Amygdala

The amygdala is a complex of several nuclei in the rostromedial part of the temporal lobe (Figs. 1 and 2). There are several deep nuclei (lateral nucleus, basal nucleus, and accessory basal nucleus), which are substantially interconnected with the temporal, insular, and frontal cortex, the striatum, and the mediodorsal thalamus. On the surface are a number of modified cortical areas, many of which are interconnected with the olfactory system. In the dorsal part of the amygdala are the central nucleus and medial nucleus, which have connections with the hypothalamus and autonomic brain stem nuclei.

The amygdala receives input from all the sensory systems. In lower mammals, these are dominated by inputs from the olfactory system, although there are also inputs from the taste/visceral afferent system and the visual and somatosensory systems. In primates the most prominent inputs are derived from higher order sensory association cortex, especially from the visual areas in the inferior temporal cortex (i.e., the temporal visual processing stream, important for analysis of form and color and recognition of complex stimuli

Parabrachial Nucleus

Figure 1 A photomicrograph of the amygdala, hippocampus, and parahippocampal gyrus from a human brain. BNM-basal nucleus of Meynert, ac-anterior commisure. The amygdala includes the lateral nucleus (L), basal nucleus (B), accessory basal nucleus (AB), central nucleus (Ce), and medial nucleus (Me). The hippocampus, located posterior to the amygdala in the medial temporal lobe, includes the parasubiculum (Para-S), presubiculum (Pre-S), subiculum (Sub), fields CA1 and CA3, and the dentate gyrus (DG). The entorhinal cortex (EC) occupies the parahippocampal gyrus ventral to the amygdala and hippocampus.

Figure 1 A photomicrograph of the amygdala, hippocampus, and parahippocampal gyrus from a human brain. BNM-basal nucleus of Meynert, ac-anterior commisure. The amygdala includes the lateral nucleus (L), basal nucleus (B), accessory basal nucleus (AB), central nucleus (Ce), and medial nucleus (Me). The hippocampus, located posterior to the amygdala in the medial temporal lobe, includes the parasubiculum (Para-S), presubiculum (Pre-S), subiculum (Sub), fields CA1 and CA3, and the dentate gyrus (DG). The entorhinal cortex (EC) occupies the parahippocampal gyrus ventral to the amygdala and hippocampus.

Figure 2 Photographs of the amygdala and entorhinal cortex (EC) of a macaque monkey, stained for acetylcholinesterase (left) and cells (Nissl method) (right). Note the component nuclei of the amygdala, including the basal nucleus (B), lateral nucleus (L), accessory basal nucleus (AB), central nucleus (Ce), and medial nucleus (Me). Also note the cholinergic nucleus basalis of Meynert (NBM), ventral to the anterior commissure (AC), the supraoptic nucleus (SON), the optic chiasm (OC), and the third ventricle (III).

Figure 2 Photographs of the amygdala and entorhinal cortex (EC) of a macaque monkey, stained for acetylcholinesterase (left) and cells (Nissl method) (right). Note the component nuclei of the amygdala, including the basal nucleus (B), lateral nucleus (L), accessory basal nucleus (AB), central nucleus (Ce), and medial nucleus (Me). Also note the cholinergic nucleus basalis of Meynert (NBM), ventral to the anterior commissure (AC), the supraoptic nucleus (SON), the optic chiasm (OC), and the third ventricle (III).

such as faces) (Fig. 3). Recordings from the amygdala show that the neurons respond to complex sensory stimuli, including visual stimuli such as faces, as well as to other sensory modalities. In addition to the sensory aspects of the stimuli, the responses are influenced by the novelty of the stimulus or its affective sign (whether the stimulus is rewarding or aversive).

There is a complex system of intraamygdaloid connections that associate these inputs. These connect the deep amygdaloid nuclei (which interact with the cortex) with the central and medial nuclei (which connect to the hypothalamus and brain stem). There are also extensive connections to other limbic areas, including the hippocampus, parahippocampal gyrus, and the nucleus basalis of Meynert.

The outputs from the amygdala can be divided into three categories (Fig. 3):

1. Return projections back to the sensory areas that project into the amygdala: In the case of the visual system, these return projections even extend back to the primary visual cortex.

2. Descending projections to the visceral control centers of the hypothalamus and brain stem: The central amygdaloid nucleus projects to a wide variety of autonomic-related cell groups, including the lateral hypothalamus, the periaqueductal gray, the parabrachial nucleus, the nucleus of the solitary tract, the dorsal vagal nucleus, and the ventrolateral medulla. Through these projections, the amygdala can influence heart rate and blood pressure, gut and bowel function, respiratory function, bladder function, etc. For example, stimulation of the central nucleus can cause stomach ulcers as well as changes in cardiovascular function.

3. Interactions with the orbital and medial frontal cortex, both via direct amygdalocortical projections and via connections with the mediodorsal thalamic nucleus and the ventromedial parts of the basal ganglia: This circuit appears to be involved in determination of the affective "sign" of sensory stimuli (e.g., whether it is rewarding or aversive) and in setting mood.

Several additional observations may be used to illustrate the role of the amydala in the control of emotional behavior. Bilateral lesions of the amygdala and adjacent medial temporal structures in monkeys

Amygdala Nuclei And Connections

Figure 3 Schematic summary of the axonal connections of the amygdala. (Left) Inputs from sensory association cortical areas and output from the central and medial nuclei to the hypothalamus and brain stem. (Right) Connections are illustrated with the mediodorsal thalamus (MDm), the ventromedial striatum [accumbens (Acc) and caudate nuclei (Ca)], ventral pallidum (VP), and the orbital and medial prefrontal cortex (Orb/Med PFC) as well as with the hippocampal formation. The connections from Ca/Acc to VP and from VP to MDm are shown with arrowheads to indicate that they are inhibitory.

Figure 3 Schematic summary of the axonal connections of the amygdala. (Left) Inputs from sensory association cortical areas and output from the central and medial nuclei to the hypothalamus and brain stem. (Right) Connections are illustrated with the mediodorsal thalamus (MDm), the ventromedial striatum [accumbens (Acc) and caudate nuclei (Ca)], ventral pallidum (VP), and the orbital and medial prefrontal cortex (Orb/Med PFC) as well as with the hippocampal formation. The connections from Ca/Acc to VP and from VP to MDm are shown with arrowheads to indicate that they are inhibitory.

produce ''psychic blindness'' (Kluver-Bucy syndrome) in which the animals can see objects but are apparently unable to distinguish their significance. Such animals are not capable of appropriate social behavior, and in group settings they become isolated and solitary.

In rats, bilateral lesions of the amygdala block the acquisition of conditioned fear responses. For example, if a light is consistently coupled with a painful foot shock, the animal comes to associate the two stimuli, and the light becomes a ''fearful'' stimulus that can produce autonomic and behavioral responses characteristic of fear. After bilateral lesions of the amygdala, the shock produces the same response as before, but the light never becomes a fearful stimulus. Recent observations indicate that a similar deficit is present in humans who have a rare disease that results in bilateral amygdaloid destruction.

Stimulation of the amygdala in awake cats produces a defense reaction in which there are integrated behavioral and visceral changes that prepare the animal for fight or flight. Stimulation of the amgydala or hippocampus in human patients (during exploratory surgery for severe epilepsy) evokes complex sensory and experiental phenomena, often involving fear and including sensory hallucinations, feelings of deja vu, and memory-like episodes that resemble static dreams.

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