Ascending projections from the medial pontine reticular formation, the mesencephalic reticular formation, and the median raphe nucleus were examined using the autoradiographic technique. The majority of the ascending fibers labeled after injections of [3H]-leucine into the nucleus pontis caudalis (RPC) course through the brainstem within the tracts of Forel (tractus fasciculorum tegmenti of Forel) and directly ventral to them. At the caudal diencephalon, Forel's bundle divides into dorsal and ventral components bound primarily for the dorsal thalamus and the subthalamus, respectively. RPC fibers project to several regions involved in oculomotor/visual functions. These include the abducens nucleus, the intermediate gray layer of the superior colliculus (SCi), the anterior pretectal nucleus (APN), the ventral lateral geniculate nucleus (LGNv), and regions of the central gray directly bordering the oculomotor nucleus, the interstitial nucleus of Cajal, and the nucleus of Darkschewitsch. Few, if any, fibers from RPC (or from nucleus pontis oralis-RPO) terminate within the oculomotor nucleus proper. Other sites receiving heavy projections from the RPC include adjacent regions of the pontomesencephalic reticular formation (RF), the parafascicular (PF) and central lateral (CL) nuclei of the thalamus and the fields of Forel/zona incerta (FF-ZI). RPO fibers also ascend predominantly in Forel's bundle. Other ascending tracts for these fibers are the medial longitudinal fasciculus and the central tegmental tract (CTT). RPO fibers distribute significantly to the same structures of the oculomotor/visual system receiving projections from RPC. The RPO projections to the SCi and the APN are particularly pronounced. RPO fibers terminate heavily in several nuclei located ventrally within the rostral midbrain/caudal diencephalon. These include major dopamine-containing cell groups (the retrorubral nucleus, the ventral tegmental area, and the substantia nigra-pars compacta) as well as the interpeduncular nucleus, the lateral mammillary nucleus, and the supramammillary nucleus. Other prominent targets for RPO fibers include the mesencephalic RF, specific regions of the central gray, the PF, the CL, the paracentral and central medial nuclei of the thalamus, and the FF/ZI. The major bundle of the ascending fibers labeled after injections of the mesencephalic reticular formation (MRF) travels within the CTT in a position just lateral to the central gray, but a significant number of labeled axons also course in Forel's bundle.(ABSTRACT TRUNCATED AT 400 WORDS)
Superficial lesions of the superior colliculus produced deficits in form discrimination, while deeper lesions produced, in addition, an inability to track objects. These two syndromes were related to an anatomical subdivision: Superficial lesions resulted in anterograde degeneration in the visual thalamus, whereas lesions confined to the deeper layers produced degeneration in the nonvisual thalamus and in brainstem motor areas.
The large, well developed superior colliculus of the tree shrew with its highly differentiated layers is ideal for analyzing the connections of individual layers. Our most significant finding concerns the differences between the projections of the superficial and deep layers. Lesions limited to those strata which receive projections from the retina and striate cortex (superficial 3 layers) result in terminal degeneration almost exclusively within the pulvinar, the pretectal area, and the dorsal and ventral lateral geniculate nuclei. In each case, the greatest amount of degeneration was present in the pulvinar, supporting previous suggestions that the tecto-pulvinar pathway conveys visual information. In sharp contrast, lesions limited to the deep layers which receive multimodal input from nonstriate areas of the neocortex and from a variety of subcortical centers, produce terminal degeneration in entirely different thalamic nuclei -certain intralaminar nuclei, the subthalamic region, and a region homologous to the posterior nuclear group of Rose and Woolsey ('49). The deep lesions also result in terminal degeneration within the inferior colliculus, the parabigeminal nucleus, the reticulo-tegmental nucleus, and the inferior olivary nucleus, as well as in the brainstem reticular formation. Finally, deep lesions produced scattered degenerating fibers in the motor facial nucleus. Our results favor a division of the tree shrew superior colliculus into superficial and deep portions based on strikingly different projection patterns and may be useful in resolving certain problems of thalamic homology.The traditional view that the superior colliculus is merely a "reflex center" has long since been abandoned in the face of behavioral and anatomical evidence to the contrary. For example, the superior colliculus was ablated in a number of studies and the resulting symptoms implicate this structure in the perception of pattern, visual localization, orientation and tracking and other functions that cannot be covered by "reflex" i n Sherrington's ('06) sense of the term (Denny-Brown, '62; Sprague and Meikle, '65; Jane et al., '69; Schneider, '69; Sprague et al., '70; Berlucchi et al., '72; Casagrande et al., '72). A further role of the superior colliculusthat of a relay center in a pathway to cortex -was suggested by results from ablation studies of the visual cortex in the tree shrew: the cortex adjacent to the striate J. COMP. NEUR., 148: 361-386.
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