How Thalamus Connects to Spiny Stellate Cells in the Cat's Visual Cortex

Abstract

In the cat's visual cortex, the responses of simple cells seem to be totally determined by their thalamic input, yet only a few percent of the excitatory synapses in layer 4 arise from the thalamus. To resolve this discrepancy between structure and function, we used correlated light and electron microscopy to search individual spiny stellate cells (simple cells) for possible structural features that would explain the biophysical efficacy of the thalamic input, such as synaptic location on dendrites, size of postsynaptic densities, and postsynaptic targets. We find that thalamic axons form a small number of synapses with the spiny stellates (188 on average), that the median size of the synapses is slightly larger than that of other synapses on the dendrites of spiny stellates, that they are not located particularly proximal to the soma, and that they do not cluster on the dendrites. These findings point to alternative mechanisms, such as synchronous activation of the sparse thalamic synapses to boost the efficacy of the thalamic input. The results also support the idea that the thalamic input does not by itself determine the cortical response of spiny stellate cells, allowing the cortical microcircuit to amplify and modulate its response according to the particular context and computation being performed.

Large-scale automatic reconstruction of neuronal processes from electron microscopy images

“…Without staining these cell organelles, identification of synapses relies entirely on geometrical features, like the apposition of spines and boutons. da Costa and Martin (2011) have shown that geometrical features are not sufficient to identify synapses. In this paper we provide long range reconstructions with conventional osmium stained images, preserving all structural information for biological analysis of the data, such as synapse identification or automatic mitochondria reconstruction (Lucchi et al, 2012; Giuly et al, 2012).…”

Generation of dense statistical connectomes from sparse morphological data

“…Often, these approaches are combined with labeling the recorded neurons, allowing for reconstruction of the respective soma locations, dendrite morphologies and putative contact sites at light-microscopic levels (Feldmeyer et al, 1999, 2002; Sun et al, 2006; Frick et al, 2008; da Costa and Martin, 2011). Paired recording/reconstruction approaches are however limited to acute brain slices in vitro , where slice thicknesses of 300 μm result in substantial cutting of dendrites (Oberlaender et al, 2012a) and axons (Oberlaender et al, 2011), limiting these measurements to close-by neurons.…”

Input clustering in the normal and learned circuits of adult barn owls

“…Eight groups found evidence consistent with the predictions (McBride et al, 2008, Kleindienst et al, Makino and Malinow, 2011, Chen et al, 2012, Fu et al, 2012, Takahashi et al, 2012, Rah et al, 2013, Druckmann et al, 2014), while two groups did not (Jia et al, 2010, Chen et al, 2011, da Costa and Martin, 2011, Varga et al, 2011). This issue is not resolved and multiple interpretations are still valid (reviewed in DeBello et al, 2014).…”

“…Immunoelectron microscopy of representative contacts in owl ICX revealed true synapses made between tracer-labeled axons and the dendrites of CaMKII+ neurons (Rodriguez-Contreras et al, 2005). In a larger-scale study of thalamic synapses made onto layer 4 spiny stellate cells in cat visual cortex, contacts were identified at the LM level whenever a gap could not be discerned between axon and dendrite (da Costa and Martin, 2011). At the EM level, fully half of the false positives were found not to be in contact.…”