Limbic system Flashcards
What two main functions is the limbic system concerned with?
Emotion and memory
Limbic system components…
Amygdala
Hippocampus
Parahippocampal gyrus
Cingulate gyrus
Hypothalamus (mammillary bodies)
Orbital and medial prefrontal cortex (orbitofrontal cortex)
Basal forebrain (nucleus accumbens; parts of basal ganglia)
Some thalamic nuclei (anterior, dorsomedial)
Septal nuclei
Insula
What are the two parts of the limbic lobe?
Cingulate gyrus
Parahippocampal gyrus
What is the cortical group of the amygdala associated with?
Olfactory
What is the centromedial group of the amygdala associated with?
Visceral responses
What is the basolateral group of the amygdala associated with?
Emotional response to stimuli, necessary for emotional (reward), memory.
What is Papez circuit connecting (brief)?
Demonstrated that the hypothalamus, via the anterior nucleus of the thalamus, connects to cingulate cortex, which in turn projects to hippocampus that then projects back to hypothalamus.
Explain the pathways in Papez’ circuit?
Mammillary bodies - (mammillothalamic tract) - anterior thalamic nucleus - (internal capsule) - cingulate gyrus - parahippocampal gyrus (entorhinal cortex)- perforant pathway - dentate gyrus - subiculum - fornix - mammillary bodies
Inputs to Amygdala
All sensory association cortex (also direct sensory input from the thalamus)
Cortical amygdala (part of primary olfactory cortex)
Entorhinal cortex, hippocampus (memory)
Cingulate (conscious emotional experience (feeling), top-down control),
Prefrontal (reward processing, top-down control),
Septal (reward and reinforcement) areas
Mediodorsal thalamus (memory)
Brain stem, visceral sensory
Where are memory inputs to the amygdala from?
Entorhinal cortex, hippocampus
Mediodorsal thalamus
Where are do the olfactory inputs to amygdala?
Cortical
How does the amygdala receive sensory input?
From thalamus directly and from sensory association cortex
What are the two amygdala output pathways?
Amygdalofugal pathway
Striae terminalis
Where does the striae terminalis go?
(looping) From medial nuclei of amygdala to septal area, hypothalamus and nucleus accumbens
What is the primary output for basolateral and central nuclei of the amygdala?
Amygdalofugal pathway
What is the primary output for medial nuclei of the amygdala?
Striae terminalis
Where does the amygdalofugal pathway go?
Direct to the hypothalamus, septal area and thalamus
Klüver-Bucy syndrome
Loss of amygdala
Visual agnosia (inability to recognize objects without general loss of visual discrimination)
Excessive oral tendency (oral exploration of objects)
Hypermetamorphosis (excessive visual attentiveness)
Placidity with loss of normal fear and anger responses
Altered sexual behavior (hypersexuality) –
Changes in eating
What is the role of the amygdala in emotion?
Processing social signals of emotions (especially fear)
Facial expressions as well as vocal expressions
What is the role of the amygdala in conditioning?
Emotional fear conditioning/ feeling of fear
Receives sensory stimulus (i.e foot shock stimulus from somatosensory thalamus and cortex or sound stimulus from auditory thalamus and cortex)
Lateral nucleus receives, sends to central nucleus.
Outputs for control sent to central grey, lateral hypothalamus and PVN of hypothalamus.
What is the role of the amygdala in memory?
Unusually enhanced memory for emotional aspects of stories compared with non-emotional ones.
What is the role of the amygdala in olfactory processing?
Olfactory stimulation activates the amygdala and frontal cortex
Corticomedial amygdala distinguishes intensity of odours.
Where is the hippocampus?
Medial part of the temporal lobe “tucked into” the inferior horn of the lateral ventricle.
What are the parts of the hippocampus?
Hippocampus
Cornu Ammonis (Ammon’s horn CA1-4)
Dentate gyrus
Subiculum
Entorhinal cortex (in parahippocampal gyrus)
Hippocampus inputs
Cingulate gyrus via cingulum - All association cortex via entorhinal cortex (perforant path)
Septal cortex (ACh) and hypothalamus mammillary bodies via fornix
Amygdala
What is the main hippocampal output tract?
Fornix
Where does the fornix travel?
Hippocampus to septal area and mammillary bodies
How does the hippocampus output to the association cortex?
Parahippocampal gyrus
What cells are in the dentate gyrus?
Granule cells
What connections do granule cells form?
Excitable connections with CA3 neurons (mossy fibres)
What do CA3 mossy fibres form connections with?
CA3 pyramidal cells form monosynaptic connections with CA1 pyramidal cells (Schaffer collaterals)
All excitatory (glutamate release)
What are the CA3-CA1 connections called?
Schaffer collaterals
What feeds into the granule cells of the DG?
Perforant path from the entorhinal cortex
Where do CA1 cells project?
CA1 cells project to the subiculum, fornix, then output
What is the dorsal/posterior hippocampus associated with?
Memory and spatial navigation
Place cells: Support cognitive map of known location (taxi drivers!). Also episodic memory in humans
Time cells: Fire at successive moments in temporally structured experiences (flow of time)
What is the ventral/anterior hippocampus associated with?
Anxiety-related behaviours
In primates, amygdala projecting neurons are focally restricted to most anterior CA1 and prosubiculum
How is the entorhinal cortex topographically arranged associated with?
Infralimbic and prelimbic areas (cingulate cortex areas involved in emotional regulation) - ventral/anterior parts
Retrosplenial cortex (spatial processing in cingulate cortex) - dorsal/posterior parts
What are the inputs to the mammillary bodies?
Fornix (hippocampus)
What are mammillary body outputs?
Anterior nucleus of thalamus, via mammillothalamic tract (thence to cingulate gyrus)
To midbrain tegmental motor structures (pedunculopontine nucleus)
What is the mammillothalamic tract?
Mammillary bodies to anterior nucleus of thalamus
What is the mammillotegmental tract?
To midbrain tegmental area pedunculopontine nucleus
What midbrain nucleus does the mammillary bodies project to?
Pedunculopontine nucleus
Inputs to the nucleus accumbens
Prefrontal cortex (particularly the prelimbic cortex and infralimbic cortex)
Basolateral amygdala
Ventral hippocampus,
Thalamic nuclei (specifically the midline thalamic nuclei and intralaminar nuclei of the thalamus)
Glutamatergic projections from the ventral tegmental area (VTA) (dopaminergic inputs from the ventral tegmental area, which connect via the mesolimbic pathway)
What loop is the nucleus accumbens associated with?
The nucleus accumbens is often described as one part of a cortico-basal ganglia-thalamo-cortical loop.
Nucleus accumbens outputs
The output neurons of the nucleus accumbens send axonal projections to the basal ganglia and the ventral globus pallidus.
The VP, in turn, projects to the medial dorsal nucleus of the dorsal thalamus, which projects to the prefrontal cortex as well as the striatum.
What makes up the NAcc?
The NAcc is made up mainly of medium spiny neurons containing mainly D1-type or D2-type dopamine receptors.
Role of NAcc
The D1-type medium spiny neurons mediate reward-related cognitive processes, whereas the D2-type medium spiny neurons mediate aversion-related cognition.
DA in nucleus accumbens
Does NOT directly influence encoding of specific goals or outcomes
May influence brain regions that evaluate stimuli and carry out goal selection by influencing saliency of specific options
What is the orbitofrontal cortex?
Part of the pre-frontal cortex which is very important for behavioural inhibition, inhibitory self-control and emotional regulation.
Lateral OFC
Evaluation of options independently of each other, (i.e. each option on its own, given sensory and emotional context).
Looks at instantaneous, subjective value.
Takes into account information from amygdala, hypothalamus, insular cortex (taste, disgust), DA neurons in midbrain (can heighten value attributed to a signal), sensory info from thalamus.
Medial OFC
Compares and contrasts options in order to make a choice, (i.e. value comparison)
OFC inputs
Amygdala (via uncinate fasciculus) Hypothalamus Hippocampus NAcc Association cortex (all sensory inputs) Medio-dorsal (DM) thalamus
OFC outputs
VTA
Dorsolateral prefrontal cortex (and from there to premotor cortex)
The major output pathway from the amygdala is the
Striae terminalis
The major target of output from the amygdala is the
Hypothalamus
The anterior thalamus receives a major input from the
Mammillary body which is linked functionally to the output of the hippocampus
The fornix has
Both afferent and effect fibres to the hippocampus
