Olfaction and the limbic system Flashcards
What the three cell types that make 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?
Inferior to the frontal lobe, on top of the cribriform plate
Which cells synapse with which within the olfactory bulb?
The bipolar cells pass their axons through the cribriform plate to synapse with the second order neurones (olfactory bulb mitral cells) in the glomerulus (since multiple branches and synapses) within the olfactory bulb
What structure do the second order neurones form and what does this structure split into?
Olfactory tract
It travels along the inferior surface of the frontal lobe and then it splits to form the medial and lateral olfactory stria
Where does higher processing of smell take place?
Piriform Cortex (in the temporal lobe) Orbitofrontal Cortex (area of the prefrontal cortex that sits just above the orbits)
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
Explain thesignificance of this with regards to epileptic patients.
Epilepsy is often focused in the temporal lobe
This means that some people with epilepsy will experience PRODROMAL AURA (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
What structures make up the limbic system and how would you describe this system?
Hippocampus, amygdala, mammilary bodies, fornix, cingulate gyrus (and also forebrain structures)
=> Structurally and functionally interrelated areas considered as a single functional complex
State some roles of the limbic system?
- Homeostasis (mainly hypothalamic functions such as regulation of food intake and pituitary hormone release)
- Agonistic behaviour (defence and attack)
- Sexual and reproductive behaviour
- Memory
Which parts of the limbic system are found within the temporal lobes.
Hippocampus and Amygdala
Where is the cingulate cortex located?
The cingulate cortex is situated in the medial aspect of the cerebral cortex. The cingulate cortex includes the entire cingulate gyrus, which lies immediately above the corpus callosum
Describe/draw the papez circuit.
Hippocampus –> Fornix –> Mammillary Bodies –> Mammillo-Thalamic Tract (MTT) –> Anterior Nucleus of the Thalamus –> Cingulate Cortex –> Cingulum Bundle –> Hippocampus
What is the function of the Papez circuit?
It is a neural circuit for the control of emotional expression
What feeds into this circuit via the cingulate cortex to provide emotional ‘colouring’?
Neocortex
What form of imaging can be 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 (aslo in the medial temporal lobe) is linked to the hippocampus via the afferent pathway (perforant pathway)
The entorhinal cortex receives input from all other parts of the neocortex
State the efferent pathway of the hippocampus
Fimbria –> Fornix
What are the functions of the hippocampus?
Memory and learning
Describe the spatial relations of the hippocampus and the fornix to other important brain structures.
The hippocampus is found on the floor of the lateral ventricles
Each fornix comes out of the hippocampus (R and L respectively) posteriorly and passes under the corpus callosum connected to each other by a thin sheet of transverse fibres called the commisure of the fornix.
It then dives inferior and anteriorly towards the mammillary bodies
State the salient features of advanced Alzheimer’s disease on a CT head scan in the coronal plane.
- Extensive cortical atrophy
- Ventricles appear enlarged
- Widening of sulci
- Shrinkage of the hippocampus
Describe two microscopic hallmarks of neurodegeneration.
- Tau Immunostaining:
- Intracellular buildup of abnormal tau protein which cause neurofibrillary tangles (normal tau functions to stabilise microtubules) - Senile Plaques
- Extracellular deposits of amyloid beta in the grey matter of the brain associated with degenerative neural structures
Describe the anatomical progression of Alzheimer’s disease, including the symptoms experienced (from early to moderate to late)
Early: - Hippocampus and entorhinal cortex affects - Short-term memory problems Moderate: - Parietal lobe (where you have your procedural memory) - Dressing apraxia Late: - Frontal lobe - Loss of executive skills
Where is the amydala found?
A nucleas located in the white matter of the anterior medial 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 emotion/response is the amygdala involved in?
Fear & anxiety
Fight or flight
State a syndrome involving the amygdala. What is the cause and what are the symptoms?
Kluver-Bucy Syndrome
Bilateral lesions of the anterior temporal lobe (including amygdaloid nucleus) =>
- Hyperorality
- Hypersexuality
- Loss of Fear
- Visual Agnosia
State three structures shown experimentally to be associated with aggression. What is the main neurotransmitter involved?
Hypothalamus
Brainstem (periaqueductal grey matter)
Amygdala
5-HT (serotonin)
What are the main afferent connections of the septal nuclei?
Amygdala
Olfactory Tract
Hippocampus
Brainstem
What function do the septal nuclei provide? Where are they found?
Reinforcement & reward
Located on the lower posterior part of the frontal lobe; the septal area refers to the nearby septum pellucidum
Name another structure that is important in the reward system
Nucleus accumbens
Name and describe another dopaminergic pathway other than the nigro-striatal pathway that is affected in Parkinson’s disease.
Mesolimbic pathway
Ventral Tegmental Area (VTA) of the midbrain –> (via Median Forebrain Bundles) –> Cortex + Nucleus Accumbens + Amygdala
Drugs of abuse e.g. opioids, nicotine, amphetamines, ethanol and cocaine have what effect on the nucleus accumbens
Increase dopamine release in nucleus accumbens
