The primate oculomotor system. I. Motoneurons. A synthesis of anatomical, physiological, and clinical data.

The aim of this paper is to relate recent physiological and anatomical data from Rhesus monkeys to normal function and pathology in the human oculomotor system. 1. Anatomical location of motoneurons in the oculomotor nuclei was investigated with retrograde labelling techniques, and medial rectus motoneurons were found within three different subgroups. 2. Physiological characteristics of motoneurons show a continuous distribution of different types from tonic to phasic, with tonic-phase neurons representing the majority. 3. Quantitative analysis of the firing pattern of motoneurons leads to the construction of iso-frequency curves. They show all possible eye positions for a given firing frequency in a single neuron. They give a quantitative measure for the innervation of a muscle when acting in synergy with all other muscles during fixation. The eye displacement during saccades can be described by a vector represented by a burst discharge in motoneurons, the integral of which can be used to hold the eyes in position. 4. Clinical syndromes will be discussed that can be related to anatomy and physiology.