Limbic system Flashcards
How many different odours can you smell?
2000-4000
What is the molecular mechanism?
Largely unknown
What happens to smell with age?
There is progressive loss
What are the 3 types of olfactory epithelium?
Bipolar Olfactory Neurones
Sustentacular Cells – support cells mainly providing metabolic support
Basal Cells – there is some regeneration in olfactory neurones
Where is the olfactory bulb found?
Sitting on top of the cribriform plate
Where is the olfactory epithelium?
In the upper part of the nose
What is the passage of information from olfactory receptor cells?
Axons of bipolar cells pass through the cribriform plate in the base of the skull up into olfactory bulb
These bipolar cells synapse at glomerulus with second order olfactory neuron which sends axons down olfactory tract toward brain
What can second order olfactory neurons be called?
olfactory bulb mitral cells
What happens to olfactory tract?
It splits to form the medial and lateral olfactory stria
Where does higher processing of smell occur?
piriform and orbitofrontal cortices
What is the relevance of connections to the brainstem?
Connections of the olfactory system to the brainstem because odours can promote autonomic responses e.g. salivate when you smell
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What is a clinical deficit in olfactory system known as?
Anosmia
What is a common cause of anosmia?
Mid-face trauma
Fracture of the skull can break cribriform plate and shear off neurons going from epithelium
Where is epilepsy focused?
Temporal lobe
piriform cortex is also in temporal lobe
What will people with epilepsy experience that may help to indicate arrival of epileptic event?
Prodromal aura
They’ll smell something that isn’t there prior to onset of a seizure
What is a general common cause of anosmia?
neurodegenerative disease
Give two diseases in which pathology in olfactory bulb is an early aspect/presentation
Parkinson’s - abnormal protein accumulation (from my notes)
Alzheimer’s
Describe the genetic aspect of Parkinsons
It’s sporadic
less than 5% due to autosomal dominant/recessive inheritance
Environmental trigger
How can environmental trigger cause some of the early symptoms of Parkinsons?
Stimulation may be through the gut (vagus to brainstem) or through nose, loss of smell and diarrhoea
Not useful as these are common symptoms
What was Broca’s definition of limbic system?
rim or limbus of cortex adjacent to corpus callosum and diencephalon
Structurally and functionally interrelated areas considered as a single functional complex
What functions is limbic system responsible for?
Maintenance of homestasis via:
Homeostasis (mainly hypothalamic functions such as regulation of food intake and pituitary hormone release)
Agonistic behaviour (fight or flight)
Sexual and reproductive behaviour
Memory - the basis of all emotional responses to world related to previous experience
What are the two key parts and where do they sit?
Amygdala ( anteriorly)
Hippocampus (immediately behind the aygdala on floor of inferior horn of lateral ventricle)
Temporal lobe
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Which two structures are part of the Papez circuit?
Hippocampus and Amygdala
What is the cortical representation of the limbic system?
Cingulate Cortex
Which is just above the corpus callosum
Describe papez circuit
Hippocampus (floor of inferior horn of lateral ventricle) Fornix(fibre pathway that comes out of the inferior horn and under the corpus callosum, dives anteriorly to synapse in…) Mammillary Bodies Mammillo-Thalamic Tract (MTT) Anterior Nucleus of the Thalamus Cingulate Cortex(via thalamocortical projections) Cingulum Bundle Hippocampus
Neocortex has output to cingulate cortex based on previous experience
What is particularly damaged in alcoholism and Wernicke-Korsakoff syndrom?
Mammillary bodies
What can the Papez circuit be broken down into in words and which parts are in which?
Emotional colouring: Neocortex
Emotional experience
Emotional expression: hypothalamus
What form of imaging is used to study the limbic system?
Digital Tensor Imaging – shows co-instant activity in different parts of the brain thus showing which parts of the brain are working together
What are the main connections of the hippocampus?
afferent: perforant pathway (adjacent cortex/enterohinal cortex) and that adjacent cortex receives input receives from every other neocortical area.
Efferent : fimbria/fornix
What are functions of hippocampus?
Memory and learning
Medial temporal lobe
What happens to hippocampus in Alzheimer’s?
It shrinks severely
Describe the spatial relations of the hippocampus and the fornix to other important brain structures.
The hippocampus is found on the floor of the lateral ventricles
The fornix comes out of the hippocampus and passes under the corpus callosum
It then dives inferior and anteriorly towards the mammillary bodies
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Is amygdala in ventricle?
Nope
White matter at front of temporal lobe
What is the main point to grasp from the coronal section of the hippocampus?
Hippocampus and enterohinal cortex are nearby and are connected by perforant pathway, enterohinal cortex receives input from all other neocortices.
Describe the appearance of advanced Alzheimer’s disease on a CT head scan in the coronal plane.
There will be extensive cortical atrophy
The ventricles would appear enlarged
There will also be widening of sulci
Shrunken hippocampus
Frontotemporal lobe tends to be damaged more than occipital
State two microscopic hallmarks of neurodegeneration in alzheimers.
Tau Immunostaining
• Intracellular pathology – the cytoskeleton has been compromised
• The tau proteins show up in the staining and allow the damaged neurones to be seen, they are microtubule associating proteins
Senile Plaques
• Extracellular pathology
• Lumps of protein sitting in between cells in the neuropil
Pathology starts at trans enterohinal cortex
Describe the anatomical progression of Alzheimers
Early • Hippocampus and entorhinal cortex affects • Short-term memory problems Moderate • Parietal lobe (where you have your procedural memory) • Example of effects: dressing apraxia Late • Frontal lobe • Loss of executive skills
Where is the amydala found?
In the white matter of anterior temporal lobe
What are the afferent connections of the amygdala?
Olfactory Cortex Septum (septal nuclei) Temporal Neocortex Hippocampus Brainstem
What is the main output pathway of the amygdala?
Stria terminalis
What is the function of the amygdala
Fear and Anxiety (fight or flight)
In Alzheimer’s and Parkinson’s disease, the amygdala starts showing pathology early on. What are the possible outcomes of this?
Patients could either become terrified of everything or they could become totally disinhibited and become quite aggressive
What is Kluver-Bucy syndrome?
Bilateral lesions of the anterior temporal lobe including amygdaloid nucleus
What are the symptoms of KB
- Hyperorality - start exploring things with mouth again like a baby
- Hypersexuality
- Loss of Fear - can be accompanied by aggression
- Visual Agnosia - can’t recognise objects
State three structures associated with aggression.
Hypothalamus
Brainstem (periaqueductal grey matter)
Amygdala
5-HT in raphe nuclei of brainstem
What are main pathways of septal nuclei?
Afferent: Amygdala, olfactory tract, hippocampus, brainstem
Efferent: Stria medularis thalami, hippocampus, hypothalamus
What are functions of septal nuclei?
Reinforcement & reward
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Name another structure that is important in the reward system.
Nucleus accumbens
Describe another dopaminergic pathway other than the nigro-striatal pathway that is affected in Parkinson’s disease.
Ventral Tegmental Area (VTA) of the midbrain Median Forebrain Bundles Cortex + Nucleus Accumbens + Amygdala
What happens in Parkinsons?
Loss of dopaminergic cells in the substantia nigra in the midbrain projecting up to the basal ganglia.
Where is VTA relative to substantia nigra?
Ventral tegmental area more medial than substantial nigra
Generally what do drugs of abuse do?
Increase dopamine release in nuclue accumbens
stimulate midbrain neurons
promote DA release
Inhibit DA reuptake e.g. cocaine MAO uptake blocker
