Olfaction and the limbic system

Question 1

What the three cell types that make up the olfactory epithelium?

Answer 1

1. Bipolar Olfactory Neurones
2. Sustentacular Cells – support cells mainly providing metabolic support
3. Basal Cells – there is some regeneration in olfactory neurones

Question 2

Where is the olfactory bulb found?

Answer 2

Inferior to the frontal lobe, on top of the cribriform plate

Question 3

Which cells synapse with which within the olfactory bulb?

Answer 3

The bipolar cells pass their axons through the cribriform plate to synapse with the second order neurones (olfactory bulb mitral cells) in the glomerulus (since multiple branches and synapses) within the olfactory bulb

Question 4

What structure do the second order neurones form and what does this structure split into?

Answer 4

Olfactory tract
It travels along the inferior surface of the frontal lobe and then it splits to form the medial and lateral olfactory stria

Question 5

Where does higher processing of smell take place?

Answer 5

Piriform Cortex (in the temporal lobe)
Orbitofrontal Cortex (area of the prefrontal cortex that sits just above the orbits)

Question 6

What is a clinical deficit in the olfactory system called?

Answer 6

Anosmia

Question 7

What is a common cause of anosmia?

Answer 7

Mid-face trauma
Impact with enough force could cause a fracture of the cribriform plate, shearing the neurones going from the olfactory epithelium to the olfactory bulb

Question 8

Explain thesignificance of this with regards to epileptic patients.

Answer 8

Epilepsy is often focused in the temporal lobe
This means that some people with epilepsy will experience PRODROMAL AURA (they are made aware of an imminent seizure because they’ll smell something that’s not there)

Question 9

Neurodegenerative disease is a relatively common cause of anosmia. State two neurodegenerative diseases that could cause anosmia.

Answer 9

Alzheimer’s disease

Parkinson’s disease

Question 10

What structures make up the limbic system and how would you describe this system?

Answer 10

Hippocampus, amygdala, mammilary bodies, fornix, cingulate gyrus (and also forebrain structures)
=> Structurally and functionally interrelated areas considered as a single functional complex

Question 11

State some roles of the limbic system?

Answer 11

• Homeostasis (mainly hypothalamic functions such as regulation of food intake and pituitary hormone release)
• Agonistic behaviour (defence and attack)
• Sexual and reproductive behaviour
• Memory

Question 12

Which parts of the limbic system are found within the temporal lobes.

Answer 12

Hippocampus and Amygdala

Question 13

Where is the cingulate cortex located?

Answer 13

The cingulate cortex is situated in the medial aspect of the cerebral cortex. The cingulate cortex includes the entire cingulate gyrus, which lies immediately above the corpus callosum

Question 14

Describe/draw the papez circuit.

Answer 14

Hippocampus –> Fornix –> Mammillary Bodies –> Mammillo-Thalamic Tract (MTT) –> Anterior Nucleus of the Thalamus –> Cingulate Cortex –> Cingulum Bundle –> Hippocampus

Question 15

What is the function of the Papez circuit?

Answer 15

It is a neural circuit for the control of emotional expression

Question 16

What feeds into this circuit via the cingulate cortex to provide emotional ‘colouring’?

Answer 16

Neocortex

Question 17

What form of imaging can be used to study the limbic system?

Answer 17

Digital Tensor Imaging – shows co-instant activity in different parts of the brain thus showing which parts of the brain are working together

Question 18

Describe the afferent pathway of the hippocampus.

Answer 18

Afferent Pathway = Perforant Pathway
The entorhinal cortex (aslo in the medial temporal lobe) is linked to the hippocampus via the afferent pathway (perforant pathway)
The entorhinal cortex receives input from all other parts of the neocortex

Question 19

State the efferent pathway of the hippocampus

Answer 19

Fimbria –> Fornix

Question 20

What are the functions of the hippocampus?

Answer 20

Memory and learning

Question 21

Describe the spatial relations of the hippocampus and the fornix to other important brain structures.

Answer 21

The hippocampus is found on the floor of the lateral ventricles
Each fornix comes out of the hippocampus (R and L respectively) posteriorly and passes under the corpus callosum connected to each other by a thin sheet of transverse fibres called the commisure of the fornix.
It then dives inferior and anteriorly towards the mammillary bodies

Question 22

State the salient features of advanced Alzheimer’s disease on a CT head scan in the coronal plane.

Answer 22

• Extensive cortical atrophy
• Ventricles appear enlarged
• Widening of sulci
• Shrinkage of the hippocampus

Question 23

Describe two microscopic hallmarks of neurodegeneration.

Answer 23

1. Tau Immunostaining:
   - Intracellular buildup of abnormal tau protein which cause neurofibrillary tangles (normal tau functions to stabilise microtubules)
2. Senile Plaques
   - Extracellular deposits of amyloid beta in the grey matter of the brain associated with degenerative neural structures

Question 24

Describe the anatomical progression of Alzheimer’s disease, including the symptoms experienced (from early to moderate to late)

Answer 24

Early:
- Hippocampus and entorhinal cortex affects
- Short-term memory problems
Moderate:
- Parietal lobe (where you have your procedural memory)
- Dressing apraxia
Late:
- Frontal lobe
- Loss of executive skills

Question 25

Where is the amydala found?

Answer 25

A nucleas located in the white matter of the anterior medial temporal lobe

Question 26

What are the afferent connections of the amygdala?

Answer 26

Olfactory Cortex
Septum (septal nuclei)
Temporal Neocortex
Hippocampus
Brainstem

Question 27

What is the main output pathway of the amygdala?

Answer 27

Stria terminalis

Question 28

What emotion/response is the amygdala involved in?

Answer 28

Fear & anxiety

Fight or flight

Question 29

State a syndrome involving the amygdala. What is the cause and what are the symptoms?

Answer 29

Kluver-Bucy Syndrome

Bilateral lesions of the anterior temporal lobe (including amygdaloid nucleus) =>

• Hyperorality
• Hypersexuality
• Loss of Fear
• Visual Agnosia

Question 30

State three structures shown experimentally to be associated with aggression. What is the main neurotransmitter involved?

Answer 30

Hypothalamus
Brainstem (periaqueductal grey matter)
Amygdala

5-HT (serotonin)

Question 31

What are the main afferent connections of the septal nuclei?

Answer 31

Amygdala
Olfactory Tract
Hippocampus
Brainstem

Question 32

What function do the septal nuclei provide? Where are they found?

Answer 32

Reinforcement & reward

Located on the lower posterior part of the frontal lobe; the septal area refers to the nearby septum pellucidum

Question 33

Name another structure that is important in the reward system

Answer 33

Nucleus accumbens

Question 34

Name and describe another dopaminergic pathway other than the nigro-striatal pathway that is affected in Parkinson’s disease.

Answer 34

Mesolimbic pathway

Ventral Tegmental Area (VTA) of the midbrain –> (via Median Forebrain Bundles) –> Cortex + Nucleus Accumbens + Amygdala

Question 35

Drugs of abuse e.g. opioids, nicotine, amphetamines, ethanol and cocaine have what effect on the nucleus accumbens

Answer 35

Increase dopamine release in nucleus accumbens