Olfaction and the Limbic system Flashcards
how many different odours can we smell?
2000-4000
what is contained in the olfactory epithelium [3]?
o Bipolar olfactory neurones (primary neurone)
o Sustentacular cells – metabolic/support cells.
o Basal cells – regenerative stem cells.
through which bone does the olfactory bulb project its neurones
ethmoid bone
- it has fenestrations
how does loss of olfactory epithelium change?
progressive with age
olfactory pathway to promote autonomic responses
- bipolar olfactory neurones (primary neurone)
- via cribriform plate
- olfactory bulb has mitral cells (secondary neurone)
- olfactory tract
- olfactory stria (medial and lateral)
- Piriform cortex (lateral stria), Orbitofrontal cortex (medial stria)
connections then promote autonomic responses.
clinical deficit in smell
anosmia
prodromal auras
In some epilepsy patients, prior to seizures they begin to smell something that isn’t there , indicative on pending seizure
example of disease that targets the olfactory bulbs
Parkinsons
what is the limbic system responsible for?
survival instinct: the 4 Fs
o Fighting (fight or flight) - Agonistic behaviour – defence and attack, aggression or fear o Fucking - Sexual behaviour. o Forgetting - Memory o Feeding
what are the components of the limbic system?
what is the function of the limbic system?
olfactory bulbs hypothalamus amygdala hippocampus thalamus frontal lobe
function:
- feeding
- fucking (sexual behaviour and reproduction)
- fighting (flight or flight, fear and aggression)
- forgetting (memory)
emotional experience section of Papez Circuit
- Hippocampus to..
- hypothalamus (mam. bodies) to…
- Anterior nucleus of Thalamus too…
- Cingulate cortex back to the hippocampus
what connects the hippocampus to the hypothalamus?
fornix
what connects the mamillary bodies of the hypothalamus to the anterior nucleus of the thalamus?
mamillothalamic tract (MTT)
projects from mamillary bodies of the hypothalamus to the anterior nucleus of the thalamus
what connects the cingulate cortex to the hippocampus?
cingulum bundle
runs along the midline of the corpus callosum
what does the neocortex contribute to in the Papez Circuit (control of emotional expression)?
emotional colouring using previous memory
what is DTI used for?
Diffusion Tensor Imaging used for visualising the limbic system:
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 is the afferent pathway of hippocampus?
perforant pathway
coming from the enterohinal cortex
what is the efferent pathway of hippocampus?
fimbria/ fornix
towards the mammilary bodies
fimbria (path attached to hippocampus)
fornix (detached, when it leaves)
function of hippocampus
memory and learning
what are 2 pathologies of the hippocampus?
1) Alzheimers
2) Epilepsy
remember hippocampus is in the medial temporal lobe
What do Alzheimers patients present with?
cortical atrophy affecting the hippocampus particularly
- loss of neurones
- ventricular enlargement
- widening sulci
- frontotemporal lobe more atrophied than occipital (unique to Alzheimers)
what can be seen in tau immunostaining of the brain in Alzheimers?
in Alzheimer’s disease, sporadic tau proteins in the brain become hyper-phosphorylated and this stops them functioning so they die.
what causes the formation of senile plaques in Alzheimers?
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.
what is affected in early stage Alzheimers?
Affected – hippocampus & entorhinal cortex.
Symptoms – short-term memory loss.
then parietal
then frontal
what is affected moderate stage Alzheimers?
Affected – Parietal lobe.
Symptoms
– Dressing apraxia (inability to perform purposive actions) loss of procedural memory
what is affected in late stage Alzheimers?
Affected – Frontal lobe.
Symptoms – Loss of executive skills (e.g. recognition).
what are the afferent connections of amygdala?
Olfactory cortex septum temporal neocortex hippocampus brainstem.
efferent is stria terminalis
what is the efferent connection of amygdala?
Stria terminalis.
functions of amygdala
o Fear and anxiety (and opposite).
o Fight or flight.
bilateral lesion–> Kluver Bucy Syndrome
pathology of amygdala
o Kluver-Bucy Syndrome – post-trauma, medial temporal lobes are sheared off middle cranial fossa and amygdala damaged.
o Symptoms – hyperorality (explore things by mouth) , loss of fear, visual agnosia, hypersexuality.
what structured are associated with aggression?
hypothalamus
brainstem (periaqueductal grey)
amygdala
what chemical stimulates aggression?
from where?
5-HT (serotonin)
from the raphe nuclei (brainstem)
other structures involved in aggression are:
- hypothalamus
- periaqueductal grey
afferent connection of septum
Amygdala, olfactory tract, hippocampus, brainstem.
efferent connection of septum
Stria medularis thalami, hippocampus, hypothalamus.
function of the septum
reinforcement and reward (nucleus accumbens)
pathways in the median forebrain bundle
midbrain (VTN) to the cortex, nucleus accumbens and amygdala via the median forebrain bundle using mesocortical and mesolimbic fibres
transmitter used in mesolimbic pathway
dopamine
where do neurones to the nucleus accumbens project from in the midbrain
Ventral Tegmental Nucleus
via mesolimbic fibres of the medial forebrain bundle
which drugs increase dopamine in the nucleus accumbens
o Opioids. o Nicotine. o Amphetamines. o Ethanol. o Cocaine.
how is the increase in dopamine achieved in the Nucleus accumbens
- stimulating the midbrain neurones to release more DA
- inhibiting its reuptake
which drug inhibits the reuptake of dopamine
cocaine
opioids
which lobe is epilepsy focused on?
temporal
how does epilepsy affect smell sensation?
what does it give rise to?
- piriform cortex (lateral stria) and orbitofrontal cortex (medial stria) are located in the temporal lobe
- gives rise prodromal auras:
due to an overactive piriformis cortex leading to a sensation of smell before seizures
what are the causes of anosmia?
1) mid-face trauma–> fracture to cribriform plate, shearing off the bipolar olfactory neurones
2) neurodegenerative disease e.g. Parkinson’s
