Olfaction and the limbic system Flashcards by Rocky Fella

What makes up the olfactory epithelium?

Bipolar olfactory neurones–bipolar
Sustentacular cells- support cells
Basal cells- regenerate olfactory neurones

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How do olfactory neurones change with age?

We lose them

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Where is the olfactory bulb found?

Sitting just above the cribriform plate- part of ethmoid bone

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Where is the olfactory epithelium found?

Upper part of the nose

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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

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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

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What are the second order olfactory neurones called?

Olfactory bulb mitral cells

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What happens to the olfactory tract?

It splits to form two olfactory stria (medial and lateral) each going to either piriform or orbitofrontal cortex

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What does damage to piriform or orbitofrontal cortices result in?

anosmia

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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

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What is a clinical deficit in the olfactory system?

Anosmia

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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

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Where does most of the olfactory processing take place?

Piriform cortex and orbitofrontal cortex

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Where is epilepsy often focussed?

Temporal lobe

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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

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What is pathology in the olfactory bulb an early aspect of?

Parkinson’s and Alzheimer’s disease

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What is the definition of the limbic system?

A rim of cortex adjacent to the corpus callosum and diencephalon

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What is the main aim of limbic processes?

Survival of the individual

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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

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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)

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What circuit do the amygdala and hippocampus belong to?

Papez circuit

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What cortical representation of the limbic system is there?

Mainly the cingulate cortex which is just above the corpus callosum

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What is the Papez circuit?

A neural circuit for the control of emotional expression

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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

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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 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-sitting above corpus callosum

What completes the loop from the cingulate cortex in the Papez circuir? Circuit doesn't act in isolation-- affected by

It is complete by fibres projecting back to the hippocampus via the cingulum bundle neocortex and cortical areas

How is your reaction to the world affected by what you've seen before?

It is coloured by the neocortex

In which part of the brain does emotional colouring occur Emotional experience Emotional expression What does digital tensor imaging show?

Neocortex Cingulate cortex Emotional expression Fibre tract activity in functional imaging-may aid diagnoses of psychatric disorders e.g bipolar

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) | Connected in one structure to fornix to mamillary bodies

What does the fornix lie immediately below?

The corpus callosum

Where is the amygala found?

in front of hippocampus.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 major pyramidal cells and dentate gyrus 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

3 Key features to identify alzheimers What is the very stereotypical progression of cortical atrophy?

Atrophy of hippocampus, cortical areas and enlarged ventricles Loss of neurones and ventricles are much larger than they should be and widening of the sulci

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-can't do up button Late: Frontal lobe Loss of executive skills--decision making

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? Functions of amygdala Appearance in scans

Stria terminalis- going to anterior parts of the hypothalamus fear and anxiety fight or flight grey matter nucl emb in white matter ant tmp lobe in front of ventricles

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? main neurotransmitter involved

Hypothalamus Brainstem (periaqueductal grey matter) Amygdala Serotonin (aka 5-HT) in the raphe nuclei

What are the main afferent connections of the septal nuclei?

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 pathway lead to in Parkinson's?

Tremulous presentation or akinetic, rigid presentation

What do projections from the VTA dopaminergic nucleus go via? where is vta where is na

The medial forebrain bundle to the cortex, amygdala and nucleus accumbens- important in drug dependence midbrain midbrain

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 other neurotransmitters also modify this system