Excitation by geniculocortical synapses is not 'vetoed' at the level of dendritic spines in cat visual cortex.

1. We used anatomical methods to examine whether the geniculocortical afferent input to dendritic spines could be gated or 'vetoed' by an inhibitory input to the same spine. 2. Physiologically identified X- and Y-type afferents were injected intra-axonally with horseradish peroxidase (HRP), processed, and drawn under the light microscope. Selected regions of the terminal arbors were then serially sectioned for examination under the electron microscope. 3. Three-dimensional reconstructions of thirty-nine HRP-filled terminal boutons forming fifty asymmetric (type 1) synapses showed that thirty-one synapses were on the heads of dendritic spines. Only two of thirty-one spine heads received an additional symmetric (type 2) synapse, which is presumed to be inhibitory. 4. Examination of twenty-three boutons from two clutch cells (a GABA (gamma-aminobutyric acid)-ergic smooth cell) that form symmetric (type 2) synapses on spines indicated that their preferred location was opposite the asymmetric synapse on the head of the spine. Synaptic input to the necks of spines appears rare. 5. We conclude that most of the excitation provided by the geniculocortical afferent input to the heads of spines cannot be gated or vetoed by inhibition at the level of the spine.