Limbic system Flashcards
What is the limbic system?
Several functionally and anatomically interconnected brain structures that are concerned with emotions and memories
What are the components of the limbic system?
- 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
What is the cingulate gyrus?
Curved fold covering the corpus callosum
What is the parahippocampal gyrus?
Grey matter cortical region of the brain that surrounds the hippocampus
What is the Papez circuit?
Hippocampal formation → fornix → mammillary bodies → mammillothalamic tract → anterior thalamic nucleus → cingulum → entorhinal cortex → hippocampal formation
What does the amygdala consist of?
Three main groups of nuclei:
- Centromedial - visceral responses
- Cortical - forming part of the olfactory cortex
- Basolateral - emotional response to stimuli
Discuss inputs to the amygdala
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
Outputs from the amygdala
- 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 - 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
What is the amygdala responsible for?
- Processing the social signals of emotions (eg. fear)
- Facial expressions as well as vocal expressions - Emotional conditioning (fear conditioning)
- Consolidation of emotional memories
- Usually enhanced memory for emotional aspects of stories compared with non-emotional ones - Actual feeling of fear?
5 OLFACTORY PROCESSING → distinguishes the intensity of odours
What part of the limbic system distinguishes pleasant and unpleasant odours and projects back to the amygdala?
Orbitofrontal cortex
What structures is the amygdala therefore linked ti?
olfactory system, association cortex, hypothalamus and brainstem (via
amygdalofugal tract and stria terminalis)
What does the hippocampus consist of?
Cornu Ammonis - CA1-4
Dentate gyrus
What is the hippocampal formation?
Hippocampus – Dentate gyrus – Subiculum – Entorhinal cortex (in parahippocampal gyrus) See diagram on OneNote
Hippocampal inputs
All association cortex via entorhinal cortex
(perforate path)
Cingulate gyrus via cingulum
Septal cortex (ACh) and hypothalamus via fornix
Amygdala
Hippocampal outputs
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
Describe hippocampal circuitry
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
What is LTP?
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
What does hippocampal LTP play a role in?
Spatial or episodic memory
Describe functional organisation of hippocampus
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
Inputs to mammillary bodies
from hippocampus via fornix – has a number of
different nuclear groups
Outputs of mammillary bodies
To anterior nucleus of thalamus, via mammillothalamic
tract (thence to cingulate gyrus)
To midbrain tegmental motor structures
(pedunculopontine nucleus)
Describe Korsakoff’s syndrome
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)
Functions of orbitofrontal cortex
behavioural inhibition, inhibitory self-control and emotional regulation, discrimination and
decision-making (part of PFC)
Inputs to orbitofrontal cortex
Amygdala (via uncinate fasciculus) Hypothalamus Hippocampus Nucleus accumbens Association cortex (all sensory inputs) Medio-dorsal (DM) thalamus
Outputs of orbitofrontal cortex
VTA
Dorsolateral prefrontal cortex (and from there to
premotor cortex)
What does OFC activity signal?
- hedonic experience of reward
- valueof reward
- errors
What do OFC lesions result in?
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
