Olfactory and limbic system Flashcards
Where are the olfactory bulbs?
They are on the interior surface of the frontal lobes
Where are the primary olfactory neurones?
In the mucosa at the top of the nose
Where is the olfactory epithelium found?
In the upper part of the nose
What are the different cells involved in olfaction?
- Primary olfactory neurones are bipolar in structure
- Sustentacular cells support these olfactory neurones
- Basal cells exist in the nasal mucosa – we can produce new olfactory cells during life
What may loss of smell be associated with?
May be more insidious than just age
- dementia or alzheimers
What is the cribriform plate part of?
What passes through it?
The ethmoid bone - it is a very fine layer with small holes
Projecting through the holes are olfactory receptor cells
Describe the pathway of the olfactory neurones
olfactory receptor neurones project into olfactory bulb forming glomerular like like structures where they interact with the second order olfactory neurones in the bulb. The cells in the olfactory bulbs are called mitral cells. The second order cells then pass to the brain.
What do the secondary order neurones split into?
- Mitral cells project back to the olfactory tract, and they split into medial and lateral olfactory stria
Where do the medial and lateral stria project to?
Two cortical olfactory processing areas:
- Piriform cortex of the temporal lobe
- Orbitofrontal cortex
Where do the olfactory neurones going to the brainstem do?
Evoke autonomic responses e.g. if you smell something you start salivating
What is anosmia?
complete loss of smell
What is a prodormal aura?
- In temporal lobe epilepsy, the electrical activity before a seizure may provoke it
- This is a smell which they experience before a seizure
Do Parkison’s and Alzheimer’s have genetic forms?
Both of these diseases have genetic forms (<5% are autosomal dominant inherited diseases – 95% are sporadic)
What are the two routes of potential environmental triggers in Parkison’s?
- Up the nose
- Through the gut
Where is some of the earliest pathology seen in Parkison’s?
In the medulla, in the motor nucleus of the vagus
What is the limbic system?
- The limbic system: rim or limbus of the cortex, adjacent to the corpus callosum and diencephalon
- These are structurally and functionally interrelated areas considered as a single functional complex
What are the roles of the limbic system?
- Maintenance of homeostasis (hypothalamic function) via activation of visceral effector mechanisms, modulation of pituitary hormone release and initiation of feeding and drinking
- Agonistic (defence & attack) behaviour (fight or flight)
- Sexual & reproductive behaviour
- Memory – vital in terms of emotional response to stimuli in the environment
What is the cortical atrophy seen in Alzheimer’s?
- The temporal lobe atrophy
- The gyri have got thinner and the sulci has got wider – there is less brain substance
- In later stages of Alzheimer’s, the frontal lobe also starts to undergo atrophy
- The primary motor cortex, primary sensory cortex and occipital lobe are largely unaffected
How is the hypothalamus connected to the thalamus?
Via the mamillo-thalamic tract
What is the Papez circuit?
- A neural circuit for the control of emotional expression
- The circuit connecting the hypothalamus to the limbic lobe was the basis for emotional experiences
What are the different parts of the limbic system?
The amygdala, hippocampus, thalamus, hypothalamus, basal ganglia, and cingulate gyrus
How does information in the Papez circuit get to the hypothalamus?
Which part is involved in emotional expression and how does it do this?
- The neocortex received information from outside -> cingulate cortex and this is integrated at the hippocampus (via the cingulum bundle)
- This provokes a response, which stimulates the hypothalamus to change normal homeostatic activity
- The reactions you have are influenced by memories and previous experiences
In the Papez circuit how are the hippocampus and the hypothalamus connected?
- Main output pathway of the hippocampus is the fornix
- These fibres go up, forward and down into the mammillary bodies of the hypothalamus
What else is the thalamus connected to?
There are also projections from the thalamus to the cingulate cortex
Which parts of Papez circuit are involved in emotional colouring, emotional experience and emotional expression?
colouring - neocortex
experience - cingulate cortex, hippocampus and anterior nucleus of thalamus
expression - hypothalamus
What is the cortical atrophy seen in Alzheimer’s?
- The temporal lobe atrophy
- The gyri have got thinner and the sulci has got wider – there is less brain substance
- In later stages of Alzheimer’s, the frontal lobe also starts to undergo atrophy
- The primary motor cortex, primary sensory cortex and occipital lobe are largely unaffected
- Hippocampus atrophies
- Ventricles enlarge
Where is the amygdala?
Embedded in the white matter of the anterior part of the temporal lobe (medial temporal love structure)
What are the afferent pathways to the amygdala?
olfactory cortex, septum, temporal neocortex, hippocampus, brainstem
What are the efferent pathways from the amygdala?
Stria terminalis
What are the functions of the amygdala?
- Fear and anxiety (damage to the amygdala can lead to Kluver-Bucy syndrome)
- Fight or flight
What is the septum?
The septum pellucidum is a thin, triangular, vertical double membrane separating the anterior horns of the left and right lateral ventricles of the brain. It runs as a sheet from the corpus callosum down to the fornix
What is the fornix of the brain?
- Part of the limbic system
- C-shaped bundle of nerve fibers that act as the major output tract of the hippocampus
- Also carries some afferent fibres to the hippocampus from structures in the diencephalon and basal forebrain
What is kluver-bucy syndrome?
- There is a reversion to the basis survival instincts, and exploration of the environment
- You become completely fearless
- Can result from localised bilateral lesions in the anterior temporal lobe
- Lead to hyperorality (putting things in your mouth), loss of fear, visual agnosia, hypersexuality
What are the structures in the brain that are associated with aggression?
- Anterior parts of the hypothalamus
- Brainstem (periaqueductal grey matter)
- Amygdala
Which neurotransmitter is involved in aggression?
serotonin
What are afferent pathways to the septum?
Amygdala, olfactory tract, hippocampus, brainstem
What are efferent pathways to the septum?
Stria medularis thalami, hippocampus, hypothalamus
What is the functions of the septal nuclei?
Reinforcement and reward
What are retrograde fibres?
The fornix (going from the hippocampus to the mammillary bodies), also picks up some reverse fibres, going back from the septum to the hippocampus.
If an electrode is placed in the septal nuclei what will happen?
- When stimulated the animal becomes happy
- The animal has a foot lever which they can self-stimulate
- They keep pressing the button
What is the mesolimbic pathway involved in?
- Drug dependance
- Involves dopamine
What is the pathway in drug dependence (describe it)?
Dopaminergic neurones are situated in the midbrain. Fibres project via the median forebrain bundle and go to the cortex, nucleus accumbens and amygdala).
What do different drugs do the DA system and what is the effect of this?
- Opioids, nicotine, amphetamines, ethanol and cocaine all increase DA release in nucleus accumbens
- Different drugs stimulate midbrain neurons, promote DA release or inhibit DA reuptake (main mechanism: increase DA longevity)
- The reward centres are stimulated -> person feels happy
What must doctors be vary of when treating Parkinson’s with dopamine agonists?
They must look out for compulsive reward-based behaviour. Patients treated with dopamine agonists may become compulsive gamblers.
Where does the hippocampus get a large afferent input from?
What is the pathway called?
From the cortex sitting next to it -> entorhinal cortex
- perforant pathway
Where does the entorhinal cortex get input from?
From every other cortical area - feeds into hippocampus for memory encoding
What is the main output from the hippocampus?
Fimbria/fornix
What is the function of the hippocampus?
What are some clinical problems involved with the hippocampus?
- STM memory learning
- Alzheimer’s disease, epilepsy
What kind of atrophy do patients with Alzheimer’s have?
- Radiologists look at the medial temporal area for atrophy
- They have localised atrophy which progresses over time
- The ventricles get larger
- The hippocampus atrophies
Which cells are seen in the hippocampus?
The inter-locking cells of the dentate gyrus and the pyramidal cells are in the hippocampus
