2022 年 81 巻 2 号 p. 67-78
It is generally accepted that voluntary quick eye movements (saccades) are organized in the horizontal and vertical eye movement systems. Therefore, oblique saccades are considered to be generated by the vector sum of the outputs from the coordinated horizontal and vertical saccade systems, but the precise neural mechanism of generation of oblique saccades remains unresolved. The superior colliculus (SC) is known to be the center for saccades. Our previous studies show that many efferent neurons in the SC project to the midbrain and the pontine reticular formation, but their branching patterns and exact terminal areas in the brainstem are not known. In this study, we used an electrophysiological method for investigating branching patterns of single tectofugal neurons that project to various last-order premotor neurons for horizontal and vertical saccades in the brainstem of the cat. We recorded antidromic spikes and examined the effects of stimulation of the vertical and horizontal last-order premotor neuron areas (Forel's field H, FFH; excitatory burst neuron (EBN) region and inhibitory burst neuron (IBN) region for horizontal saccades). The results showed that there are four types of branching patterns of single tectofugal neurons that project to the last-order horizontal and vertical premotor neuron areas, and the upper cervical spinal cord. Among these, two types of the tectofugal neurons (about 69%) have axonal branches that project to the ipsilateral FFH and the contralateral EBN region, and furthermore, about 69% of them have another axonal branch that extends to the spinal cord. This finding indicates that single tectofugal neurons innervating both horizontal and vertical saccade generators with their axonal collaterals can easily synchronize the onsets of the horizontal and vertical saccade generating systems, and provides evidence that the branching patterns of single tectofugal neurons determine the functional synergies for coordinated oblique eye and head movements.
