16. Olfactory and limbic system Flashcards
Where are the 2 olfactory bulbs located?
Inferior surface of the frontal lobes
Describe the olfactory epithelium
• Upper part of the nose • Bipolar primary olfactory neurones - sustentacular cells support these • Basal cells exist - produce new olfactory cells - lost with age
What projects through the cribriform plate?
Bipolar olfactory receptor cells project through it, into the olfactory bulb
What are the cells in the olfactory bulb called?
- Mitral cells
* Glomerular like structures int their interactions with the first order olfactory neurones
Where do the second order olfactory neurones project to?
(mitral cells)
• Olfactory tract lies on the inferior surface of the frontal lobe
• Split into medial and lateral olfactory stria
• Project to the olfactory processing centres
- piriform cortex of the temporal lobe
- orbitofrontal cortex
(promotes autonomic responses)
What is a prodromal aura?
- A smell that gives an indication of the onset of a disease
- e.g. a seizure in temporal lobe epilepsy
What is a common cause of anosmia?
Mid-face trauma
What are the theories for an environmental trigger of Parkinson’s?
- Stimulation through the gut
- Up the vagus nerve to the brainstem
- Diarrhoea is a presenting symptom
- Through the nose
- Anosmia is a presenting symptom
- Pathology in the olfactory bulb is an early aspect of the disease
What is the limbic system?
- Rim of the cortex adjacent to the corpus callosum and diencephalon
- Consists of structurally and functionally interrelated areas considered as a single functional complex
The limbic system is responsible for which processes aimed at the survival of an individual?
- Maintenance of homeostasis (hypothalamic function), modulation of pituitary hormone release and feeding + drinking
- Agonistic behaviour (fight or flight)
- Sexual and reproductive behaviour
- Memory - vital in terms of emotional response to stimuli
Describe the Papez circuit
Neural circuit for the control of emotional expression
• Fornix is the main output pathway of the hippocampus - comes out of the inferior horn of the lateral ventricle
• Fibres go under the corpus callosum and synapse in the mammillary bodies (in the hypothalamus)
• Mammillo-thalamic tract (MTT) projects to the anterior nucleus of the thalamus
• Thalamo-cortical projections go to the cingulate cortex - emotional experience
• Loop completed by fibres projected back to the hippocampus via the cingulate bundle
• Neocortex has input - emotional colouring based on previous experience
Describe the afferent connections of the hippocampus
- Called the perforant pathway
- Main connections from the adjacent (entorhinal) cortex, which receives input from every other neocortical area itself
- Part of memory encoding process
Describe the efferent connections of the hippocampus
- Called the fimbria/fornix
* Part of the Papez circuit
What are the functions of the hippocampus?
- Memory
* Learning
Where is the amygdala located?
- Nucleus buried in the white matter
* In the anterior part of the temporal lobe
Where are the inter-locking cells of the dentate gyrus and the pyramidal cells located?
In the hippocampus
Where is the hippocampus located?
Medial temporal lobe
What happens to the hippocampus structure in Alzheimer’s disease?
Severe shrinking (atrophy)
Describe the pathology of Alzheimer’s disease at a cellular leve?
- Tangles - abnormal cytoskeleton proteins
- Cytoskeleton breaks down and neuronal cell function breaks down
- Different susceptibilities - some cells not affected
- Senile plaques - protein disturbs memory function
Describe the progression of Alzheimer’s disease?
- Early on - spreads to hippocampus/entorhinal cortex => short term memory problems
- Moderate - spreads to parietal lobe => dressing apraxia
- Late - spreads to frontal lobe => loss of executive skills
Where does the efferent of the amygdala go to?
Hypothalamus (Stria terminalis pathway)
What are the functions of the amygdala and what can damage lead to?
- Fear and anxiety
- Fight or flight
- Damage => Kluver-Bucy syndrome
What is Kluver-Bucy syndrome?
- Reversion to the basis survival instincts and exploration of the environment
- Become completely fearless
- Hyperorality (putting things in mouth), visual agnosia, hypersexuality, compulsive eating
Where is the septum/septal nuclei and what is its function?
- Septum is the membrane between the two lateral ventricles anteriorly
- Septal nuclei are at the base of the septum
- Function - reinforcement and reward
What are retrograde fibres
Fornix (hippocampus => mammillary bodies)
This pathway also picks up some reverse fibres going from the septum to the hippocampus
If you implant an electrode in the septal nuclei in a rodent and stimulate it, how will its mood change, and why is this clinically significant in humans?
• Rodent becomes happy
• Humans have an analogous areas
- potential area for deep brain stimulation
- used to reduce tremor in Parkinson’s
Which structures have been shown to experimentally associated with aggression?
• Hypothalamus
• Brainstem (periaqueductal grey matter)
• Amygdala
(• Serotonin in the raphe nuclei of the brainstem)
Describe the mesolimbic pathway (dopamine)
• A second dopaminergic pathway
• Comes from the ventral tegmental area (VTA) - also in the midbrain
• Projections go via medial forebrain bundle (MFB) to:
- cortex
- amygdala
- nucleus accumbens (important in drug dependece)
Where is the ventral tegmental area located compared to the substantia nigra?
VTN is more medial
Where do the dopaminergic neurones of the substantia nigra project to?
Basal ganglia
How do all drugs of abuse affect the nucleus accumbens?
Increase dopamine release in the nucleus accumbens
• Stimulate midbrain neurones
• Promote DA release
• Inhibit DA reuptake e.g. cocaine
