VPB 121: Veterinary Physiology - II (2+1)

OLFACTION

• It is defined as the detection of chemicals that are carried in the air.  It is the sense of smell. 
• The vertebrates have greater capacity in distinguishing the variety of odorants (chemicals detected by the olfactory system).  

Olfactory system

• It comprises of the olfactory receptors, supporting cells, and basal cells. 
• The supporting layer, Bowman’ glands secrete mucus which covers the nasal passage. 
• The basal cells are precursors for new olfactory receptors, which are replaced once in  every two months. 
• The olfactory receptors found in dogs are about 50 million in number. 

Process

The olfactory system of the vertebrates is located in the roof of the nasal cavity.

• Olfaction starts when the odorant enters the nasal cavity and comes into contact with the mucus layer that lines the olfactory epithelium of the nose.
• These mucus contain odorant binding proteins which allow lipophilic odorants to dissolve in the aqueous mucus layer.
• The olfactory receptor cells are bipolar neurons where one end of the neuron ends in the olfactory epithelium and the other ends as a synaptic terminal with the neurons of the olfactory bulb of the brain.
• The olfactory receptor cells are covered by cilia which project into the nasal passage.  These are non-motile, and contain odorant receptor proteins.
• The odorant receptor proteins are  G-proteins.  These proteins are of multigene family and contain many numbers of genes coding for odorant receptors.  In mouse about 1000 potential odorant receptor genes have been identified.
• Each receptor expresses single receptor protein.  Thereby the ability to distinguish even the slightest change in the odorants.
• As the odorant molecules bind with the receptor, the receptor undergoes conformational change that sends signals to the G-protein associated with it.  The activated protein act through the adenylate cylase pathway and cause generator potential.  There are also  other mechanism of causing action potential like the IP3 pathway.
• If the depolarization is large enough, then the action potential is triggered which travel to the bipolar neuron.  In this, the action potential moves toward the cell body and not away from it as in other motor neurons.
• The action potential is transmitted from the bipolar neuron to the synaptic terminal with the neurons of the olfactory bulb and they carry the information to the olfactory bulb.
• The olfactory bulb is a complex neural structure.  It has a lining of ball-like neural junction known as glomeruli.  Each glomerulus receives signals only from a single type of receptor cell.  Thereby, these glomeruli act as smell files to sort out the type of smell of the odorant.
• The fibers leaving the olfactory bulb pass through two routes.
  • Subcortical route:  It goes to the regions of the limbic system mainly to the medial sides of the temporal lobe which is concerned with olfaction and known as primary olfactory cortex.  It includes hypothalamic involvement and coordinates smell with behavioral reactions. 
  • Thalamic-cortical route:  This route is important for conscious perception and fine discrimination of smell.
  • The odorants are cleared from the nasal passage very quickly and the olfactory system adapts quickly, so the sensitivity to the smell of a particular odorant diminishes after a short exposure to the smell.  The odorants are cleared by enzymes named as odorant-clearing enzymes.  They resemble the detoxification enzymes in liver. 

Pheromone

• It is a chemical that causes a natural behavioral response from the another member of the same species (especially reproductive behaviour).
• These pheromones are detected by separate organ known as Vomeronasal organ.  It is also known as Jacobson’s organ.  It is found on each side of the base of the nasal cavity near the nasal septum.  A narrow tube is found between the vomeronasal organ and the nasal cavity.  The vomeronasal organ has receptors that act via the phospholipase C-based signal transduction.

Variation in olfaction

• Animals differ in their ability to detect odours. Animals like dogs, are very sensitive in detection of odour and are called as macrosmatic; those that can detect odour but with less sensitivity like birds are termed microsmatic; those that lack olfactory apparatus like dolphins and whales are termed anosmic.
• Ungulates and rhesus monkeys detect the female in heat by the odour of the vaginal secretions. The olfactory system involved in maternal recognition, regulation of reproduction, mother-young interaction, establishment of dominance.