Olfaction & The Limbic System Flashcards
What is the olfactory system?
Sense of smell!
Can smell between 2000-4000 different odours
Anatomy of the olfactory system?
The epithelium contains:
o bipolar olfactory neurones
o sustentacular cells
o basal cells
There is progressive loss of olfactory epithelium with age
The olfactory bulb projects neurones through fenestrations in the ethmoid bone
What is the olfactory pathway?
- Olfactory bulb (mitral cells)
- Olfactory tract
- Olfactory stria (medial & lateral)
- Piriform cortex (lateral stria) and orbitofrontal cortex (medial stria)
- Connections to brainstem - promote autonomic responses
What is the clinical defect in smell known as?
Anosmia
Relationship between epilepsy and smell?
In SOME patients, prior to seizures they begin to smell something that isn’t there - knows as
PRODROMAL AURAS
What is an early target for many pathologies in the olfactory pathway?
Olfactory bulbs
i.e. in people with Parkinson;s
Locations of the Olfactory pathway?
ONENOTE!!
Why is the Limbic System named ‘limbic’?
Due to close adherence to the limb of the corpus callosum (Ludleys!!)
Functions of the Limbic System
Responsible for processes aimed at survival of the individual
o Homeostasis
- activation of visceral effector mechanisms, modulation of pituitary hormone release, initiation of feeding & drinking
o Agonistic behaviour - defence & attack
o Sexual & reproductive behaviour
o Memory
What is the Limbic System compsoed of?
o Olfactory bulbs
o Hypothalamus
o Amygdala
o Hippocampus
o Thalamus
o Frontal lobe
Describe the Papez Circuit
ONENOTE!!
Hippocampus –> hypothalamus –> Anterior nucleus of thalamus –> cingulate cortex (neocortex inputs here)
o Neocortex - contributes things such as previous memory (emotional colouring)
o Cingulate cortex (emotional experience)
o Hypothalamus (emotional expression)
Tracts that take neural information to different parts of the Papez Circuit (output tract)?
o Cingulate cortex –> hippocampus
via. Cingulum bundle
o Hippocampus –> Hypothalamus
via. Fornix
o Hypothalamus –> anterior nucleus of thalamus
via. MTT (mammilla-thalamic tract)
o ANofT –> CC
via. thalamo-cortical projections
Fornix = specifically projects to the mammillary bodies of the hypothalamus
Cingulum bundles run along the midline of the corpus callosum
Function of Papez Circuit?
Neural circuit for the control of emotional expression
DTI?
Diffusion Tensor Imaging
Used to theorise the pathways/networks in the brain
o to highlight parts of the brain that are interconnected when performing certain actions/functions (e.g. crying)
Main connections of the Hippocampus?
Afferent = Preforant pathway
o Entorhinal cortex receives input from all other part of the brain for the hippocampus
Efferent = Fimbria –> fornix
o known as the fimbria
o while the path is attached to the hippocampus, when it leaves, becomes the fornix
ONENOTE!!
Function of the Hippocampus?
Memory & learning
Clinical importance of Hippocampus?
Alzheimer’s & epilepsy
Where is the Hippocampus?
Medial temporal lobe
How does Alzheimer’s Disease present
Presents with severe CORTICAL ATROPHY with a particularly affected hippocampus
Tau immunostaining?
In Alzheimer’s
o Sporadic tau proteins in the brain become hyper-phosphorylated
o HENCE this stops the proteins from functioning so they die
Senile plaques?
Alpha-beta proteins build up in Alzheimer’s
o when there is too much, the cells excrete it
AND
o the proteins form toxic plaques
Anatomical progression of Alzheimer’s?
Early:
o Affected - hippocampus & entorhinal cortex
o Symptoms - short-term memory loss
Moderate:
o Affected - Parieral lobe
o Symptoms - dressing apraxia (inability to perform purposive actions)
Late:
o Affected - Frontal lobe
o Symptoms - loss of executive skills (e.g. recognition)
Main connections of the Amygdala?
Afferent o Olfactory cortex o Septum o Temporal neocortex o Hippocampus o Brainstem
Efferent
o Stria terminalis
Function of the Amygdala?
Fear and anxiety (and opposite)
Fight or flight
Location of the Amygdala?
This is buried in the white matter of the anterior temporal lobe
Clinical importance of the Amygdala?
Kluver-Bucy Syndrome
o post-trauma, temporal lobes are sheared off middle cranial fossa & amygdala is damaged
Symptoms o hyperorality o loss of fear o visual agnosia o hypersexuality
What is an issue brought by the Amygdala when seeing sections of the brain?
Can NOT see the horns of the ventricles in coronal sections of the brain with the amygdala
What are the structures associated with aggression?
o Hypothalamus
o Brainstem - specifically, periaqueductal grey
o Amygdala
What can aggression be stimulated by?
5-HT and serotonin release from the Raphe nuclei
Main connections of the Septum?
Afferent: o Amygdala o Olfactory tract o Hippocampus o Brainstem
Efferent:
o Stria medularis thalami
o Hippocampus
o Hypothalamus
Functions of the Septum?
Reinforcement & reward (from the nucleus accumbens)
ONENOTE!!
Main pathway that can be affected by drug dependence?
Mesolimbic pathway (using dopamine)
o MFB - Median Forebrain Bundle
o Midbrain projects dopaminergic neurons into the cortex, nucleus accumbens & amygdala
Where does the Mesolimbic pathway project to?
Shows the VTN (ventral tegmental nucleus)
o projects neurons into the nucleus accumbens
ONENOTE!!
What is the other dopaminergic pathway?
Dopaminergic neurons from the substantia nigra (in the midbrain) project to the basal ganglia
Drugs that increase DA in the nucleus accumbens?
o Opioids o Nicotine o Amphetamines o Ethanol o Cocaine
What are the different ways that the drugs can increase DA?
o Stimulating midbrain neurons to release DA
OR
o Inhibiting DA re-uptake (e.g. cocaine)
Other NTs can also modify the system
