PSYB51H3 Lecture Notes - Lecture 12: Angular Velocity, Fetus, Quinine

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7 Sep 2013
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Lecture 12
Odors: Olfactory sensations
Chemical compounds that are ...
Volatile: high vapor pressure at ordinary, room-temperature conditions
Small
Hydrophobic
But not all.
The human olfactory apparatus:
Nose: Small ridges (nasal cycle)-> one nostril will always be smaller than the other (it will switch back and
forth because the air flow has different speeds)
Sniffing
olfactory cleft, olfactory epithelium
Secondary purpose of the nose: olfaction; primary purpose: breathing to survive
The olfactory epithelium: the ―retina‖ of the nose contains 3 types of cells:
1. Supporting cells
2. Basal cells (precursors to...)
3. Olfactory sensory neurons (OSNs)cilia protruding into mucus covering olfactory
epithelium
Olfactory receptors (ORs): Interact with odorants action potentials along...
Olfactory nerve (cranial nerve I; thin axons, slow)
Lesions to olfactory nerve cause anosmia:
Head trauma (cribriform plate)
Infections: permanent damage and olfactory loss called anosmia
Olfactory loss can cause great suffering:
Sense of taste/flavour:you can‘t sense what you‘re eating and whether it‘s turned bad, etc
Danger warning
Quite common
Early symptom for Alzheimer‘s, Parkinson‘s
The olfactory bulb:
Ipsilateral projection
Glomeruli: spherical structures in which OSNs synapse with mitral cells and tufted cells
Chemo-topography: glomeruli sort according to ORs
-> Olfactory cortex
-> Amygdala-hippocampal complex
-> Entorhinal cortex
• The genetic basis of olfactory receptors:
~1000 different olfactory receptor genes (Each single receptor has it‘s own gene; we have 300
types of receptors), each codes for single type of OR
Pseudogenes: dormant, don‘t produce proteins that are necessary to create these olfactory receptors
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(i.e., ORs), 20% in dogs, 60-70% in humans
Trade-off between vision and olfaction? (that we rely more on vision and dogs are more olfactory)
Multisensory perception: a feel of scent:
Odorants can stimulate somatosensory system (touch, pain, temperature receptors)
These sensations are mediated by the trigeminal nerve (cranial nerve V)
Also see taste!
From Chemicals to Smells
Shape-pattern theory
Odor mixtures
Shape-pattern theory: Match between shapes of odorants and odor receptors (key & lock); dominant
biochemical theory
we‘re talking about 2 molecules: one is one that flows on your nose and the other one sits in and
fits like a lock
Recent molecular research: Scents are detected by means of combinatorial codes
The molecules lock onto the odorous receptors in all sorts of combinations
The importance of patterns:
We detect a multitude of scents based on „only‟ 300-400 olfactory receptors ... how?
We can detect the pattern of activity across various receptor types
Similar to metameres in colour vision: phenyl ethyl alcohol = rose
Odor mixtures:
We rarely smell ―pure odorants,‖ rather we smell mixtures
How do we process the components in a mixture of odorants?
Two possibilities:
1. Analysis (audition: hi/low pitched tones)
2. Synthesis (metameres in colour vision)
Olfaction rather synthetic but can be trained
Olfactory Psychophysics
Differences between Detection
Discrimination
Recognition
How do we adapt to smells?
Olfactory detection thresholds: Depend on several factors, e.g., length of carbon chains (vanilla!)
Women: lower thresholds than men, depending on menstrual cycles but not pregnancy
Professionals can distinguish up to 100,000 odors (e.g., professional perfumers, wine tasters
Recognition: Smell and memory
Durability: Our recognition of smells is durable even after several days, month, or year
Identification: smell and language
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