Efferent vestibular neurons. Electrophysiological evidence for axon collateralization to cristae ampullares in the pigeon (Columba livia).

Extracellular spikes were recorded under general anaesthesia from the cell bodies of efferent vestibular neurons located in the caudal pontine reticular nucleus of the pigeon. Discrete electrical stimuli, applied directly to the three ampullary nerve branches in one labyrinth and to the anterior ramus of the vestibular nerve trunk in the other labyrinth, evoked antidromic spikes which served to identify efferent neurons. Most cells could be antidromically driven only by stimuli to the vestibular nerve trunk (anterior ramus). The majority of cells exhibiting direct axonal connections to one individual semicircular canal crista ampullaris showed axon collateralization to one or two other cristae as well. Sixty percent of the efferent neurons responded with antidromic spikes to ipsilateral labyrinthine stimuli, 34% to contralateral stimuli, and 6% to both. Synaptic activation was observed in a few efferent and adjacent unidentified neurons. It is concluded that efferent neurons often send collaterals to various cristae in one labyrinth, and less frequently, to both labyrinths. Such projections are incompatible with the assumption that vestibular efferents provide a simple control mechanism which is related to the direction of head movement.