Question

Describe the structure and function of the olfactory epithelium. How does the sense of smell come about? (pages \(309-10\) ).

Short answer

The olfactory epithelium, located in the nasal cavity, is responsible for detecting odorant molecules and is composed of olfactory receptor neurons, supporting cells, and basal cells. Olfactory receptor neurons, each expressing a unique odorant receptor (OR), bind to specific odorant molecules and initiate a signal transduction pathway. This process involves the activation of a G-protein, G_olf, which increases cyclic AMP (cAMP) production, leading to the opening of cyclic nucleotide-gated (CNG) ion channels, calcium and sodium influx, and the generation of action potentials. These action potentials travel to the olfactory bulb and higher brain regions, such as the piriform cortex and amygdala, for the perception and identification of different odors.

Step-by-step solution

Olfactory Epithelium: Location and Structure

The olfactory epithelium is a specialized tissue located within the nasal cavity. It is responsible for detecting odorant molecules from the surrounding air. The epithelium is made up of three main cell types: olfactory receptor neurons, supporting cells, and basal cells.

Olfactory Receptor Neurons

Olfactory receptor neurons are specialized, bipolar, sensory neurons that express unique odorant receptors (ORs) on their cilia. Olfactory cilia extend into the mucus layer covering the epithelium, where they are able to bind to odorant molecules. Each olfactory receptor neuron only expresses one type of OR, allowing for a tremendous range of odor specificity.

Detection of Odorant Molecules

When an odorant molecule binds to its specific OR on the olfactory cilia, it triggers a signal transduction pathway that leads to the generation of an action potential. This process involves the activation of an intracellular signaling cascade, ultimately leading to the opening of ion channels and the influx of ions which depolarize the neuron.

Signal Transduction Pathway

In the signal transduction pathway, the binding of the odorant molecule to its OR activates a G-protein called G_olf. This G-protein activates the enzyme adenylate cyclase, which increases the production of the second messenger molecule cyclic AMP (cAMP). The increase in cAMP levels leads to the opening of cyclic nucleotide-gated (CNG) ion channels, allowing for calcium and sodium influx and the generation of an action potential.

Olfactory Signal Processing and Perception

The action potentials generated in the olfactory receptor neurons travel along their axons and converge at the olfactory bulb, where the information is processed and relayed to higher brain regions, such as the piriform cortex and the amygdala. This higher brain processing allows for the perception and identification of different odors. In summary, the sense of smell arises from the activation of olfactory receptor neurons within the olfactory epithelium by specific odorant molecules. This activation generates action potentials, which are transmitted to the olfactory bulb and then to higher brain regions for processing and perception.