Limbic System

Question 1

What are the 3 main roles of the limbic system?

Answer 1

• 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

Question 2

What are the telencephalic, diencephalic and brainstem components of the limbic system?

Answer 2

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

Question 3

What mnemonic can be used to remember the functions of the hemispheric limbic structures?

Answer 3

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*

Question 4

Identify the structures of the limbic system

Answer 4

Question 5

What is the dentate gyrus?

What group of structures is this part of?

Answer 5

• 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

Question 6

What are the 4 bundles of white matter fibres associated with the limbic system?

Answer 6

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

Question 7

What do the stria terminalis and fornix have in common?

Where do they pass to and from?

Answer 7

• 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

Question 8

What is the significance of higher mammals having a neocortex that is larger than their archicortex (emotional brain)?

Answer 8

• 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

Question 9

Where is the limbic lobe located?

What is the isthmus and what is this continuous with?

Answer 9

• 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

Question 10

What is the role of the cingulum?

Answer 10

• 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

Question 11

What is the hippocampal formation?

What are the 3 main components?

Answer 11

• a nuclear complex located in the medial temporal cortex that is mainly involved in memory
• the 3 main components are:

1. hippocampus proper (seahorse shaped)
2. dentate gyrus
3. subiculum

Question 12

What is the subiculum?

Answer 12

• 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

Question 13

Why is the dentate gyrus separate from the other structures of the hippocampus?

Answer 13

• 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

Question 14

How are the dentate gyrus and hippocampus proper positioned relative to each other?

What is significant about the anterior region of the hippocampus proper?

Answer 14

• 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

Question 15

What is the alveus?

Answer 15

• 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

Question 16

What is the role of the fornix?

Answer 16

• 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

Question 17

What are the different parts of the fornix and where do they travel?

Answer 17

• 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

Question 18

Why is a lesion to the hippocampus on one side not too serious?

Answer 18

• the hippocampal commissure (commissure of the fornix) allows for exchange of information between the hippocampi on both sides

Question 19

Complete the labels

Answer 19

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)

Question 20

What needs to be opened up in order to see the hippocampus?

Answer 20

• 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

Question 21

What is the function of the precommissural portion of the fornix?

What about the mamillothalamic fasciculus?

Answer 21

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

Question 22

What is the role of the Circuit of Papez?

Answer 22

• it is involved with the reinforcement of emotion, memory and olfaction with each other

Question 23

What are the 2 different inputs to the hippocampus involved in the circuit of Papez?

What happens at the level of the hippocampus?

Answer 23

• 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

Question 24

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?

Answer 24

• 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

Question 25

What are the origins and terminations of the pre-commissural and post-commissural fibres of the fornix?

Answer 25

**_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

Question 26

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?

Answer 26

* 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*

Question 27

What is the autonomic response associated with the circuit of Papez and why does it occur?

Answer 27

* the mammillary bodies are part of the **hypothalamus**, which is the major autonomic centre * a **particular memory** is able to trigger an **autonomic response** - e.g. salivation, tearing or an increase in HR

Question 28

What are the main functions of the hippocampal formation? What happens if there is damage to the hippocampal formation?

Answer 28

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_**

Question 29

What happens in Korsakoff syndrome and what is the typical cause?

Answer 29

* results from **damage to the _hippocampus**_ or _**mammillary bodies_** due to **_chronic alcoholism_** * there is a **_loss of recent memory_** and tendency to **fabricate false accounts** of recent events * patients may insert remembered events from the remote past into **fluent but untrue stories**

Question 30

Other than short-term memory, what is the other role of the hippocampus?

Answer 30

it has an **_endocrine role_** * inputs to the hypothalamus, which is connected to the pituitary gland, from the HF can exert a modulatory role on endocrine function

Question 31

Where is the amygdala located? What is it composed of?

Answer 31

* 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**: 1. **large _basolateral_** nuclei 2. **small _corticomedial**_ and _**central_** nuclei

Question 33

What are the roles of the amygdala?

Answer 33

***it is involved in the _emotional_ and _behavioural response to stimuli_*** * it is associated with **pain, pleasure** and **rage** * it provides an **emotional component to learning** * it can relate **environmental inputs** to coordinated **autonomic and endocrine responses** (species preservation): 1. sex and nurturing 2. feeding and drinking 3. respose to stress

Question 34

What is shown by the arrows?

Answer 34

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

Question 35

What is uncal herniation?

Answer 35

* 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**

Question 36

What are the 3 inputs into the amygdala?

Answer 36

* 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

Question 37

What are the 3 outputs from the amygdala?

Answer 37

**_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_**

Question 38

Describe the path of the dorsal and ventral amygdalofugal fibres

Answer 38

**_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_***

Question 39

What is the source of the main input into the amygdala from the brainstem?

Answer 39

* there are several inputs from the brainstem, but these are mainly from the **_solitary nucleus_** * this monitors the **internal state of the viscera**

Question 41

What are the 4 main characteristics of Kluver-Bucy syndrome? What causes this?

Answer 41

* caused by **_bilateral_ destruction of the amygdala** * *e.g. following temporal lobectomy* * characterised by: 1. **absence of emotional responses** (fear / aggression are absent) 2. **overly attentive to sensory stimuli** (tendency to place objects in mouth) 3. **hypersexuality** 4. **visual agnosia** (objects not recognised) * visual agnosia is likely due to **damage to the ventral "what" stream** in the temporal lobe