Mapping the oculomotor system

Over the last three decades and together with Bernard Cohen, Volker Henn, Ulrich Büttner, and Anja Horn, it has been possible to morphologically identify several functional cell groups in the oculomotor system: the medium-sized horizontal excitatory and inhibitory burst neurons (EBNs, IBNs) in the paramedian pontine reticular formation (PPRF), the more sparsely scattered vertical EBNs in the rostral interstitial nucleus of the MLF (RIMLF), and the typically elongated omnipause neurons (OPNs) in nucleus raphé interpositus--all essential for the generation of saccades. In contrast, the role of the central mesencephalic reticular formation (cMRF) in saccades is more complex, as is the morphological outlining of its borders. A detailed study of the extraocular motoneurons showed that they can be divided into two separate types: those for singly innervated (twitch) muscle fibres (SIFs) and those for multiply innervated (non-twitch) muscle fibres (MIFs). The two motoneuron types receive different premotor afferents, proving that MIF and SIF motoneurons have different functions. The cell groups were outlined by different tract tracing methods including rabies virus. The localization and histochemical characterization of all these functional cell groups in monkey formed the basis for the identification of the homologous groups in the human brainstem. Taken together these studies provide a neuroanatomical background for understanding clinical eye movement disorders.