Influence of the labyrinth on unitary discharge of the oculomotor nucleus and some adjacent formations

Penicillin-induced paroxysmal activity in brainstem neurons

The spontaneous electrical activity of single mesencephalic and bulbar neurons was recorded in hemispherectomized rats, following topical application of the GABA-antagonist penicillin-G on the mesencephalon or on the rhombencephalon, to investigate whether these structures could develop a specific penicillin paroxysmal activity independently of the upper structures. Twenty minutes following penicillin-G, the mesencephalic neurons developed paroxysmal activity characterized by a significant increase in the spontaneous electrical activity, the appearance of multiunit activity and, frequently, phasic activity with rhythmical outbursts. The paroxysmal activity at bulbar level appeared later than that observed in the mesencephalon and was characterized by a significant increase of the spontaneous firing rate of the neurons, single short bursts and sometimes rhythmical outbursts. The bulbar outbursts always discharged at lower frequency than those at the mesencephalic level. Following a midcollicular transection the paroxysmal bulbar activity abruptly disappeared. This phenomenon might be explained by a loss of facilitation from superior structures on the bulbar neurons which in roditors show a poor GABA-receptor distribution. In other words, penicillin alone, due to the scarcity of GABA receptors, might not be sufficient to induce paroxysmal activity in bulbar neurons but the simultaneous presence of both the superior facilitation and the drug might enhance neuronal excitability to a critical level. However, the diffusion of the drug upwards to the mesencephalon, with consequent activation of a system allowing the downward propagation of paroxysmal activity, cannot be excluded. In conclusion, while the mesencephalic neurons demonstrate a proper ability to develop penicillin paroxysmal discharge, the bulbar neurons must be sustained by intact connections with upper structures to be able to do so.

Motor responses of the esophagus and lower esophageal sphincter (LES) to vestibular stimulation in man

Pressure recordings in the esophageal body, LES and stomach were performed in 10 healthy subjects before and after caloric stimulation of the labyrinth. While esophageal and stomach responses varied or were absent, all subjects showed variations at the LES level. Duration of LES relaxation was increased and both time and amplitude of LES augmentation were significantly reduced. The physiological mechanism of labyrinthine-digestive connections and the possibility of clinical applications are discussed.

Labyrinthine projection to the hypoglossal nucleus

Evoked potentials and responses of single hypoglossal neurons were recorded in response to electrical stimulation of the labyrinth. In addition, the spontaneous electrical activity of hypoglossal neurons was significantly modified in response to ipsi- and contralateral static tilt of the whole animal and thermic stimulation of the labyrinth. The experiment showed that the labyrinth modulates the electrical activity of hypoglossal neurons with phasic inputs in response to ampullar stimulation and with tonic inputs in response to macular stimulation. The vestibular phasic influence of hypoglossal neurons represents the most adequate functional pattern to obtain a quick, short lasting response of the tongue muscles instantly modifiable with every abrupt head displacement. On the contrary, the vestibular tonic influence of hypoglossal neurons represents the most adequate functional pattern to obtain not only adjustment but also maintenance of the muscular lingual response to static displacement of the head.

Horizontal vestibular nystagmus. II. Activity patterns of medial vestibular neurones during nystagmus

1. Identified medial vestibular neurones were studied before, during and after nystagmogenic labyrinthine stimulation in the Encéphale isolé cat. Motor discharges were simultaneously recorded from the contralateral abducens nerve. 2. 87.9% of the recorded neurones showed changes in tonic firing frequency during repetitibe labyrinthine stimulation with no nystagmic modulation in behaviour. 3. The secondary vestibular neurones projecting monosynaptically to the contralateral abducens motoneurones were included in this non-rhythmic population. 4. Only 2% of the recorded population fired rhythmically both during nystagmogenic stimulation and poststimulation nystagmus. These neurones showed plasticity in behaviour regarding the phases of nystagmus recorded from the contralateral abducens nerve. 5. The remaining 10.1% of the medial vestibular neurones were excited or inhibited by repetitive stimulation of the labyrinth but showed burst firing pattersn correlated with poststimulation nystagmic discharges.