Accessory abducens nucleus and conditioned eye retraction/nictitating membrane extension in rabbit.

The role of accessory abducens nucleus neurons in the conditioned eye retraction/nictitating membrane extension response was defined in the rabbit. Horseradish peroxidase injections into the retractor bulbi muscle showed that accessory abducens nucleus is the principal location of its motor-neurons. Single and multiple unit recording in accessory abducens indicated that these motor neurons show a marked responsiveness to corneal and periorbital stimulation and fire in close correlation with conditioned, unconditioned, or spontaneous eye retraction/nictitating membrane extension. Complete lesions of accessory abducens showed, at most, a partial reduction of the conditioned and unconditioned eye retraction response. Section of the extraocular muscles, other than retractor bulbi, also caused a partial reduction of the eye retraction response. Accessory abducens lesions, combined with extraocular muscle section, were necessary to dramatically reduce the eye retraction response permanently. These experiments demonstrated that accessory abducens is a primary controller of eye retraction through its axons to retractor bulbi. The other extraocular muscles act in concert with retractor bulbi to elicit conditioned and unconditioned eye retractions.