Limbic system Flashcards

1
Q

What is the limbic system?

A

Several functionally and anatomically interconnected brain structures that are concerned with emotions and memories

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2
Q

What are the components of the limbic system?

A
  • amygdala
  • hypothalamus
  • parahippocampal gyrus
  • cingulate gyrus
  • hypothalamus (mammillary bodies)
  • orbital and medial PFC (orbitofrontal cortex)
  • BF (nucleus accumbens, parts of basal ganglia)
  • some thalamic nuclei (anterior dorsomedial)
  • septal nuclei; insula
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3
Q

What is the cingulate gyrus?

A

Curved fold covering the corpus callosum

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4
Q

What is the parahippocampal gyrus?

A

Grey matter cortical region of the brain that surrounds the hippocampus

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5
Q

What is the Papez circuit?

A

Hippocampal formation → fornix → mammillary bodies → mammillothalamic tract → anterior thalamic nucleus → cingulum → entorhinal cortex → hippocampal formation

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6
Q

What does the amygdala consist of?

A

Three main groups of nuclei:

  1. Centromedial - visceral responses
  2. Cortical - forming part of the olfactory cortex
  3. Basolateral - emotional response to stimuli
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7
Q

Discuss inputs to the amygdala

A

They are to the basolateral and central nuclei
They are from:
1. All sensory association cortex (also direct sensory input from thalamus) - thus direct and indirect inputs from thalamus to amygdala
2. Cortical amygdala (part of primary olfactory cortex) - olfactory stimulation activates the amygdala
and frontal cortex
3. Entorhinal cortex, hippocampus (memory)
4. Cingulate (conscious emotional experience - feeling; top-down
control), prefrontal (reward-processing; top-down control), septal
(reward and reinforcement) areas
5. Mediodorsal thalamus (memory)
6. Brainstem - visceral sensory

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8
Q

Outputs from the amygdala

A
  1. One is direct to the hypothalamus (for autonomic and endocrine
    components of emotional responses), called the ventral
    amygdalofugal pathway – this is the primary output for the
    basolateral and central nuclei
  2. The other is via the stria terminalis a long looping path that
    courses over diencephalon to terminate in the septal region and
    nucleus accumbens (also projects to hypothalamus) – the primary
    output for the medial nuclei
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9
Q

What is the amygdala responsible for?

A
  1. Processing the social signals of emotions (eg. fear)
    - Facial expressions as well as vocal expressions
  2. Emotional conditioning (fear conditioning)
  3. Consolidation of emotional memories
    - Usually enhanced memory for emotional aspects of stories compared with non-emotional ones
  4. Actual feeling of fear?

5 OLFACTORY PROCESSING → distinguishes the intensity of odours

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10
Q

What part of the limbic system distinguishes pleasant and unpleasant odours and projects back to the amygdala?

A

Orbitofrontal cortex

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11
Q

What structures is the amygdala therefore linked ti?

A

olfactory system, association cortex, hypothalamus and brainstem (via
amygdalofugal tract and stria terminalis)

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12
Q

What does the hippocampus consist of?

A

Cornu Ammonis - CA1-4

Dentate gyrus

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13
Q

What is the hippocampal formation?

A
Hippocampus
– Dentate gyrus
– Subiculum
– Entorhinal cortex
(in parahippocampal gyrus)
See diagram on OneNote
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14
Q

Hippocampal inputs

A

 All association cortex via entorhinal cortex
(perforate path)
 Cingulate gyrus via cingulum
 Septal cortex (ACh) and hypothalamus via fornix
 Amygdala

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15
Q

Hippocampal outputs

A

The mammillary bodies, septum and hypothalamus via the fornix

  • The fornix is the main output tract of the hippocampus and follows a C shaped curve caused by cortical expansion in development.
  • The fornix also carries cholinergic fibers from the basal forebrain to the hippocampus, which degenerate in Alzheimer’s.

The association corticies via the parahippocampal gyrus

The Nucleus accumbens via the limbic striatum

The hippocampal commisure connects the two hippocampi

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16
Q

Describe hippocampal circuitry

A

Axons for perforant pathway → granule cells of dentate gyrus → axons of the granule cells (mossy fibres) → pyramidal cells of CA3 → axons of CA3 pyramidal cells branch to form commissural fibers to the contralateral hippocampus, collaterals to same or neighboring CA3 cells (recurrent collaterals) or to CA1 pyramidal cells (Schaffer collaterals) which go back to the entorhinal cortex AND efferents to the mammillary bodies of the hypothalamus via the fornix

17
Q

What is LTP?

A

LTP is the persistent strengthening of synaptic connectivity, based on recent patterns of activity of that
synapse - when a neurone is stimulated
many times, its response becomes
stronger - that the number of action potentials in the presynaptic neuron required to elicit the same response in the post-synaptic neuron is reduced

18
Q

What does hippocampal LTP play a role in?

A

Spatial or episodic memory

19
Q

Describe functional organisation of hippocampus

A

Dorsal/posterior – memory and spatial navigation:
 Place cells – support cognitive map of known location, also episodic memory in humans
 Time cells – fire at successive movements in temporally-structured experiences (flow of time)
- Ventral/anterior – anxiety-related behaviours:
 In primates, amygdala-projecting neurons are focally restricted to most anterior CA1 and
prosubiculum
- Entorhinal cortex – cortical inputs are topographically arranged into:
 Infralimbic and prelimbic areas – (cingulate cortex areas involved in emotional regulation) project
to more ventral/anterior parts
 Retrosplenial cortex (spatial processing in cingulate cortex) projects to more dorsal/posterior parts

20
Q

Inputs to mammillary bodies

A

from hippocampus via fornix – has a number of

different nuclear groups

21
Q

Outputs of mammillary bodies

A

 To anterior nucleus of thalamus, via mammillothalamic
tract (thence to cingulate gyrus)
 To midbrain tegmental motor structures
(pedunculopontine nucleus)

22
Q

Describe Korsakoff’s syndrome

A

Lesions in mammillary bodies and dorsomedial
thalamus
 Thiamine (B 1 ) deficiency in chronic alcoholism combined with poor diet, common cause of amnesia
 Cannot form new associated memories – make it up (confabulation)

23
Q

Functions of orbitofrontal cortex

A

behavioural inhibition, inhibitory self-control and emotional regulation, discrimination and
decision-making (part of PFC)

24
Q

Inputs to orbitofrontal cortex

A
  Amygdala (via uncinate fasciculus)
 Hypothalamus
 Hippocampus
 Nucleus accumbens
 Association cortex (all sensory inputs)
 Medio-dorsal (DM) thalamus
25
Q

Outputs of orbitofrontal cortex

A

 VTA
 Dorsolateral prefrontal cortex (and from there to
premotor cortex)

26
Q

What does OFC activity signal?

A
  • hedonic experience of reward
  • valueof reward
  • errors
27
Q

What do OFC lesions result in?

A

Lesions result in a deficit in rapid reversal of learning – ‘perseveration’
 Inability to change goals, tasks or activities
 Uncontrolled and extended maintenance of a thought, activity, emotion, problem-solving strategy,
or topic in conversation
 The significance of stimuli (eg. pain) is lost
 Marked behavioural changes; disregard for laws