Olfaction and the Limbic System Flashcards
What the 3 types of cell that makes up the olfactory epithelium?
Bipolar Olfactory Neurones
Sustentacular Cells: support cells mainly providing metabolic support
Basal Cells: for regeneration in olfactory neurones
Where is the olfactory bulb found?
Sitting on top of the cribriform plate
Which cells synapse within the olfactory bulb?
Bipolar cells pass their axons through the cribriform plate to synapse with 2nd order neurones (olfactory bulb mitral cells) in the glomeruli within the olfactory bulb
What structure do the 2nd order neurones form and what does this structure split into?
Olfactory tract
It splits to form the medial + 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
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
Some people experience PRODROMAL AURA (they are aware of an imminent seizure because they’ll get a distinct smell beforehand)
Neurodegenerative disease is a relatively common cause of anosmia. State 2 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 + diencephalon
Structurally + functionally interrelated areas considered as a single functional complex
What are 4 roles of the limbic system?
Processes aimed at survival of individual:
Homeostasis (hypothalamic functions e.g. reg. of food intake + pituitary hormone release)
Agonistic behaviour (fight or flight)
Sexual + reproductive behaviour
Memory
State 2 important parts of the limbic system that are found within the temporal lobes.
Hippocampus
Amygdala
What circuit are the Hippocampus + Amygdala 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.
Hippocampus –> (Fornix) –> Mammillary Bodies of Hypothalamus –> (Mammillo-Thalamic Tract (MTT)) –> Anterior Nucleus of the Thalamus –> Cingulate Cortex –> (Cingulum Bundle) –> Hippocampus
What is our emotional expression ‘coloured’ by?
Neocortex
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
Entorhinal cortex is linked to the hippocampus via the perforant pathway
Entorhinal cortex receives input from all other parts of the neocortex
What is the efferent pathway of the hippocampus called?
Fornix
Fimbria when still attached hippocampus
What are the functions of the hippocampus?
Memory
Learning
What happens to the hippocampus in Alzheimer’s disease?
It shrinks severely
Describe the spatial relations of the hippocampus and the fornix to other important brain structures.
Hippocampus lies on the floor of the lateral ventricles
Fornix comes out of the hippocampus + passes under the corpus callosum
It then dives inferior + anteriorly towards the mammillary bodies
Describe the appearance of advanced Alzheimer’s disease on a CT head scan in the coronal plane.
Extensive cortical atrophy
Ventricles enlarged
Widening of sulci
State 2 microscopic hallmarks of neurodegeneration.
Tau Immunostaining
Intracellular pathology: cytoskeleton has been compromised
Tau proteins staining highlights the damaged neurones
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 + entorhinal cortex effects Short-term memory problems Moderate Parietal lobe (where you have your procedural memory) Effects: dressing apraxia Late Frontal lobe Loss of executive skills + personality change
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 + 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 + 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 3 structures associated with aggression + the neurotransmitter involved
Hypothalamus
Brainstem (periaqueductal grey matter)
Amygdala
5-HT (Serotonin)
What are the main afferent connections of the septum?
Amygdala
Olfactory Tract
Hippocampus
Brainstem
What are the functions of the septum?
Reinforcement
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 PD
Mesolimbic pathway
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?
Increase DA release in the NA
Stimulate midbrain neurones, promote DA release or inhibit NA re-uptake
Where does the neocortex feed into the Papez Ciruit? What influence does it have?
Feeds to the Cingulate cortex
Emotional colouring
