FIG. 1.

IL regulation of fear memory extinction. Schematic drawing of layer II/III and layer V pyramidal neurons within the IL. Based on cortical neuroanatomy, layer II/III and layer V neurons receive limbic inputs (from the amygdala, hippocampus, entorhinal cortex, and cingulate cortex) to the distal portions of their apical dendrites, whereas thalamic input is received on proximal apical and basal dendrites. Spines (filled red circles) cover the dendrites, an enlarged view of which is shown to the right. During extinction, layer II/III neurons fire (indicated by train of action potentials) upon exposure to the CS alone. The excitatory output of these neurons directly and/or indirectly activate GABAergic neurons that comprise the intercalated cells of the amygdala (ITC). The ITC, in turn, inhibits the central nucleus of the amygdala (CeA). Reduced CeA activity (indicated by action potentials) results in less stimulation of the lateral hypothalamus (LH), the paraventricular nucleus (PVN), and the central gray (CG), thereby reducing the physical manifestations of fear. BLA, basolateral amygdala; IL, infralimbic cortex.