Structure & Function of the Limbic System Flashcards
What is the limbic system?
Anatomical concept
Group of cortical and subcortical nuclei on medial aspect of frontal/parietal/temporal lobes
All interconnected by cortico-cortical pathways
What is the blood supply to the limbic cortex in the frontal and parietal lobes?
Anterior cerebral artery
What is the blood supply to the limbic cortex in the temporal lobe?
Posterior Cerebral artery
What part of the limbic cortex does the middle cerebral artery supply?
Tip of the temporal lobe and orbital cortex
What cortices make up the limbic cortex in the frontal and parietal lobes?
Orbito-frontal cortex
Cingulate cortex
What cortex makes up the third part of the limbic cortex?
Parahippocampal cortex in the medial temporal lobe
What does the limbic system do?
- mediates sense of reward which you get from promoting survival (eating/drinking/sex)
- mediates sense of unhappiness/misery when decrease chance of survival (pain when injured/depressed when you fail)
What is the limbic system fundamentally involved in?
Memory processes
- to be rewarded/punished for something, it comes after the act so you have to remember the experience
What else is the limbic system involved in?
Learning - when an act is rewarded you tend to do it again vs. if it is punished you avoid doing it again
What are motivational processes?
Processes which link reward or punishment to changes in behaviour
What are emotional processes?
Subjective experience of motivational processes
Why do some smells make you feel hungry/nauseous?
Limbic system is intimately connected to the olfactory system
What is the association of the limbic system with the olfactory system?
Some smells are hard wired to pleasure/unpleasant emotions and don’t have to learn
What are reward circuits?
Neuronal circuits that are active when you have sensation of pleasure/joy/happiness
What are negative reinforcement circuits?
Essentially punishment circuits but cannot call them this
- neuronal circuits which are active when you are unhappy/depressed/in pain
When is the anterior cingulate cortex active?
During experience of pain/feelings of misery or depression
What is the pathway of pain/
Nociceptive pain info travels up lateral SPT
- goes to parabrachial nucleus to amygdala
- via anterior thalamic nucleus to anterior cingulate cortex and insula
What is the function of the anterior cingulate cortex?
Cortical area responsible for unpleasantness of pain and its depressing emotional consequences
What is the function of the prefrontal cortex?
Activated to do something about the pain you experience
What is cingulotomy?
cutting into anterior cingulate gyrus to disrupt fibres which pass rosro-caudally though it
- reduces emotional distress of pain
What is the function of the orbitofrontal cortex?
- anterior part of cingulate merges into it
- associated with behaviour when a threat is anticipated
- fits in with frontal love role as involved in planning future actions
- eg. actions to avoid pain
What happens if making a decision when a threat is anticipated is very difficult?
- no win situation
- unable to chosoe between actions so freezes
- prolonged pressure to make impossible choices = extreme stress
- release of stress hormones (cortisol and adrenaline)
- leads to sense of hopelessness and clinical depression
What is different about the orbito-frontal limbic cortex in patients with OCD?
MRI shows excessive activity in orbito-frontal limbic cortex
What is the posterior cingulate cortex involved in/
Emotional significance of memories
Where does the parahippocampal gyrus lie?
Medially and inferiorly in temporal lobe
What is the role of the parahippocampal gyrus?
Primary involve with acquisition of new memories
Closely connected to subcortical parts of limbic system inside temporal lobe
What are the subcortical parts of the limbic system?
- hippocampus and amygdala
- around walls of inferior horn of lateral ventricles in temporal lobe
Where does the hippocampus lie?
Along medial wall of inferior horn of lateral ventricle
- output fibres form fornix
Where is fornix?
Curves upwards and round over the top of the 3rd ventricle
- fibre tract leaving hippocampus
- underneath corpus collosum
- dives down into hypothalamus
When the fornices converge what do they attach to?
Septum pellucidum
Where do the fornix axons end?
In the septal nuceli and the mammillary body of the hypothalamus
How does information flow between the hippocampus and the limbic cortex?
- from cingulate cortex to parahippocampal gyrus and entorhinal cortex
- to hippocampus
- along fornix to mamillary bodies of hypothalamus
- to anterior thalamus
- back to cingulate cortex
IN A LOOP
What is the entorhinal cortex?
Tail of parahippocampal cortex
What is the name given to the series of connections in the limbic system forming a closed loop?
Papez’s circuit
How does severe anterograde amnesia present?
When someone can no longer store new experiences/memories
As soon as they stop concentrating on it they forget it
Can retrieve stuff from memory before condition arised but cannot put new stuff in it
What is the role of the hippocampus?
Has a mechanism for storage memory
Not where memory is stored
How does the hippocampus provide labels?
Has a spatial map as well as a time of day enabling you to retrieve memories from these labels/possibly other parameters such as who you were with/what the weather was like
Essentially gives new experiences a context meaning they are properly stored
What is declarative memory?
When you ask someone what they remember and they tell you
What is procedural memory?
When someone learns to ride a bicycle/drive a car
This bypasses the hippocampus and so if you have severe anterograde amnesia you may still have limited motor ability
Depends on cerebellum and other brain regions instead
What does loss of hippocampal function cause?
Failure to transfer new experiences into long term memories
Anterograde amnesia
Does not affect ability to learn new motor skills
What is anterograde amnesia?
Loss of ability to store new experiences
What is the Kluver-Bucy syndrome?
Bilateral removal of amygdala and adjacent temporal cortex:
- psychic blindness
- oral tendencies
- altered sexual behaviour
- emotional changes
What does the Kluver-Bucy syndrome suggest?
- electrical stimulation to amygdala = panic/fear/terror responses
- amygdala is a fear centre in the brain
- Made up of subnuclei with different connections and functions
Where is the amygdala?
Embedded in entorhinal cortex in anterior medial temporal lobe
What is the function of the amygdala?
- continuously analyses sensory input to brain
- activates fight or flight response of SNS
- activates behavioural responses (running away/fighting)
- aids ability to recognise emotion in others and therefore empathise/conduct normal social relationships
- complements the hippocampus in labelling new experiences telling you whether they were painful/pleasant etc.
How does the amygdala activate the SNS?
- sends commands to hypothalamus -> reticular formation -> activates reticulospinal tracts -> activates SNS
Where are the septal nuclei?
Lie at the base of the septum pellucidum
merge into basal nucelus of Meynert
What is the ventral striatum
Septal nuclei + basal nucleus of meynert + accumbens nucleus
What is the accumbens nucleus?
Anatomically part of basal ganglia
Functionally part of limbic system
Receives dopaminergic fibres as part of mesolimbic dopaminergic pathway
What is the mesolimibic pathway?
Set of dopamine neurons which project from brainstem next to substantia nigra to accumbens nucleus
What is the function of the accumbens?
Part of ventral striatum
- involved in initiation and termination of behaviours (motor actions) which activate reward pathways
- dopamine input to accumbens is necessary for pleasurable behaviour
What happens if you block dopamine receptors in accumbens?
May help stop motor actions involved in addictive behaviours
What is the dorsal striatum?
- caudate and putamen
- involved in selection of actions based on cognitive plans created in dorsal frontal cortex
- feed into motor cortex via globus pallidus and motor thalamus
What is the ventral striatum?
- nucelus accumbens
- involved in selection of actions based on rewards/threats
- feeds into motor system via ventral pallidum and globus pallidus
What are the similarities between the dorsal and ventral striatum?
- require activity in dopamine axons from midbrain work
