Drawing of the brainstem of a monkey, showing the locations of areas discussed in the main text. III, oculomotor nucleus; IV, trochlear nucleus; VI, abducens nucleus; Med. RF, medullary reticular formation; MRF, midbrain reticular formation; NIC, interstitial nucleus of Cajal; NPH, nucleus prepositus hypoglossi; PPRF, paramedian pontine reticular formation; riMLF, rostral interstitial nucleus of the medial longitudinal fasciculus; SC, superior colliculus. Modified, with permission, from Ref. 13 © 1982 Springer-Verlag. b | A diagram of the connections of the cell types that are crucial components of models of the horizontal burst generator. VI, abducens motor neuron; EBN, excitatory burst neuron; IBN, inhibitory burst neuron; LLBN, long-lead burst neuron; NPH/MVN, cells in nucleus prepositus hypoglossi or medial vestibular nucleus; OPN, omnipause neuron; SC, superior colliculus. Note the crucial role of the OPNs. These cells inhibit the EBNs, which innervate the motor neurons, and the IBNs, which inhibit motor neurons that innervate antagonistic muscles. Saccades are initiated by a trigger signal (Tr) that inhibits the OPNs. The OPNs are prevented from resuming their tonic discharge during the generation of the saccade command by the activity of latch neurons (La), inhibitory interneurons that relay an inverted output of the EBN burst. Modified, with permission, from Ref. 14 © 1996 Elsevier Science.