Basolateral amygdaloid nuclei

The basolateral amygdaloid nuclei (a.k.a. the basolateral nuclear complex or BNC) form a major part of the amygdala and lie deep to the cortical and central amygdaloid nuclei. They consists principally of the lateral, basolateral (basal) and basomedial (accessory basal) nuclei, each of which may be further subdivided.

Functional role of basolateral amygdaloid nuclei

A major distinguishing feature of the basolateral amygdaloid nuclei is their extensive reciprocal connectivity with higher-order areas of the cerebral cortex, particularly the prefrontal, temporal and insular cortices, as well as the hippocampal region. They are a major recipient of highly processed sensory information. Visual, auditory, somatosensory, gustatory and visceral information reaches different parts of the basolateral nuclear complex predominantly from higher-order sensory association cortices, allowing sensory experiences to acquire emotional and motivational significance. The olfactory information reaches the basolateral amygdaloid nuclei mainly indirectly through olfactory-related cortical and amygdaloid regions rather than directly from the olfactory bulb.

The basolateral amygdaloid nuclei contain two major neuronal populations.

• Approximately 80–90% are pyramidal or semipyramidal principal neurons, which are predominantly glutamatergic projection neurons.

• The remaining population consists largely of GABAergic non-pyramidal interneurons, which regulate the excitability and synaptic activity of the principal neurons.

Functionally, the basolateral amygdaloid nuclei act as an important interface between sensory processing and emotional responses. Sensory and contextual information can converge upon their principal neurons, where changes in synaptic strength allow associations to develop between a stimulus and its emotional significance. Their outputs reach other components of the amygdala, particularly the central nucleus, as well as the bed nucleus of the stria terminalis and ventral striatum. These structures can subsequently influence hypothalamic and brainstem systems responsible for the somatomotor, autonomic and endocrine components of emotional behaviour.

Consequently, basolateral amygdaloid nuclear circuits have important roles in emotional learning and memory, particularly fear conditioning, fear extinction and anxiety, but they are not restricted to aversive responses and also participate in appetitive conditioning. For example, during auditory fear conditioning, auditory and aversive inputs converge on principal neurons of the lateral nucleus, and strengthening of relevant excitatory inputs contributes to formation of the learned association. Thus, rather than simply generating an emotional response itself, the basolateral complex helps determine what a sensory experience means emotionally and communicates that information to systems that organise the appropriate behavioural and physiological response.