Olfactory System - Perception - Lecture Slides, Slides of Brain and Cognitive Science

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The Olfactory System

Olfactory System

Chemical sensing system with receptor organs in the nasal passages

Receptors synapse directly into the brain; heavy connections with the limbic system

Different from other sensory systems in many ways

Olfactory System: Peripheral Structure Olfactory receptors are located on the olfactory (or nasal) epithelium. The epithelium hangs down from the roof of the nasal sinus. The epithelium contains olfactory receptor cells and supporting cells.

Dendrites of olfactory receptor cells extend into the mucus coating of the epithelium; odorant molecules bind to receptors on the dendrites. Axons of the olfactory receptor cells enter the brain and synapse on cells in the olfactory bulb.

BRAIN

SINUS

Olfactory sensory neurons No circuitry or synapses in the epithelium; receptors have axons (thin, unmyelinated, slow) which project directly to the brain. Receptors die and are replaced about every 60 days.

Stem cell

To olfactory bulb Docsity.com

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/O/Olfaction.html

Olfactory receptors use a G-protein coupled transduction mechanism similar to visual receptors

What exactly do receptors code?

How odors are encoded by the olfactory receptors was a long-standing mystery

Early olfactory researchers suggested that a small number of receptor types could encode a large number of natural odors, similar to 3 cones coding all perceived colors: The “Prime Odor” theory (7 primes was a popular number)

Difficult to determine what those “prime odors” might be and how they would be combined to give the smell of a natural substance

Richard Axel and Linda Buck used molecular techniques to determine the number of different olfactory receptor types. The concept and strategy:

1. SPECIFICITY WOULD BE BASED ON STRUCTURE OF RECEPTOR-G PROTEIN COMPLEX; THEREFORE, IF YOU DETERMINE THE NUMBER OF DIFFERENT RECEPTOR STRUCTURES, YOU KNOW THE NUMBER OF DIFFERENT FUNCTIONAL TYPES, AND THEREFORE THE NUMBER OF DIFFERENT “PRIME ODORS” 2. STRUCTURALLY DIFFERENT RECEPTOR PROTEINS WOULD BE CODED BY DIFFERENT GENES; CLONE, SEQUENCE, CHARACTERIZE GENES EXPRESSED IN THE OLFACTORY EPITHELIUM, LOOK FOR SYSTEMATIC VARIATION ON G-PROTEIN TYPES 3. LOCALIZE THE EXPRESSED GENES BACK TO THE OLFACTORY RECEPTOR CELLS

http://nobelprize.org/medicine/laureates/2004/buck-slides.pdf

The olfactory

epithelium is

“mapped”, but not in

a familiar way

The 4 gene families are expressed in different zones of the epithelium

Within a zone, different receptor types appear to be randomly scattered

http://nobelprize.org/medicine/laureates/2004/buck-slides.pdf

A combinatorial code means that receptors can contribute to the perception of very different smells

Output of the olfactory epithelium goes to the Olfactory Bulb: Olfactory bulb is a three layered structure. Mitral cells are the principal neurons of the olfactory bulb.

Cells expressing a single type of receptor are widely scattered across the olfactory epithelium.

Axons of all these cells converge on a single place (glomerulus) in the olfactory bulb.

All the axons terminating in a glomerulus are from the same type olfactory receptors. Therefore each glomerulus codes one odorant type.

Axons from each olfactory receptor type terminate in very few (maybe only 1 or 2) glomeruli at one point in the olfactory bulb.