The vibration-sensitivity of larval lamprey Mauthner (Mth) neurones is dependent on behavioural state. Animals are maximally vibration-sensitive when at rest and less so when active or aroused. To demonstrate this effect in freely behaving larvae, we provided repeated vibratory or electrical stimuli to the vestibular labyrinths while animals made transitions between rest and activity. Stimuli which were adequate to elicit Mth spikes 100% of the time in a resting animal (recorded extracellularly from the spinal cord) were consistently subthreshold while the animal was swimming. The same effect was seen in semi-intact preparations, both moving and curarized, while recording intracellularly from Mth cell bodies. Mth vibration-sensitivity decreased abruptly with the onset of ‘arousal’, defined here by the presence of tonic, descending spinal cord discharge. During arousal, the Mth soma exhibited a slight depolarization (2–8 mV), an increased membrane conductance, and a strong depression of vibration-evoked excitatory postsynaptic potential (EPSP) amplitude. This Mth PSP depression (MPD) appears to underlie altered vibration-sensitivity.

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