Lecture 12: The Limbic System

Limbic System

- Bridges autonomic survival behavior (hypothalamus) and conscious control

- Between Hypothalamus and neocortex

Limbic Lobe/Cortex

- Border of evolutionarily older cortex surrounding the diencephalon

- 3 to 5 cortical layers

Limbic Lobe/Cortex Includes

- Cingulate gyrus

- Parahippocampal gyrus (includes uncus)

- Orbitofrontal cortex (ventromedial PFC, gyrus recta)

- Insula

- Olfactory and Gustatory cortices (smell and taste closely tied to emotion)

Key Functions of the Limbic System

- Emotion

- Learning/memory

- Behavior motivation

Limbic System -- Two Main Subsystems

- Amygdala --> emotion

- Hippocampus --> memory

Hippocampus

- Located in medial temporal lobe

- 3 layered cortex, major sites for memory formation

Subcortical Structures

- Amygdala

- Basal Forebrain

Amygdala

Emotional processing hub, near the uncus and hippocampus

Basal Forebrain

Produces acetylcholine (cholinergic system for attention/memory)

Basal Forebrain Key Nuclei

- Nucleus basalts (of meynert)

- Septal nuclei

- Nucleus of the diagonal band (of Broca)

Cortical Areas Used by Amygdala -- Anterior Limbic Lobe

- Anterior cingulate cortex

- Ventromedial prefrontal cortex

- Anterior parahippocampal gyrus

- Anterior insula

- Amygdala connects to neocortex via dorsomedial nucleus of the thalamus

Amygdala has 12 nuclei Grouped into 3 Main Functional Groups

- Medial Group

- Central Group

- Basolateral Group

Medial Group

- Connected to olfactory system (input)

- Processes smells and related memories

Central Group

- Input of visceral sensory information

- Connects to hypothalamus via stria terminalis and ventral amygdalofugal pathway (VAF)

- Connects to Reticular formation and periaqueductal gray (PAG) via medial forebrain bundle (MFB)

Basolateral Group

- Largest Group

- Inputs from somatosensory, visual, auditory, gustatory, and insula (emotional comfort/discomfort)

Basolateral Group Connects to

- Hippocampus (emotional memory encoding)

- Ventral striatum (emotional --> behavioral motivation)

Amygdala Reward Pathways

Amygdala and Hippocampus

Amygdala and Hippocampus Pathway sends info to

- Ventral striatum (nucelus accumbens)

- Gets dopaminergic input from ventral segmental area (VTA)

- Forms loop

Amygdala and Hippocampus Pathway Loop

Amygdala --> nucelus accumbens --> back via dorsomedial thalamus

Amygdala and Hippocampus Pathway Important For

- Learning reward associations

- Addiction: drugs increase D3 dopamine receptor expression

Amygdala and Fear/Emotions

- Electrical Stimulation of amygdala in animals

- Destruction of amygdala

- Pain and empathy responses

Electrical Stimulation of amygdala in animals

- Causes fight or flight, rage, or fear

- In humans: triggers fear, increased heart rate, pupil dilation

--> activation of sympathetic system

Destruction of amygdala

- Loss of fear

- Monkey: tries to mate with lion instead of escaping

Pain and empathy responses

- Pain activates amygdala

- Even watching someone else in pain activates: amygdala, insula, medial prefrontal cortex, occipitotemporal cortex (face recognition)

Emotional Behavior and Prefrontal Interplay

- Amygdala --> prefrontal cortex --> emotional regulation

- Affect: outward expression of emotion (facial muscles)

- Emotions influence decision making

- Feeling/sensing types vs judgmental/logical types (Myers-Briggs)

Conscious Emotional Control

- Prefrontal cortex (PFC) can suppress emotional responses

- Study of men watching erotic films

Study of men watching erotic films

- Normal viewing showed an increase of activity in amygdala and hypothalamus

- Suppressed viewing showed decrease in amygdala and hypothalamus activity and an increase in dorsolateral PFC and Anterior cingulate cortex (ACC)

Emotional Blindsight

- Patient with cortical blindness (bilateral occipital damage:

--> shows amygdala activation of emotional faces (fear, anger)

--> shows emotion processing doesn't require conscious vision

Amygdala Outputs

- Mirror inputs (reciprocal)

- Key Targets: Hypothalamus, PAG, Cortex, Hippocampus, Ventral Striatum

Hypothalamus Output

Autonomic responses

PAG Output

Pain modulation (descending pain inhibition)

Cortex Output

Emotional experience/awareness

Hippocampus Output

Emotionally charged memories

Ventral Striatum (nucleus accumbens)

Reward based behavior

Amygdala Function -- Fear

Stimulation triggers fight or flight (sympathetic)

Amygdala Function -- Lesions cause

Emotional Blunting or Danger Neglect

Amygdala Function -- Emotional Learning

Forms associations between emotions and stimuli (especially fear/pain)

Amygdala Function -- Emotion Memory Link

Basolateral amygdala --> hippocampus = strong emotional memories

Amygdala and Prefrontal Cortex Have Reciprocal Connections With:

- Dorsolateral prefrontal cortex

- Anterior cingulate cortex

Amygdala and Prefrontal Cortex Have Conscious Control Over Emotion:

- fMRI: effortful emotional suppression activates prefrontal areas, suppresses amygdala

- EX: subjects told to suppress sexual arousal --> no amygdala/hypothalamus activity, but high prefrontal cortex activity

fMRI Studies Found

Watching others in pain activates insult + amygdala

Reward Circuit Studies

- Amygdala --> ventral striatum (nucelus accumbens)

Amygdala Groups Summary

- Medial = olfactory

- Central = autonomic

- Basolateral = sensory + hippocampus

Inputs and Outputs

Relays from Amygdala to hypothalamus, PAG, cortex, hippocampus, and ventral striatum

Amygdala Outputs Utilize

Dorsomedial nucleus of the thalamus

Insula

Emotional awareness, gustatory input, comfort/discomfort