Olfaction and the limbic system FINAL Flashcards
What makes up the olfactory epithelium?
Bipolar olfactory neurones- these have two processes
Sustentacular cells- support cells mainly providing metabolic support
Basal cells- regenerate olfactory neurones
How do olfactory neurones change with age?
We lose them
Where is the olfactory bulb found?
Sitting just above the cribriform plate, underneath the frontal lobe
Where is the olfactory epithelium found?
Upper part of the nose
How do the axons of the bipolar cells travel?
They pass through the cribriform plate in the base of the skull up into the olfactory bulb
What happens when these bipolar cells reach the olfactory bulb?
They synapse at a glomerulus with a second order olfactory neurone which send their axons down the olfactory tract towards the brain
What are the second order olfactory neurones called?
Olfactory bulb mitral cells
What happens to the olfactory tract?
It splits to form two olfactory stria (medial and lateral)
Where does higher processing of smell occur?
In the piriform and orbitofrontal cortices
What does damage to piriform or orbitofrontal cortices result in?
Inhibition of normal smell processes
What effect does connections of the olfactory system to the brain stem have?
Odours can promote autonomic responses e.g. salivating when you smell foot
What is a clinical deficit in the olfactory system?
Anosmia
What is a common cause of anosmia and how?
Mid-face trauma- if you get smacked in the nose that is serious enough to cause a fracture of the skull you can break the cribriform plate and shear off the neurones going from the olfactory epithelium
Where does most of the olfactory processing take place?
Piriform cortex and orbitofrontal cortex
Where is epilepsy often focussed?
Temporal lobe
What does the fact that epilepsy is often centred in temporal lobe lead to?
Some people with epilepsy will experience prodromal aura where they are made aware that they’re going to have a seizure because they’ll smell something that isn’t there
What is pathology in the olfactory bulb an early aspect of?
Parkinson’s and Alzheimer’s disease
What is thought to be the cause of Parkinson’s
It is a very sporadic disease with less than 5% of cases being autosomal dominant or recessive inheritance. It is thought that there could be some environmental trigger
What is thought to be the environmental trigger of Parkinson’s?
In the gut (up the vagus nerve to the brainstem) or through the nose (little between top of nose and brain)
What is the definition of the limbic system?
A rim of cortex adjacent to the corpus callosum and diencephalon
What is the main aim of limbic processes?
Survival of the individual
What behaviour is the limbic system responsible for?
Maintenance of homeostasis via: -Activation of visceral effector mechanisms -Modulation of pituitary hormone release -Initiation of feeding and drinking Agnostic behaviour Sexual and reproductive behaviour Memory
What are the two key parts to the limbic system and where are they found?
They are found in the temporal lobe:
Amygdala (anteriorly)
Hippocampus (immediately behind the amygdala on the floor of the inferior horn of the lateral ventricle)
What circuit do the amygdala and hippocampus belong to?
Papez circuit
What cortical representation of the limbic system is there?
Mainly the cingulate cortex which is just above the corpus callosum
What is the Papez circuit?
A neural circuit for the control of emotional expression
Where is the hippocampus and what is its main output pathway?
It is found on the floor of the inferior horn of the lateral ventricle and its main output pathway is the fornix
What is the fornix?
The fibre pathway that comes out of the inferior horn, passes under the corpus callosum and dives down anteriorly to synapse in the mamillary bodies (in the hypothalamus)
What are the mamillary bodies particularly damaged in?
Chronic alcoholism and Wernicke-Korsakoff syndrome- these diseases involve amnestic tissues
What is the pathway from the mamillary bodies in the Papez circuit?
The fibre pathway called the mammillo-thalamic tract (MTT) which projects to the anterior nucleus of the thalamus
What projects from the anterior nucleus of the thalamus?
There are thalamocortical projections in this case going to the cingulate cortex.
What completes the loop from the cingulate cortex in the Papez circuir?
It is complete by fibres projecting back to the hippocampus via the cingulate vundle
How is your reaction to the world affected by what you’ve seen before?
It is coloured by the neocortex
What does digital tensor imaging show?
This form of imaging shows co-instant activity in different parts of the brain hence shows which parts of the brain are working together
What is the main afferent connection of the hippocampus?
The perforant pathway- main connections are from the adjacent cortex (entorhinal) through the perforant pathway and that adjacent piece of cortex receives input from every other neocortical area. Every other piece of cortex sends some projections to the entorhinal crotex and from there via the perforant pathway you have input to the hippocampus
What is the main efferent connection of the fimbria/fornix?
This is the output pathway from the hippocampus- part of the Papez circuit
What is the function of the hippocampus?
Memory and learning
What clinical conditions affect the hippocampus?
Alzheimer’s and epilepsy
Where is the hippocampus?
In the medial temporal lobe
What happens to the hippocampus in Alzheimer’s?
There is severe shrinking of the hippocampus
What do you find on the floor of the lateral ventricles?
The hippocampus on either side (look a bit like penises)
What does the fornix lie immediately below?
The corpus callosum
Where is the amygala found?
Not in the ventricle but in the white matter at the front of the temporal lobe
What does the hippocampus look like in a coronal section?
Seahorse shape of the two interlocking Cs of the primary cells - this is why it is called hippocampus because it means seahorse
What key area is next to the hippocampus?
Entorhinal cortex which receives inputs from every other cortical area- prohections into hippocampus from entorhinal cortex is via the perforant pathway
What is cortical atrophy a general feature of?
Alzheimer’s
What is the very stereotypical progression of cortical atrophy?
Loss of neurones and ventricles are much larger than they should be and widening of the sulci
What type of staining allows you to see damaged neurones?
Tau immunostaining- tau protein which shows intracellular pathology
What do senile plaques show?
Extracellular pathology- lumps of protein sitting in between cells in the neuropil
What is the normal anatomical progression of Alzheimer’s?
Early: Hippocampus and entorhinal cortex- leads to short term memory problems
Moderate:
Parietal lobe- procedural memory e.g. dressing apraxia
Late:
Frontal lobe
Loss of executive skills
Where is the amygdala found?
It is buried in the white matter of the anterior temporal lobe and it has connections to basically everything
What are the main afferent connections of the amygdala?
Olfactory cortex Septum Temporal neocortex Hippocampus Brainstem
What is the main efferent connection of the amygdala?
Stria terminalis- fear and anxiety- focus of degenerative disease
How does the amygdala show pathology early on in Alzheimer’s and Parkinson’s?
It can go in two directions: people who are tired of everything or people who are totally disinhibited and can become quite agressive
What does Kluver-Bucy syndrome result from?
Bilateral lesions of the anterior temporal lobe (including amygdaloid nucleus)
What are the symptoms of Kluver-Bucy syndrome?
Hyperorality- exploring things with your mouth
Loss of fear
Visual agnosia- inability to recognise objects
Hypersexuality
What structures are associated with aggression?
Hypothalamus
Brainstem (periaqueductal grey matter)
Amygdala
Serotonin (aka 5-HT) in the raphe nuclei of the brainstem appear to also be involved in agression
What are the main afferent connections of the septum?
Amygdala
Olfactory tract
Hippocampus
Brainstem
What are the main efferent connections of the septum?
Stria medularis thalami
Hippocampus
Hypothalamus
What are the functions of the septum?
Reinforcement and reward
What do the motor problem’s in Parkinson’s disease arise from?
Loss of dopaminergic cells in the substantia nigra in the midbrain projecting up to the basal ganglia
What does the breakdown of the dopaminergic (mesolimbic) pathway lead to in Parkinson’s?
Tremulous presentation or akinetic, rigid presentation
What do projections from the VTA dopaminergic nucleus go via?
The medial forebrain bundle to the cortex, amygdala and nucleus accumbens- important in drug dependence
How does the VTN relate to the substantia nigra?
VTN is more medial
Where do the dopaminergic neurones of the substantia nigra project to?
They project to the basal ganglia
Where do the dopaminergic neurones of the ventral tegmental area project to?
Nucleus accumbens
What effect do all drugs of abuse have in dopaminergic neurones?
They all increase dopamine release in nucleus accumbens
How do drugs of abuse increase dopamine release in nucleus accumbens?
They stimulate midbrain neurones
Promote DA release
Inihbit DA reuptake
