Anatomy and physiology of goldfish oculomotor system. I. Structure of abducens nucleus

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Abstract
The posterior rectus muscle and the abducens nucleus, by which it is innervated, were examined by light microscopy and EM in the goldfish, Carassius auratus. The muscle is divided into 2 bands, one containing red, tonic fibers, the other containing white, phasic fibers. The abducens nucleus is also divided into 2 cell groups, containing roughly equal numbers of cells. Cells in the caudal group, whose phasic activity precedes rapid eye movements, commonly receive synaptic endings that make gap junction contacts. Cells in the rostral group, which fire only tonically during slow movements, receive few gap junction endings. Although the sizes of cells vary within each group, the distribution of cell sizes are the same for the 2 groups. Therefore, the physiological differences between the 2 groups cannot depend on differences in cell size, but, more likely, on specific differences in the distribution of synaptic input, such as demonstrated for the gap junction endings.