Sensation Of Smell Physiology Notes

Olfactory Receptors

• Olfactory receptors are situated in the olfactory mucous membrane, which is the modified mucous membrane that lines the upper part of the nostril. The olfactory mucous membrane consists of 10–20 million of olfactory receptor cells supported by the sustentacular cells. The mucosa also contains mucus-secreting Bowman’s glands.
• The olfactory receptor cell is a bipolar neuron. The dendrite of this neuron is short. The expanded end of the dendrite is called the olfactory rod. From the rod, about 10–12 cilia arise. Cilia are unmyelinated, with a length of 2 p and a diameter of 0.1 p. The cilia project to the surface of the olfactory mucous membrane.
• The mucus secreted by Bowman’s glands continuously lines the olfactory mucosa. The mucus contains some proteins, which increase the actions of odoriferous substances on receptor cells.

Read And Learn More: Medical Physiology Notes

1. Vomeronasal Organ:

• The vomeronasal organ is an accessory olfactory organ found in many animals, including mammals. This organ was discovered in 1813 by Danish physician Ludwig Jacobson hence it is also called Jacobson’s organ. It is enclosed in a cartilaginous capsule which opens into the base of the nasal cavity.
• The olfactory receptors of this organ are sensitive to nonvolatile substances such as cents and pheromones. The vomeronasal organ helps the animals to detect even trace quantities of chemicals. The impulses from this organ are sent to the amygdala and hypothalamus via the accessory olfactory bulb.
  • Vomeronasal Receptors in Human Beings:
    • In human beings, the vomeronasal organ was considered as vestigial or nonfunctional. Recently, it is found that this organ is present in the form of vomeronasal pits on the anterior part of the nasal septum. The receptors of the vomeronasal pit detect odorless human pheromones or vomeropherins at a very low concentration in the air. The subconscious detection of odorless chemical messengers in air is considered as the sixth sense in human beings.

Olfactory Pathway

• Axons of the bipolar olfactory receptors pierce the cribriform plate of ethmoid bone and reach the olfactory, bulb. Here, the axons synapse with the dendrites of mitral cells. Different groups of these synapses form globular structures, called olfactory glomeruli.
• The axons of mitral cells leave the olfactory bulb and form the olfactory tract. The olfactory tract runs backward and ends in the olfactory cortex through the intermediate and lateral olfactory stria.
• The olfactory cortex includes the structures, which form a part of the limbic system. The structures are the anterior olfactory nucleus, prepyriform cortex, olfactory tubercle, and amygdala.

Generator Potential In Olfactory Receptor

• The odoriferous substance stimulates the olfactory receptors, only if it dissolves in mucus, covering the olfactory mucous membrane. The molecules of the dissolved substance, bind with receptor proteins in the cilia.
• The substance receptor complex activates adenyl cyclase that causes the formation of cyclic AMP. The cyclic AMP in turn causes the opening of sodium channels, leading to an influx of sodium and the generation of receptor potential. The receptor potential causes the generation of action potential in the axon of the bipolar neuron.

Classification Of Odor

The odor is classified into various types:

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.

Threshold For Olfactory Sensation

Ethyl ether            :   5.8 mg/L of air
Choroform            :   3.3 mg/L of air
Peooermint Oil      :  0. 02 mg/L of air
Butyric musk         :  0. 009 mg/L of air
Artifical musk        :  0.00004 mg/L of air
Methyl mercaptan :  0.0000004 mg/L of air

Thus, methyl mercaptan produces an olfactory sensation even at a low concentration of 0.0000004 mg/L of air.

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

1. 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 the nose which occurs during the common cold, nasal sinus, and allergic conditions. Permanent anosmia occurs during lesions in the olfactory tract, meningitis, and degenerative conditions such as Parkinson’s disease and Alzheimer’s disease.

2. 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 the same causes of anosmia.

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