Nerves, Taste, Touch
BIOL241
Last lecture
Taste
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Tastants
taste receptor cells
taste buds
five primary taste sensations
Properties of the taste system
Na+, H+, Ca++
80% Smell
Taste Bud & Receptors
Connective
tissue
Gustatory
hair
Taste fibers
of cranial
nerve
Basal Gustatory Taste
cells (taste) cells pore
Stratified
squamous
epithelium
of tongue
(c) Enlarged view of a taste bud.
Figure 15.23c
Primary Taste Sensations
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salty
sour
sweet
bitter
Umami
Dissolve in saliva, diffuse into the taste
pore, and contact the gustatory hairs
Taste Components
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Thermoreceptors
Mechanoreceptors
Nociceptors
Hot
Ansomias
Uncinate Fits
Papillae
Olfactory
epithelium
Olfactory tract
Olfactory bulb
Nasal
conchae
(a)
Route of
inhaled air
Figure 15.21a
Epiglottis
Palatine tonsil
Lingual tonsil
Foliate papillae
Fungiform papillae
(a) Taste buds are associated with fungiform,
foliate, and circumvallate (vallate) papillae.
Figure 15.23a
Circumvallate papilla
Taste bud
(b) Enlarged section of a
circumvallate papilla.
Figure 15.23b
Properties of the taste system
• A single taste bud contains 50–100 taste cells
representing all 5 taste sensations (so the
classic textbook pictures showing separate
taste areas on the tongue are wrong).
• Each taste cell has receptors on its apical
surface. These are transmembrane proteins
which
– admit the ions that give rise to the sensation of
salty;
– bind to the molecules that give rise to the
sensations of sweet, bitter, and umami.
Properties of the taste system,
cont.
• A single taste cell seems to be restricted to
expressing only a single type of receptor (except
for bitter receptors).
• Taste receptor cells are connected, through an
ATP-releasing synapse, to a sensory neuron
leading back to the brain.
• However, a single sensory neuron can be
connected to several taste cells in each of
several different taste buds.
• The sensation of taste — like all sensations —
resides in the brain
Gustatory cortex
(in insula)
Thalamic nucleus
(ventral posteromedial
nucleus)
Pons
Solitary nucleus in
medulla oblongata
Facial nerve (VII)
Glossopharyngeal
nerve (IX)
Vagus nerve (X)
Figure 15.24
Gustatory Pathway
• Cranial nerves VII and IX carry impulses
from taste buds to the solitary nucleus of
the medulla
• Impulses then travel to the thalamus and
from there fibers branch to the:
– Gustatory cortex in the insula
– Hypothalamus and limbic system
(appreciation of taste)
Salty
• In mice, perhaps humans, the receptors for table salt
(NaCl) is an ion channel that allows sodium ions (Na+) to
enter directly into the cell. This depolarizes it allowing
calcium ions (Ca2+) to enter [Link] triggering the release
of ATP at the synapse to the attached sensory neuron
and generating an action potential in it.
• In lab animals, and perhaps in humans, the hormone
aldosterone increases the number of these salt
receptors. This makes good biological sense: The main
function of aldosterone is to maintain normal sodium
levels in the body.
• An increased sensitivity to sodium in its food would help
an animal suffering from sodium deficiency (often a
problem for ungulates, like cattle and deer).
Sour
• Sour receptors are transmembrane ion
channels that detect the protons (H+)
liberated by sour substances
• (Why?)
Sweet
• Sweet substances (like table sugar — sucrose)
bind to G-protein-coupled receptors (GPCRs)
at the cell surface.
• Each receptor contains 2 subunits designated
T1R2 and T1R3 and is
• coupled to G proteins.
• The complex of G proteins has been named
gustducin because of its similarity in structure
and action to the transducin that plays such an
essential role in rod vision.
Gustducin
• Activation of gustducin triggers a cascade
of intracellular reactions:
– activation of adenylyl cyclase
– formation of cyclic AMP (cAMP)
– the closing of K+ channels that leads to
depolarization of the cell.
• The mechanism is similar to that used by
our odor receptors [View].
Leptin
• The hormone leptin inhibits sweet cells by
opening their K+ channels. This
hyperpolarizes the cell making the
generation of action potentials more
difficult.
• Could leptin, which is secreted by fat cells,
be a signal to cut down on sweets?
Bitter
• The binding of substances with a bitter taste,
e.g., quinine, phenylthiocarbamide [PTC],
also takes place on G-protein-coupled
receptors that are coupled to gustducin.
• In this case, however, cyclic AMP acts to
release calcium ions from the endoplasmic
reticulum [Link], which triggers the release of
neurotransmitter at the synapse to the
sensory neuron.
Bitter, cont.
• Humans have genes encoding 25 different
bitter receptors ("T2Rs"), and each taste cell
responsive to bitter expresses a number (4–
11) of these genes. (This is in sharp contrast
to the system in olfaction where a single
odor-detecting cell expresses only a single
type of odor receptor.)
• Despite this — and still unexplained — a
single taste cell seems to respond to certain
bitter-tasting molecules in preference to
others.
Bitter, cont.
• The sensation of taste — like all sensations —
resides in the brain.
• Transgenic mice that express T2Rs in cells that
normally
– express T1Rs (sweet) respond to bitter
substances as though they were sweet;
– express a receptor for a tasteless substance in
cells that normally express T2Rs (bitter) are
repelled by the tasteless compound.
• So it is the activation of hard-wired neurons that
determines the sensation of taste, not the
molecules nor the receptors themselves.
Umami
• Umami is the response to salts of glutamic
acid — like monosodium glutamate
(MSG) a flavor enhancer used in many
processed foods and in many Asian
dishes. Processed meats and cheeses
(proteins) also contain glutamate.
• (What is Glutamic Acid?)
Umami
The binding of amino acids, including glutamic
acid, takes place on G-protein-coupled
receptors that are coupled to heterodimers of
the protein subunits T1R1 and T1R3.
• Another umami receptor (at least in the rat's
tongue) is a modified version of the glutamate
receptors found at excitatory synapses in the
brain.
Taste Receptors in Other Locations
• Taste receptors have been found in
several other places in the body.
• Examples: ?
Taste Receptors in Other
Locations
• Bitter receptors (T2Rs) are found on the cilia
and smooth muscle cells of the trachea and
bronchi [View] where they probably serve to
expel inhaled irritants;
• Sweet receptors (T1Rs) are found in cells of
the duodenum. When sugars reach the
duodenum, the cells respond by releasing
incretins. These cause the beta cells of the
pancreas to increase the release of insulin.
• So the function of "taste" receptors appears
to be the detection of chemicals in the
environment — a broader function than
simply taste.
Touch
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What?
How?
Where?
Cells
Nerves
4 distinct somatic modalities:
– touch
– proprioceptive sensations
– pain
– thermal sensations
Central somatic pathways
• 2 major pathways to the somatosensory
cortex
– dorsal column-medial lemniscal system -tactile sensation and arm proprioception
– anterolateral system -- pain and temperature
a bit of tactile information
• the body surface is represented in the
brain in an orderly fashion
Central somatic pathways
Somatosensory Cortex
Hair shaft
Epidermis
Papillary
layer
Dermis
Reticular
layer
Hypodermis
(superficial fascia)
Nervous structures
• Sensory nerve fiber
• Pacinian corpuscle
• Hair follicle receptor
(root hair plexus)
Dermal papillae
Subpapillary
vascular plexus
Pore
Appendages
of skin
• Eccrine sweat
gland
• Arrector pili
muscle
• Sebaceous
(oil) gland
• Hair follicle
• Hair root
Cutaneous vascular
plexus
Adipose tissue
Figure 5.1
Layers of the Dermis: Papillary
Layer
• Papillary layer
– Areolar connective tissue with collagen and
elastic fibers and blood vessels
– Dermal papillae contain:
• Capillary loops
• Meissner’s corpuscles
• Free nerve endings
Functions of the Integumentary
System
2. Body temperature regulation
– ~500 ml/day of routine insensible
perspiration (at normal body temperature)
– At elevated temperature, dilation of dermal
vessels and increased sweat gland activity
(sensible perspirations) cool the body
3. Cutaneous sensations
– Temperature, touch, and pain
Mechanoreceptor
Sensory Neurons
• Meissner's corpuscles detect changes in
texture (vibrations around 50 Hz) and
adapt rapidly. small receptive field: 2-4mm
• Pacinian corpuscles detect rapid vibrations
(about 200–300 Hz).
• Merkel's discs detect sustained touch and
pressure. small receptive field: 2-4mm
• Ruffini's corpuscle
Meissner's corpuscles
• Meissner's corpuscles are located at the
tips of the dermal papillae. Each corpuscle
consists of a number of flattened layers of
cells, each with an elongated nucleus. The
neuron within is coiled among these cells,
but is not easily seen. When the corpuscle
is deformed by pressure, the nerve
endings are stimulated, registering the
sensation of touch
Meissner's corpuscles
Meissner's corpuscles
Meissner's corpuscles
Pacinian corpuscles
• Each corpuscle is an egg-shaped structure
consisting of many concentric layers of
tissue. Embedded within this structure is a
free nerve ending. When the corpuscle is
deformed by pressure, an action potential is
initiated in the nerve ending. Pacinian
corpuscles are found in many areas of the
body, including the skin, the mesenteries
surrounding the gut, and joint capsules. The
Pacinian corpuscles in joints provide the CNS
with information on the position of the joints.
As such they play an important role as
proprioceptors.
Pacinian corpuscles
Pacinian corpuscles
Merkel's discs
• found in the deepest layer of the epidermis
(stratum basale).
• Always found associated with a sensory
receptor nerve ending for touch.
Merkel's discs
Merkel's discs
Ruffini endings
Ruffini endings
Non-hairy vs. hairy Receptors
Nervous System Cells
• Sensory neurons:
– afferent neurons of PNS
• Motor neurons:
– efferent neurons of PNS
• Interneurons:
– association neurons
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Ependymal cells
Astrocytes
Microglia
Oligodendrocytes
Satellite cells (amphicytes)
Schwann cells (neurilemmacytes)
Neuroglia are supporting
cells
2. Astrocytes
• Maintain blood–brain barrier (isolates
CNS)
• Create 3-dimensional framework for CNS
• Repair damaged neural tissue
• Guide neuron development
• Control interstitial environment
Astrocytes
Astrocytes
Astrocytes
3. Microglia
• Migrate through neural tissue
• Clean up cellular debris, waste products,
and pathogens
• Not of neural origin; related to
macrophages (like osteoclasts)
Microglia
Microglia
4. Oligodendrocytes
• Processes contact other neuron cell bodies
• Wrap around axons to form myelin sheaths
Oligodendrocytes
1. Ependymal Cells
• Form epithelium called ependyma
• Line central canal of spinal cord and
ventricles of brain:
– secrete cerebrospinal fluid (CSF)
– have cilia or microvilli that circulate CSF
– monitor CSF
– contain stem cells for repair
Ependymal Cells
Ependymal Cells
1. Schwann Cells
• Form myelin sheath around peripheral
axons (nerves)
• 1 Schwann cell sheaths 1 segment of
axon:
– many Schwann cells sheath entire axon
Schwann Cells
Schwann Cells
Schwann Cells
Schwann Cells
(“Unmyelinated”)
1. Satellite Cells
• Surround ganglia
• Regulate environment around neuron
Satellite Cells
Satellite Cells
Neuron
Neural Cells
Purkinje Cells
Interneurons
• Most are located in brain, spinal cord, and
autonomic ganglia:
– between sensory and motor neurons
• Are responsible for:
– distribution of sensory information
– coordination of motor activity
• Are involved in higher functions:
– memory, planning, learning
Some questions
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Is the Cerebellum part of the Brain Stem?
Not really (similar location)
Into what muscles are Injections given?
Gluteus minimus, Deltoid, Vastus lateralis,
Ventrogluteal (Dorsalgluteal – “avoid”)
• Association Areas vs. Interneurons?
• association areas of the brain are important because they
integrate incoming (sensory) information coming into
various parts of the sensory cortex, they also compare it
with existing memory of the same sensations; the motor
association areas refine movements by coordinating the
signals from various parts of the motor cortex before
initiating the movement.
Cerebellum
Association Areas
Interneuron