Olfactory Receptors and there functions

 OLFACTORY RECEPTORS

Olfactory receptors are situated in olfactory mucus membrane, which is the modified mucus membrane that lines upper part of nostril. Olfactory mucus membrane consists of 10 to 20 millions of olfactory receptor cells supported by the sustentacular cells. Mucosa also contains mucus-secreting Bowman glands.

Olfactory receptor cell is a bipolar neuron. Dendrite of this neuron is short and it has an expanded end called olfactory rod. From olfactory rod, about 10 to 12 cilia arise. Cilia are non-myelinated, with a length of 2 μ and a diameter of 0.1 μ. These cilia project to the surface of olfactory mucus membrane.

Mucus secreted by Bowman glands continuously lines the olfactory mucosa. Mucus contains some proteins, which increase the actions of odoriferous substances on receptor cells.

VOMERONASAL ORGAN

Vomeronasal organ is an accessory olfactory organ found in many animals including mammals. This organ was discovered in 1813, by a Danish physician Ludwig Jacobson, hence it is also called Jacobson organ. It is enclosed in a cartilaginous capsule, which opens into the base of nasal cavity. Olfactory receptors of this organ are sensitive to non-volatile substances such as scents and pheromones. Vomeronasal organ helps the animals to detect even the trace quantities of chemicals. Impulses from this organ are sent to amygdala and hypothalamus via accessory olfactory bulb.

VOMERONASAL ORGAN IN HUMAN BEINGS

In human beings, the vomeronasal organ was considered as vestigial or non-functional. Recently, it is found that this organ is present in the form of vomeronasal pits on the anterior part of nasal septum. Receptors of vomeronasal pit detect odorless human pheromones or vomeropherins, at a very low concentration in air. The subconscious detection of odorless chemical messengers in air is considered as the sixth sense in human beings.

OLFACTORY PATHWAY

Axons of bipolar olfactory receptors pierce the cribriform plate of ethmoid bone and reach the olfactory bulb. Here, the axons synapse with dendrites of mitral cells. Different groups of these synapses

form globular structures called olfactory glomeruli. Axons of mitral cells leave the olfactory bulb and

form olfactory tract. Olfactory tract runs backward and ends in olfactory cortex, through the intermediate and lateral olfactory stria.

Olfactory cortex includes the structures, which form a part of limbic system. These structures

are anterior olfactory nucleus, prepyriform cortex, olfactory tubercle and amygdala.

OLFACTORY TRANSDUCTION

Olfactory transduction is the process by which olfactory receptor converts chemical energy into

action potentials in olfactory nerve fiber. The odoriferous substance stimulates the olfactory

receptors, only if it dissolves in mucus, covering the olfactory mucus membrane. Molecules of dissolved

substance, bind with receptor pro teins in the cilia and form substance-receptor com plex. Substancereceptor complex activates adenyl cyclase that causes the formation of cyclic AMP. Cyclic AMP in turn, causes opening of sodium channels, leading to influx of sodium and generation of receptor potential. Receptor potential causes generation of action potential in the axon of bipolar neuron.

CLASSIFICATION OF ODOR

Odor is classified into various types. Substances producing different types of odor are:

1. Aromatic or resinous odor: Camphor, lavender, clove and bitter almonds

2. Ambrosial odor: Musk

3. Burning odor: Burning feathers, tobacco, roasted coffee and meat

4. Ethereal odor: Fruits, ethers and beeswax

5. Fragrant or balsamic odor: Flowers and perfumes

6. Garlic odor: Garlic, onion and sulfur

7. Goat odor: Caproic acid and sweet cheese

8. Nauseating odor: Decayed vegetables and feces

9. Repulsive odor: Bed bug.

ADAPTATION

Olfactory receptors are phasic receptors and adapt very rapidly. Within one second, the adaptation occurs up to 50%.

APPLIED PHYSIOLOGY – ABNORMALITIES OF OLFACTORY SENSATION

ANOSMIA

Anosmia refers to total loss of sensation of smell, i.e. inability to recognize or detect any odor. It may

be temporary or permanent. Temporary anosmia is due to obstruction of nose, which occurs during

common cold, nasal sinus and allergic conditions. Permanent anosmia occurs during lesion in olfactory

tract, meningitis and degenerative conditions such as Parkinson disease and Alzheimer disease.

HYPOSMIA

Hyposmia is the reduced ability to recognize and to detect any odor. The odors can be detected only at

higher concentrations. It is the most common disorder of smell. Hyposmia also may be temporary or permanent. It occurs due to same causes of anosmia.

HYPEROSMIA

Hyperosmia is the increased or exaggerated olfactory sensation. It is also called olfactory hyperesthesia. It occurs in brain injury, epilepsy and neurotic conditions.