Limbic System Flashcards
What is olfaction?
Detection of chemicals as odours
2000-4000 different odours distinguishable
Molecular mechanism unknown
What the three types of cell that makes up the olfactory epithelium?
Bipolar Olfactory Neurones - Travel to cribriform plate
Sustentacular Cells – support cells mainly providing metabolic support
Basal Cells – there is some regeneration in olfactory neurones
(Numbers of neurones tends to progressively decrease in age)
Where is the olfactory bulb found?
Lies on inferior surface of frontal lobes sitting on top of the cribriform plate
Which cells synapse within the olfactory bulb?
The bipolar cells pass their axons through the cribriform plate to synapse with the second order neurones (olfactory bulb mitral cells) in the glomerulus within the olfactory bulb
What structure do the second order neurones form and what does this structure split into?
Olfactory tract
It splits to form the medial and lateral olfactory stria
Where does higher processing of smell take place?
Piriform Cortex
Orbitofrontal Cortex
What is a clinical deficit in the olfactory system called?
Anosmia
Note: Prodromal auras (smell just prior to seizure) are a rarer deficit. Also some possible involvement in Parkinsons as damage seen in bulbs - possibly toxins transported to brain nasally
What is a common cause of anosmia?
Mid-face trauma Impact with enough force could cause a fracture of the cribriform plate, shearing the neurones going from the olfactory epithelium to the olfactory bulb
The piriform cortex is found within the temporal lobe. Explain the significance of this with regards to epileptic patients.
Epilepsy is often focused in the temporal lobe This means that some people with epilepsy will experience PRODROMAL AURA (they are made aware of an imminent seizure because they’ll smell something that’s not there)
Neurodegenerative disease is a relatively common cause of anosmia. State two neurodegenerative diseases that could cause anosmia.
Alzheimer’s disease
Parkinson’s disease
What is the limbic system?
A rim of cortex adjacent to the corpus callosum and diencephalon
What are the roles of the limbic system?
Survival system
- Homeostasis (hypothalamus)
- agonistic (defense and attack) behaviour
- Sexual behaviour
- Memory to response
State two important parts of the limbic system that are found within the temporal lobes.
Hippocampus and Amygdala
These are part of the Papez Circuit (a neural circuit for the control of emotional expression)
What is the cortical representation of the limbic system?
Cingulate Cortex
Describe/draw the papez circuit.
Hippocampus –> Fornix –> Mammillary Bodies –> Mammillo-Thalamic Tract (MTT) –> Anterior Nucleus of the Thalamus –> Cingulate Cortex –> Cingulum Bundle –> Hippocampus
How are the functions of the papez circuit divided?
Cingulate cortex via cingulum bundle to hippocampus (memory coding)
Hippocampus via fornix to mammillary bodies
Mammillary bodies via Mamillary thalamic tract to anterior nucleus of thalamus (emotional expression)
From thalamus to cingulate cortex (experience emotional)
What conditions could damage the mammillary bodies leading to amnestic issues?
Chronic Alcoholism
Wernicke-Korsakoff Syndrome
What is our emotional expression ‘coloured’ by?
Neocortex
Describe the afferent pathway of the hippocampus.
Afferent Pathway = Perforant Pathway The entorhinal cortex is linked to the hippocampus via the afferent pathway (perforant pathway) The entorhinal cortex receives input from all other parts of the neocortex
What is the efferent pathway of the hippocampus called?
Fimbria/Fornix
What are the functions of the hippocampus?
Memory and Learning (In Alzheimer’s the hippocampus severely atrophies and ventricles are enlarged)
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
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
State two microscopic hallmarks of neurodegeneration.
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
Senile Plaques
Extracellular pathology
Lumps of protein sitting in between cells in the neuropil
Describe the anatomical progression of Alzheimer’s disease, including the symptoms experienced.
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
State and describe a syndrome affecting the amygdala.
Kluver-Bucy Syndrome - Bilateral lesions of the anterior temporal lobe (including amygdaloid nucleus)
Symptoms Hyperorality Hypersexuality Loss of Fear Visual Agnosia
State three structures associated with aggression.
Hypothalamus
Brainstem (periaqueductal grey matter)
Amygdala
What are the main afferent connections of the septum?
Amygdala
Olfactory Tract
Hippocampus
Brainstem
What are the functions of the septum?
Reinforcement and Reward
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
Name a structure that is important in drug dependence.
Nucleus Accumbens
What effect do all drugs of abuse have on the nucleus accumbens?
They all increase dopamine release in the nucleus accumbens
