In this article we will discuss about the Smell Receptors and Taste Receptors.
The receptors for taste and smell are classified as chemoreceptors as these respond to special chemicals in aqueous solution. In each case, the chemicals must go into solution in the film of liquid coating the membranes of the receptor cells before these can be detected.
The taste receptors are specialized cells that detect chemicals present in quantity in the mouth itself, while smell receptors are modified sensory neurons in the nasal passage which detect the volatile chemicals that get wafted up the nostrils from distant sources.
These two types of receptors complement each other and often respond to the same stimulus. We can now guess why a very strong perfume leaves a peculiar taste in your mouth. The smell receptors can be as much as 3,400 times more sensitive than the taste receptors.
Sense of Smell (Olfaction)/Smell Receptors:
The receptors of smell occur in a small patch of olfactory epithelium (pseudo stratified epithelium) located in the roof of the nasal cavity.
The olfactory epithelium is yellowish in colour and consists of three types of cells.
(i) Olfactory Receptor Cells:
They act as sensory receptors as well as conducting neurons. The olfactory receptor cells are “unusual” bipolar neurons. Each cell is spindle shaped and has a thin apical dendrite that terminates in a knob which bears non motile cilia called olfactory hairs. Olfactory receptor cells are unique in that they are the only neurons that undergo turnover throughout adult life.
(ii) Supporting Cells:
These are columnar cells which lie between the olfactory receptor cells to support them. They have brownish yellow pigment (similar to lipofuscin) which gives the olfactory epithelium its yellowish colour. Anatomical cells that support other cells are called sustentacular cells.
(iii) Basal Cells:
These are short cells that do not reach the surface. They give rise to new olfactory receptor cells to replace the worn out ones. This is an exception to the fact that neurons are not formed in the postnatal (after birth) life. The olfactory receptor cells survive only for about two months.
Olfactory glands (Bowman’s glands):
Many olfactory glands occur below the olfactory epithelium that secrete mucus to spread over the epithelium to keep it moist. The mucus also protects the cells from dust and bacteria.
The dissolved chemicals stimulate the olfactory receptors by binding to protein receptors in the olfactory hairs (cilia) membranes and opening specific Na+ and K+ channels. This leads ultimately to an action potential that is conducted to the first relay station in the olfactory bulb.
The fibres of the olfactory nerves synapse with mitral cells (second- order neurons) in complex structures called glomeruli (balls of yam). When the mitral cells are activated, impulses from the olfactory bulbs via olfactory tracts to main destinations (e.g., temporal lobe of the cerebrum).
Women often have a keener sense of smell than men, especially at the time of ovulation. Smoking damages the olfactory receptors. With ageing the sense of smell deteriorates. Hyposmia (hypo- less, osmi- smell) is a reduced ability to smell.
Sense of Taste (Gustation)/Taste Receptors:
The receptors for taste are found in the taste buds, mostly located on the tongue but also found on the palate, pharynx and epiglottis and even in the proximal part of oesophagus. The number of taste buds declines with age.
Each taste bud is an oval body consisting of three kinds of cells.
(i) Gustatory Receptor Cells:
They bear at the free end microvilli projecting into the taste pore. The microvilli have special protein receptor sites for taste-producing molecules and come in contact with the food being eaten.
Nerve fibres of the cranial nerves VII (facial), IX (glossopharyngeal) or X (Vagus) end around the gustatory receptor cells, forming synapses with them. The gustatory receptor cells (taste cells) survive only about 10 days and are then replaced by new cells.
(ii) Supporting Cells:
These cells lie between the gustatory receptor cells in the taste bud. They bear microvilli but lack nerve endings.
(iii) Basal Cells:
These cells are found at the periphery of the taste bud. They produce supporting cells, which then develop into gustatory receptor cells.
Specific chemicals in solution pass into the taste bud through the taste pore to come in contact with the protein receptor sites on the microvilli of the gustatory receptor cells. The latter set up nerve impulses in the sensory nerve fibres. The nerve fibres transmit the impulses to the taste centre in the brain (e.g., parietal lobe of the cerebrum) where the sensation of taste arises.
The facial nerve (VII) serves the anterior two-thirds of the tongue, the glossopharyngeal nerve (IX) serves the posterior one-third of the tongue and the vagus nerve (X) serves the pharynx and epiglottis.
Basic Taste Areas:
Human tongue has four basic taste areas: sweet, salty, sour and bitter as shown in the figure 21.43.