Figure 2. Amygdala–medial prefrontal cortex networks engaged during resting conditions, during fear learning, following consolidation and extinction.

Different activity states of fear-related structures during fear learning and extinction evokes a distinct pattern of synaptic activity. In control conditions, fear expression is absent and activity in the medial central nucleus (CeM) neurons is low. During the acquisition of fear conditioning, conditioned (CS) and unconditioned stimulus (US) information converges on neurons in the basolateral group (BLA), leading to potentiation of CS inputs to neurons in the BLA. In turn, CeL ‘on’ cells are activated with ultimate disinhibition of CeM output neurons and an enhanced fear response. In the consolidation phase of fear conditioning, hippocampal (HPC) and prelimbic prefrontal cortex (PLPFC) activity increases and is thought to add context dependency (HPC) as well as to increase the basolateral amygdala activity for stronger fear expression. Extinction leads to suppression of the fear response through an additional pathway that suppresses the fear circuit. PLPFC activity not only decreases, but infralimbic prefrontal cortex (ILPFC) activity increases, which is thought to directly activate intercalated neurons (ITCs). This in turn leads to feed-forward inhibition of CeM output neurons and the suppression of the fear response. Interneurons are represented as black and white circles, indicating the large variety of interneurons, whereas pyramidal cells are shown as triangles. The strength of connections is represented by the width of the connecting lines with wider lines representing stronger connections. The activity of specific neuronal types is indicated as action potential spikes next to the individual regions, with two spikes representing low activity and multiple spikes representing increased activity.