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Neurobiological theories of memory
suggest that neural plasticity underlying memory storage requires not only
neural activity representing the to-be-stored information, but also some
nonspecific arousal inputs. This proposition is attested by the findings
that affective laden experience often leaves robust memory. Extensive
evidence shows that various hormones released under arousal play a critical
role in facilitating storage of newly learned information. However, it is
intriguing that these hormones, circulating in blood stream of periphery,
have no access to the central nervous system due to the blood-brain-barrier.
In the past twenty years, findings on rodents and humans from several
laboratories have shed lights on this issue. Epinephrine is a hormone
released from the adrenal medulla in response to an arousing event. It is
elevated in circulation by learning experience. Peripheral administration of
epinephrine shortly after training has biphasic effects: Medium doses
enhance but high doses impair retention of the learned response. The effect
is due to binding of epinephrine to peripheral receptors on the viscera,
drugs antagonizing the peripheral action of epinephrine can abolish the
effect. The influence is conveyed into the brain by vagal afferents,
transection of the vagus attenuates the effect of epinephrine on memory.
Vagal afferents innervate the nucleus of solitary tract mono-synaptically
and the locus coeruleus multi-synaptically, which are implicated in visceral
processing and vigilance, respectively. Suppressing these two brainstem
nuclei abolishes influence of epinephrine on memory. Neurons in the two
nuclei issue noradrenergic fibers to innervate widespread forebrain regions,
including the amygdala and hippocampus, implicated in mnemonic function.
Further studies revealed that treatments suppressing noradrenergic functions
in the amygdala attenuate the memory enhancing effects of epinephrine and
other peripheral humoral factors. This structure entail memory trace formed
in other brain regions, such as the hippocampus or various cortical regions,
modulated by emotional saliency of an event. These findings suggest that
bodily feedback activated by an emotional incidence helps to forge the
memory trace for that incidence, just as it helps to forge our awareness of
emotion for that incidence proposed by William James and Carl Lange a
century ago. |
