OLFACTORY SENSATION or

SENSE OF SMELL

What is olfactory?

-the sense of smell, in which  involves the detection and perception of chemicals floating in the air. Chemical molecules enter the nose and dissolve in mucous within a membrane called the olfactory epithelium. In humans, the olfactory epithelium is located about 7 cm up and into the nose from the nostrils.

The Olfactory system

Olfactory Hair cell- Hair cells are the receptors in the olfactory epithelium that respond to particular chemicals. These cells have small hairs called cilia on one side and an axon on the other side.

The electrical activity produced in these hair cells is transmitted to the olfactory bulb. The information is then passed on to mitral cells in the olfactory bulb.

The olfactory tract transmits the signals to the brain to areas such as the olfactory cortex, hippocampus, amygdala, and hypothalamus. Many of these brain areas are part of the limbic system. The limbic system is involved with emotional behavior and memory. That's why when you smell something, it often brings back memories associated with the object.

3 Kinds of cell of Olfactory Epithelium

Receptor - The receptor cells contain sensory receptors on their epithelial surface and extend their axons directly to the brain.

Supporting cells- whose function is unknown, but which are thought to resemble glial or Schwann cells in the nervous system, are interdigitated between the receptor cells. (supporting cell between them)

Basal cell- divided regularly producing a fresh sensory neurons to replace those that die.

Olfactory neuron - contain over a thousand receptor genes,

there is only a single enhancer capable of binding to the promoters of these genes and turning them on.

Carl Linnaeus - first did the serious effort of identification

of primary olfactory sub modalities. - Swedish botanist, in 1756. These are the primary odors in the sense

of primary taste: Sweet, sour, salty and bitter.

Odor substances have in common that they are gases or volatile liquids. This is the form in which the odorant reaches the sensory epithelium , either to the nostrils with inspired air or by the back door through the mouth and throat.

How does the sense of smell work? Everything you smell, therefore, is giving

off molecules– whether it is bread in the bakery, onions, perfume, a piece of fruit or whatever. Those molecules are generally light, volatile (easy to evaporate) chemicals that float through the air into your nose.

The molecules are generally light, volatile (easy to evaporate) chemicals that float through the air into your nose.

At the top of your nasal passage behind your nose, a patch of special neurons ( about the size of the postage stamp) that are unique in that they are out in the open where they can come into contact with the air. They have hair-like projection called cilia that increase their surface area. An odor molecule binds to these cilia to trigger the neuron and cause you to perceive a smell.

Smell, like taste is a chemical sense detected by sensory cell called chemoreceptor. When an odorant stimulates the chemoreceptor in the nose that detect smell, they pass through electrical impulse to the brain. The brain interprets the electrical activity as specific odors and olfactory sensation becomes perception.

Brain recognize, categorize, and memorize the huge variety of odors.

Richard Axel and Linda Buck - published a ground breaking paper that

shed light on olfactory receptor and how the brain interprets smell.

- discover large gene family. 1000 genes , 3% of human total that are coded for olfactory receptor types.

- every olfactory cell has only one type of receptor.

- each receptor type can detect a small number of related molecule and respond to some with greater intensity than others.

- they also found each olfactory receptor types sends its electrical impulse to a particular microregion of the olfactory bulb.

The Process of smelling:1. Vaporized odor molecules (chemicals) floating in the air

reach the nostrils and dissolve in the mucus.2. Underneath the mucus, I the olfactory epithelium,

specialized receptor neurons detect the odor. These neurons are capable of detecting thousands of different odors.

3. The olfactory receptor neurons transmit the information to olfactory bulbs, which are located at the back of the nose.

4. The olfactory bulbs has sensory receptors that are actually part of the brain which send message directly to:

* The most primitive brain centers where they influence emotions and memories (limbic system structures), and

*Higher centers where they modify conscious thought (neo-cortex).

5. These brain centers perceive odors and access memories to remind us about people, places, or events associated with these alfoctorysensations.