This document discusses taste and smell (gustation and olfaction). It describes the anatomy and physiology of taste buds and the taste pathway from receptors to the cerebral cortex. The primary taste sensations are sweet, salty, sour, bitter and umami. For smell, it outlines the olfactory mucosa, pathway from receptors to olfactory cortex, and transduction process. Disorders of taste include ageusia, hypogeusia and dysgeusia. Anosmia, hyposmia and hyperosmia are abnormalities of smell sensation.

1. TASTE AND SMELL
Dr. Nabeel Beeran
MBBS, MD
Department of Physiology
30/03/2020
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2. Specific Learning Objectives
At the end of this class, student should be able to:
• Name the primary taste sensations.
• Explain the process of taste transduction.
• Trace the taste pathway.
• List the disorders of taste sensation.
• Trace the olfactory pathway.
• Enumerate the disorders of olfaction.
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3. SENSATION OF TASTE
OR
GUSTATION
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4. TASTE BUDS
• Taste buds are the sense organs for taste or gustatory sensation.
• Most of the taste buds are present on the papillae of tongue.
• 3 types of papillae are located on the tongue:
a) Filiform papillae
b) Fungiform papillae
c) Circumvallate papillae.
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5. Structure of Taste Bud
• Taste bud is a bundle of taste receptor
cells, with supporting cells embedded in
the epithelial covering of the papillae.
• Each taste bud contains about 40 cells,
which are the modified epithelial cells.
• Divided into 4 groups:
i. Type I cells or sustentacular cells
ii. Type II cells
iii. Type III cells
iv. Type IV cells or basal cells.
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6. TASTE PATHWAY
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7. Receptors
• Receptors for taste sensation are the type III cells of taste buds.
• Each taste bud is innervated by about 50 sensory nerve fibers.
First order neurons
• Dendrites of the neurons are distributed to the taste buds.
• Fibers arising from taste buds, reaches the cranial nerve nuclei by:
1. Chorda tympani fibers of facial nerve- anterior 2/3rd of tongue
2. Glossopharyngeal nerve fibers- posterior 1/3rd of tongue
3. Vagal fibers, which run from taste buds in other regions.
• Axons runs together in medulla oblongata and terminate in the nucleus of
tractus solitarius.
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8. Second order neuron
• Nucleus of tractus solitarius.
• Axons of 2nd order neurons run through medial lemniscus and terminate
in posteroventral nucleus of thalamus.
Third order neuron
• Posteroventral nucleus of thalamus.
• Axons of 3rd order neurons project into parietal lobe of the cerebral
cortex.
Taste center
• Opercular insular cortex (lower part of postcentral gyrus).
• Taste fibers do not have an independent cortical projection.
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9. PRIMARY TASTE SENSATIONS
• Sweet
• Salt
• Sour
• Bitter
• Umami
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10. TASTE TRANSDUCTION
• The process by which taste receptor converts chemical energy into action
potentials in the taste nerve fiber.
• Receptors are chemoreceptors, stimulated by substances dissolved in
mouth by saliva.
• The dissolved substances act on microvilli of taste receptors
Development of receptor potential Generation of action potential
in the sensory neurons.
• Taste receptor is generally a G-protein coupled receptor (GPCR)- also
called G protein gustducin.
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11. ABNORMALITIES OF TASTE SENSATION
1. Ageusia: Loss of taste sensation.
2. Hypogeusia: Decrease in taste sensation.
3. Taste blindness: Inability to recognize substances by taste due to
genetic disorder.
4. Dysgeusia: Disturbance in taste sensation like hallucinations of taste.
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12. SENSATION OF SMELL
OR
OLFACTION
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13. OLFACTORY PATHWAY
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14. • Olfactory receptors are situated in olfactory mucus membrane.
• Olfactory mucus membrane consists of 10 to 20 millions of olfactory
receptor cells supported by the sustentacular cells.
• Mucosa is lined by mucus-secreting Bowman glands.
• Mucus contains some proteins, which increase the actions of
odoriferous substances on receptor cells.
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15. Olfactory receptor cell is a bipolar neuron.
Dendrite of this neuron is short and has an expanded end called olfactory rod.
From olfactory rod, about 10 to 12 cilia arise, projecting to the mucus
membrane
Axons pierces cribriform plate of ethmoid bone and reach the olfactory bulb
The axons synapses with dendrites of mitral cells
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16. 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 anterior olfactory nucleus, prepyriform cortex,
olfactory tubercle and amygdala -forms part of limbic system.
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17. OLFACTORY TRANSDUCTION
• The process by which olfactory receptor converts chemical energy into
action potentials in olfactory nerve fiber.
• The odoriferous substance stimulates the olfactory receptors resulting
in generation of receptor potential.
• The receptor potential causes generation of action potential in the
axon of bipolar neuron.
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18. ABNORMALITIES OF OLFACTORY SENSATION
1. Anosmia: Total loss of sensation of smell.
2. Hyposmia: Reduced ability to recognize and to detect any odor.
3. Hyperosmia or olfactory hyperesthesia: Increased or exaggerated
olfactory sensation.
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19. References
1. Textbook of Medical Physiology by Guyton and Hall 13th edition.
2. Essentials of Physiology for Dental Students by K Sembulingam and
Prema Sembulingam 2nd edition.
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20. Summary
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