Ultrastructure of the ventral lateral geniculate nucleus (LGNv) receiving tectal afferents in the cat.

The ultrastructure of the ventral lateral geniculate nucleus (LGNv) associated with the superior colliculus was investigated. In addition to normal synaptic morphology tectal input was examined following unilateral ablation of the superior colliculus at survival times of 2 to 12 days. Eight cats were used (six experimental, two control). Examination of the ventromedial LGNv revealed a prevalence of Flat-Symmetric (FS) synapses (58.2 +/- 1.9%), relative to the usually more abundant, Round-Asymmetric (RA) contacts (41.8 +/- 1.8%). The functional morphology of terminating tectal fibres (although infrequent) was suggestive of the FS type. Since FS connections usually serve an inhibitory purpose, it is suggested that the tectal LGNv connection has a negative modulation function in the fixation and tracking process.