Map of the synapses onto layer 4 basket cells of the primary visual cortex of the cat

Abstract

The pattern of excitatory and inhibitory inputs to the inhibitory neurons is largely unknown. We have set out to quantify the major excitatory and inhibitory inputs to layer 4 basket cells from the primary visual cortex of the cat. The synapses formed with the soma, and proximal and distal dendrites, were examined at the light and electron microscopic levels in four basket cells, recorded in vivo and filled with horseradish peroxidase. The major afferents of layer 4 have been well characterised, both at the light and electron microscopic levels. The sizes of the synaptic boutons of the major excitatory inputs to layer 4 from the thalamic relay cells, spiny stellate cells, and layer 6 pyramidal neurons are statistically different. Their distributions were compared to those of the boutons forming asymmetric contacts onto the basket cells, which were assumed to be provided by the same set of excitatory afferents. The best-fit results showed that about equal numbers of synapses were provided by the layer 6 pyramids (43%) and the spiny stellates (44%), whereas the thalamic afferents contributed only 13%. A similar analysis on the symmetric synaptic input to the basket cells indicated that as much as 79% of the symmetric synapses could have originated from layer 4 basket cells. Thalamic and spiny stellate synapses were preferentially located on the soma and proximal dendrites, regions that also had 76% of all the symmetric contracts.

Multiple Clusters of Release Sites Formed by Individual Thalamic Afferents onto Cortical Interneurons Ensure Reliable Transmission

“…8), consistent with ultrastructural studies (Ahmed et al, 1997; White et al, 1984; White and Rock, 1981). Together with structural data showing that ~15% of thalamic inputs to interneurons may be axosomatic (Ahmed et al, 1997; Staiger et al, 1996), and recent findings suggesting a similar proximal bias for thalamic inputs onto cortical excitatory neurons (Richardson et al, 2009), our data highlight one of the key parameters likely to contribute to the relative strength of thalamocortical inputs despite their numerical sparseness (Gil et al, 1999; Stratford et al, 1996).…”

Spine Dynamics: Are They All the Same?

“…In the adult, very few excitatory synapses onto excitatory neurons are located on the dendritic shaft, and the vast majority of spines contain a single type 1 excitatory synapse (Harris et al, 1992; LeVay, 1973). Excitatory synapses that have been observed on the shaft are generally formed on the dendrites of inhibitory interneurons that are typically aspiny (Ahmed et al, 1997; Bock et al, 2011; Bopp et al, 2014; Buhl et al, 1997). The minority of inhibitory interneurons that contain spines, are less spiny than pyramidal neurons (Kawaguchi et al, 2006; Kuhlman and Huang, 2008).…”

Axon topography of layer 6 spiny cells to orientation map in the primary visual cortex of the cat (area 18)

“…Electron microscopy of L6 cells estimated that they account for 20–40 % of all synapses on L4 spiny stellate cells (Ahmed et al 1994, 1997; Binzegger et al 2004; Tanaka et al 2011; Pichon et al 2012). In addition to this, L6 outputs were shown to activate inhibitory neurons too (West et al 2006; Olsen et al 2012; Bortone et al 2014), although the relative proportion of the targeted excitatory and inhibitory cells is still a matter of debate (McGuire et al 1984; Somogyi 1989; Ahmed et al 1994, 1997; Staiger et al 1996). Importantly, the functional consequence of these findings is that facilitatory and suppressive inputs must go hand in hand and mutually modify the activity level of the entire L4 circuitry through which they may contribute to emerging novel receptive field properties.…”