Amygdala

The amygdala lies deep to the uncus, in front of the hippocampus and the tip of the temporal horn of the lateral ventricle. It is a collection of nuclei in three main groups:
• the basolateral group, which receives highly processed input from all sensory association cortices, the thalamus and the prefrontal cortex;
• the centromedial group, the main output nucleus to the hypothalamus and brainstem;
• the cortical (superficial) group, which receives olfactory input.

Sensory information reaches the amygdala by a fast "low road" (directly from the thalamus) and a slower but detailed "high road" (through the cortex). The amygdala attaches emotional value to these stimuli, especially threat. In Pavlovian fear conditioning, pairing a neutral stimulus with an aversive one strengthens synapses in the basolateral nucleus, so the neutral stimulus alone later evokes fear. The amygdala also strengthens hippocampal encoding of emotionally charged events, which is why such memories are vivid.

Output from the central nucleus produces the physiological components of fear:
• to the hypothalamus: sympathetic activation and CRH/cortisol release;
• to the periaqueductal grey: freezing and analgesia;
• to the parabrachial nucleus: rapid breathing;
• to the locus coeruleus: vigilance.
The main efferent tracts are the stria terminalis, which arches with the caudate to the hypothalamus and the bed nucleus, and the ventral amygdalofugal pathway. The ventromedial prefrontal cortex inhibits the amygdala during the extinction of fear.