Neuro 14: Olfaction and limbic system Flashcards
Cells in the olfactory epithelium
bipolar olfactory neurons (primary olfactory neurone)
sustentacular cells (support cells)
basal cells (potential for regeneration… unique)
Effect of age on smell
progressive loss with age
What cells pass throught the cribriform plate. What neurons do these synapse onto
olfactory receptor cells (bipolar)….
these then synapse onto the second order olfactory neurons (=mitral cells) in the olfactory bulb)
Pathway for olfactory system
Olfactory bulb (mitral cells)
Olfactory tract
Olfactory stria
Piriform and orbitofrontal cortex
Connections to brainstem promote autonomic responses (i.e. smell something you want to eat –> salivation)
What is clinical deficit of smell
Anosmia
Where is the piriform cortex
Temporal lobe (inferomedially)
What is a prodromal aura
If they have an epilepsy and the focus is near the piriform cortex, they can smell an odour that isn’t there, just before a seizure
Nose in parkinson’s
smell lost early…. easiest route of disease causing agents into the brain
Function of limbic system
Aimed at survival:
maintenance of homeostasis
agonistic (defence & attack) behaviour
sexual & reproductive behaviour
memory
or
HOMES (homeostasis, olfaction, memory, emotion, sexual and reproductive behaviour)
How does limbic system help to achieve homeostasis
via activation of visceral effector mechanisms, modulation of pituitary hormone release and initiation of feeding and drinking
Components of limbic system
Frontal lobe, thalamus, hippocampus, amygdala, hypothalamus, olfactor bulb and cingulate nucleus
What is the Papaz ciruit
Neocortex –> cingulate cortex
Cingulate cortex –> hippocampus (via cingulum bundle)
Hippocampus –> hypothalamus (via fornix)
Hypothalamus (mamillary bodis) –> anterior nuleus of thalamus (via mamillo-thalamic tract)
Thalamus –> cingulate cortex
Which structure achieves the following:
- emotional colouring
- emotional experience
- emotional expression (response)
neocortex,
cingulate cortex
hypothalamus
Connections of hippocampus
Afferent: Perforant pathway
Efferent: Fimbria (=fornix)
Function of hippocampus
memory and learning
Clinical conditins involving problems with hippocampus
Alzheimer’s (short term memory loss), epilepsy
Where do the hippocampi sit
Lying in the floor of the inferior horn of the lateral ventricle
Where does the fornix project
From hippocampus, up and underneath corpus callosum, forward to the mamillary bodies
Where does the amygdala sit
In the WHITE MATTER of the temporal lobe, medial to the hipoocampi ,which are in the floor of the latera lventircle
Where does the perforant pathway come from
entorhinal cortex
Signs of alzheimer’s
enlgarment of the ventricles, cortical atrophy (in particular the temporal lobe and the parietal lobe...but strangely NOT in the motor or somatosensory cortex... weirdly? occipital lobe also not affected)
so motor, sensory and vision mostly left unaffected
What are tangles in Alzheimers and what are plaques… which is thought to be cause
Through its isoforms and phosphorylation tau protein interacts with tubulin to stabilize microtubule assembly. All of the six tau isoforms are present in an often hyperphosphorylated state in paired helical filaments from AD. and you get cytoskeleton effects leading to tangles in neuorons (some neurons susceptible, some are not)….
then the dead material from within this cell can be dumped outside the cell leading to plaques
Outline the anatomical progression of Alzheimer’s
Early
Hippocampus and entorhinal cortex
Short-term memory problems
Moderate
Parietal lobe
Dressing apraxia
Late
Frontal lobe
Loss of executive skills
Connections of the amygdala
Afferent: Olfactory cortex, septum, temporal neocortex, hippocampus, brainstem
Efferent: Stria terminalis (going to hypothalamus, thalamus and septal nuclei)
Function of amygdala
Fear & anxiety
Fight or flight
In which clinical situituation is amygdala implicated
Kluver-Bucy syndrome
Symptomy of kluver bucy
Hyperorality
Loss of fear
Visual agnosia
Hypersexuality
Sex
Mouth
Agnosia
Loss of fear
(due to bilateal temporal lobe contusion)
Strcutures related to aggression
Hypothalamus
Brainstem (periaqueductal grey)…. seratonin in raphe nuclei involved (as well as other nuclei)
Amygdala
What is the septal nuclei
Nuclei at the base of the septum between the anterior lateral ventricles
Main connections of septal nuclei
Afferent: Amygdala, olfactory tract, hippocampus, brainstem
Efferent: Stria medularis thalami, hippocampus, hypothalamus
NOT REALLY INVOLVED IN MESOLIMBIC PATHWAY THOUGH
Outline mesolimbic pathway
Dopaminergic cells projecting from midbrain (ventral tegmental area, VTN) to the cortex, nucleus accumbens and amygdala.
Travel via the MFB (medial forebrain bundle)
Relevance of mesolimbic pathway
Pathway from VTN to nucleus accumbens has increased dopamine release in drugs such as:
Opioids, nicotine, amphetamines, ethanol and cocaine all
How do drugs affecting the mesolimbic pathway work.
Specifically how does cocaine work
Stimulate midbrain neurons, promote DA release or inhibit DA reuptake
Other neurotransmitters also modify this system
Cocaine inhibits reuptake of dopamine
Problems for parkinsons treatment
usually start with L-DOPA, but when this wears off, you use
dopamine agonists for the nigra-striatal pathway…
BUT as an unwanted effect you get also increased dopamine to the nucleus accumbens through the mesolimbic pathway, which can lead to obsessive compulsive activity (i.e. online gambling)
What are the functions of the medial and lateral olfactory stria
The olfactory tract travels posteriorly on the inferior surface of the frontal lobe. As the tract reaches the anterior perforated substance (an area at the level of the optic chiasm) it divides into medial and lateral stria:
Lateral stria – carries the axons to the primary olfactory cortex, located within the uncus of temporal lobe.
Medial stria – carries the axons across the medial plane of the anterior commissure, where they meet the olfactory bulb of the opposite side.
The primary olfactory cortex sends nerve fibres to many other areas of the brain, notably the piriform cortex, the amygdala, olfactory tubercle and the secondary olfactory cortex. These areas are involved in the memory and appreciation of olfactory sensations.
Where are the hippocampi located
floor of the lateral ventricle..
What does the hippocampus connect to and how
Connects to the mamillary bodies…. via the fornix which runs underneath the corpus collosum
Outline the connection between the cingulate cortex and the hippocampus
Cingulate cortex –> cingulum bundle –> entorhinal cortex –> perforant pathway –> hippocampus
Which areas of the brain are affected in alzheimers and which are not
ARE: atrophy of the temporal lobe…. hippocampus affected a lot (leading to memory loss)
Occipital lobe largely unaffected so visual symptoms not prevalnet
Primary somatosensory and motor cortices unaffected. (apart from dressing apraxia in the moderate stage due to some parietal cell damage)
Where are mitral cell bodies located
Cell bodies are arranged in a thin mitral cell body layer between the granule cell layer and the external plexiform layer
State the nuclei in the amygdala
Cortico-medial nuclei
Central nucleus
Basolateral nuclei
Why is the olfactory nerve special
Because it contains basal cells which can regenerate, so you can get new neurons here over a life time.
However, with age, they can be lost faster than be replaced by the basal cells.
Loss of neurons here is not inevitable and may be associated with neurodegenerative disease.
What is the limbic system
rim or limbus of cortex adjacent to corpus callosum and diencephalon
Structurally and functionally interrelated areas considered as a single functional complex
