Editing Sensory Systems/Zebrafish (section)

===The Zebrafish Olfactory System===
The zebrafish (Danio rerio) is a freshwater teleost native in Southeast Asia [1]. Water flow through
the nose is laminar and unidirectional. Even when a zebrafish is not moving, water flow is provided
by motile cilia such that a constant odourant supply is provided. Hence, a zebrafish constantly
screens the odour space by moving through the environment. The first relay station of odour
information is the olfactory bulb. Information passing in the olfactory bulb is an extremely complex
process which includes multiple steps of transformation performed by the underlying circuitry. For
instance, an odour consisting of different molecules activates a specific set of odourant receptors
on olfactory sensory neurons, which terminate in the olfactory bulb in an array of glomeruli. Hence,
an odour is encoded in a combinatorial fashion of glomerular activation patterns. An adult zebrafish
olfactory bulb contains about 140 stereotyped glomeruli [2]. A glomerulus is a functional unit
consisting of synaptic connections within three different cell classes(Figure 1)[3].

* The incoming olfactory sensory neurons expressing the same odorant receptor. All synaptic connections are excitatory.
* Inhibitory interneurons responsible for multiple transformations of the odour signal. In Zebrafish, interneurons can be subdivided into periglomerular cells, granule cells and short axon cells, each of which has distinct morphological features.
* Mitral cells, which relay the signal out of the olfactory bulb to higher brain areas. In adult Zebrafish there are about 1’500 mitral cells. 70% of these mitral cells receive input from one distinct glomerulus [4].

[[File:ZebrafishOB schematic.jpg|thumb|Figure 1: Schematic view of cell types in the Zebrafish olfactory bulb. Short axon cells (SAC), Olfactory sensory neurons (OSN), Granule cells
(GC), Periglomerular cells (PGC).]]

For the olfactory system, the broad concept of receptive fields present in the visual system is only
valid in a very general sense. As described above, a glomerulus receives input from olfactory
sensory neurons expressing the same odourant receptor. As a result, a rough spatial chemotopic
map is spanned across the olfactory bulb. In other words, different classes of natural odours of the
Zebrafish (amino acids, bile acids, nucleotides) activate different anatomical domains of the
olfactory bulb [1].