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
All other sensory systems first pass through the thalamus before projecting to the
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 organ