The responses to vestibular stimulation of brain stem neurons that regulate sympathetic outflow and blood flow have been studied extensively in decerebrate preparations, but not in conscious animals. In the present study, we compared the responses of neurons in the rostral ventrolateral medulla (RVLM), a principal region of the brain stem involved in the regulation of blood pressure, to whole body rotations of conscious and decerebrate cats. In both preparations, RVLM neurons exhibited similar levels of spontaneous activity (median of ∼17 spikes/s). The firing of about half of the RVLM neurons recorded in decerebrate cats was modulated by rotations; these cells were activated by vertical tilts in a variety of directions, with response characteristics suggesting that their labyrinthine inputs originated in otolith organs. The activity of over one-third of RVLM neurons in decerebrate animals was altered by stimulation of baroreceptors; RVLM units with and without baroreceptor signals had similar responses to rotations. In contrast, only 6% of RVLM neurons studied in conscious cats exhibited cardiac-related activity, and the firing of just 1% of the cells was modulated by rotations. These data suggest that the brain stem circuitry mediating vestibulosympathetic reflexes is highly sensitive to changes in body position in space but that the responses to vestibular stimuli of neurons in the pathway are suppressed by higher brain centers in conscious animals. The findings also raise the possibility that autonomic responses to a variety of inputs, including those from the inner ear, could be gated according to behavioral context and attenuated when they are not necessary.

We have investigated the temporal and spatial profiles of apoptotic cells in an experimental transection spinal cord injury by the terminal deoxynucleotidyl transferase-mediated biotin-16-2'-deoxyuridine-5'-triphosphate nick-end labeling (TUNEL) method. Twenty-four hours postinjury, a numerous TUNEL-positive cells appeared both rostrally and caudally to the transection site. Those positive cells, however, gradually diminished in number by several days postinjury. In contrast, other TUNEL-positive cells were found scattered within the white matter remote from the lesion by the third day postinjury. These cells were typically embedded in or among vacuolated fibers, where they were identified in close proximity to the vacuolated space enclosed by myelin basic protein (MBP)-positive structures confirmed by TUNEL-MBP double staining. Because of their linear arrangement, these TUNEL-positive cells were considered interfascicular oligodendrocytes, a fact that was confirmed by the finding that some TUNEL-positive cells were also stained with CCI, a cell marker for oligodendrocyte. Electron microscopic studies revealed that the cells expressing apoptotic morphology were invariably encased in a space formed by myelin splitting. Although the biological significance of apoptotic interfascicular oligodendrocytes in the process of wallerian degeneration is yet to be determined, the finding of such profiles localized within degenerating myelin structures suggests that; oligodendrocytes may be "trapped" within rapidly swollen and disintegrating myelin lamellae, which isolates and perhaps predisposes them to death.

In addition, the PBN/KF region receives inputs from the vestibular system and likely mediates the malaise associated with motion sickness. However, previous studies have not considered whether GI and vestibular inputs converge on the same PBN/KF neurons, and if so, whether the GI signals alter the responses of the cells to body motion. The present study, conducted in decerebrate cats, tested the hypothesis that intragastric injection of copper sulfate, which elicits emesis by irritating the stomach lining, modifies the sensitivity of PBN/KF neurons to vertical plane rotations that activate vestibular receptors. Intragastric copper sulfate produced a 70% median change in the gain of responses to vertical plane rotations of PBN/KF units, whose firing rate was modified by the administration of the compound; the response gains for 16 units increased and those for 17 units decreased. The effects were often dramatic: out of 51 neurons tested, 13 responded to the rotations only after copper sulfate was injected, whereas 10 others responded only before drug delivery. These data show that a subset of PBN/KF neurons, whose activity is altered by a nauseogenic stimulus also respond to body motion and that irritation of the stomach lining can either cause an amplification or reduction in the sensitivity of the units to vestibular inputs. The findings imply that nausea and affective responses to vestibular stimuli may be modified by the presence of emetic signals from the GI system.

Anatomical studies have demonstrated that the vestibular nuclei project to nucleus tractus solitarius (NTS), but little is known about the effects of vestibular inputs on NTS neuronal activity. Furthermore, lesions of NTS abolish vomiting elicited by a variety of different triggering mechanisms, including vestibular stimulation, suggesting that emetic inputs may converge on the same NTS neurons. As such, an emetic stimulus that activates gastrointestinal (GI) receptors could alter the responses of NTS neurons to vestibular inputs. In the present study, we examined in decerebrate cats the responses of NTS neurons to rotations of the body in vertical planes before and after the intragastric administration of the emetic compound copper sulfate. The activity of more than one-third of NTS neurons was modulated by vertical vestibular stimulation, with most of the responsive cells having their firing rate altered by rotations in the head-up or head-down directions. These responses were aligned with head position in space, as opposed to the velocity of head movements. The activity of NTS neurons with baroreceptor, pulmonary, and GI inputs could be modulated by vertical plane rotations. However, injection of copper sulfate into the stomach did not alter the responses to vestibular stimulation of NTS neurons that received GI inputs, suggesting that the stimuli did not have additive effects. These findings show that the detection and processing of visceral inputs by NTS neurons can be altered in accordance with the direction of ongoing movements.

A central pattern generator (CPG) for swallowing in the medulla oblongata generates spatially and temporally coordinated movements of the upper airway and alimentary tract. To reveal the medullary neuronal network of the swallowing CPG, we examined the cytoarchitecture of the swallowing CPG and axonal projections of its individual neurons by extracellular recording and juxtacellular labeling of swallowing-related neurons (SRNs) in the medulla in urethane-anesthetized and paralyzed guinea pigs. Three major types of neuronal discharge patterns were identified during fictive swallowing induced by stimulation of the superior laryngeal nerve: early (burst-like activation during the pharyngeal stage), late (activation after the pharyngeal stage), and inhibited (inhibition during the pharyngeal stage) types. Sixteen neurons were successfully labeled in the nucleus tractus solitarii (NTS) and in the medullary reticular formation (RF). No motoneuron was labeled. The SRNs in the NTS had axons projecting to the NTS, RF, nucleus ambiguus, nucleus hypoglossus, and dorsal motor nucleus of the vagus on the ipsilateral side. Some NTS SRNs projected only within the NTS. The axons of SRNs in the RF projected also to the NTS, RF, motor nuclei on the ipsilateral side, and to the other side RF. These findings show anatomic substrates for the neuronal network of the CPG for swallowing, which consists of complex neuronal connections among SRNs in the NTS, RF, and motor nuclei.