Anatomical and electrophysiological identification of motoneurones supplying the cat retractor bulbi muscle.

Motoneurones innervating the retractor bulbi muscle in the cat have been identified by retrograde labelling with horseradish peroxidase, by intracellular recording and by intracellular staining with horseradish peroxidase. Their somata are found in an accessory abducens nucleus, analogous to that described in some other species, which consists of a narrow column of cells situated in the lateral tegmental reticular field, above the superior olive and medial to the facial nerve. The column of cells extends over approximately 1.5 mm from P 5.5 to P 7. The retractor bulbi motoneurones number from 80 to 120 and have large, elongated somata which give rise to five or six major dendrites. Their axons cross the reticular formation in a dorso-medial direction to pass through the principal abducens nucleus before turning to leave the brain stem in the 6th nerve. Antidromic latencies ranged from 0.4 to 0.7 ms. Some retractor bulbi motoneurones could also be activated antidromically by stimulation of the lateral rectus muscle nerve.