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.

Key concepts

Olfactory Receptor Neurons

Within the olfactory epithelium, you'll find the star players of the sense of smell: the olfactory receptor neurons (ORNs). These are specialized neurons with the primary role of detecting different smells. Each ORN has a unique property—it only responds to specific odorant molecules, thanks to the presence of unique odorant receptors (ORs) found on their cilia. Imagine these cilia as tiny hair-like structures extending into the nasal cavity. They ensure the neurons are just the right level of sensitive to detect minute traces of odor in the air. ORNs are bipolar neurons, meaning they have two extensions. One extends to interact with odorants, and the other sends signals to the brain.

• Every ORN expresses only one type of OR, granting exquisite selectivity and sensitivity to a vast array of smells.
• These neurons are continually regenerated, maintaining the integrity of the olfactory system.

This ability to regenerate is quite unique and essential, as ORNs are exposed directly to the external environment and suffer frequent damage.

Signal Transduction Pathway

Once an odorant molecule binds to its corresponding olfactory receptor on an ORN, a complex process known as the signal transduction pathway begins. This is where the magic of smell perception really starts. Initially, the binding of an odorant activates a G-protein called G_olf, a kind of molecular switch. This triggers adenylate cyclase, an enzyme that ramps up the production of a secondary messenger molecule called cyclic AMP (cAMP).

• cAMP acts like a messenger, opening cyclic nucleotide-gated ion channels.
• These channels allow calcium and sodium ions to flow into the neuron, leading to depolarization.

Depolarization is a change in the cell’s electric state, crucial for carrying the smell signal further along to the brain. This entire chain of events ensures that the smallest whiff of an odor can be detected and processed effectively.

Olfactory Signal Processing

Once the signal transduction pathway initiates an action potential, this electrical pulse travels along the axon of the neuron to the olfactory bulb. Here in the olfactory bulb, the information undergoes the first major sorting and processing stage. The olfactory bulb is like the brain's reception area for smells, organizing and relaying odor information to other brain regions.

• Action potentials from multiple ORNs converge onto structures called glomeruli in the olfactory bulb.
• Each glomerulus processes specific scent information, acting like an organizational hub.
• This hub-sorted information then gets relayed to higher brain regions for further processing.

These higher brain regions include areas crucial for emotional memory and odor recognition, helping us identify and remember different smells.

Odorant Molecules

Odorant molecules are the chemical compounds that provoke the sense of smell. They are diverse and complex, often comprising volatile compounds that can float through the air and reach our nasal cavity. Once inhaled, these molecules encounter the olfactory epithelium, finding specific olfactory receptors that match their shape and chemical properties.

• The structure and composition of odorant molecules determine their interaction with olfactory receptors.
• Only molecules with certain characteristics can bind to the receptors efficiently, initiating the sense of smell.

This specificity ensures that the olfactory system can distinguish between thousands of potential odors, contributing to our nuanced perception of the environment.

Action Potential

The action potential is the electrical signal generated by the olfactory receptor neurons when stimulated by an odorant molecule. It represents a crucial step in transforming a chemical signal—the odor—into an electrical one that the brain can interpret.

• The influx of ions through opened channels causes a quick change in voltage, known as depolarization.
• This change triggers an action potential, which travels along the neuron's axon.
• Upon reaching the olfactory bulb, this electrical signal initiates further processing.

The action potential is a fundamental aspect not just of smell, but of how neurons communicate in general. It ensures the rapid transmission of information across networks of neurons, enabling swift and accurate sensory perception.