As I understand it, exposure to a painful stimulus triggers
the amygdala, which has widespread efferent connections to the
rest of the brain. Neurotransmitters from the amygdala enhance
the long-term potentiation of whatever neurons are firing at the
time, so that only one exposure to the painful stimulus is
required for other stimuli associated with it (e.g. visual
stimuli) to be "remembered." The neural paths involved
include reverse connections to the amygdala. Thus when the
associated stimuli appear again -- or maybe only some of them --
the amygdala triggers again to produce the response appropriate
to the original painful stimulus.
At the same time, inhibitory connections to the amygdala
from the prefrontal cortex can suppress the amygdala's response;
i.e. we can "control" our fear.
REFERENCES:
Michael S. Gazzaniga (ed.), "The Cognitive Neurosciences," MIT
Press, Cambridge, MA (1995) -- has a whole section on emotion.
Several presentations at the third annual meeting of the
Cognitive Neuroscience Society, March 31 - April 2, 1996, in San
Francisco California dealt with emotion:
Neurobiology of Emotion -- Antonio Damasio (U. of Iowa, College
of Medicine)
Functional Organization of the Primate Amygdala -- David Amaral
(UC Davis)
Emotion, Memory, and the Brain -- Joseph LeDoux