Unit 10 - Limbic System Flashcards
limbic system - general location
“Limbus” means rim or border – and in this case it’s referring to the set of connected structures along the medial surface of the cerebrum, including cingulate gyrus, and connecting through continuously, to the medial temporal lobe (parahippocampal gyrus)
Function of limbic system
behaviours it is involved in
The limbic system is the seat of emotion, pain, reward and basic survival functions
For example, it functions critically in survival behaviours such as feeding, fight or flight response, aggression and the expression of emotion, as well as pleasure, fear, affection, and memory
plays a role in autonomic, behavioural and endocrine aspects of sexual response
features of some subregions of nuclei
3 layered - phylogenetically older cortices
likely involved in less evolved behaviours
External input to papez circuit
Via our sensory cortices (olfactory, somatosensory, vision, auditory)
+ cortical areas that integrate sensory modalities - insula
functions influenced by limbic system
Social cognition, facial recognition or valence appraisal (i.e. whether something is positively or negatively valanced), can all be processed and interact with the limbic system to determine our subjective emotional experience and dictate our expressive behaviours in response
Papez circuit as an anatomical substrate
anatomical substrate for convergence of cognitive, cortical activities, emotional experience, and expression
What runs in the cingulate gyrus
Carries info via ______ to the ______
Cingulum bundle
carries information via the lingual gyrus to the parahippocampal gyrus (just medial to hippocampus proper)
Where is the amygdala
Anterior and slightly superior to the head (or anterior-most part) of the hippocampus
Main output tract of the hippocampus
Fornix
Main output tract of amygdala
While it has a more direct route to the frontal cortex, it is stria terminalis
Where do the outputs of the hippocampus and amygdala converge
Where do these structures feed info to
These outputs converge on an inferior frontal area, including structures of the hypothalamus (including mammillary bodies, septal area, and the bed nucleus of the stria terminalis)
These structures then feed to the sub-genual cingulate cortex closing the circuit
Overview of Papez circuit
Cingulum → parahippocampal gyrus and entorhinal cortex
Hippocampus → via fornix terminating in the septal area and mamillary bodies
Mamillary bodies → via mamillothalamic tract to the anterior nucleus of the thalamus
Amygdala → via stria terminalis to the BNST
BNST, amygdala & septal area and hypothalamus all relay to the subgenual cingulate closing the circuit
Name the parts of the cingulate gyrus
Where does the cingulate gyrus receive input from
wide-spread areas of association cortices, and is consistently demonstrated to be involved in pain related activation through measurements made of regional cerebral blood flow derived from water PET, or from measuring the blood oxygen-level dependent (BOLD) signal derived from functional MRI
Cingulum bundle
where do the fibres project to
refers to the underlying pair of association white matter tracts beneath each cingulate gyrus, or deep to them, in each hemisphere
fibres project to the entorhinal cortex of the temporal lobe and the hippocampal formation
Cingulotomy
a lesion surgically made in the cingulum tract
This procedure is used to treat severe depression and OCD
What is the subgenual part of cingulate implicated in
BA 24
mood regulation, and a successful site of deep brain stimulation in treatment refractory depression
DBS for bipolar disorder
Between anterior and dorsal cingulate
What gyrus is found in close proximity to the cingulate
Curves continuous with the lingual gyrus inferior to the splenium and ultimately with the parahippocampal gyrus
Location of hypothalamus
hippocampus proper is a long sausage shaped structure lying in the inferior horn of the lateral ventricle, in an antero-posterior orientation (in green)
Input to hippocampus proper
derives from fibres emanating from the parahippocampal gyrus, passing through and area more specifically referred to as the entorhinal cortex and finally, the subiculum, before forming CA1 subregion of the hippocampus
Separation between ventricle space and cisternal CSF
Thin layer of pia mater
parts of hippocampus
CA1
CA2
CA3
CA4
Dentate gyrus
What is hippocampus implicated in
disruptions to memory – amnesia
Output of hippocampus
gathers fibres and thickens as you move posteriorly to form the alveus
Alveus forms fimbria which eventually forms the fornix
Short term memory
Usually refers to minutes or hours
An example of short-term memory is working memory – refers to ability to hold in mind or pay attention to or manipulate several pieces of information at once
Long term memory
Long term memories can be declarative (can put them into words), or non-declarative (there is a certain amount of learning that you’re only partially aware of or are that partially accessible to consciousness)
Types of declarative memory are semantic or episodic
Semantic are facts and information and episodic would be your personal record of your day-to-day events
An example of a non-declarative or implicit memory, would be procedural memory – important when learning a motor skill
Bilateral hippocampal lesions
Anterograde amnesia
Lose the ability to form new memories with a particularly large deficit in episodic memories
procedural memory is usually normal, which means it is still possible to learn new skills, for instance it would be possible to train a person with medial temporal lobe amnesia to play the piano, but they would not be aware that they had received any lessons
Retrograde amnesia
Some pre existing memories may also be lost following hippocampal damage
occasionally extending over several years
Possible function of hippocampus in storage of memories
hippocampus may therefore store memories temporarily prior to consolidation in the neocortex
Alzheimer’s disease
there is severe degeneration of the hippocampus and entorhinal cortex, which is associated with profound disturbance of episodic memory and spatial navigation – forgetting recent conversations or getting lost in familiar places
fornix
parts
where does it arise from
Alveus, fimbria, crura, commissure, body, columns
Arises from hippocampus
Alveus → fimbria → crura → commissure → body → columns
origin of the fornix is the alveus (slim white matter band running into the fimbria)
The fimbria of the fornix (the small flap in the image) runs along the length, antero-posteriorly, of the hippocampus, thickening as it forms the crura of the fornix
Crura (meaning “legs”) are connected to the hippocampus laterally in the temporal lobes and running superiorly, meet at a superior location where they connect in the form of the commissure of the fornix (hippocampal commissure)
This commissure lies superior to the thalami nuclei and inferior to the isthmus of corpus callosum
Anterior to the commissure it the body of the fornix and is more medial in its location, and descends mid-sagittally before separating into columns – one pair more anterior to the anterior commissure and one pair posterior to the anterior commissure
The columns of the fornix run along the base of septum pellucidum
Finally as it descends, the columns of the fornix branch to the septal area, when anterior to anterior commissure or to the mamillary bodies when they are posterior to the anterior commissure
Where do the columns of the fornix divide
In a location superior to anterior commissure
Septal area is anterior to lamina terminalis - contains the anterior commissure
Anterior commissure
tightly packed ovoid bundle of fibres – they cross the midline in the upper portion of lamina terminalis to interconnect the olfactory bulbs and the temporal lobes
Input to septal area
Role of septal area
septal area receives input from the habenula via stria medullaris and also via the fornix
It plays a key role in regulating the hippocampal activity associated with memory and spatial navigation, as well as being involved in pleasure and reward, along with the nucleus accumbens
Where do the mamillary bodies run
Role of mamillary bodies
project onward toward the anterior nucleus of the thalamus via the mammillothalamic tract – represents the main way in which the amygdala and hippocampus are connected to the thalamus
The tract appears to also play a role in spatial navigation
Bilateral lesioning of amygdala results in
Docile and passive behaviour
Amygdalum =
where does the amygdala lie
Almond shaped
lies near the anterior pole of the temporal lobe
It is slightly medial and anterior to the anterior limit of the hippocampus. It is both anterior and superior to the inferior horn of the lateral ventricle (locate ventricle space first)
What is the amygdala implicated in
Anxiety disorders
Disorders of olfaction, detecting danger, sexual behaviour and Kluver-Busy Syndrome
Kluver Busy Syndrome symptoms
Involves lesions of the medial temporal lobe that include the amygdala
In humans, this syndrome involves docility, hypersexuality and visual agnosia (inability to recognise objects)
Role of amygdala in healthy human cognition
amygdala appears to be vital to learning the emotional significance of a stimulus, such as a negative stimulus in the environment, and includes healthy responses of anger, fear, recognition of facial expressions and social cognition
Where is the amygdala located
Structure of amygdala
13 subnuclei
Large lateral subnucleus of amygdala
receives input from sensory cortices
Basal subnucleus of amygdala
acts as an output to the peri-aqueductal grey of the brainstem
Peri-aqueductal gray
gray area encircling the cerebral aqueduct of the midbrain and somewhat in the pons as well, involved in ascending anxiety related system and pain system
layers of subnuclei
paleocortex – 3 layered cortical areas as opposed to the usual 6 layered cortex
This 3-layered cortical areas were present 250million years prior to the emergence of mammals
It’s a structure that is well-developed at birth with phylogenetically older nuclei showing synaptogenesis earlier
, it is though that the interplay balance with orbitofrontal cortex plays a role in anxiety disorders
Range of anxiety disorders
broad range and include generalised anxiety specific phobias, such as agoraphobia (fear of being outdoors), panic disorder, PTSD
Output tract of the amygdala
Where does it project to
Stria terminalis
Projects to bed nucleus of stria terminalis (BNST)
amygdala does have a secondary output tract that more directly reaches the frontal cortices (green in the image) – called ventral amygdalofugal pathway
- more ventral route and more direct and has connections with the periaqueductal gray, thalamus, hypothalamus, septal nuclei, nucleus accumbens and basal forebrain
Location of BNST
bilateral, lateral to columns of fornices, and inferior and lateral to the inferior tip of the lateral ventricle, and superior to the anterior commissure
Projects onwards to anterior hypothalamus
