limbic system Flashcards
What are the two divisions of the limbic system? Which areas make up these two divisions?
Cortical division - ACC and the parahippocampal gyrus
Subcortical division - hippocampus, amygdala, anterior thalamus and hypothalamus
What is the limbic system’s evolutionary relationship with the PFC?
Across species and time, the larger the PFC, the smaller the limbic system -> inverse relationship
What is the role of the amygdala? What are the three nuclei in the amygdala, and their function?
The amygdala is the motional epicentre, receiving and responsing to information from the thalamus and the sensory cortices.
Three nuclei ->
- basolateral nuclei -> encodes valence and emotion
- medial nuclei -> recieves olfactory input
- central nuclei -> main output nuclei, primarily to the hypothalamus and brainstem
What are the efferent pathways of the basolateral amygdala?
Fear (negative valence) pathways -> connection to central amygdala and hippocampus (mediates fear response)
Reward (positive valence) pathway -> connection to nucleus accumbens
Describe fear learning in the basolateral amygdala.
Before learning, the stimulus activates cells in the BL amygdala from both valences. During learning, coactivation of the stimulus and shock cells strenghtens the fear response. During extinction, the stimulus alone results in activation of the fear cells, sending a response to the central amygdala. The central amygdala then sends signals to the dorsal anterior cingulate cortex and the PFC (formation of long term fear memory)
Describe reward learning in the basolateral amygdala.
Before learning, the stimulus activates cells in the BL amygdala from both valences. During learning, coactivation of the stimulus and rewrard cells strenghtens the reward response. During extinction, the stimulus alone results in activation of the reward cells, sending a response to the nucleus accumbens.
Describe the pathways for fear learning projecting from the amygdala
The anterior cingulate cortex and the amygdala have reciprocal connections, whihc facilitates in learning high fear states -> if you silence this connection in rats their fear response reduces
The ventromedial PFC and amygdala are involved in low fear states, and learning what not to fear. This is strengthened in exposure therapy
How does the ventral tegmental area influence reward learning?
The VTA receives dopaminergic input, and responds to the prediction of rewarding stimuli
Unpredicted reward -> increase in dopamine
Predicted reward -> increase in dopamine only at the simulus
Unrewarded prediction -> increase in dopamine at the stimulus, decrease at no reward
The VTA also sends dopaminergic signals to the nucleus accumbens and the PFC
Describe the VTA’s efferent connections and what they do.
The VTA sends dopaminergic signals to the nucleus accumbens. Sensory (drugs, etc.) and abstract (gambling) rewards increases the dopamine release in the nucleus accumbens/ striatum. The anticipation of pleasant rewards increases the nucleus accumbens dopamine in magnitude with the reward and the probability of the reward -> important in the motivation for a reward.
The VTA sends dopaminergic connections to layers 5 and 6 (pyramidal and polymorphic) of the PFC -> the amount of dopamine determines whether thalamic or cortical innervation dominates:
- high DA inhibits cortical connectivity, thalamic input dominates
- low DA, cortical input dominates
This connection is important in reward signalling, ie. goal-directed behaviour (operant conditioning) and long term reward memories
What syndrome occurs after removal of the amygdala? What are the symptoms?
Kluver-Bucy syndrome
- fearless and placid, no response to social threats or gestures
- hyperorality
- hypersexuality
- highly attentive to sensory stimuli
Describe a disorder of the fear/ reward circuit?
PTSD - brain circuits shown in normal fear and reward response shown to be disrupted, particularly those that link the PFC to subcortical structures
Describe the top-down regulation of emotion/ reward?
The PFC sends input to the amygdala and hippocampus, regulating their function. This was seen in the pornographic film study, in which men were tasked with watching pornographic films. When told to suppress sexual arousal, the activation in the amygdala and hippocampus decreased, whilst activity was seen in the dorsal PFC and the anterior cingulate cortex, indicating a top down suppression of arousal
Describe emotional blindness.
Patients with strokes in the occipital lobe are cortically blind, however they can distinguish pictures of different facial emotions -> activates the right amygdala
Describe the PFC’s role in emotion regulation.
The PFC sends input to the amygdala and hippocampus, regulating their function. This was seen in the pornographic film study, in which men were tasked with watching pornographic films. When told to suppress sexual arousal, the activation in the amygdala and hippocampus decreased, whilst activity was seen in the dorsal PFC and the anterior cingulate cortex, indicating a top down suppression of arousal.
What are the four sections of the hippocampus?
Hippocampus proper, dentate gyrus, subiculum and the entorhinal cortex.
