PHGY 210 Lecture Notes - Lecture 11: Cerebral Cortex, Olfactory Receptor, Neural Coding

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31 Jan 2013
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Central Olfactory Pathway
Olfactory receptor neurons send axons into the two olfactory bulbs (see Fig. 8.14). The input
layer of each bulb contains about 2000 spherical structures called glomeruli. Each glomerulus
is the site of about 25 000 primary olfactory axon endings converging and terminating on
dendrites of about 100 second-order olfactory neurons.
The mapping of receptor cells onto glomeruli is precise. Each glomerulus receives receptor
axons from a large region of the olfactory epithelium. This mapping is also consistent across the
two olfactory bulbs each bulb has only two P2 (receptor gene)-targeted glomeruli, in
symmetrical positions (see Fig. 8.15). It seems that each glomerulus receives input from only
receptor cells of one particular type. This means that the array of glomeruli within a bulb is a
very orderly map of the receptor genes expressed in the olfactory epithelium (see Fig. 8.16).
Many brain structures receive olfactory connections. Among the most important targets are the
primitive region of cerebral cortex called the olfactory cortex. This anatomy makes olfaction
unique.
All other sensory systems first pass through the thalamus before projecting to the
cerebral cortex
This produces an unusually direct and widespread influence on the parts of the forebrain
that have roles in odour discrimination, emotion, motivation, and certain kinds of memory
Spatial and Temporal Representations of Olfactory Information
In olfaction, there is an apparent paradox similar to the one in gestation. Individual receptors are
broadly tuned to their stimuli but when we smell, we can easily tell these stimuli apart. How?
Each odour is represented by the activity of a large population of neurons
The neurons responsive to particular odours may be organized into spatial maps
The time of action potentials may be an essential code for particular odours
Olfactory Population Coding
The olfactory system uses the responses of a large population of receptors to encode a specific
stimulus (see Fig. 8.13).
Olfactory Maps
A sensory map is an orderly arrangement of neurons that correlates with certain features of the
environment. In terms of olfaction, neurons in a specific place in the bulb respond to particular
odours. Receptor neurons are distributed across a wide area of the olfactory epithelium and will
respond to the presentation of a single odorant.
Experiments reveal that while many bulb neurons are activated by one odour, the neurons’
position form complex but reproducible spatial patterns (see Fig. 8.18). Thus, the smell of a
particular chemical is converted into a specific map within the ‘neural space’ of the bulb.
Furthermore, the form of the map depends on the nature and concentration of the odorant.
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