The Olfactory System

The olfactory system can generally be divided into three parts:

• Sensation
• Processing
• Association and identification

Sensation - the olfactory epithelium

The first stage of olfaction - sensation - is handled by the olfactory sensory neurons (OSNs) in the olfactory epithelium, situated in the nasal cavity. The olfactory epithelium contains sensory neurons, Schwann cells and basal cells (stem cells), and its surface is covered with mucosa. The epithelium has in many organisms been observed to be divided into zones. Each receptor type is expressed in one zone only, randomly dispersed among receptors of other types.

The mucosa

The mucosa is believed to keep ion concentration outside the cilia on a suitable level. It also contains odorant binding proteins (OBPs). The exact role of OBPs is still unclear. They have been hypothesised to

• transport odorants to receptor sites
• scavenge (remove) odorants from receptor sites
• protect sensory neurons from poisoning by binding to or altering harmful substances

Olfactory Sensory Neurons

Olfactory sensory neurons have an elongated shape with a dendrite terminated by a dendritic knob from which 5-20 cilia protrude into the mucosa that covers the surface of the olfactory epithelium. The olfactory receptors are situated on the cilia.

The average life-length of an OSN is 20 days, and OSNs are constantly regenerated from stem cells. Each sensory neuron is thought to express only one type of receptor. This is known as the "one cell - one receptor" hypothesis. All OSNs expressing the same receptor type project to the same glomerulus in the olfactory bulb.

Processing - the olfactory bulb

The first processing of olfactory stimuli is achieved by the olfactory bulb, a small part of the brain situated closely above the olfactory epithelium. Here, sensory neuron input from neurons expressing the same receptor type is summed is summed in specialized structures called glomeruli.

Glomeruli

Glomeruli are roughly spherical structures with a diameter somewhere between 20-40 micrometer (fish and amphibians) to 100-200 micrometer (rabbits and cats), depending on the organism. In the glomeruli, the axon terminals of OSNs meet dendrites from mitral cells and periglomerular cells.

Important cell types in the olfactory bulb

• Mitral cells: The output neurons of the olfactory bulb. They recieve input from OSNs.
• Granule cells: Inhibitory interneurons that recieve input from and project onto mitral cells. They are important for synchronization of mitral cell firing.
• Periglomerular cells: Inhibitory interneurons that connect glomeruli.

Association and identification - the olfactory cortex

The olfactory cortex is a collection of higher areas that recieve input from the olfactory bulb. Among these are areas that have close ties to feeling and memory. The piriform cortex is usually considered to be the main, most important area.Its principal neurons are the pyramidal cells, and it contains both feed-forward and feed-backward inhibitory systems.

Our olfactory systemresearch projects are performed within the context of the GOSPEL Network of Excellence. A brief introduction to the projects:

BioPatAna - Biomimetic Pattern Analysis

The BioPatAna project is performed within the context of the GOSPEL Network of Excellence. The project aims to increase our understanding of the olfactory system, and to find better analysis algorithms and methods for gas sensor array data. The project has several interconnecting parts:

• Computational modelling of the zebrafish olfactory bulb, based on experimental data. This work is performed by the Rainer Friedrich Lab at the Max Planch Institute in Heidelberg.
• Scaling up the original olfactory bulb model to a size resembling that of the zebrafish olfactory bulb, and to study the effects of this increase in size. Construction of a large-scale model of the olfactory cortex, and connection of the two models. This work is performed within our group.
• Construction of an abstract model of the olfactory cortex, to study pattern processing. This work is performed within our group.
• Construction of an abstract mathematical (and analytically tractable) model from the original olfactory bulb model. This work is performed by the Martinez Lab at LORIA/INRIA in France.
• Search for principles used by the biological olfactory system, and comparison of these principles with current data analysis methods. This work is performed by several groups: our group, the Martinez group at LORIA/INRIA, the SIC group at Universitat de Barcelona and the IMM-CNR group in Lecce.

ORNABI - Modelling the Olfactory Receptor Neuron Array providing Bulb Input

The ORNABI project is likewise performed within the context of the GOSPEL Network of Excellence, and in collaboration with Dr Jean-Pierre Rospars at INRA in France. The aim is to construct a large-scale computational model of the olfactory receptor neuron (ORN) layer, and to gain insight about the ''sensory image'' presented to the olfactory bulb by the ORN layer. The project consists of four parts:

• Construction of a a model of a single ORN with an adequate spiking behaviour. Much of this work is or has been performed by Dr. Rospars, but it will be added to by our group..
• Construction of a large-scale model of an ORN array, using the single-ORN model. This work is performed in collaboration between our group and Dr. Rospars.
• Evaluation and theoretical analysis of the information carried in the large scale ORN array model. This work will be performed by researchers from Universitat de Barcelona and Linköping University.
• Characterization of variability in GPCR-protein mediated responses, using a melanophore-based assay. This work will be performed by Prof. Ingemar Lundström and Dr. Anke Suska from Linköping University.