Posted by jayaram in OLFACTORY RECEPTORS
Olfactory receptors are situated in olfactory mucus membrane, which is the modified mucus membrane lining upper part of nostril. The olfactory mucus membrane consists of 10 to 20 millions 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 is short. The expanded end of the dendrite is called olfactory rod. From the rod, about 10 to 12 cilia arise. Cilia are non-myelinated with a length of 2 micron and a diameter of 0.1 micron. The cilia project to the surface of olfactory mucus membrane.
The mucus secreted by Bowman’s glands continuously lines the olfactory mucosa. This mucus contains some proteins, which increase the actions of odoriferous substances on receptor cells.
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 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 olfactory tract. The olfactory tract runs backwards and ends in olfactory cortex through the intermediate and lateral olfactory stria.
The olfactory cortex includes the structures, which form a part of limbic system. The structures are anterior olfactory nucleus, prepyriform cortex, olfactory tubercle and amygdale.
GENERATOR POTENTIAL IN OLFACTORY RECEPTOR
The odoriferous substance stimulates the olfactory receptors, only if it dissolves in mucus, covering the olfactory mucus 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 cyclic AMP. This cyclic AMP causes opening of sodium channels, leading to influx of sodium and generation of receptor potential.
This receptor potential causes generation of action potential in the axon of the bipolar neuron.
ADAPTATION
Olfactory receptors are phasic receptors and adapt very rapidly. Within first second, the adaptation occurs up to 50%.
APPLIED PHYSIOLOGY – ABNORMALITIES OF OLFACTORY SENSATION
- Anosmia – loss of sensation of smell.
- Hyposmia – Reduction in olfactory sensation due to constant exposure to a particular odor (like excessive perfume)
- Hyperosmia – Increased olfactory sensation