collective term denoting a heterogeneous array of brain structures at or near the edge ("limbus") of the medial wall of the cerebral hemisphere (forms rim around core of cerebrum), in particular the hippocampus, amygdala, and fornix. The term is often used so as to include also the interconnections of these structures, as well as their connections with the septal area, the hypothalamus, and a medial zone of mesencephalic tegmentum. By way of the latter connections, the limbic system exerts an important influence upon the endocrine and autonomic motor system's; its functions also appear to affect motivational and mood states.
Emotional behavior is mediated by various structures, one of which is the limbic structure.
Two components of emotions:
(1) behavioral response: somatic, visceral and hormonal
(2) the cognitive emotional experience (I’m sad, I’m angry, etc.)
Scientist named Papez came up with a circuit he thought carried emotions. He was only partially right.
Papez’ Circuit
: Hippocampal formation
Þ fornixÞ mamillary bodyÞ ant nuc of dorsal thalamusÞ cingulate gyrus
Breakthrough came with discovery of amygdala by LeDoux.
AMYGDALA
– complex of nuclei at anterior portion of hippocampus, in temporal lobe. Determines cognitive experience + emotional response.
basolateral amygdala – emotional behavior
corticomedial amygdala – appetite and feeding
Fear Conditioning
Conditioned fear learning is quick and long-lasting. Fear conditioning leads to: defensive behavior, autonomic arousal, reflex potentiation, activation of pituitary-adrenal axis.
How does sensory information get to the amygdala to bring about all of these behaviors?
Two Pathways
:
(1) "Low Road" – Fast Pathway – from sensory thalamus directly to amygdala, bypassing sensory cortex
(2) "High Road" – Slow Pathway – from sensory thalamus through sensory cortex to amygdala
Experiment
: Rat was conditioned (by electrocution) to fear an auditory tone. Lesion in cortex did not completely disrupt rat’s fear conditioning because "low road" was still functional (but rat could no longer tell two very similar auditory tones apart). Lesion in sensory thalamus (specifically medial geniculate) had a much greater effect in disrupting rat’s fear conditioning. This showed that: the amygdala mediates behavioral responses to the conditioned stimulus, and that defensive behavior comes first, and analysis comes second (seeing a rope on the road will immediately make you scared because you are conditioned to think it’s a snake, but a few moments later your cortex will tell you it is just a rope). Association between a stimulus and fear is learned in the amygdala but the cortex can override this if the stimulus is innocuous.
Contextual Fear Conditioning
– rat will be scared just being put near electrocuting device. Contextual fear conditioning is created by the Hippocampus which receives multimodal sensory input from sensory, auditory, and somatic cortex. It then stores this info as an explicit memory. Upon a stimulus it sends signal to amygdala which defines context and thereby forms implicit emotional memory. Lesions of the hippocampal input to the amygdala abolish contextual fear conditioning.
Summary: The amygdala forms "hub" of the fear system
sensory thalamus - stimulus features
sensory cortex - objects
rhinal (transition) cortex - memories
hippocampus - memories and contexts
medial prefrontal cortex - extinction
Cognitive Component of Emotion
Amygdala’s influence on sensory areas of the cortex is greater than the influence of the same areas on the amygdala!
The amygdala projects directly to the specific sensory cortices and to the non-specific cortical activating system (nuclei that turn up the autonomic system, etc).
Neural ingredients of a conscious emotional experience:
(1) Amygdala activates cortical areas and arouses cortex in general by talking to diffuse systems (like nucleus basalis).
(2) Visceral organs respond to situation via signal from amygdala. FEELING OF FEAR!!! Heart rate
Ý , breathing Ý , etc because visceral neurons Ý firing. This creates afferent signal from viscera to brain thus reinforcing this response. Info goes to nucleus solitarius Þ reticular formation Þ reinforcing arousal.
Activation of pituitary-adrenal axis by amygdala causes release of glucocorticoids. While this is adaptive in the short term, chronic activation of this system (post-traumatic stress disorders) may be harmful.
What do glucocorticoids do? turn on hippocampus which is supposed to down-regulate axis.
Chronic stimulation of hippocampus can lead to its degeneration
. When hippocampus degenerates person will feel fear and anxiety all the time.