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.
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