Page 86 - 84 human physiolofy part-1

Chapter 5

olfactory nerve to the olfactory bulb.

Many tiny hair-like cilia protrude from the olfactory receptor cell's dendrite and into the mucus
covering the surface of the olfactory epithelium. These cilia contain olfactory receptors, a type of G
protein-coupled receptor. Each olfactory receptor cell contains only one type of olfactory receptor, but
many separate olfactory receptor cells contain the same type of olfactory receptor. The axons of
olfactory receptor cells of the same type converge to form glomeruli in the olfactory bulb.

Olfactory receptors can bind to a variety of odor molecules. The activated olfactory receptor in
turn activates the intracellular G-protein GOLF, and adenylate cyclase and production of Cyclic AMP
opens ion channels in the cell membrane, resulting in an influx of sodium and calcium ions into the
cell. This influx of positive ions causes the neuron to depolarize, generating an action potential.

Individual olfactory receptor neurons are replaced approximately every 40 days by neural stem
cells residing in the olfactory epithelium. The regeneration of olfactory receptor cells, as one of the
only few instances of adult neurogenesis in the central nervous system, has raised considerable interest
in dissecting the pathways for neural development and differentiation in adult organisms.

In the brain

The axons from all the thousands of cells expressing the same odor receptor converge in the
olfactory bulb. Mitral cells in the olfactory bulb send the information about the individual features to
other parts of the olfactory system in the brain, which puts together the features into a representation of
the odor. Since most odor molecules have many individual features, the combination of features gives
the olfactory system a broad range of odors that it can detect.

Odor information is easily stored in long term memory and has strong connections to emotional
memory. This is possibly due to the olfactory system's close anatomical ties to the limbic system and
hippocampus, areas of the brain that have long been known to be involved in emotion and place
memory, respectively.

Pheromonal olfaction

Some pheromones are detected by the olfactory system, although in many vertebrates pheromones
are also detected by the vomeronasal organ, located in the vomer, between the nose and the mouth.
Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make
a face called flehmen to direct air to this organ. In humans, it is unknown whether or not pheromones
exist.

Olfaction and Gustation

Olfaction, taste and trigeminal receptors together contribute to flavor. It should be emphasized that
there are no more than 5 distinctive tastes: salty, sour, sweet, bitter, and umami. The 10,000 different
scents which humans usually recognize as 'tastes' are often lost or severely diminished with the loss of
olfaction. This is the reason why food has little flavor when your nose is blocked, as from a cold.

The key nutrition players in our taste is the olfactory function, 80-90% of what we consider taste is

86 | Human Physiology