guyton and hall lectures medical physiology

1. Topic-050 Chemoreceptors: Taste and Smell Receptors
Chemoreceptors are receptor cells that are specialized for acquiring information about the chemical
environment and transmitting it to other neurons.
Chemoreceptors can be divided into two categories:
• • Gustatory (taste) receptors
• • Olfactory (smell) receptors
These receptors operate quite differently from one another.
Gustatory (taste) receptors
The gustatory (taste) receptors respond to dissolved
molecules that come in direct contact with the receptive
structure.
Taste receptors in insects
The organs of taste in insects are sensory sensilla that are
located on the feet and mouthparts.
Every sensillum contains several receptor cells, each of which
is sensitive to a different chemical stimulus e.g., water,
cations, anions, or carbohydrates.
The receptor cellsof sensillaappear to be hair-like due to
longer dendrites. The dendrites are sent to the cuticle.
The cuticle around dendrites has minute pores that allow
stimulant moleculesto contact the dendrites.
The stimulus is converted into electrical signal by the soma of sensory cells.
Taste receptors in vertebrates
• • Many aquatic vertebrates have taste receptors on different locations of the body.
For example some fishes have modified pectoral fins that have taste receptors at the tips of the fin rays
that are used to locate food in the muddy bottom.
• • In terrestrial vertebrates, taste receptors are
located at the anterior region of digestive tract e.g. on
the tongue and epiglottis, in the back of the mouth,
in the pharynx and upper esophagus.
Taste buds
• • The gustatory organs of vertebrates are
called taste buds.
• • The taste bud is composed of about 50
modified epithelial cells including supporting cells
(sustentacular cells), basal cells and taste receptor
cells.
• • The basal cells are progenitor cells that give
rise to new taste receptors. They regularly
• generate new sensory taste receptor cells
which have an active life of only 10 days.
Taste Receptor Cells

2. • • The outer tips of the taste cells are arranged around a minute taste pore.
• • From the tip of each taste cell, several microvilli, or taste hairs, protrude outward into the taste
pore to approach the cavity of the mouth.
• • These microvilli provide the receptor surface for taste.
• • Interwoven around the bodies of the taste cells is a branching terminal network of taste nerve
fibers that are stimulated by the taste receptor cells.
Olfactory (Smell) Receptors
The olfactory (smell) receptors respond to
airborne molecules that stimulate the
receptor from distance.
They detect odorants and pheromones.
• • In insects, olfactory sensilla are
• present on their antennae.
• • In vertebrates, the olfactory
receptors
• are present in the nasal cavity.
Olfactory system has two anatomically distinct organs:
• Main Olfactory Epithelium (MOE) that is responsible for the detection of odorants
• Vomeronasal Organ (VNO) that detects pheromones.
Topic-051 Mechanism of Taste Reception
Sense of Taste
The sense of taste can be grouped into five primary sensations of taste: sweet, salty, sour, bitter and
umami.
All perceived tastes depend on various combinations of these fundamental sensations.
Taste Receptors
Each individual taste cell expresses a single receptor type and transmits action potentials to the brain
representing only one of the five tastes.
In humans, there are more than 30 different receptors for bitter taste, each able to recognize multiple
bitter tastants. However there is only one type receptor each for sweet and umami tastes.
The receptors of sweet, umami, and bitter tastes are G protein-coupled receptors, while the receptors for
salty and sour tastes are ion channels.
Mechanisms of Taste Reception
Salty Taste Reception
• • Salty stimuli such as NaCl, readily dissociate in water into Na+ and Cl- ions.
• • The Na+ ions enter receptors through Na+ channels in the membrane to directly depolarize the
receptor cell membrane.
• • These Na+ channels are distinctive because they can be blocked by the drug amiloride unlike
the voltage-gated Na+ channels that mediate most APs.

3. Salty taste reception Sour taste reception
Sour Taste Reception
• • Sour stimuli are characterized by excess H+ ions.
• • They act either through the Na+ channels or by blocking the
K+ channels.
• • In either case, the membrane is depolarized.
Sweet Taste Reception
• Many sweet compounds and the amino acid alanine (Ala)
bind to receptors and activate a G protein. The activated G protein
activates adenylate cyclase that forms cAMP. Increased conc. of
cAMP closes the K+ channels in the basolateral membrane,
depolarizing the receptor.
Bitter Taste Reception
• Some bitter substances, such as quinine,bind
to the receptor and activate a G protein that is coupled
with phospholipase C. Phospholipase C converts
Phosphatidyl inositol bisphosphate to inositol
triphosphate (InsP3).Increased intracellular inositol
triphosphate (InsP3) causes the release of Ca2+ from the
intracellular stores. Increased Ca2+ conc. causes the cell
to depolarize.
Umami Taste Reception
The receptors for umami (savory or delicious) taste were discovered in the taste buds in year 2000. This
taste is produced by the amino acid glutamate and its salt monosodium glutamate. This taste is
manifested by meat and aged cheese.
The receptor for MSG is a G-protein coupled receptor that results in a cellular cascade causing the
release of Ca2+ ions and depolarization.
Release of Neurotransmitters

4. In all cases, depolarization in the receptor cell eventually results in the release of neurotransmitters which
propagate the signal in nervous system.
Transmission of Taste Signals
Taste receptors generate APs, but they have no axons, so they cannot themselvescarry information
to the central nervous system.
Instead, they synapse with, and modulate activity in, neurons whose axons run in the facial,
glossopharyngeal, and vagus nerves (seventh, ninth, and tenth cranial nerves).
Labeled Line Coding
Each receptor subtype for five kinds of taste sensations is connected to a particular set of axons.
In that arrangement, for example,information about "sweetness" would be carried by some
specific subset of axons. Such a pattern is called labeledline coding.
Topic-052 Mechanism of Olfactory Reception
Olfactory Receptors
• • The olfactory receptors of vertebrates are located
inside the nasal cavity.
• • The sensory receptor cells involved in olfaction are
actually neurons.
• • These neurons have long axons. All these axons are
packed together in the olfactory nerve. Nerve impulses are sent
directly to the olfactory bulb of the brain, along the axons.
• • Each receptor neuron has a long thin dendrite that
terminates in a small knob at the surface.
• • The knob has 4 to 25 olfactory hairs (also called
olfactory cilia) that are about 200 μm long. These cilia are covered by a protein solution called mucus.
Olfactory Transduction
• • The portion of each olfactory cell that responds to the olfactory chemical stimuli is the
olfactory cilia.

5. • The odorant substance first diffuses into the mucus that covers the cilia.
• Then it binds with receptor protein in the membrane of each cilium.
• This protein is coupled to a G-protein.
• The G-protein activates adenylyl cyclase that, in turn, converts ATP into
cAMP.
• This cAMP opens channels in the plasma membrane that are permeable to
both Na+ and Ca2+.
• Ca2+ inflow also triggers opening of Cl- channels allowing Cl- ion outflow.
• The flow of these ions causes depolarization that results in the excitation of
the olfactory neuron and generation of action potential.
• The action potentials are transmitted to the central nervous system through
the olfactory nerve.
This mechanism of transduction ensures amplification of excitatory effect of even the weakest odorant,
thereby increasing the sensitivity of the olfactory receptors.
Basis of Differentiating Smells
The receptor protein in the olfactory cilia actually belongs to a very large family of proteins that are
expressed only in olfactory epithelial cells. All these proteins have small variation in their structure,
giving rise to large number of subtypes. Each subtype is associated with a different odorant. This forms
the basis of ability to distinguish a wide variety of smells.