The limbic system, emotion, the hypothalamus, appetites Flashcards
What does the limbic system process?
Emotion and related brain activity, memories, enviromental cues and state of the individual and acts on this information to maximise survival strategies
What are the core componenets and effectors of the limbic system?
Core components:
- Amygdala - emotion
- Hippocampus )+cortex) (H. formation) - memory
- Limbic cortex - cingulate gyrus/insula
Effectors (output)
- Hypothalamus
- Brain stem structures
What make up the limbic cortex?
Cingulate gyrus + insula
Where is the amygdala located?
- Anterior temporal lobe at the tail of caudate nucleus and rostal to hippocampus
- Not a basal ganglia structure
- Close to lateral ventricle
What syndrome can give an insight into amygdala function?
Kluver-Bucy syndrome (rare syndrome in humans which produces behavioural impairment)
What are some of the symptoms of Kluver-Bucy syndrome?
- Placidity (lack of fear)
- Hyperorlaity
- Hypersexuality
- Angnosia
- Memory loss
What are some pf the causes of Kluver-Bucy syndrome?
- Herpes encephalitis
- Trauma
- Tumours
- Hypoxia
- Pick’s disease (type of FTD tau-opathy)
What disease is a rare genetic recessive disorder which induces calcification in both temporal lobes?
Urbach-Wiethe disease
- Lacks fear
What does fear activate in the brain?
- Amygdala generates and is activated by fear
- Fear activates central nucleus (and bed nucleus of the stria terminalis (BST))
- Centres are enhanced in anxiety disorders
What are the 3 types of inputs into the amygdala which cause a fear response?
- Stimulus (e.g view of a snake)
- Concept (e.g idea of a snake/ memory)
- Context (e.g snake in the room)
What are the 2 pathways fear stimulus can take to reach the amygdala and therefore cause a response?
- Take a fast track pathway via the thalamus - doesn’t reach consciousness (e.g. subliminal short route)
- Longer route via cerebral cortex (long route e.g. visual cortex)
The concept of a fear (e.g idea of a snake) reaches the lateral amygdala via what?
Cerebral cortex
The contex of the fear (e.g snake in the room) reaches the amygdala via what?
Hippocampus
Where do inputs of fear go to?
Lateral nucleus of amygdala
Where do outputs of fear come from?
Central nucleus of amygdala
How can the amygdala learn a modified fear response (emotional memory)?
Synapse strengthening
What does the output to the hypothalamus from the amygdala travel via?
Stria terminalis (another C shaped structure)
What does the amygdala output to in the brainstem?
- Periaqueductal grey matter (PAG)
- Locus Coeruleus (LC)
- Parasympathetics (solitary nucleus, dorsal vagal nucleus)
- Ventral tegmental area
What is the function of the amygdala?
Controls emotional reactions (particularly threat/fear) via effectors (hypothalamus, brain stem through to the autonomic nervous system, endocrine and motor system)
What are the 3 areas the cingulate gyrus can be broken up into?
- Anterior cingulate cortex
- Midde cingulate cortex
- Posterior cingulate cortex
What part of the cingulate gyrus is involved in emotion and what is it called?
Anterior Cingulate cortex and anterior portion of middle involved in emotion make up what is called:
- Limbic Cortex
What does the anterior cingulate encode?
Basic emotions, happiness, sadness and fear and emotional memory (so more than just fear)
What are the functions of the anterior cingulate?
- Top down influence - works to recode the amygdala
- Computes relevance/outcomes - drives appropriate behaviour
- Provides conflict resolution (dorsal ACC e.g. self vs non-se;f in threat)
- Part of pain network - thalamus, primary somatosensory area and insula
What are the ways in which the limbic cortex can affect the motor control?
- Motor reactions complex and context dependant
- Specific zone for driving face muscles
- Direct outputs from ACC to autonomic system
What is considered to be the primary limbic cortex?
The anterior cingulate cortex (can influence the amygdala)
What is the function of the insula?
- Works with ACC in evaluating emotional contexts - role in empathy
- Encodes emotional awareness
- Classified as limbic “sensory” cortex
How does PTSD affect the limbic system?
- Part of ACC is hypo-responsive in PTSD
- Reduction in size of ACC
- Amygdala becomes hyper-responsivene to trauma related stimuli
- Top down control of amygdala by ACC is therefore missing in PTSD
What are the main functions of the cingulate gyrus?
- Emotion response regulation coordinated via connections with the amygdala
- Make prediction about negative consequences resolves conflicts
- Top down regulation “recodes” the amygdala processing/responses
- Drives conscious responses to unpleasant experiences
- Drives avoidance behaviours and motor such as facial expression (from MCC to motor cortex)
- Also direct influence on limbic effectors (ANS) by-passing amygdala
- Works with insula which provides wider context of awarenwss to unpleasant especially visceral stimuli (empathy; pain)
What are the 4 main functions of the Hypothalamus?
- Autonomic - CV, resp, piloerection sweating etc.
- Endocrine - e.g stress response
- Behavioural - General level of activity up to and including rage for pursuit of food, water, sex etc; ingestion/disinterest in food; fear behaviour; increased sexual drive
- Basic homeosasis - Fluid electrolyte, feeding energy, thermoregulation, reproduction, sleep wake
What part of the limbic system promotes alertness and wakefulness?
Locus coeruleus
What is the locus coruleus and what is its function?
- Midbrain nucleus involved in physiological responses to panic and stress
- Input from CG, amygdala, PAG and hypothalamus
- NA fibres projecting from nucleus to widespread brain structures
- Promotes alertness and wakefulness (widespread connections)
- Promotes anxiety + formation and retrieval of emotional memories (amygdala) - dense projections from LC to amygdala
- Projections to hypothalamus from LC maintains arousal and also effect ANS output
- Projections FROM PAG selects fight or flight mode
What is the function of the PAG?
- Part of the pathway that mediates LIMBIC autonomic reactions (fear, pain, analgesia) inputs from cingulate gyrus and amygdala
- Projects to nucleus of solitary tract, dorsal nucleus of vagus, intermediolateral column. Projections from PAG to LC selects fight or flight mode
What are the 2 defensive fear network paths the PAG fibres can take?
- Amygdala -> ventrolateral (vl) PAG -> vagal paths (parasympathetic in freezing)
- Amygdala/CG -> dorsolateral (dl) PAG -> LC and Bstem (flightorflight switching)
How is fear processed in the brain if the stimulus is far away?
Cingulate gyrus processes response through basolateral amygdala to striatum (PAG independantly)
How is fear processed in the brain if the stimulus is close by?
Central amygdaala drives PAG and other brainstem centres to panic mode (AS)
Describe the function of the Dorsal Raphe Nucleus (DRN)
- Midbrain nucleus projects an asceding system
- Projectd to and receives from amygdala and ACC
- 5HT (serotonin) neurons
- Determines tonic limbic activity and dynamic mood state
- Also processes descending pain paths from PAG to DRN to spinal cord
What is the theory of 5HT and depression/mood?
- High levels of 5HT = Mainia
- Low levels of 5HT = Depression
What disease are variant polymorphisms in 5HT transporter gene thought to predispose for?
Depression
What happens to the limbic system in established depression?
- Reduced metabolism in ACC
- Reduced size of ACC
- Amygdala reduced size and hyperactive
What are the functions of the hippocampus in terms of memory?
- Combines spatio temporal information with emotional, sensory and cognitive information to establsih new episodic memory
- Including memories for locations and events that take place at those locations
Where is the hippocampus and what does it look like?
- Deep in temporal lobes
- Elongate structure in the floor of the lateral ventricle
- Name reflects similarity to a seahorse
- Rolled appearence
- Evolutionarilly old cortex
What makes up the hippocampal formation?
- Hippocampus and associated cortex (parahippocampal gyrus) together
What structure is thought to creat new memories?
Hippocampus
What are the 2 types of declerative memory?
- Episodic memory (recollection about a specific event in one’s past tied to a specific time and place)
- Semantic memory (general knowledge about the world)
What is the function of the hippocampus in terms of memory?
Plays a critical role in memory formation by providing the brain with a spatiotemporal framework within which the various sensory, emotional, and cognitive components of an experience are bound together
- This framework allows the experience to be stored in such a way that it can be later retriebed as a conscious recollection of that experience
Where in the parahippocampal gyrus is associated with scenes?
Parahippocampal cortex
Where in the parahippocampal gyrus is associated with objects?
Perirhinal cortex
Where in the parahippocampal gyrus is associated with faces?
Entorhinal cortex
What are diseases in the hippocampus associated with?
- Sensitive to stress (glucocorticoids)
- Cushing’s disease and glucocorticoid administration
- Ischaemia sensitive
- Alzheimer’s (lead to atrophy)
- Hypertension
- Depresion
- Epilepsy
- Stress (PTSD)
- Genetic disorders
- Chronic alcohol use (all lead to atrophy)
What is one of the first regions to be affected by Alzheimer’s?
Hippocampus
What is the fornix?
A major output tract of the hippocampus involved in memory
- Fornix projects to the mammillary bodies which are connected to the anterior nucleus of the thalamus (damage to the fornix and these 2 structures cause memory problems)
How is language and visiospatial output divided in the fornix?
Left/Right segregation
L - Language
How is emotional/motivational learning divided from scene learning in the fornix?
Output sgregated according to hippocampus processing (rostral/caudal)
