The Amygdala and Emotional Memory

The amygdala is a complicated collection of nuclei found in the medial portion of the temporal lobe. It is a phylogenetically ancient structure having been identified in the brains of amphibians, reptiles and birds as well as in all mammals.

Developmental origin of the amygdala

The amygdala arises at the lateral edge of the cortex. It is the fusion of 3 very different regions: olfactory cortex (pallium), basal ganglia (sub-pallium) and claustrum. The claustrum is also called ventral pallium and its embryological origin is not well understood.

Anatomy of the amygdala 1

Anatomy of the amygdala 2

Functional organization of the amygdala

Fear conditioning and the startle reflex

Many studies in primates and rodents suggested that the amygdala was involved in the expression of emotions: specifically fear and aggression.

Two behavioral paradigms were developed to study this in a controlled manner: fear conditioning and fear potentiated startle.

Fear conditioning 1. When a rat is given an electrical shock to its feet it will freeze (UCS and UCR).2. A tone (CS) is then paired with the shock.3. After the pairing the tone will by itself elicit freezing (CR).This effect is blocked by lesions of the amygdala (LA). The learning is prevented by

blocking NMDA receptors in the amygdala.Fear potentiated startle1. A rat will startle in response to a noise tone- baseline startle response.2. Now the rat is presented with a light paired with foot shock as above.3. Finally the tone is presented in the presence of the light.4. The startle response to the tone is greatly increased.This effect is again dependent on the amygdala (lateral, basal and central) and the

learning is blocked by antagonists of the NMDA receptor.

So the amygdala is essential for learned fear and synaptic plasticity mediated by NMDA receptors is vital for this learning.

The amygdala and Fear conditioning 1

A schematic of how auditory input will cause emotional responses of various kinds after pairing with an aversive stimulus.

The amygdala and Fear conditioning 2

A more detailed circuit for the routes by which auditory input can be linked to emotional behavior and the sites of plasticity within the amygdala.

Cellular substrates for amygdalar plasticity 1

Pairing tone and shock causes increased firing of basolateral amygdala neurons to the tone presented by itself. These recordings are after 1 trial learning: a single pairing of the tone and footshock was given and still produced these dramatic effects.

Cellular substrates for amygdalar plasticity 2

High frequency stimulation of the thalamic inputs to the amygdala induces LTP. The BL stimulation serves as a control.So the cellular basis for learning and memory are present in the amygdala.