Taste and Smell & Equilibrium

• Gustation (taste)


  Sensation that results from action of chemicals on taste buds
• There are approximately 4000 taste buds
• Taste buds


  • Mainly on tongue in papillae (fungiform, foliate, and vallate)
  • Others are present inside cheeks, and on soft palate, pharynx, and epiglottis
• Structure of a taste bud

  Lemon-shaped groups of 40 – 60 taste cells, supporting cells, and basal cells
• Taste cells


  • Have tuft of apical microvilli (taste hairs) that serve as receptor surface for taste molecules
  • Taste pores – pit in which the taste hairs project
  • Taste hairs are epithelial cells not neurons
  • Synapse with and release neurotransmitters onto sensory neurons at their base
  • Basal cells replace taste cells every 7-10 days
• Physiology of taste


  To be tasted, molecules must dissolve in saliva and flood the taste pore
• Five primary taste sensations

  • Salty – produced by metal ions (sodium and potassium)
  • Sweet – associated with carbohydrates and other foods of high caloric value
  • Sour – acids such as in citrus fruits
  • Bitter – associated with spoiled foods and alkaloids such as nicotine, caffeine, quinine, and morphine
  • Umami – 'meaty' taste of amino acids in chicken or beef broth
• Taste sensations on the tongue

  • Tip is most sensitive to sweet, edges to salt and sour, and rear to bitter
  • However, all taste buds can detect all 5 primary sensations
  • Each taste cell can only respond to one type of taste
• Mechanisms of taste


  1. Activate 2nd messenger systems – sugars, alkaloids, and glutamate bind to receptors which activates G proteins and second-messenger systems within the cell

  2. Depolarize cells directly – sodium and acids penetrate cells and depolarize it directly

  3. Either mechanism results in release of neurotransmitters that stimulate dendrites at base of taste cells
• Salty taste sensation


  • Due to presence of Na or other cations
  • Na+ or K+ passes through ion channels into taste cells
  • Taste cells are depolarized which triggers calcium influx
  • Calcium influx triggers release of NT on CN VII and IX, X
• Sour taste sensation


  • Due to presence of H+ (all acids are sour)
  • Hydrogen ions move through apical channels in receptor cell
  • Causes depolarization and calcium influx
  • Ca triggers NT release onto CN VII and IX, and X
  • Degree of sourness corresponds to quantity of NT released
• Sweet and umami taste

  • Sensations are coupled to membrane receptors coupled to G proteins
  • The G proteins, called Gustducins, in turn, trigger 2nd messenger systems
  • 2nd messengers close K+ channels preventing the efflux of K+
  • This leads to depolarization and release of NT to trigger CN VII and IX, X
• Bitter taste


  • Sensations are coupled to membrane receptors coupled to G proteins
  • The G proteins, called Gustducins, in turn, trigger 2nd messenger systems
  • 2nd messengers trigger internal release of calcium which triggers release of NT to trigger CN VII and IX, and X
• Projection pathways for taste

  1. Facial nerve collects sensory information from taste buds over anterior two-thirds of tongue

  2. Glossopharyngeal nerve from posterior one-third of tongue

  3. Vagus nerve from taste buds of palate, pharynx and epiglottis
• Projection pathways for taste

  1. First order neurons from CN VII, IX, X terminate in the nucleus tractus solitarius (med oblongata)

  2. Second order neurons terminate in the ventral posteromedial nucleus of thalamus

  3. Third order neurons terminate in the Gustatory cortex (in insula) where taste is perceived

  4. Gustatory cortex relays to other cortical areas dealing with vision and smell to generate a more complex taste experience

  5. Some second order neurons also project to hypothalamus and limbic system to generate emotional and visceral response to taste (gagging, salivation, pleasure)
• Olfaction (smell)


  Sense of smell
• Olfactory mucosa


  • Contains 10 to 20 million olfactory neurons which make up the olfactory nerve(CN I)
  • Also contain supporting cells, basal stem cells
• Olfactory cells


  • Bipolar neurons
  • Head bears 10 – 20 cilia called olfactory hairs
  • Have binding sites for odorant molecules
  • Covered by mucus secreted by olfactory glands
  • Fascicles are collectively regarded as Cranial Nerve I
  • Only neurons in the body directly exposed to the external environment
  • Have a lifespan of only 60 days
  • Basal cells continually divide and differentiate into new olfactory cells
• Smell - physiology


  • Odorant molecules bind to membrane receptor on olfactory hair
  • We have 350 kinds of olfactory receptors
  • Each olfactory cell has only one receptor type
  • But each receptor can respond to multiple odors
  • Each odorant can bind to multiple receptors
• Olfactory physiology


  1. All odorant receptors are G-protein coupled

  2. Binding of receptor causes dissociation of G-protein subunits

  3. Dissociated subunits bind and activate adenylate cyclase

  4. Adenylate cyclase converts ATP to cAMP and PPi (pyrophosphate)

  5. cAMP opens ion channels that allow inward flux of Na and Ca which depolarize the bipolar neuron to create action potentials that is propagated down its axon into the brain
• Olfactory projection pathways


  1. Olfactory cells synapse in olfactory bulb on dendrites of mitral and tufted cells

  2. Dendrites meet in spherical clusters called glomerulus

  3. Each glomerulus dedicated to single aspect of the odor because all fibers leading to one glomerulus come from cells with same receptor type

  4. A particular odor may trigger activity in several glomerulus to create an odor signature that has several characteristics

  5. Tufted and mitral cell axons form olfactory tracts that reach primary olfactory cortex in the inferior surface of the temporal lobe

  6. Secondary destinations include hippocampus, amygdala, hypothalamus, insula, and orbitofrontal cortex to identify odors, integrate smell with taste, perceive flavor, evoke memories and emotional responses, and visceral reactions

  7. Fibers reach back to olfactory bulbs where granule cells inhibit the mitral and tufted cells, which is why odors change under different conditions (e.g. food smells more appetizing when you are hungry perhaps because there is less inhibition of mitral and tufted cells)
• Smell - physiology


  • Olfactory receptors adapt quickly due to synaptic inhibition in olfactory bulbs
  • Some odorants act on nociceptors of the trigeminal nerve (ammonia, menthol, chlorine, and capsaicin of hot peppers)
• Human pheromones


  • Human body odors may affect sexual behavior
  • A person's sweat and vaginal secretions affect other people's sexual physiology
  • Dormitory effect - presence of men seems to influence female ovulation
  • Ovulating women's vaginal secretions contain pheromones called copulines, that have been shown to raise men's testosterone level
• Equilibrium


  Coordination, balance, and orientation of body in three-dimensional space
• Equilibrium receptors


  • Located in the vestibular apparatus
  • Three semicircular canals
  • Utricle and Saccule
  • These structures contain receptors that generate nerve impulse to reflect movement of the body (and head)
• Semicircular canals


  • Three semicircular canals oriented perpendicular to each other
  • Anterior canal detects movement of head up or down
  • Posterior canal detects movement of head up and down to the side
  • Lateral Canal detects movement of head from side to side ('no')
• Semicircular canal anatomy


  • Receptor of the semicircular canals are hair cells located in the crista ampulla
  • Hair cells have stereocilia that are graded in height toward the tallest one called a kinocilium
  • Stereocilia project upward into gelatinous material called the cupula
• Generating an impulse


  1. Bending of the hair cells cause ion channels to open or close resulting in a change in the membrane potential of the hair cells

  2. Hair cells release NT to generate action potential associated vestibular nerve
• Coding for acceleration


  • At rest or constant motion hair cells fire at constant rate
  • When rotation causes stereocilia to bend away from kinocilium, ion channels are closed which hyperpolarizes the hair cell and less AP generated
  • When rotation causes stereocilia to bend toward kinocilium, ion channels are open to depolarize hair cell and generate more AP
• Utricle and saccule


  • Bulges between semicircular canals and cochlea
  • Oriented to detect linear acceleration
  • Utricle—detects forward and backward motion
  • Saccule—detects up and down motion
• Anatomy: utricle and saccule

  • Hair cells of the macula in the utricle and saccule act as the receptor cells
  • Hair cells have stereocilia embedded in gelatinous layer containing otoliths (calcium carbonate crystals)
  • Hair cells of utricle and saccule are oriented perpendicularly to each other
• Coding for linear acceleration


  1. Movement of body creates drag on otolithic membrane which bends stereocilia toward or away from kinocilium to open or close ion channels in the hair cells (similar to the mechanism of the semicircular canals)

  2. Bending toward kinocilium, opens ion gates to increase firing

  3. Bending away from kinocilium closes ion gates and decreases firing

  4. Linear acceleration will cause movement of stereocilia toward or away from kinocilium in the saccule (vertical) and/or utricle (horizontal)
• Coding for head tilting

  • Utricle is also used to detect head tilting
  • Tilting head forward, bends stereocilia toward kinocilium which increases AP in afferent nerve
  • Bending head backward, bends stereocilia away from kinocilium to decrease AP in afferent nerve
• Equilibrium projection pathways


  1. Hair cells of saccule, utricle and semicircular canals triggers impulse in the vestibular nerve (part of CN VIII)

  2. First order neurons travel through CN VIII to synapse in Vestibular nuclei (pons)

  3. Secondary neurons synapse in Thalamus

  4. Tertiary neurons synapse in Vestibular cortex (Parietal lobe)
• Equilibrium projection pathways


  • Vestibular nerve ends in vestibular nuclei which relays information to five target areas
  • Left and right nuclei receive input from both ears
  • Other targets include cerebellum, nuclei of oculomotor, trochlear, and abducens nerves (CN III, IV, and VI) to produce vestibulo-ocular reflex, reticular formation, and spinal cord
• Vestibular nystagmus


  • Involuntary oscillations of the eyes that occurs when spinning motion is suddenly stopped
  • During a spin, bending cupula causes smooth movement of eyes in a direction opposite to that of head movement so that stable visual fixation point can be maintained
  • When spinning is suddenly stopped, cupula moves in opposite direction and the eyes move in that direction as well, causing the eyes to involuntarily oscillate until cupula is stable
  • Gives you a feeling that you or the room is spinning
  • Observed as symptom of an inner ear disease called Meniere's disease
• Vertigo


  • Loss of equilibrium
  • May occur naturally from spinning due to the physiology of the vestibular apparatus
  • Pathologically due to anything that alters the firing rate of one of the vestibular nerve (right or left) compared to the other, usually due to viral infection or Meniere's syndrome (affects internal ear)