2.4 Olfaction & the limbic system

Question 1

The human olfactory system can differentiate between 2000 – 4000 odours (much less than other animals where the proportion of brain devoted to olfactory function is larger):
• Molecular mechanism is largely unknown; sensory olfactory neurones are located in the ______________ (in the upper part of the nose)
o Consists of bipolar olfactory neurones, ___________ (supporting the neurones) and__________ (some regeneration of olfactory neurones during life)
• Progressive loss of olfactory neurones occurs with age

Answer 1

olfactory epithelium;

sustentacular cells ;

basal cells

Question 2

ANATOMY OF OLFACTION In the nasal cavity, the olfactory neurones are located in the epithelium in the upper part:
• Axons of bipolar olfactory receptor cells pass through ____________ to synapse with second-order olfactory neurones in _________ (glomerular-type formations)
o Olfactory bulbs (mitral cells) lie at the _______________
• Second-order olfactory neurones pass back in the olfactory tract to the brain
• Olfactory tracts divide into 2 olfactory stria (medial and lateral); one of which goes to the ______________ and further connects to the ________________
o Other connections to brainstem (promote autonomic responses to stimuli)

Answer 2

cribriform plate;

olfactory bulb ;

inferior surface of orbitofrontal cortex ;

piriform cortex (temporal lobe); orbitofrontal cortex (highest cortical area in olfactory function);

Question 3

Anosmia ( loss of smell ): clinical deficit of the olfactory system; may be caused by various factors
 Most commonly caused by trauma/ fracture to the _____________ –> shears off olfactory neurones as they pass through the base of the skull
 Feature of neurodegenerative diseases (e.g. Alzheimer’s, Parkinson’s) –> early sign

Answer 3

base of skull/midface

Question 4

The limbic system is a rum/limbus of cortex adjacent to _________________ comprising structurally and functionally interrelated areas (single functional complex).
It is responsible for processes aimed at the survival of the individual including:
1. Maintenance of homeostasis via activation of visceral effector mechanisms, modulation of pituitary hormone release and initiation of feeding and drinking
2. Agonistic (defence and attack) behaviour
3. Sexual and reproductive behaviour
4. Memory

Answer 4

corpus callosum and diencephalon

Question 5

What are the main components of the Papez circuit?

Answer 5

hippocampus, amygdala (fear and aggression), thalamus, hypothalamus, olfactory bulb and higher cortical function (frontal lobe), which are connected by the Papez circuit

Question 6

Papez circuit
• Main outflow from the hippocampus is via the ________________ to the ____________
• Hypothalamus is then linked to the ______________ by the mammillothalamic tract
• Projections from the anterior nucleus extend to the _______________, from which a single bundle reinnervates the hippocampus

Answer 6

fornix (fibre pathway);

mammillary bodies of the hypothalamus;

anterior nucleus of thalamus;

cingulate cortex (lying just above the corpus callosum)

Question 7

Hypothalamus is the area involved in ___________________:
• Emotional experience is built up, and a previous experience can colour the emotion to an external stimulus (memory from ___________ → emotional colouring)
• Diffusion tensor imaging investigates coincident activity in different parts of the brain (suggests where the functional networks are) → may be used for limbic system

Answer 7

emotional expression (reaction based on emotion);

neocortex

Question 8

The hippocampus is one of the main components of the limbic system, and possesses connections to all the other structures:
• Largely involved in ______________ (clinical pathologies include Alzheimer’s disease and epilepsy)

Pathways

• Afferent: Perforant pathway (originates in the __________________ lying immediately adjacent to the hippocampus)
• Efferent: Fimbria/fornix (same fibres → known as fimbria when still associated with the hippocampus)

Answer 8

memory and learning;

parahippocampal gyrus/entorhinal cortex

Question 9

The hippocampus sits in the inferior horn of the lateral ventricle:
• Fornix goes ____________ following the ventricles → terminates in the ______________ (base of diencephalon)
• Amygdala is another key structure, which is buried in the white matter of the anterior pole of the temporal lobe

Answer 9

posteriorly then anteriorly;

mammillary bodies

Question 10

Alzheimer’s disease: chronic neurodegenerative disease involving memory loss and starts in the temporal lobe next to the hippocampus (affects projec tions into the hippocampus
- Characterised by accumulation of ___________ which triggers neuronal degeneration (induction of apoptosis) + tissue inflammation
- Pathological changes: extreme shrinkage of ______________ , severely
enlarged _________, deepened __________
- Early disease : _______________ –> short term memory loss
- Moderate disease : ___________ –> dressing apraxia (neglect of left side when dressing)
- Late disease : _________ –> loss of executive skills

Answer 10

beta amyloid peptide and tau protein;

cerebral cortex and hippocampus; ventricles; sulci

hippocampus and entorhinal cortex;

parietal lobe;

frontal lobe

Question 11

The amygdala is buried in white matter in the ___________________, just anterior to the hippocampus (in the floor of the ventricle):
• Involved in fear and anxiety and the fight-or-flight response (clinical pathologies include a range of psychiatric issues like the Kluver-Bucy syndrome)
• Structures associated with aggression: hypothalamus (_________ part), brainstem (______________), amygdala
o Neurotransmitter: serotonin (5-HT) found in ________________

• Afferent: Olfactory cortex, septum, temporal neocortex, hippocampus, brainstem
• Efferent: ______________ (to hypothalamus)

Answer 11

anterior part of the temporal lobe;

anterior;

periaqueductal grey matter;

raphe nuclei of the brainstem;

Stria terminalis

Question 12

Kluver Bucy syndrome: seen in patients with _______________
- Clinical presentation: loss of fear , visual agnosia (can see but not interpret visual information), hyperorality (insertion of inappropriate objects in mouth), hypersexuality

Answer 12

bilateral temporal lobectomy

Question 13

The septum consists of septal nuclei (labelled 13 in image below) beneath fibrous septum dividing the lateral ventricles:
• Involved in reinforcement and reward behaviour

• Afferent: Amygdala, olfactory tract, hippocampus, brainstem
• Efferent: ____________, _______, _________

Answer 13

Stria medullaris thalami, hippocampus, hypothalamus

Question 14

The \_\_\_\_\_\_\_\_\_\_\_ (labelled 15 in image below) has similar functions in reinforcement and reward behaviour → researched for drug therapy in patients with depression:
• Serves as the pathway for the mesolimbic pathway (dopamine), which is particularly active in drug-dependent individuals
• Series of dopaminergic cells in the midbrain in the \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ which projects via the medial forebrain bundle (MFB) to various areas (including the nucleus accumbens in the basal forebrain) 

Drugs (e.g. opioids, nicotine, amphetamines, ethanol, cocaine) all increase dopamine release in the nucleus accumbens via the following mechanisms:
• Stimulation of ______________
• Promotion of dopamine release at the axonal terminals
• Inhibition of dopamine reuptake at synaptic terminals (e.g. cocaine)
*Other neurotransmitters may also modify this system (but dopamine is the main one).

Answer 14

nucleus accumbens ;

ventral tegmental area (VTA),;

midbrain neurones;