Olfaction and the Limbic System Flashcards
What the three types of cell that makes up the 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
Describe how sensory information is initially processed in the olfactory system.
The bipolar cells pass their axons through the cribriform plate to synapse with the second order olfactory neurones (which have a glomerular structure - called mitral cells) within the olfactory bulb.
Axons of the second order neurons project down the olfactory tract to the brain.
What structure do the second order olfactory neurones form and what does this structure split into?
2nd order olfactory neurons –> olfactory tract –>
It splits to form the medial and lateral olfactory stria
Where does higher processing of smell take place?
- Piriform Cortex
- Orbitofrontal Cortex
These areas have connections to the brainstem and can promote autonomic responses.
What is a clinical deficit in the olfactory system called?
Anosmia - loss of smell usually due to trauma but a degenerative process can cause it too
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
Label:
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 and when it is patients can have PRODROMAL AURA
= if epilectic focus is close to the piriform cortex they may smell something just prior to a seizure
(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 - it has been found that only 5% of PD is inherited so 90% is possibly due to environmental exposures so there is an interest in whether there are environmental toxins that go up the nose to later initiate PD e.g. organophosphates
What is the limbic system?
A rim of cortex adjacent to the corpus callosum and diencephalon = single functional complex
What are the roles of the limbic system?
- Homeostasis maintenance by activation of visceral effector mechanisms, modulation of pituitary hormone release and initiation of feeding and drinking
- Agonistic behaviour (defense and attack/fight or flight)
- Sexual and reproductive behaviour
- Memory - everything you do is coloured by previous experience
State two important parts of the limbic system that are found within the temporal lobes.
Hippocampus and Amygdala
See below: cingulate nucleus is not marked (sits just above the corpus callosum)
What circuit are limbic structures a part of?
Papez Circuit
What is the cortical representation of the limbic system?
Cingulate Cortex
What is the function of the Papez circuit?
It is a neural circuit for the control of emotional expression
Describe/draw the papez circuit.
Cingulate cortex receives information about an emotional experience and passes it onto the hippocampus via the cingulum bundle
Hippocampuss main output is to the hypothalamus via the fornix
Hypothalamus (particularly MAMMILARY BODIES) project to the anterior nucleus of the thalamus via the MTT (mammilo-thalamic tract)
This projects back to the cingulate cortex
NB: memories and personality colour the emotional response. This is largely due to the NEOCORTEX.
What conditions could damage the mammillary bodies leading to amnestic issues?
Chronic
Alcoholism
Wernicke-Korsakoff Syndrome
What is our emotional expression ‘coloured’ by?
Neocortex
What part of the circuit is responsible for EXPRESSION of the emotional response.
Largely 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
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 - fibres coming from the hippocampus, one is still visibly attached to the hippocapus (fimbira) but as soon as it breaks away it is called the fornix.
What are the functions of the hippocampus?
Memory (encoding) and Learning
Long term memories however are stored elesewhere - probably in the parietal cortex.
What happens to the hippocampus in Alzheimer’s disease?
It shrinks severely –> short term memory loss
Describe the spatial relations of the hippocampus and the fornix to other important brain structures.
- Hippocampus is in the medial temporal lobe and is sitting in the ventricle
- 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
- Hippocampus sits in the ventricles surrounded by CSF
Image below: most to the left of the image are the mammillary bodies; the two dots are the amygdala which is usually buried in the white matter of the anterior temporal lobe.
Describe the “seahorse” structure of the hippocampus.
Note:
- Perforant pathway coming from the entorhinal cortex which is immediately adjacent to the hippocampus
- This goes directly into the hhippocampus in the middle
- Processing occurs and fibres pass to the fibria, fornix and through the mammillary bodies.
So perforant IN, fimbria and fornix OUT.
Describe the appearance of advanced Alzheimer’s disease on a CT head scan in the coronal plane.
- There will be specific cortical atrophy - especially in TEMPORAL lobe whereas primary motor and somatosensory cortices aren’t affected much
- The ventricles would appear enlarged - especially seen in the inferior part of brain where ventricles are very obvious and hippocampus is atrophic
- There will also be widening of sulci, shrinkage of gyrii
On the left is an age match control.
State two microscopic hallmarks of neurodegeneration.
Tangles
- Intracellular pathology – the breakdown of cytoskeleton of neurones
- Tangles are made of up Tau protein which becomes hyperphosphorylated
Senile Plaques
- Extracellular pathology
- Lumps of amyloid beta protein sitting in between cells in grey matter
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)
- Dressing apraxia (forgetting how to do everyday action)
Late
- Frontal lobe
- Loss of executive skills
Where is the amydala found?
In the white matter of anterior temporal lobe, in front of the hippocampus
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 - emotions which colour behaviour. Patients who have a pathology of the amygdala are terrified. In psychiatry we are starting to see that these are largely due to neurotransmitter imbalances.
- 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. What are the symptoms?
Kluver-Bucy Syndrome
- Bilateral lesions of the anterior temporal lobe (including amygdaloid nucleus). Largely due to trauma.
Symptoms - revert to developmental behaviour
- Hyperorality
- Hypersexuality
- Loss of Fear
- Visual Agnosia
State three structures associated with aggression.
- Hypothalamus
- Brainstem (periaqueductal grey matter)
- Amygdala
What neurotransmitter is thought to be related to aggression?
5-HT (serotonin) in raphe nuceli below the periaqueductal grey
Where are the septal nuclei located?
Septum is a membrane that divides the two lateral ventricles anteriorly.
At the base of the membrane are septal nuclei.
What are the main afferent connections of the septum?
- Amygdala
- Olfactory Tract
- Hippocampus
- Brainstem
What are the main efferent connections from the septal nuclei?
- Stria medularis thalami
- Hippocampus
- Hypothalamus
What are the functions of the septum?
Reinforcement and Reward
(if you stimulate the septal nuclei in a rodent they will sit and do what you tell them to all day)
Name another structure that is important in the reward system.
Nucleus Accumbens
It is also involed in obsessive compulsive behaviour.
What type of pathway is the mesolimbic pathway?
Dopaminergic - this is probably the pathway of addiction
Describe the course of the dopaminergic pathway involved in drug dependance.
Dopaminergic neurons of the VTA of the midbrain –> via Median Forebrain Bundles (MFB) –> Cortex + Nucleus Accumbens + Amygdala
Name some substances which increase the DA release in the nucleus accumbens. What are their mechanisms?
- Opioids
- Nicotine
- Amphetamines
- Ethanol
- Cocaine (DA reuptake inhibitor)
Mechanisms: midbrain neurons are stimulated, DA release is promoted or there is inhibition of DA reuptake.
What effect do all drugs of abuse have on the nucleus accumbens?
They all increase dopamine release in the nucleus accumbens
In what condition might drugs be used therapeutically which cause the upregulation of the mesolimbic pathway? What are the risks of this?
Parkinson’s Disease
L-DOPA is first line = greater precursor availability. This eventually starts to wear off.
Next line is dopaminergic receptor antagonists which are very effective in some patients but you have to make the family aware of the SE which stem from the overactivity of the mesolimbic pathway –> obsessive compulse activities e.g. internet gambling,
