Physiological and anatomical evidence for an inhibitory trigemino-oculomotor pathway in the cat.

During blink down-phase, the levator palpebrae superioris (levator) muscle is inactivated, allowing the orbicularis oculi muscle to act. For trigeminal reflex blinks, the excitatory connections from trigeminal sensory nuclei to the facial nucleus have been described, but the pathway whereby the levator is turned off have not. We examined this question by use of both physiological and anatomical approaches in the cat. Intracellular records from antidromically activated levator motoneurons revealed that periorbital electrical stimulation produced bilateral, long latency inhibitory postsynaptic potentials (IPSPs). Central electrical stimulation of the principal trigeminal nucleus produced shorter latency IPSPs. Intracellular staining revealed that these motoneurons reside in the caudal central subdivision and have 10 or more poorly branched dendrites, which extend bilaterally into the surrounding supraoculomotor area. Axons penetrated in this region could be activated from periorbital and central electrodes. Neurons labeled from tracer injections into the caudal oculomotor complex were distributed in a crescent-shaped band that lined the ventral and rostral aspects of the pontine trigeminal sensory nucleus. Double-label immunohistochemical procedures demonstrated that these cells were not tyrosine hydroxylase-positive cells in the Kölliker-Fuse area. Instead, supraorbital nerve afferents displayed a similar crescent-shaped distribution, suggesting they drive these trigemino-oculomotor neurons. Anterograde labeling of the trigemino-oculomotor projection indicates that it terminates bilaterally, in and above the caudal central subdivision. These results characterize a trigemino-oculomotor pathway that inhibits levator palpebrae motoneurons in response to blink-producing periorbital stimuli. The bilateral distributions of trigemino-oculomotor afferents, levator motoneurons, and their dendrites supply a morphological basis for conjugate lid movements.