Direct projections from the anterior portions of the parietal cortex of the cat to the brain stem nuclei, especially those sending fibers to the cerebellum, were investigated by the Nauta-Gygax and Fink-Heimer methods.Following unilateral lesions of the anterior portions of the middle suprasylvian and/or lateral gyri, a significant amount of pencellular degeneration was found almost entirely ipsilaterally in the rostra1 levels of the red nucleus and its vicinities, and in the pontine nuclei. Projection fibers to the pontine nuclei appeared to extend over several longitudinal, columnar zones in the pontine gray. Fibers from the anterior portion of the lateral gyrus were observed mainly in the paramedian and lateral nuclei, and those from the middle suprasylvian gyrus in the ventral, paramedian and lateral nuclei. Degeneration in the nucleus reticularis tegmenti pontis of Bechterew was slight, and found bilaterally with ipsilateral predominance.The significance of the anterior portion of the parietal cortex of the cat as a link of cerebro-cerebellar loops was discussed.It is well known that the interpositus and lateral nuclei of the cerebellum project to the motor cortex of the cerebrum through the nucleus ventralis lateralis of the thalamus, and that the motor cortex in turn projects back to the cerebellum via several brain stem nuclei (Bell and Dow, '67). Recently, Sasaki and his associates ('71, '72a, b ) have shown that electrical stimulation of the interpositus or the lateral nucleus of the cerebellum in the cat elicits evoked potentials in the parietal cortex (anterior portions of the middle suprasylvian and lateral gyri) as well as in the sensorimotor cortex. Although fiber projections from the sensorimotor cortex to those brain stem nuclei that send fibers to the cerebellum have been studied in the cat with silver impregnation techniques (Rinvik and Walberg, '63; Mabuchi and Kusama, '66; Brodal, Margala and Brodal, '67; Brodal, '68a; '69; '70; Brodal and Brodal, '71), projection fibers from the parietal cortex so far have not been investigated systematically.In this context the present study was J. COMP. NEUR., 147: 511-522.carried out to investigate projection fibers from the parietal areas to the brain stem nuclei, in particular, to the nuclei projecting to the cerebellum, in the hope of shedding more light upon the organization of the cerebro-cerebellar loops. MATERIALS AND METHODSThe experiments were carried out on nine young adult cats, weighing between 2 and 3.5 kg. With aseptic precautions, unilateral cortical lesions were produced in the anterior portions of the middle suprasylvian and/or lateral gyri by thermocauterization under general anesthesia (sodium pentobarbital 40 mg/kg i.p.). The animals were allowed to survive five to eight days before being perfused intravitally through the ascending aorta with 1000 ml of 10% neutral formalin under deep anesthesia. The brains and spinal cords were subsequently removed in toto and immediately placed into a large volume of 10% neutral formali...

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The crossed rubrobulbar fibers coursing in association with the classical rubrospinal tract in the rabbit were investigated by means of the Nauta and the Fink-Heimer methods. The synaptic organization within the terminal areas of the rubrobulbar fibers were also studied electron microscopically.The crossed rubrobulbar fibers are distributed to the ventral portion of the reticular area intercalated between the motor and the main sensory nuclei of the trigeminal nerve, to the ventrolateral part of the lateral parvocellular reticular formation, the dorsal region of the facial nucleus, the subtrigeminal portion of the lateral reticular nucleus, and the rostrolateral part of the main portion of the lateral reticular nucleus. Small to medium-sized, electron-dense. degenerated synaptic knobs were observed in the dorsal region of the facial nucleus and in the rostrodorsolateral part of the lateral reticular nucleus. All of the synaptic vesicles contained in the degenerated synaptic bags were spherical. Almost all of the degenerated synaptic terminals were in contact with dendritic profiles. Sporadic electron-dense synaptic knobs contacting the soma of nerve cells were encountered only in the dorsal aspect of the facial nucleus.Crossed rubrobulbar fibers descending through the brain stem in association with classical rubrospinal tract have been demonstrated in the monkey (Carpenter and Pines, '57; Kuypers et al., '62), cat (Walberg, '58a,b; Hinman and Carpenter, '59; Courville, '66b) and opossum (Martin and Dom, '70). Although the tract has been well documented by these authors, no agreement has been reached with respect to the synaptic distribution of the rubrobulbar fibers.The present study was primarily conducted by light and electron microscope methods to examine the distribution pattern and the precise mode of termination of the rubrobulbar fibers in the rabbit, a species has ,frequently used than cat and monkey in experimental-anatomical studies. Special attention was directed to the following questions :1. Do rubrobulbar fibers terminate in the motor nuclei of cranial nerves, particularly the masticatory, facial and ambiguous nuclei? 2. In which subareas of the cranial motor nuclei are rubronuclear fibers, if any, distributed?3. Do the rubrobulbar fibers terminate in any area of the brain stem reticular formation?4. How is the synaptic organization of the rubrobulbar fibers within their terrninal areas?Some of the results here reported have been published in a preliminary form elsewhere (Mizuno and Nakamura, '71a,b; Mizuno and Matsushima, '72). MATERIALS AND METHODSThe experiments were carried out on 1 1 young adult rabbits (weight 2500 gm); seven for light microscopic and four for electron microscopic study. With aseptic precautions, stereotaxic lesions were placed in the red nucleus of animals under sodium pentobarbital anesthesia ( were made electrolytically using a fine needle electrode insulated except at the tip. After vertical insertion of the electrode through a small trephine opening in the skul...