Neural mechanisms involved in the acquisition and extinction of conditioned fear. During fear acquisition, sensory information regarding the CS+ and US enters the basolateral amgydaloid (BLA) complex via the cortex and thalamus; the BLA is where CS-US associations are formed. The BLA sends excitatory projections to the central nucleus (CE) of the amygdala. The central nucleus controls fear expression via its projections to a number of effector sites. These include the lateral hypothalamus (LH), periaqueductal gray (PAG), and reticularis pontis caudalis (RPC), which are important for autonomic components of the fear response, freezing behavior, and startle-potentiation, respectively. The VMPFC mediates extinction of conditioned fear, possibly through its connections with intercalated cell masses (ITC). The VMPFC sends excitatory projections (+) to the ITC, which in turn send inhibitory projections (−) to the CE. Thus, the net effect of vmPFC activity is inhibition of both CE activity and the fear response. The hippocampus also sends projections to the amygdala, and has been implicated in contextual control of extinction.