Limbic System Flashcards
What are the 3 main roles of the limbic system?
- responsible for consolidation of short-term memories into long term memories
- controls aggression and rage behaviour
- the neocortex allows us to be rational and suppress this behaviour
- underlies behaviour and emotional expression by way of the autonomic nervous system
What are the telencephalic, diencephalic and brainstem components of the limbic system?
Telencephalic components:
- cingulate gyrus
- parahippocampal gyrus
- hippocampal formation
- amygdala
- septal area
Diencephalic components:
- hypothalamus
- mammillary bodies
- anterior nucleus of thalamus
- habenular nucleus
Brainstem components:
- nuclei of the reticular formation
What mnemonic can be used to remember the functions of the hemispheric limbic structures?
HOME
- H - Homeostasis (in hypothalamus)
- O - olfaction (in paraolfactory / septal area)
- M - memory (in hippocampus)
- E - emotions (in amygdala)
- the term “limbic” means “ring / border” and the structures of the limbic system form a ring around the diencephalon*
Identify the structures of the limbic system
What is the dentate gyrus?
What group of structures is this part of?
- it is a thin layer of grey matter with a tooth-like appearance that is associated with the hippocampus proper (purple)
- the dentate gyrus, hippocampus proper and parahippocampal gyrus are part of the hippocampal formation
- the parahippocampal gyrus is continuous with the cingulate gyrus
What are the 4 bundles of white matter fibres associated with the limbic system?
Cingulum:
- band of white matter fibres that underlies the cingulate gyrus
Fornix:
- band of white matter fibres connecting the fornix to the hypothalamus
Stria terminalis:
- white matter fibres connecting the amygdala to the thalamus, hypothalamus & septal area
Stria medullaris thalami:
- connects the habenular nuclei to the septal area
What do the stria terminalis and fornix have in common?
Where do they pass to and from?
- they both follow the curves of the lateral ventricles
- the fornix runs from the hippocampus, arches over the thalamus and projects medially to the mammillary bodies of the hypothalamus
- the stria terminalis runs from the amygdala and arches over the thalamus, but more laterally than the fornix, to reach the hypothalamus & septal area
What is the significance of higher mammals having a neocortex that is larger than their archicortex (emotional brain)?
- lower species rely on their emotional brain to drive their behaviour
- higher species have a neocortex which allows them to make more rational decisions as this has an influence over behaviour / emotions
Where is the limbic lobe located?
What is the isthmus and what is this continuous with?
- the limbic lobe is the cortical component of the limbic system that is located above the corpus callosum** and **below the cingulate sulcus
- the isthmus is the part of the cingulate gyrus that arches behind the splenium of the corpus callosum
- it is also called the retrosplenial cortex
- the isthmus is continuous with the parahippocampal gyrus, which overlies the hippocampus
- the parahippocampal gyrus hooks upon itself to form the uncus, which overlies the amygdala
What is the role of the cingulum?
- this is a bundle of white matter fibres that underlies the cingulate gyrus
- it retrieves information from many different areas of the brain and brings it all together to reach the hippocampus (centre for memory) and amygdala (centre for emotion)
- the hippocampus can store this information as short-term memories
- these memories can be redistributed to other areas of the cortex to consolidate them as long-term memories
What is the hippocampal formation?
What are the 3 main components?
- a nuclear complex located in the medial temporal cortex that is mainly involved in memory
- the 3 main components are:
- hippocampus proper (seahorse shaped)
- dentate gyrus
- subiculum
What is the subiculum?
- a large area of grey matter that acts as a transitional area between the hippocampus proper and the entorhinal cortex
- the entorhinal cortex is the grey matter of the parahippocampal gyrus
Why is the dentate gyrus separate from the other structures of the hippocampus?
- during embryological development, all components of the hippocampus begin as one
- the hippocampal sulcus forms due to folding during development, which separates the dentate gyrus from other structures
- fimbriae and alveus are white matter structures arising from the hippocampus
How are the dentate gyrus and hippocampus proper positioned relative to each other?
What is significant about the anterior region of the hippocampus proper?
- the dentate gyrus is located more medial than the hippocampus proper
- the fimbriae of the fornix is the white matter layer lying between these 2 structures
- the anterior dilation of the hippocampus proper is the pes hippocampi
- this region has foot-like projections
What is the alveus?
- the alveus is the white matter surrounding the hippocampus
- cell bodies within the hippocampus proper send axons via the alveus, which continue as the fimbriae of the fornix
What is the role of the fornix?
- it contains the main efferent fibres of the hippocampus
- it follows a C-shape over the thalamus and then some fibres travel to the mamillary bodies and some travel to the septal area
What are the different parts of the fornix and where do they travel?
- the fimbriae are the collection of white matter fibres that leave the hippocampus
- these originate from the alveus (white matter surrounding the hippocampus)
- the fimbriae lead to the crus, which leaves the temporal lobe ventral to the splenium of the corpus callosum
- the body is the part that arches over the thalamus
- the columns are the part that dives deep into the mammillary bodies
Why is a lesion to the hippocampus on one side not too serious?
- the hippocampal commissure (commissure of the fornix) allows for exchange of information between the hippocampi on both sides
Complete the labels
Coronal section:
- the fimbriae of the fornix are seen inferiorly as they emerge from the hippocampus
- the body of the fornix is seen more superiorly arching over the thalamus
Horizontal section:
- the columns of the fornix are seen diving towards the mammillary bodies
- we know these are not the mamillary bodies as we would also be able to see the midbrain (more inferior section)
What needs to be opened up in order to see the hippocampus?
- the hippocampus can be seen when the inferior horn of the lateral ventricle is opened up
- the hippocampus lies medial to the inferior horn and also forms some of its roof
What is the function of the precommissural portion of the fornix?
What about the mamillothalamic fasciculus?
Precommissural portion of the fornix:
- the majority of fibres enter the columns of the fornix and dive deep to reach the mammillary bodies
- some fibres split from the columns of the fornix before they reach the MBs and enter the precommissural portion of the fornix
- this travels to the septal area
Mamillothalamic fasciculus:
- this describes the fibres of the fornix that travel to the anterior nucleus of the thalamus
What is the role of the Circuit of Papez?
- it is involved with the reinforcement of emotion, memory and olfaction with each other
What are the 2 different inputs to the hippocampus involved in the circuit of Papez?
What happens at the level of the hippocampus?
- fibres reach the hippocampal formation via the cingulum
- some fibres will also reach the hippocampal formation from the entorhinal cortex of the parahippocampal gyrus
- the entorhinal cortex is also connected to the cingulum
At the level of the hippocampus, memories are stored
What is the efferent pathway of the hippocampus involved in the circuit of Papez?
How does this travel and how does it divide as it descends?
- the efferent pathway of the hippocampus is the fornix
- cell bodies within the hippocampus send projections as the fimbriae of the fornix, which leave the hippocampus via the alveus
- the fimbriae continue as the crura, then the body and then the columns
- as the fornix descends as the columns, it will divide into pre-commissural and post-commissural fibres
- these are located anterior and posterior to the anterior commissure
What are the origins and terminations of the pre-commissural and post-commissural fibres of the fornix?
Pre-commissural fibres:
- originate in the hippocampus proper and terminate in the septal area
Post-commissural fibres:
- originate in the subiculum and terminate in the mammillary bodies, where they synapse
What is the role of the mammillothalamic tract (MTT) of the circuit of Papez?
What does this connect to and how does it complete the circuit?
- the MTT travels from the mammillary bodies to the anterior nucleus of the thalamus
- the anterior nucleus sends thalamocingulate fibres which reach the cingulate gyrus via the genu of the internal capsule
- these fibres join the cingulum, which can then reach many specific areas of the cortex
- including the prefrontal cortex, parietal/temporal association areas & entorhinal cortex
What are the main functions of the hippocampal formation?
What happens if there is damage to the hippocampal formation?
the hippocampal formation is vital for the formation of new memories** and **learning
- the hippocampal formation receives all types of information via the cingulum
- when specific items of information need to be remembered, the HF releases signals that enable these items to be rehearsed repeatedly until they are permanently stored in areas of the cortex associated with long-term memory
Damage to the hippocampus:
- leads to inability to recall recent events (short-term memory) and problems with learning
- memories of the distant past (long-term memory) and intelligence remain intact
Where is the amygdala located?
What is it composed of?
- the amygdala is an almond-shaped nuclear complex located in the medial temporal lobe
- it is deep to the uncus and anterior to the hippocampus
- the amygdala is not a uniform mass of grey matter - it is composed of subnuclei:
- large basolateral nuclei
- small corticomedial** and **central nuclei
What is shown by the arrows?
coronal section demonstrating the amygdala
- the hippocampal formation cannot be seen as the section is taken too far anteriorly
- the mammillary bodies, posterior cerebral artery and basilar artery can be seen
- the columns of the fornix are visible more superiorly
What is uncal herniation?
- the amygdala is underlying the uncus
- a mass in the brain can compress the brain and lead to different types of herniation
- in uncal herniation, the uncus herniates underneath the tentorium cerebelli and compresses the midbrain
What are the 3 inputs into the amygdala?
- the olfactory tract is connected to the central and corticomedial nuclear groups
- the solitary tract of the brainstem is connected to the central nucleus
- the cingulate and parahippocampal gyri are connected to the basolateral nuclear group
What are the 3 outputs from the amygdala?
Corticomedial nuclear group:
- projects to the septal area and hypothalamus via the stria terminalis
Basolateral nuclear group:
- projects to the hypothalamus and thalamus via ventral amygdalofugal fibres
Central nucleus:
- projects to the hypothalalmus, thalamus and nuclei of the ANS via ventral amygdalofugal fibres
Describe the path of the dorsal and ventral amygdalofugal fibres
Dorsal amygdalofugal fibres (stria terminalis):
- this arches over the thalamus lateral to the fornix
- it projects to the septal area and hypothalamus
this can elicit autonomic responses based on emotional state at the time
Ventral amygdalofugal fibres:
- band of fibres that extends from the amygdala to the thalamus, hypothalamus or autonomic nuclei of the brainstem
- projections are mainly to the dorsal motor nucleus of X
this allows for stimulation of the vagus nerve depending on emotional state
What is the source of the main input into the amygdala from the brainstem?
- there are several inputs from the brainstem, but these are mainly from the solitary nucleus
- this monitors the internal state of the viscera
