Olfaction Process and Olfactory Pathway

Olfactory System

• Each glomerulus receives input from only one type of receptor cells, creating a map of odor information in the olfactory bulb.
• Odorant signals are segregated into broad categories, independent of strength and interference from other odorants.
• Each type of receptor corresponds to a distinct subset of glomeruli, and when an odorant binds to its receptor, it activates a specific pattern of glomeruli.

Glomeruli and Spatial Patterns

• Individual glomeruli contain information from the same type of olfactory cell and receptor.
• Differentially activated spatial responses are distinct for each odorant, reducing the response to different glomeruli.
• Axons of each receptor type synapse to one glomerulus.

Temporal Patterns

• The precise timing of action potentials in mitral and tufted cells relative to the inhalation cycle shapes the temporal dynamics of odorant coding.
• Odorants evoke distinct patterns of neural activity across different phases of the inhalation cycle.

Receptor Types and Coupling

• G-protein-coupled receptors (e.g., olfactory receptors) have long-lasting effects, signal amplification, and diverse postsynaptic actions, but are slower due to the signaling cascade.
• Ion-channel-coupled receptors (e.g., photoreceptors) are faster but have limited control and diversity of postsynaptic actions.

Olfactory Receptor Activation

• Odorants bind to membrane odorant receptor proteins, stimulating G-proteins, adenylyl cyclase, and cAMP, which opens cation channels and allows influx of Na+ and Ca2+.
• This triggers a Cl- current, amplifying the olfactory receptor potential and causing membrane depolarization and action potential propagation.

Olfactory Epithelium and Olfactory Bulb

• There are approximately 350 odorant receptor proteins in humans.
• The olfactory epithelium is organized into zones, each containing a different subset of receptor cells, each expressing a single olfactory receptor protein.
• Olfactory receptor neurons send axons into the two olfactory bulbs, with 2000 glomeruli in each bulb, and about 25,000 primary olfactory axons in each glomerulus.