Anatomical schemes for the synthesis of upward and downward movements. From the vertical semicircular canals, primary afferents from CN VII synapse of the VN and ascend into the median longitudinal fasciculus (MLF) and brachium conjunctivum (not shown) to contact the CN II and CN IV nuclei and the interstitial nucleus of Cajal (INC). (For clarity, only excitatory vestibular projections are shown). Excitatory burst neurons in the rostral interstitial nucleus of the medial longitudinal fasciculus (nMLF) project to the motoneurons of the CNs III and IV and send axon collaterals to the INC. Each rlMLF neuron sends axon collaterals to yoke-pair muscles (Hering's law), III nucleus = oculomotor nerve (CN III) nucleus; IV nucleus = trochlear nerve (CN IV) nucleus; PC = posterior commissure.