Olfaction and the Limbic System Flashcards
What does the olfactory epithelium contain?
- Bipolar olfactory neurones
- Sustentacular cells – sustaining cells
- Basal cells – regenerative stem cells
What is the olfactory pathway?
Olfactory bulb (mitral cells) -> olfactory tract -> olfactory stria (medial and lateral) -> Piriform cortex (lateral stria), Orbitofrontal cortex (medial stria) -> connections then promote autonomic responses
What is anosmia?
clinical defecit of smell
What is prodromal auras?
In some epilepsy patients, prior to seizures they begin to smell something that isn’t there
What is the limbic system?
Named due to its close adherence to the limb of the corpus callosum
Responsible for general survival instinct:
- Homeostasis – activation of visceral effector mechanisms, pituitary hormone release and initiation of feeding and drinking
- Agonistic behaviour – defence and attack
- Sexual behaviour
- Memory
Composed of:
- olfactory bulbs
- hypothalamus
- amygdala
- hippocampus
- thalamus and frontal lobe
What is the Papez circuit?
Hippocampus -> (via formix) hypothalamus -> (via MTT) Anterior nucleus of Thalamus -> Cingulate cortex [back to hippocampus via cingulum bundle]
MTT = Mammilla-Thalamic Tract
The fornix specifically projects to the mammillary bodies of the hypothalamus
The cingulum bundle runs along the midline of the corpus callosum
The neocortex contributes things such as previous memory
What is DTI used for?
diffusion tensor imaging imaging is used to theorise the pathways/networks in the brain to highlight parts of the brain that are interconnected when performing certain actions/functions (e.g. crying)
What are the functions of the hippocampus?
memory and learning
What are the connections in the hippocampus?
Afferent: Perforant pathway
Efferent: Fimbria-> Fornix
*The efferent is known as the fimbria while the path is attached to the hippocampus, when it leaves, it becomes the fornix
** The Entorhinal cortex receives input from all other parts of the brain for the hippocampus
What is alzheimer’s disease?
Presents with severe cortical atrophy with a particularly affected hippocampus (left picture note the small hippocampus)
Tau immunostaining – in Alzheimer’s disease, sporadic tau proteins in the brain become hyper-phosphorylated and this stops them functioning so they die
Senile plaques – alpha-beta proteins build up in the brain in Alzheimer’s and when there is too much, the cells excrete it and the proteins form toxic plaques
How does alzheimer’s progress?
Early:
- Affected – hippocampus & entorhinal cortex
- Symptoms – short-term memory loss
Moderate:
- Affected – Parietal lobe
- Symptoms – Dressing apraxia (inability to perform purposive actions)
Late:
- Affected – Frontal lobe
- Symptoms – Loss of executive skills (e.g. recognition)
What are the functions and connections of the amygdala?
Functions:
- Fear and anxiety (and opposite)
- Fight or flight
Connections:
- Afferent: Olfactory cortex, septum, temporal neocortex, hippocampus, brainstem
- Efferent: Stria terminalis
What is the clinical significance of the amygdala?
Kluver-Bucy Syndrome
- post-trauma, temporal lobes are sheared off middle cranial fossa and amygdala damaged
Symptoms: hyperorality loss of fear visual agnosia hypersexuality
cannot see horns of ventricles in coronal sections of the brain with the amygdala
What structures are associated with aggression?
- Hypothalamus
- Brainstem – specifically, periaqueductal grey
- Amygdala
Aggression may be stimulated by 5-HT and serotonin release from the Raphe nuclei.
What are the functions and connections of the septum?
Functions:
Reinforcement & reward (from the nucleus accumbens)
Connections: Afferent: - Amygdala - olfactory tract - hippocampus - brainstem Efferent: - Stria medularis thalami - hippocampus - hypothalamus
