Fig. 3. Anatomic schemes for the synthesis of upward, downward, and torsional eye movements. From the vertical semicircular canals, primary afferents on the vestibular nerve (vn) synapse in the vestibular nuclei (VN) and ascend in the medial longitudinal fasciculus (MLF) and brachium conjunctivum (not shown) to contact neurons in the trochlear nucleus (CN IV), oculomotor nucleus (CN III), and the interstitial nucleus of Cajal (INC). The rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), which lies in the prerubral fields, contains saccadic burst neurons. It receives an inhibitory input from omnipause neurons of the nucleus raphe interpositus (rip), which lie in the pons (for clarity, this projection is only shown for upward movements). Excitatory burst neurons in riMLF project to the motoneurons of CN III and CN IV, and also send an axon collateral to INC. Each riMLF neuron sends axon collaterals to yoke-pair muscles (Hering's law). Projections to the elevator subnuclei (innervating the superior rectus and inferior oblique muscles) may be bilateral due to axon collaterals crossing at the level of the CN III nucleus. Projections of inhibitory burst neurons are less well understood, and are not shown. The INC provides a gaze-holding signal, and projects to vertical motoneurons via the posterior commissure. Signals contributing to vertical smooth pursuit and eye–head tracking reach CN III from the y-group via the brachium conjunctivum and a crossing ventral tegmental tract. Neurotransmitters: asp, aspartate; glu, glutamate; gly, glycine. (Modified from Leigh RJ, Zee DS. The Neurology of Eye Movements, 3rd ed. New York: Oxford University Press, 1999)