Studies on a primitive cerebellar cortex III. The projection of the anterior lateral-line nerve to the lateral-line lobes of the dogfish brain

Field potentials and unit discharges generated by electrical stimulation of the anterior lateral-line nerves were recorded from the ipsilateral lateral-line lobes of the hindbrain of decerebrate dogfish. They were absent from the cerebellar corpus and the contralateral hindbrain. The field potentials were positive-going within the molecular layer and negative-going in the underlying dorsal nucleus. They were preceded by compound action potentials of sensory fibres and of antidromically activated lateral-line efferent neurones. The earliest part of the main field probably represents the monosynaptic activation of second-order afferent cell bodies. It was closely followed by the antidromic invasion of the cell bodies and dendrites of the efferent neurones. Later portions of the potentials were created by repetitive activity of the second order afferent cells and of the efferent neurones. It is thought that the dendrites of the second order cells propagated active potentials from the cell body into the molecular layer at velocities of about 0.5 m s-1. The unit discharges were identified as arising from: (i) primary afferent fibres, (ii) second order afferent neurones, and (iii) efferent neurones. Primary afferent fibres, which frequently discharged in phase with respiratory movements, responded vigorously to natural stimulation of the lateral lines of the head. Secondary afferent cells did not show a respiratory rhythm, probably because lateral-line stimulation was followed by a sharp fall in excitability which began 15 ms after the stimulus and lasted for at least 30 ms. Complete recovery from a single lateral-line stimulus took as long as 100 ms. Some nerve cells, which could follow high-frequency stimulation at short latency without ‘jitter’, also discharged up to 4 later spikes to a single stimulus if stimuli were applied at rates of < 0.3 Hz. This discharge was evidently synaptically derived and would follow higher stimulation frequencies (1 Hz) if presented with paired stimuli. Such neurones must be efferent cells of the lateral-line system.