Amygdala

The amygdalae (Latin, corpus amygdaloideum, singular, amygdala) are almond-shaped groups of neurons located deep in the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.

Anatomical subdivisions

The regions described as amygdalae encompass several nuclei with distinct functional traits. Among these nuclei are the basolateral complex, the centromedial nucleus and the cortical nucleus. The basolateral complex can be further subdivided in to the lateral, the basal and the accessory basal nuclei.

Connections

The amygdalae send impulses to the hypothalamus for activation of the sympathetic nervous system, to the reticular nucleus for increased reflexes, to the nuclei of the trigeminal nerve and facial nerve for facial expressions of fear, and to the ventral tegmental area, locus ceruleus, and laterodorsal tegmental nucleus for activation of dopamine, norepinephrine and epinephrine.

The cortical nucleus is involved in the sense of smell and pheromone-processing. It receives input from the olfactory bulb and olfactory cortex. The lateral amygdalae, which send impulses to the rest of the basolateral complexes and to the centromedial nuclei, receive input from the sensory systems. The centromedial nuclei are the main outputs for the basolateral complexes, and are involved in emotional arousal in rats and cats.

Emotional learning

In complex vertebrates, including humans, the amygdalae perform primary roles in the formation and storage of memories associated with emotional events. Research indicates that, during fear conditioning, sensory stimuli reach the basolateral complexes of the amygdalae, particularly the lateral nuclei, where they form associations with memories of the stimuli. The association between stimuli and the aversive events they predict may be mediated by long-term potentiation, a lingering potential for affected synapses to react more readily. J Am Acad Child Adolesc Psychiatry,42:5,612-615 May 2003 "The amygdala is the primary brain region involved in fear-conditioned learning"

Memories of emotional experiences imprinted in reactions of synapses in the lateral nuclei elicit fear behavior through connections with the central nucleus of the amygdalae. The central nuclei are involved in the genesis of many fear responses, including freezing (immobility), tachycardia (rapid heartbeat), increased respiration, and stress-hormone release. Damage to the amygdalae impairs both the acquisition and expression of Pavlovian fear conditioning, a form of classical conditioning of emotional responses. ibid

The amygdala are also involved in apetitive (positive) conditioning. It seems that distinct neurons respond to positive and negative stimuli, but there is no clustering of these distinct neurons into clear anatomical nucleiNature Volume 439, 16 February 2006, page 865 "The primate amygdala represents the positive and negative value of visual stimuli during learning.

Memory modulation

The amygdalae also are involved in the modulation of memory consolidation. Following any learning event, the long-term memory for the event is not instantaneously formed. Rather, information regarding the event is slowly assimilated into long-term storage over time, a process referred to as memory consolidation, until it reaches a relatively permanent state.

During the consolidation period, the memory can be modulated. In particular, it appears that emotional arousal following the learning event influences the strength of the subsequent memory for that event. Greater emotional arousal following a learning event enhances a person's retention of that event. Experiments have shown that administration of stress hormones to individuals immediately after they learn something enhances their retention when they are tested two weeks later.

The amygdalae, especially the basolateral nuclei, are involved in mediating the effects of emotional arousal on the strength of the memory for the event, as shown by many laboratories including that of James McGaugh. These laboratories have trained animals on a variety of learning tasks and found that drugs injected into the amygdala after training affect the animals' subsequent retention of the task. These tasks include basic Pavlovian tasks such as inhibitory avoidance, where a rat learns to associate a mild footshock with a particular compartment of an apparatus, and more complex tasks such as spatial or cued water maze, where a rat learns to swim to a platform to escape the water. If a drug that activates the amygdalae is injected into the amygdalae, the animals had better memory for the training in the task. If a drug that inactivates the amygdalae is injected, the animals had impaired memory for the task.

Despite the importance of the amygdalae in modulating memory consolidation, however, learning can occur without it, though such learning appears to be impaired, as in fear conditioning impairments following amygdalar damage.

Evidence from work with humans indicates that the amygdala plays a similar role. Amygdala activity at the time of encoding information correlates with retention for that information. However, this correlation depends on the relative "emotionalness" of the information. More emotionally-arousing information increases amygdalar activity, and that activity correlates with retention.

In popular culture

In the television series Firefly, the character of River Tam is portrayed as having had her amygdalae 'stripped'. This operation, according to characters on the show, has caused her to be unable to suppress emotions, giving her a hypersensitivity that can come across as creepy and unsettling. She also exhibits possible signs of autism, a condition linked to the malfunctioning of the amygdalae.

References

Kandel E. R., Schwartz J. H., Jessell T. M. Principles of Neural Science, 4th ed. McGraw-Hill, New York (2000). ISBN 0838577016
Maren S. (2001). "Neurobiology of Pavlovian fear conditioning". Annual Review of Neuroscience 24, 897-931. PMID 11520922
Removal of the amygdala^* from monkeys results in Kluver-Bucy syndrome.
Stathmin gene and Amygdala: Recent research works by Dr. Gleb Shumyatsky and Prof. Eric Kandel have led to the identification of the Stathmin gene. This gene is highly enriched in the amygdala and is believed to be involved in controlling both innate and learned fear in mice. They "knocked out" the stathmin gene in the amygdala using gene knockout technology and found that mice that lacked stathmin gene lacked any kind of fear. For instance, such mice did not freeze on sighting a cat.

External links:

Cerebrum | Emotion | Limbic system

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