Chapter 8: The Chemical Senses Flashcards
2 chemical senses
- gustation
- olfaction
5 major tastes: () – common among many different cultures (box 8.1)
Saltiness, sourness, sweetness, bitterness, and umami (MSG)
Poisonous substances—often () -> taste senses offer advantage for survival
bitter
factors that contribute to distinguishing countless flavors of food
- activation of different combinations of taste receptors
- distinctive smell
- other sensory modalities: texture, temperature
Areas of sensitivity on the tongue: sweetness
tip of the tongue
Areas of sensitivity on the tongue: bitterness
back of the tongue
Areas of sensitivity on the tongue: saltiness and sourness
sides of the tongue
small structures on the upper surface of the tongue that give it its characteristic rough texture
papillae
3 different shapes of papillae
- foliate - back part of sides of tongue
- vallate - back part of tongue
- fungiform - front and middle part of tongue
major origin of taste sense
taste buds
taste buds consist of several ()
taste cells
taste cells form chemical and electrical synapses with (1) and (2)
- gustatory afferent axons
- basal cells
() are found at the base of taste buds
basal cells
() bring sensory information from the tongue to dorsal root ganglia
gustatory afferent axons
sensory neuron don’t have ()
dendrites
At the apical ends of taste buds, taste cells form (1) -> together form (2), which directly interacts with chemicals
- microvilli
- taste pore
Taste receptor cell membrane potential is usually changed by depolarization, which triggers () that are delivered to gustatory ANs
receptor potentials
Most taste receptor cells respond primarily to ().
just one of basic tastes
taste stimuli
tastants
transduction processes for tastants
- Pass directly through ion channels
- Bind to and block ion channels
- Bind to G-protein-coupled receptors and activate second messengers to open ion channels
Tastants open or block ion channels to () taste cell membranes
depolarize
If sodium arrives to surface of salt-sensitive taste cells, they pass directly through the (1)
- selective Na+ channels
the selective Na+ channels in salt-sensitive taste cells can be blocked by (1), thus they are also called (2)
- amiloride
- amiloride-sensitive Na+ channels
the NT released by synaptic vessels in salt and sour-sensitive taste cells is (); this binds to gustatory axon terminals to deliver signal to CNS
serotonin
(): causative agents of acidity and sourness
Protons
Sourness originates from (1)—>(2)
- high acidity
- low pH
Protons directly penetrate the () (usually open in sour-sensitive taste cell) to cause H+ influx -> depolarization
H+ channel
3 tastes that share common mechanism of signal transduction
bitterness, sweetness, umami
the 2 families of taste receptor proteins involved in signal transduction for bitterness, sweetness, umami
T1R and T2R
receptors involved in bitterness, sweetness, umami are all ()
GPCRs
proteins that make up the bitter receptors
homodimer of T2R receptors
proteins that make up sweet receptors
heterodimer of T1R2 and T1R3
proteins that make up the umami receptors
heterodimer of T1R1 and T1R3
ligand of umami receptors
amino acids (proteins)
portion of the thalamus that deals with sensory information in the head
ventral posterior medial (VPM) nucleus
cranial nerves (VII, IX, and X) carry () and deliver taste info to the brain
primary gustatory axons
taste axons of cranial nerves involved in gustation all enter the brain stem, bundle together, and synapse within the ()
gustatory nucleus
the VPM is synapsed by neurons of the ()
gustatory nucleus
after receiving signals from the gustatory nucleus, the VPM delivers the signal to the () in the medulla
primary gustatory cortex
() - loss of taste perception
Ageusia
summary of central taste pathways
Cranial nerves—>gustatory nucleus—>primary gustatory cortex (located in cerebral cortex)
ageusia can be caused by () in the VPM, thalamus, or gustatory cortex
localized lesions
states that individual taste receptor cells for each stimuli
labeled line hypothesis
involves coordination of large number of broadly tuned neurons -> convergence of receptor cell input onto afferent axons
population coding
Because population coding takes into account information from other neurons, it may explain the contribution of () to one’s perception of food
texture, temperature, smell
() in response to a taste/food are distinctly different
overall patterns of discharge (APs)
how do we distinguish between tastes (salt vs sour, bitter vs sweet vs umami)
each taste cell selectively expresses only 1 class of taste receptor proteins
chemicals released by the body that are important signals for reproductive behavior, territorial boundaries, identification of individuals, etc. in animals but have weak role in humans
pheromones
major organ for sensing smell
olfactory epithelium
3 types of cells in the olfactory epithelium
- olfactory receptor cells
- supporting cells
- basal cells
type of cell in olfactory epithelium: sites of transduction; one of the few types of neurons in the nervous system that are regularly replaced throughout life
olfactory receptor cells
type of cell in olfactory epithelium: similar to glia; help produce mucus (produces and maintains mucus layer)
supporting cells
why are antibodies important in the mucus layer of the olfactory epithelium
because epithelium is directly exposed to outside (viruses, bacteria, etc.)
proteins in olfactory epithelium mucus layer that help concentrate odorants in nucleus
odorant-binding proteins
bone that allows innervation of olfactory epithelium by olfactory nerve axons
Cribriform plate
activate transduction processes in olfactory neurons
odorants
inability to smell
anosmia
why do humans have a weak sense of smell
Due to small surface area of olfactory epithelium
Ways to terminate Olfactory response
- Odorants diffuse away
- breakdown of odorants by scavenger enzymes in mucus
- cAMP in receptor cell may activate other signaling pathways that end transduction process
2 unusual features of olfactory signaling
- receptor binding proteins at the beginning
- cAMP-gated channels near the end
collectively, the olfactory axons constitute the ()
olfactory nerve (cranial nerve I)
small clusters of olfactory axons penetrate the cribriform plate then course into the ()
olfactory bulb
in olfactory cells, () is unusually high
intracellular [Cl-]
Different odorants are recognized by ()
different receptor proteins
Increasing evidence indicates that the only second messenger mediating olfactory transduction in vertebrates is ()
cAMP
in most cases, only () is expressed in 1 olfactory receptor cell
1 type of receptor protein
olfactory receptor proteins belong to the () family
G protein-coupled receptor
G protein in olfactory receptor cells
G_olf
like taste, olfaction involves a () scheme to distinguish between smells
population-coding
Small spherical structures located in the input layer of olfactory bulbs; spherical forms of axon terminals
glomeruli (glomerulus)
each glomerulus contains endings of about 25k (1) that converge and terminate on the dendrites of about 100 (2)
- primary olfactory axons
- second-order olfactory neurons
Each glomerulus receives input from receptor cells of only ()
1 type
() -> molecular labeling method used to visualize mapping of receptor cells onto glomeruli
P2::LacZ knock-mouse
output axons of the olfactory bulbs connect directly to several targets, most notably the () (located in primitive region of the cerebral cortex) and some of its neighboring structures in the temporal lobes
olfactory cortex
Conscious perception of smell may be mediated by the ff path:
olfactory tubercle -> medial dorsal nucleus (of thalamus) -> orbitofrontal cortex
orderly arrangement of neurons that correlates with certain features of the environment
sensory map
while a particular odor activated many bulb neurons, the neuron’s positions form ()
complex but reproducible spatial patterns
the most critical feature of each odorant is its ()
chemical structure
each distinct odor triggers activity in ()
a different subset of neurons
depends on the timing of AP spikes, might encode the quality of odors
temporal coding
