Central trigeminal and posterior eighth nerve projections in the turtle Chrysemys picta studied in vitro

Recent electrophysiological studies in the turtle Chrysemys picta have suggested that a neural correlate of the eye-blink reflex can be evoked in an in vitro brain-stem-cerebellum preparation by electrical rather than natural stimulation of the cranial nerves. Discharge recorded in the abducens nerve, which is similar to EMG recordings from extraocular muscles during eye retraction, is triggered by a brief electrical stimulus applied to the ipsilateral trigeminal nerve. Evidence also suggests that pairing a one-second electrical stimulus applied to the posterior eighth nerve immediately prior to a single shock to the trigeminal nerve results in classically conditioned abducens nerve discharge in response to the previously neutral eighth nerve stimulus. In view of these physiological findings, the aim of the present study was to examine the central projections of trigeminal and posterior eighth nerve inputs to elucidate the anatomical substrates that may underlie the in vitro eye-blink reflex and possible pathways involved in reflex conditioning. Neurobiotin (NB) or fluorescein dextran (FD) was pressure injected into the cut end of either the trigeminal or posterior eighth nerve of the in vitro brainstem-cerebellum. Following trigeminal nerve injections, both tracers showed label in the ipsilateral trigeminal nuclear complex. Direct projections to the ipsilateral principal and accessory abducens motor nuclei were observed, suggesting that the eye-blink reflex is monosynaptic. Trigeminal nerve axons were also observed to terminate in the ipsilateral cerebellar cortex. The results of the posterior eighth nerve injections showed axonal projections and terminals in the cochlear, vestibular and principal sensory trigeminal nuclei. Terminal label was also observed in the ipsilateral cerebellar cortex, deep cerebellar nuclei, and in the principal and accessory abducens motor nuclei. Results from the NB cases suggested transneuronal transport of this tracer substance, whereas the FD cases did not. The present findings suggest that convergence of trigeminal and posterior eighth nerve inputs occurs in the ipsilateral cerebellar cortex, the principal sensory trigeminal nucleus, and the principal and accessory abducens motor nuclei. These regions of convergence may therefore be considered as potential sites of synaptic modification during in vitro studies of the conditioned abducens nerve reflex.