Lecture 16: Smell lecture 2 Flashcards
Describe the olfactory bulb projections;
ORNS (Olfactory nerve 1) Olfactory bulb (Lateral Olfactory tract) Olfactory bulb targets; - Pyriform cortex - Olfactory tubercle - Amygdala - Entorhinal cortex
Describe the projections from the olfactory primary targets
- Pyriform cortex
- Olfactory tubercle
- Amygdala
- Entorhinal cortex
All innervate the
- Hypothalamus
- Thalamus
- Orbitofrontal cortex
The entorhinal cortex especially innervates the Hippocampus
The Pyriform cortex especially innervates the
orbitofrontal cortex.
Whats the importance of the entorhinal cortex?
Provides major sensory input into the hippocampus, also receives direct sensory input from olfactory regions
Whats the importance of the limbic system?
A group of neural structures which includes;
- Pyriform cortex
- Olfactory tubercle
- Amygdala
- Entorhinal cortex
Involved in many aspects of memory and emotions
How is the olfactory sense unique among the system?
B/c of its direct and intimate contact with the limbic system
explains why smell has strong emotional associations
What does a functional MRI shown in terms of brain activity to smell?
Different parts of the brain detect pleasant (orbitofrontal cortex) and unpleasant odours (Cingulate cortex)
What are the theories of olfactory perception?
Vibrational Theory
Shape-pattern theory
What is the vibrational theory;
Every perceived molecule has a vibrational frequency, molecules that have the same vibrational Hz will smell the same.
old theory
What is the shape pattern theory;
(dominant biochem theory)
Different scents activate different arrays of ORNs in the olfactory epithelium
A combination of receptor activation is responsible for a given smell
Various arrays produce specific firing patterns of neurons in olfactory bulb which determine the scent we perceive.
Describe the shape pattern theory in terms of receptors;
A odorant can bind the receptor binding site and produce a response, but the intensity of the response will depends on the ‘pattern” matching of receptor-odorant
i.e needs same shape and colour (receptor and odourant), not just shape.
correct shape-pattern = best response
Give rise to odotopic mapping
Describe odotopic mapping using shape-pattern theory
In the olfactory bulb there is shape mapping (same) in the longitudinal plane and pattern mapping (same) in the transverse.
Why is the shape pattern theory good?
Explains why we can detect many odours with only 400 genes.
We can detect a pattern of activity across receptors.
How does intensity influence receptor activation?
Intensity of odorant changes which receptors will be activated
Weak concentrations of odourants will not smell the same as strong concentrations
Therefore specific time order of ORN activation is important
Whats a complication of odour detection?
We rarely smell pure odours, usually mixtures
How do process the components in odorants mixture?
Olfaction is primarily a synthetic sense; but some analytic properties can develop
synthesis; i.e red + green makes yellow light, but in yellow light we cannot detect red or green
Analysis: High and low notes can be played together but we can detect individual notes
How does the visual system influence olfaction?
The visual system can change the perception of a smell
Describe olfactory psychophysics;
There must be substantial cognitive function for detection, recognition and discrimination of smell
How much stimulation is required before we can perceive a smell to be there?
Olfactory detection thresholds depend on several factors including age, gender and training.
Women generally lower thresholds than men
by 65 generally 60% population are anosmic
Professional wine tasters can distinguish up to 100,000 odours.
How can we measure sense of smell?
Most common method is a 40 item scratch and sniff
Use a cards and assess how many are correctly identified (options given)
However does not indicate perception threshold
How do we measure detection threshold for a specific odour?
Staircase method
Method for determining the concentration of a stimulus for detection at a threshold level
Describe the staircase method;
- Stimulus is presented at increaseing concentrations until detection is indicate, concentrations are then decreased until detection ceases.
This is repeated several times an the average reversal point is threshold
What is another method for determining threshold concentration?
Triangle test; Participant is given three odours, two of which are the same.
Must identify the odd one
The order of the three is varied to increase accuracy
What is adaptation in relation to smell?
After continuous exposure to the odourant the receptors adapt and detection ceases.
This process is called desensitization (reversible)
I.e cant smell your own perfume
Sense of smell is typically a change detector.
What is receptor cross adaptation?
The reduction in detection of an odourant following exposure to another odourant
assumed to occur because odourants share one or more ORN in their transduction
