Sense of

smell

Localization and structure of Olfactory receptors.

Olfact = To smell Olfaction detects chemicals in solution

which is detected in the Olfactory Epithelium which is a yellow-tinged patch located in the roof of the nasal cavity.

The epithelium contains millions of olfactory receptor cells

Olfactory receptor cells are bipolar neurons surrounded by columnar epithelial cells.

Each cells has a thin apical dendrite were from which long cilia radiate.

Olfactory cilia increase the receptive surface area, covered by a coat of thin mucus.

This mucus is a solvent that “captures” and dissolves airborne odor molecules.

Stimulated olfactory receptors cells send nerve impulses along their axons which synapse with neurons located in the Olfactory bulbs.

Within the Olfactory bulbs the impulse are analyzed and the impulses travel along the olfactory tracts to the limbic system.

Which are interpreted in areas for these impulses lie within the temporal lobes, parts of the frontal lobes and hypothalamus.

Specificity of the olfactory receptors.

The idea of how we smell individual chemicals is still not known completely. Many researchers have tried to relate taste with it's 4 main chemical receptors to that of smells and have had many varied results ranging from 50 to 1000 different type of receptors. It's still a question not entirely answered yet.

Activation of the Olfactory Receptors:

For us to be able to smell a particular chemical, it must be volatile, or it must be in a gaseous state when it enters the nasal cavity. Must be water soluble to dissolve in the fluid coating olfactory epithelium. The dissolved chemicals stimulate receptors, this leads to receptor potential and to an action potential that is conducted to the first relay station in the olfactory bulb.

Mechanism of Smell Transduction:

G proteins are involved and that at least in some receptor cells 1- Cyclic AMP is a second messenger and 2- opening of Na+ channels leads to depolarization and impulse transmission. Other details of transduction are still to be fully worked out.

The Olfactory Pathway

Air has a certain path it takes when it enters the nose. First it passes over the olfactory membrane. Then chemical stimulate receptors cells. After that the information passes from the receptor neurons to the bulbs and tracks, which then goes into the frontal lobe of the brain. Each track then divides and passes to the olfactory cortex where smell is recognized.

Video!

https://m.youtube.com/watch?v=snJnO6OpjCs