POLYNEURONAL INNERVATION OF SPINY STELLATE NEURONS IN CAT VISUAL-CORTEX

Our hypothesis was that spiny stellate neurons in layer 4 of cat visua l cortex receive polyneuronal innervation. We characterised the synaps es of four likely sources of innervation by three simple criteria: the type of synapse, the target (spine, dendritic shaft), and the area of the presynaptic bouton. The layer 6 pyramids had the smallest boutons and formed asymmetric synapses mainly with the dendritic shaft. The t halamic afferents had the largest boutons and formed asymmetric synaps es mainly with spines. The spiny stellates had medium-sized boutons an d formed asymmetric synapses mainly with spines. We used these to make a ''template'' to match against the boutons forming synapses with the spiny stellate dendrite. Of the asymmetric synapses, 45% could have c ome from layer 6 pyramidal neurons, 28% from spiny stellate neurons, a nd 6% from thalamic afferents. The remaining 21% of asymmetric synapse s could not be accounted for without assuming some additional selectiv ity of the presynaptic axons. Additional asymmetric synapses may come from a variety of sources, including other cortical neurons and subcor tical nuclei such as the claustrum. Of the symmetric synapses, 84% cou ld have been provided by clutch cells, which form large boutons. The r emainder, formed by small boutons, probably come from other smooth neu rons in layer 4, e.g., neurogliaform and bitufted neurons. Our analysi s supports the hypothesis that the spiny stellate receives polyneurona l innervation, perhaps from all the sources of boutons in layer 4. Alt hough layer 4 is the major recipient of thalamic afferents, our result s show that they form only a few percent of the synapses of layer 4 sp iny stellate neurons, (C) 1994 Wiley-Liss, Inc.