The Limbic System

Question 1

Describe the location of the amygdala

Answer 1

anterior temporal lobe at the tail of the caudate nucleus, sitting above the hippocampus

Question 2

Describe Kluver-Buccy Syndrome

Answer 2

syndrome which results in behavioural impairment. Associated with damage to the anterior temporal lobes (eg. herpes encephalitis, trauma, tumours etc.) - bilateral amygdala lesions.

symptoms: hyperorality, placidity, hypersexuality, agnosia

treatment is symptomatic management and psychotropic medicines

Question 3

Describe the structure and function of the amygdala

Answer 3

• made up of grey matter
• has central, corticomedial and basolateral nuclei
• electrical stimulation results in anxiety and fear

Question 4

What input does the amygdala receive?

Answer 4

• object, imagined, contextual
• goes to lateral nucleus
• 2 tracts: fast-track through thalamus (unconscious route, non-cortical), and longer route through cerebral cortex and hippocampus

Question 5

Describe how the amygdala generates output

Answer 5

• response generated from amygdala through 2 centres: hypothalamus and brainstem
• hypothalamus through stria terminalis
• brainstem: PAG, LC, solitary and dorsal vagal nuclei and ventral tegmental area

Question 6

What structures form the limbic cortex?

Answer 6

• cingulate gyrus (anterior, middle and posterior) and insula
• ACC feeds signals (basic emotion - happiness/sadness/fear etc) to amygdala and effectors in brainstem in order to regulate the emotional response
• MCC directs signals to the motor cortex (movement/behaviour)

Question 7

Describe the actions of the ACC

Answer 7

‘re-codes’ amygdala:

• computes relevance/outcomes (to drive appropriate behaviour)
• provides conflict resolution
• involved in pain network
• feeds signal directly to ANS

Question 8

Describe the actions of the MCC

Answer 8

• motor reactions (complex/contect dependent)
• drives face muscles in context of emotion
• can feed signals to ANS indirectly through amygdala

Question 9

Describe the role and connections of the insula

Answer 9

• involved in empathy (emotional awareness)
• connected with, and its anterior portion works with ACC in evaluating emotional contexts
• also takes in visceral information including pain

Question 10

Describe how the limbic system differs in individuals with PTSD and depression

Answer 10

PTSD:

• ACC hyporesponsive and smaller in size
• amygdala is hyperresponsive to trauma-related stimuli

depression:

• lower rate of metabolism in ACC and smaller in size
• hyperreactive

Question 11

Describe the role of the hypothalamus

Answer 11

receives input from limbic cortex and amygdala, sensory systems, spinal cord, viscera, and internal systems

sends output to brainstem, spinal cord, and through hormonal control:

• autonomic
• endocrine
• behvioural
• homeostasis

Question 12

Describe the location and role of LC

Answer 12

• nucleus in midbrain
• noradrenaline fibres that connect different brain structures
• function: coordinates physiological response to panic and stress
• promotes alertness, wakefulness and anxiety

Question 13

What is the input to the LC?

Answer 13

• cingulate gyrus
• amygdala
• PAG
• hypothalamus

Question 14

Describe the function of the PAG

Answer 14

• mediates limbic autonomic reactions
• input: cingulate gyrus and amygdala
• output: nucleus solitarius, dorsal nucleus, and intermedial column
• has defensive fear network and pain network

Question 15

Describe the function of the PAG in the defensive fear network

Answer 15

• projections from amygdala to ventrolateral PAG excites vagal paths
• parasympathetics in freezing
• projections from amygdala/cingulate gyrus to dorsolateral PAG excites the LC and the brain stem (fight or flight)

Question 16

Describe the function of the PAG in the pain network

Answer 16

• pain regulation (can augment/lessen pain)
• pain signals ascend dorsal horn from stimulation of nociceptors
• signals ascend further to PAG through spinothalamic and spinobulbar tracts
• PAG activates raphe nuclei which activates encephalin neurons which inhibitory to c-fibre pain afferents
• 5-HT has anti-nociceptive effect in PAG so can be implicated in pain control as well

Question 17

Describe the location and role of the dorsal raphe nucleus

Answer 17

• in midbrain
• projects to amygdala and ACC in ascending system, also receives from there
• 5-HT neurons effective here
• determines dynamic mood state and processes descending pain from PAG to spinal cord

Question 18

Describe the location and role of the hippocampus

Answer 18

• deep in temporal lobe forming floor of lateral ventricle
• integrates spatiotemporal information with emotional, sensory and cognitive info for episodic memory so that it can be later retrieved

Question 19

Describe the functional divisions of the hippocampus

Answer 19

• parahippocampal cortex: processes spatial context from dorsal stream and sends it to caudal hippocampus
• perirhinal cortex: object recognition and sends it to caudal hippocampus
• caudal hippocampus: processes external signals for scene learning (places/location/orientation)
• rostral hippocampus: processes internal signals for emotional and motivational modulation (objects/behaviours/sounds etc)