequent to spinal, cortical, or cerebellar tracer injections, thus strengthening our hypothesis that the various brainstem reticular nuclei can be distinguished on the basis of neuronal morphology. As a whole, thalamic-projecting reticular neurons are mostly small or medium-sized cells.

The nuclear origins of projections from the brainstem reticular formation to the cerebellum were examined using four retrograde tracer substances: horseradish peroxidase, wheat germ agglutinin-horseradish peroxidase conjugate, Fluoro-Gold, and rhodamine beads. Tracer injections were made into each of the three major longitudinal zones of the cerebellar cortex (vermis, paravermal hemisphere, and lateral hemisphere) as well as into the various deep cerebellar nuclei. Counts of retrogradely labeled cells were done on a large sample of select cases. The data generated by these cell counts indicate that the strongest reticulocerebellar projections arise from the three specialized pre-cerebellar reticular nuclei: the lateral reticular nucleus, the medullary paramedian reticular nucleus, and the reticulotegmental nucleus. The presumed noradrenergic locus coeruleus (A6 cell group) was also densely packed with retrogradely labeled neurons. However, strong reticulocerebellar projections also arose from other presumed catecholamine cell groups such as those in the ventrolateral medulla (the A1/C1 complex) and the caudal pons (A5). Substantial cerebellar projections originated from most of the various presumed serotonergic brainstem raphe cell groups (particularly raphe obscurus in the medulla), as well as from the presumed cholinergic Ch5 cell group (the pedunculopontine pars compact us nucleus). Labeled cells were also seen in several nonaminergic isodendritic reticular nuclei thought to be involved in visuomotor activity (e.g. paragigantocellularis dorsalis, raphe interpositus, and the pontine dorsomedial tegmental area), as well as in the lateral reticular zone of the medulla and lower pons (reticularis dorsalis and parvocellularis). Tracer injections into the deep nuclei produced relatively greater numbers of labeled neurons in large-celled medial reticular nuclei associated with skeletomotor activity, such as gigantocellularis. magnocellularis, and pontis caudalis. Reticular nuclei conspicuous in their lack of projections to the cerebellum included reticularis ventralis in the medulla, pontis oralis, and both subdivisions of the midbrain reticular formation (cuneiformis and subcuneiformis). As a whole, the various isodendritic reticular nuclei project most strongly to midline cerebellar structures (vermal cortex or fastigial nuclei), less strongly to the paravermal cortex or interposed nuclei, and least strongly to the lateral cortex or dentate nucleus. Within individual reticular nuclei, the morphology of labeled neurons is identical to that reported previously by this laboratory subsequent to spinal or cortical HRP injections, thus strengthening this laboratory's hypothesis that the various brainstem reticular nuclei can be distinguished on the basis of neuronal morphology.

The nuclear origins of projections from the brainstem reticular formation to the cerebellum were examined using four retrograde tracer substances: horseradish peroxidase, wheat germ agglutinin-horseradish peroxidase conjugate, Fluoro-Gold, and rhodamine beads. Tracer injections were made into each of the three major longitudinal zones of the cerebellar cortex (vermis, paravermal hemisphere, and lateral hemisphere) as well as into the various deep cerebellar nuclei. Counts of retrogradely labeled cells were done on a large sample of select cases. The data generated by these cell counts indicate that the strongest reticulocerebellar projections arise from the three specialized pre-cerebellar reticular nuclei: the lateral reticular nucleus, the medullary paramedian reticular nucleus, and the reticulotegmental nucleus. The presumed noradrenergic locus coeruleus (A6 cell group) was also densely packed with retrogradely labeled neurons. However, strong reticulocerebellar projections also arose from other presumed catecholamine cell groups such as those in the ventrolateral medulla (the A1/C1 complex) and the caudal pons (A5). Substantial cerebellar projections originated from most of the various presumed serotonergic brainstem raphe cell groups (particularly raphe obscurus in the medulla), as well as from the presumed cholinergic Ch5 cell group (the pedunculopontine pars compact us nucleus). Labeled cells were also seen in several nonaminergic isodendritic reticular nuclei thought to be involved in visuomotor activity (e.g. paragigantocellularis dorsalis, raphe interpositus, and the pontine dorsomedial tegmental area), as well as in the lateral reticular zone of the medulla and lower pons (reticularis dorsalis and parvocellularis). Tracer injections into the deep nuclei produced relatively greater numbers of labeled neurons in large-celled medial reticular nuclei associated with skeletomotor activity, such as gigantocellularis. magnocellularis, and pontis caudalis. Reticular nuclei conspicuous in their lack of projections to the cerebellum included reticularis ventralis in the medulla, pontis oralis, and both subdivisions of the midbrain reticular formation (cuneiformis and subcuneiformis). As a whole, the various isodendritic reticular nuclei project most strongly to midline cerebellar structures (vermal cortex or fastigial nuclei), less strongly to the paravermal cortex or interposed nuclei, and least strongly to the lateral cortex or dentate nucleus. Within individual reticular nuclei, the morphology of labeled neurons is identical to that reported previously by this laboratory subsequent to spinal or cortical HRP injections, thus strengthening this laboratory's hypothesis that the various brainstem reticular nuclei can be distinguished on the basis of neuronal morphology.

The nuclear origins of projections from the brainstem reticular formation to the cerebellum were examined using four retrograde tracer substances: horseradish peroxidase, wheat germ agglutinin-horseradish peroxidase conjugate, Fluoro-Gold, and rhodamine beads. Tracer injections were made into each of the three major longitudinal zones of the cerebellar cortex (vermis, paravermal hemisphere, and lateral hemisphere) as well as into the various deep cerebellar nuclei. Counts of retrogradely labeled cells were done on a large sample of select cases. The data generated by these cell counts indicate that the strongest reticulocerebellar projections arise from the three specialized pre-cerebellar reticular nuclei: the lateral reticular nucleus, the medullary paramedian reticular nucleus, and the reticulotegmental nucleus. The presumed noradrenergic locus coeruleus (A6 cell group) was also densely packed with retrogradely labeled neurons. However, strong reticulocerebellar projections also arose from other presumed catecholamine cell groups such as those in the ventrolateral medulla (the A1/C1 complex) and the caudal pons (A5). Substantial cerebellar projections originated from most of the various presumed serotonergic brainstem raphe cell groups (particularly raphe obscurus in the medulla), as well as from the presumed cholinergic Ch5 cell group (the pedunculopontine pars compact us nucleus). Labeled cells were also seen in several nonaminergic isodendritic reticular nuclei thought to be involved in visuomotor activity (e.g. paragigantocellularis dorsalis, raphe interpositus, and the pontine dorsomedial tegmental area), as well as in the lateral reticular zone of the medulla and lower pons (reticularis dorsalis and parvocellularis). Tracer injections into the deep nuclei produced relatively greater numbers of labeled neurons in large-celled medial reticular nuclei associated with skeletomotor activity, such as gigantocellularis. magnocellularis, and pontis caudalis. Reticular nuclei conspicuous in their lack of projections to the cerebellum included reticularis ventralis in the medulla, pontis oralis, and both subdivisions of the midbrain reticular formation (cuneiformis and subcuneiformis). As a whole, the various isodendritic reticular nuclei project most strongly to midline cerebellar structures (vermal cortex or fastigial nuclei), less strongly to the paravermal cortex or interposed nuclei, and least strongly to the lateral cortex or dentate nucleus. Within individual reticular nuclei, the morphology of labeled neurons is identical to that reported previously by this laboratory subsequent to spinal or cortical HRP injections, thus strengthening this laboratory's hypothesis that the various brainstem reticular nuclei can be distinguished on the basis of neuronal morphology.