Rubrobulbar projections in the rabbit. A light and electron microscopic study

Abstract

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...

An Electron Microscope Study of the Locus Coeruleus in the Rabbit

“…GRAY and GUILLERY, 1966), increased osmiophilia of nerve-ending ground substance with concomitant aggregation of synaptic vesicles was most commonly encountered in the present study, as observed in other areas of the brain (MIZUNO and NAKAMURA, 1971a, b;NAKAMURA and MIZUNO, 1971;MIZUNO et al, 1972). In view of the critiques presented by COHEN and PAPPAS (1969), the absence of dark neurons in the locus coeruleus was confirmed in the carefully studied controls, and electrondense profiles filled with aggregated vesicular structures were considered as degenerative features of the synaptic bags only when they were associated with the postsynaptic membrane specialization.…”

Potentiation or diminution of discrete motor unconditioned responses (rabbit eyeblink) to an aversive Pavlovian unconditioned stimulus by two associative processes: Conditioned fear and a conditioned diminution of unconditioned stimulus processing.

“…In considering mechanisms by which a trained CS might diminish US processing in the rabbit conditioned eyeblink paradigm, we have suggested two pathways that originate in the CR pathway (Donegan et al, 1989;Wagner & Donegan, 1989). One is a projection from the magnocellular division of the red nucleus to the trigeminal spinal nuclei, or neurons adjacent to the spinal nucleus (Edwards, 1972;Mizuno, Mochizuki, Akimoto, Mastushima, & Nakamura, 1973). The second is a projection from the interpositus nucleus to the dorsal accessory olive, which is thought to be inhibitory (Andersson, Gorwicz, & Hesslow, 1987;Andersson & Hesslow, 1986).…”

Conditioned diminution of the unconditioned response in rabbit eyeblink conditioning: Identifying neural substrates in the cerebellum and brainstem.

“…In the elaborated model, we proposed that one means of associatively modulating US processing would be through inhibitory projections from elements of the CR pathway onto segments of the US pathway (Donegan et al, 1989). Two projections that we proposed as candidates for this role are the projection from the red nucleus to the trigeminal spinal nuclei (Edwards, 1972; Mizuno, Mochizuki, Akimoto, Mastushima, & Nakamura, 1973) and the projection from the interpositus nucleus to the dorsal accessory olive (e.g., Andersson & Hesslow, 1986).…”

“…Other evidence suggests that the red nucleus, which receives input from the interpositus, may produce bilateral ocular responses when activated. Electrical stimulation of the red nucleus produces bilateral postsynaptic responses in retractor bulbi motoneurons (Grant & Horcholle-Bossavit, 1986), and electrolytic lesions placed in and around the red nucleus produce bilateral terminal degeneration in the facial nucleus (Mizuno et al, 1973). Another explanation for contralaterally induced motor behavior is that stimulation of the interpositus may have activated fibers of passage from the fastigial nucleus (Brodal, 1981), which is well-known to project bilaterally to vestibular nuclei that, in turn, project to oculomotor nuclei (Batton, Jayaraman, Ruggiero, & Carpenter, 1977; Carpenter & Batton, 1982).…”