Synaptic organization of cat accessory abducens nucleus

Abstract

Intracellular records were obtained from motoneurons in the accessory abducens (AcAbd), abducens, and oculomotor nuclei following electrical stimulation of the IIIrd, Vth, VIth, VIIth and VIIIth nerves. Accessory abducens motoneurons were antidromically activated by stimulation of the VIth nerve with latencies ranging from 0.3-1.0 ms. Neither morphological nor physiological evidence for axon collaterals was observed within the brain stem. Large-amplitude disynaptic EPSP [excitatory post synaptic potentials] (10 mV) were recorded in accessory abducens motoneurons following electrical stimulation of the ophthalmic and maxillary divisions of the Vth nerve (1.6-2.0 ms), the ipsilateral cornea (2.0-2.8 ms), and the contralateral cornea (2.4-3.5 ms). By comparing central versus peripheral stimulation of the VIth nerve, it was shown that most, if not all, of these afferents reach the brain stem via the Vth nerve. Condition-test stimulation of either the ipsi- or contralateral cornea produced a prolonged 10- to 140-ms depression of the test EPSP. Trigeminal input was distributed extensively on the proximal and distal dendritic tree of accessory abducens motoneurons and its high synaptic efficacy was indicated by the frequent occurrence of partial and full-size dendritic spikes. Short-latency trigeminal excitation comparable to that recorded in accessory abducens motoneurons was never found in any oculomotor and abducens motoneurons following corneal and/or trigeminal stimulation. Weak ipsilateral disynaptic vestibular inhibition (latency 1.3-1.9 ms) was detected in accessory abducens motoneurons and confirmed by intracellular Cl application. Neither ipsilateral disynaptic utricular EPSP nor contralateral vestibular semicircular canal EPSP were observed in accessory abducens motoneurons at stimulation intensities that were effective in producing synaptic responses on oculomotor and abducens motoneurons. The clear physiological differences between vestibular and trigeminal synaptic effects in abducens, oculomotor and accessory abducens motoneurons lead to the hypothesis that the accessory abducens nucleus is uniquely employed for retraction of the globe and that motoneurons in the abducens and oculomotor nucleus are responsible for patterned (i.e., rotational) eye movement responses observed in all the extraocular muscles, including the retractor bulbi.