Synaptic Microcircuits in the Barrel Cortex

Abstract

An elementary feature of sensory cortices is thought to be their organisation into functional signal-processing units called 'cortical columns'. These elementary units process sensory information arriving from peripheral receptors; they are vertically oriented throughout all cortical layers and contain several thousands of excitatory and inhibitory synaptic connections. To understand how sensory signals are transformed into electrical activity in the neocortex it is necessary to elucidate the spatial-temporal dynamics of cortical signal processing and the underlying neurons and synaptic 'microcircuits'.In the somatosensory barrel cortex there appears to be a structural correlate for the 'functional' cortical column. Therefore, it has become an attractive model system to study the synaptic microcircuitry in the neocortex. Although many synaptic connections in whisker-related cortical 'columns' have been characterised over the past years our knowledge is far from complete, in particular with respect to inhibitory connections. In this chapter we will summarise recent data on different excitatory and inhibitory synaptic connections in a whisker-related 'column' of the somatosensory cortex and try to outline their function in the neuronal network. This requires an appreciation of the diverse types of excitatory and inhibitory neurons and their function within cortical columns and beyond. When necessary, we will also discuss the synaptic input from and to subcortical structures, in particular the thalamus. However, we will not provide a detailed description of the functional mechanisms of these connections; this is beyond the scope of this chapter.

Laminar‐specific encoding of texture elements in rat barrel cortex

“…Thus, despite apparently similar neural operations and structural anatomy in the two layers (Radnikow et al . ), the effects of a preceding active‐sensing whisker protraction are markedly different in these two layers.…”

“…; Fioravante & Regehr, ; Radnikow et al . ) and appears to act functionally to allow neurons to alter their firing rate to match statistical properties of the stimulus and to allow novel feature detection (Adibi et al . ; Webster, ).…”

“…; Fioravante & Regehr, ; Radnikow et al . ). Previous studies have used repetitive sinusoid, pulsatile and air puff stimuli (Ahissar et al .…”

“…), decrease with depth from L2 to D3, alter firing patterns via feedforward/feedback inhibition and disinhibition (Radnikow et al . ), and predominantly attenuate the incoming whisker‐driven signal. Despite the majority of L2 innervation arriving via L4 spiny neurons, most L2/3 inhibition is initiated via local neuron/interneuron feedback (Radnikow et al .…”

“…; Boloori & Stanley, ; Radnikow et al . ): pooled responses from L2/3 show stronger adaptation than cortical layers L4/L5 (Ahissar et al . ; Derdikman et al .…”