Taste & Smell Physiology Lecture SlideshowTaste & Smell Physiology Lecture Slideshow
  • Separate undesirable or lethal foods from
    pleasant & nutritious food
  • Elicit physiological responses
    – Involved in digestion & utilization of foods
  • Strongly tied to primitive emotional &
    behavioral functions of nervous systems
  • Smell
    – Animals recognize proximity of other animals or
    even individuals among animals

Sense of Taste (gustation)

  • Function of taste buds
  • Contributions
    – Sense of smell
    – Texture of food
    – Stimulation of pain endings-pepper
  • Importance
    – Food selection
  • Desires
  • Metabolic need

Taste-Primary(Elementary) Sensations

  • Sour Taste-Acids (H ion)
    – Intensity – proportional to logarithm of H ion conc
  • Salty Taste-Na ion conc
    – Quality – some salts elicit other taste sensations
    – Cations
    – Anions- lesser extent

Taste-Primary(Elementary) Sensations

  • Bitter Taste – organic substances
    – Long-chain organic substances-nitrogen
    – Alkaloids- quinine, caffeine, strychnine, &
    – Saccharin-sweet ( bitter aftertaste)
    – Bitter taste-rejection of food
  • Deadly toxins in poisonous plants-alkaloids
    – Intensely bitter taste
  • Sweet Taste
    – Organic chemicals
  • Sugars, glycols, alcohols, aldehydes, ketones, amides,
  • Amino acids, small proteins
    – Sulfonic acids, halogenated acids
    – Inorganic salts of lead & beryllium
  • Sweet to bitter
    – Slight changes in chemical structure
  • Addition of a simple radical
  • Umami Taste (delicious)- L-glutamate
    – Meat extracts & aging cheese
    – Taste receptor-related to one of glutamate
    receptors expressed in neuronal synapses of
  • Fatty
    – Long chain fatty acids

Taste Thresholds

Taste Blindness

Some people are taste blind for certain substances especially for different types of thiourea compounds.
Phenylthiocarbamide is the compound which is used to test taste blindness. It occurs in 15 to 30 % depending upon method of testing & concentration of the substance.

Distribution of Taste Sensation on the Surface of the Tongue

Sweet taste – tip of the tongue. Salty taste – lateral margin of the tongue. Sour taste – posteromedial part of upper surface of tongue. Bitter taste – posterior 1/3 portion of the tongue.

Taste Bud

  • Diameter-1/30 mm
  • Length-1/16 mm
  • Epithelial cells-50
    – Supporting cells
    – Taste cells
  • Continually being replaced
  • Mature cells-center
  • Life span-10 days in lower mammals
  • Microvilli (taste hairs)-receptor surface

Taste Buds-Location

  • Circumvallate papillae-posterior tongue
    – Large number
  • Fungiform papillae-anterior flat tongue
    – Moderate numbers
  • Foliate papillae-in folds along lateral surfaces
    – Moderate numbers
  • Additional taste buds
    – Palate, tonsillar pillars, epiglottis, proximal esophagus
  • Number of taste buds
    – Adults-3000 to 10,000 taste buds
    – Children-a few more
    – Beyond 45-degenerate decreasing taste sensitivity

Taste Buds-Specificity

  • Low concentration-taste substance
    – Single taste buds-Specific to one of five primary
    taste stimuli
  • High concentration
    – Most buds excited by two or more of primary
    taste stimuli
    – Taste stimuli other than “primary” categories

Taste Buds-Mech of Stimulation

  • Receptor Potential
    – Proportional to logarithm of conc of stimulating
  • Salty & sour sensations
    – Na & H ions-open specific ion channels in apical
  • Sweet & bitter sensations
    – Second-messenger transmitter substances
  • Intracellular chemical changes elicit taste signals

Generation of Nerve Impulses by Taste Bud

  • Adaptation
    – First application of stimulus
  • Rate of discharge of nerve fibers rises to a peak-
    fraction of a sec
  • Adapts within next few sec back to a lower, steady
    level as long as taste stimulus remains
  • Strong immediate signal
  • Weaker continuous signal

Transmission of Taste Signals into CNS

  • Anterior 2/3
    – Facial nerve
  • Back of tongue & posterior regions of
    mouth & throat
    – Glossopharyngeal nerve
  • Base of tongue & parts of pharyngeal
    – Vagus nerve
  • Tractus solitarius in medulla

3. Second order fibers pass upwards &
relay in the ventral posterior medial
(VPM) nucleus of thalamus.

4. From thalamus 3rd order fibers pass upwards to gustatory cortex which is the lower tip of post central gyrus that curls deep into the sylvian fissure & into the adjacent opercular insular area

Taste Reflexes

  • Tractus solitarius
    – Superior & Inferior salivatory nuclei
  • Submandibular
  • Sublingual
  • Parotid glands

Rapid Adaptation of Taste

  • Rapid adaptation-complete within a minute
    – Adaptation of taste buds-about half
  • Final extreme degree of adaptation-CNS

Taste Preference & Control of Diet

  • Taste preferences change-body’s need
  • Rejection of food having unpleasant affective
  • Mechanism located in CNS
    – Previous experience with unpleasant or pleasant
    tastes plays a major role in determining one’s
    taste preferences
  • Negative taste preference, or taste aversion

Abnormalities in Taste Detection

  • Ageusia
    – Absence of sense of taste
  • Adverse side effect-cisplatin & captopril
  • Deficiencies-vitamin B3 or zinc
  • Hypogeusia
    – Diminished taste sensitivity
  • Damage to lingual or glossopharyngeal nerve
  • Aging & tobacco abuse
  • Dysgeusia or parageusia
    – Unpleasant perception of taste
    – Metallic, salty, foul, or rancid taste

Sense of Smell

  • Least understood of our senses
    – Subjective phenomenon
    – Poorly developed in human beings compared to
    lower animals

Olfactory epithelium

Olfactory Membrane

  • Olfactory membrane lies in the superior part of
    each nostril.

Organization of olfactory memb & bulb, connections to olfactory tract

Olfactory receptors are specialized endings of renewable afferent neurons

Olfactory Cells

  • Receptor cells- olfactory cells-100 million
    – Bipolar nerve cells
  • surrounded by supporting cells and basal
  • The mucosal end of the olfactory cell forms a
    knob from which 4-25 olfactory hairs or cilia
    project into the mucus that coats the inner
    surface of the nasal cavity.

Physical factors affecting degree of stimulation

  • Volatile substances
  • Water solubility to dissolve through mucus
  • Slight lipid solubility

The odorant substance first diffuses
into the mucus that covers the cilia.
Then it binds with receptor proteins in
the membrane of each cilium.
On excitation of the receptor protein,
an alpha sub-unit breaks away from
the G-protein & immediately activates
adenylyl cyclase.
The activated cyclase converts many
molecules of ATP into cyclic AMP
which causes opening of gated sodium
ion channel resulting in influx of
sodium ions.
This increases electrical potential in
positive direction inside the cell
membrane & produces action
potentials which travel to CNS through
olfactory nerve.

Olfactory Cells-Memb Potentials & Action Potentials

  • Memb potential in unstimulated cells
    – -55 mv
    – Continuous action potentials-very slow rate
  • 1/20 sec up to 2-3/sec
  • Depolarization
    – From -55 mv to -30 mv
    – Action potentials
  • 20 to 30/sec
  • Rate of nerve impulses changes in proportion to
    logarithm of stimulus strength

Signal transduction in an odorant receptor

Rapid Adaptation of Olfactory Sensations

  • Olfactory receptors adaptation
    – In first sec
  • 50 %
    – After first sec
  • Very little & very slow
  • Smell sensations adapt to extinction
    – within a minute
  • Additional adaptation-within CNS

Rapid Adaptation of Olfactory Sensations

  • Postulated neuronal mechanism
    – Strong feedback inhibition by CNS
    – Centrifugal nerve fibers
  • From olfactory regions of brain to granule cells

Primary Sensations of Smell

  • Primary sensations-100
  • Receptor cells-100 million
  • Odorant receptors types-1000
    – Camphoraceous
    – Musky
    – Floral
    – Pepperminty
    – Ethereal
    – Pungent
    – Putrid

Affective Nature of Smell

  • Pleasantness
  • Unpleasantness
    – Nauseating smell
    – Selection of food
    – Powerful stimulant of human emotions

Threshold for Smell

  • Very low
  • Methylmercaptan
    – One 25 trillionth of a gram/each ml of air
  • Gradations of Smell Intensities
    – Maximum intensity of smell
  • 10 to 50 times above threshold
  • Presence or absence of odors
    – Quantitative detection of intensities
  • Eyes-500,000 to 1
  • Ears-1 trillion to 1

Olfactory Thresholds

Transmission of Olfactory Signals to CNS

From the olfactory membrane
nerve fibers pass upwards
through cribriform plate to
enter the olfactory bulb in
the cranial cavity. With in the
olfactory bulb there are
multiple globular structures
called glomeruli. Each
glomerulus is the terminus for
about 25000 axons from
olfactory cells.

Organization of olfactory memb & bulb, connections to olfactory tract

Each glomerulus is also terminus for dendrites
from about 25 large mitral cells & about 60
smaller tufted cells, the cell bodies of which lie in
the olfactory bulb superior to glomeruli.
The mitral & tufted cells send axons through the
olfactory tract (olfactory nerve) to transmit
olfactory signals to higher levels in the CNS.
The olfactory tract enters the brain at the anterior
junction b/w the mid brain & cerebrum.
The tract divides into two pathways that is one
passing medially into the medial olfactory area of
the brain stem & other passing laterally into
lateral olfactory area.
The medial olfactory area represents a very old
olfactory system, where as lateral olfactory area is
the input to; 1. A less old olfactory system & 2. a
newer system

Transmission of Smell Signals into CNS

Transmission of Smell Signals into CNS

  • Olfactory cilia
  • Olfactory cell
  • Glomerulus
  • Olfactory bulb
  • Axons of mitral cells
  • Olfactory tract
  • Olfactory area

Transmission of Smell Signals into CNS

  • Very Old Olfactory System-Medial Olfactory Area
  • Less Old Olfactory System-Lateral Olfactory Area
  • Newer Pathway

Transmission of Smell Signals into CNS

  • Very Old Olfactory System-Medial Olfactory Area
    – Olfactory reflexes
    – Septal nuclei-feed into hypothalamus and primitive
    portions of brain’s limbic system
  • Basic behavior
  • Salivation etc

Transmission of Smell Signals into CNS

  • Less Old Olfactory System-Lateral Olfactory Area
    – Automatic but partially learned control of food intake
    & aversion to toxic and unhealthy foods
    – Prepyriform & pyriform cortex
    – Cortical portion of amygdaloid nuclei
  • All portions of limbic system
    – Hippocampus-learning to like or dislike certain foods depending on
    one’s experiences with them
    » absolute aversion to foods that have caused nausea and
  • paleocortex in the anteromedial portion of the temporal lobe

Transmission of Smell Signals into CNS

  • Newer Pathway
    – Conscious perception & analysis of olfaction
    – To dorsomedial thalamic nucleus & then to
    lateroposterior quadrant of orbitofrontal cortex
  • Conscious analysis of odor

Odor Detection-Abnormalities

  • Anosmia
    – Inability to smell
  • Hyposmia
    – Diminished olfactory sensitivity
  • Nasal congestion
  • Damage to olfactory nerves-fractures of cribriform
  • Neuroblastomas or meningiomas
  • Aging-more than 75% of humans over age of 80 have
    impaired ability to identify smells

Odor Detection-Abnormalities

  • Hyperosmia
    – Enhanced olfactory sensitivity
  • Pregnancy
  • Dysosmia
    – Distorted sense of smell
  • Sinus infections
  • Partial damage-olfactory nerves
  • Poor dental hygiene

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