chemoreception: taste and smell

chemoreceptors

exteroreceptors for gustation and olfaction

roles of taste and smell

sensing food, kin, mates (pheromones), and direction

mosquitoes use chemoreceptors to

locate prey through detection of carbon dioxide and chemicals like octenol found in human sweat

ants use chemoreceptors on antennae to

sense pheromones on others

sensila

specialized projections from cuticle in insects that permits molecules to reach internal sensory endings through small pores

dendrites containing sensory receptors project to apex of each sensilium

taste buds

in higher vertebrates, the location of chemoreceptors for taste

consists of 50 long spindle-shaped receptor cells in an arrangement like slices of orange

taste bud location

human oral cavity and throat, digestive tract, lungs

taste pore

small opening on taste bud through which fluids in mouth contact receptor cell surface

vertebrate taste receptor cells

• modified epithelial cells with many surface folds (microvilli) that increase surface area

• plasma membrane of microvilli contain receptor sites that bind selectively with chemicals in environment (have to be dissolved in saliva)

tastant

taste provoking chemical, binds with receptor cell to produce depolarizing receptor potential

taste receptor lifespan

about 10 days

area of brain for taste

cortical gustatory area in parietal lobe

how would a tastant lead to an action potential?

taste bud contains clusters of receptor cells with microvilli that extend into the taste pore. Tastant interacts with the taste pore and binds to the receptor which activates the signal transduction pathway composed of electrical signals moving to the brain through cranial nerves. Specific tastants interact with equally specific receptors such as GPCRs and ENaCs which causes the cell depolarization and action potentials.

five primary tastes

salty, sour, sweet, bitter, umami

salty taste stimulated by (proximate function)

stimulated by chemical salts (Na+ in NaCl)

salty taste evolutionary function

• critical for osmotic balance and electrical signals

• direct transduction through specialized Na+ channels (ENaC) → may be leak or gated

• ion movement decreases internal negative charges, responsible for receptor potential

sour proximate function

caused by acids (have H+). Depolarization occurs by direct H+ entry or when H+ blocks K+ channels in receptor cell membranes (decreases internal negative charges)

sour evolutionary function

moderate sour taste is pleasant → ingestion of Vitamin C

Strong acid taste → warns against spoiled food or unripe fruit

sweet proximate function

evoked by small sugar molecules (G-coupled protein receptors and second messenger pathways

sweet evolutionary function

pleasant taste favors intake of primary energy form (glucose, sucrose, etc.)

bitter proximate function

elicited by chemically diverse group of tastants, many different bitter receptors

bitter evolutionary function

mainly helps avoid noxious/toxic molecules

most pathways involve G proteins (gustducin)

can be acquired taste

umami proximate function

triggered by amino acids such as glutamate. Glutamate binds to G protein coupled receptor and activates second messenger system

umami evolutionary function

nutritional, protein rich foods

nasal fossae

upper tract of respiratory airways

olfactory mucosa

located in nasal fossae has 3 cell types

3 cell types of olfactory mucosa

supporting cells (Bowman’s glands), basal cells, olfactory receptors

supporting cells (Bowman’s glands)

secrete mucus which coats nasal passages

basal cells

precursors for new olfactory receptor cells (replaced every 2 months)

olfactory receptors

specialized endings of afferent neurons, not seperate cells. Entire neuron is replaced. They are the only mammalian neurons that undergo cell division

receptor portion of olfactory receptors

large knob with several long cilia that extends to surface of mucosa. These cilia contain binding site for attaching odorants

odorants typically reach sensitive receptors by

diffusion because the olfactory mucosa is above the normal path of airflow

to be smelled, odors must be

1. volatile (easily vaporized or dissolved) so that molecules can enter nose in air or water

2. soluble to dissolve in mucus layer (coating olfactory mucosa)

smell begins with

• binding of odorant to a G-protein coupled receptor triggering a cascade of intracellular reactions → opening of Ca2+ or Na+ channels

• resulting ion movement brings about a depolarizing receptor potential that generates action potentials to the afferent fiber

reach receptor responds to

only 1 discrete component of an odor

afferent fibers in nose pass through

tiny holes in flat bone plate separating olfactory mucosa from overlying brain tissue. They immediately synapse in olfactory bulbs which are linked by glomeruli

within each glomerulus the terminals of receptor cells carrying information about a particular scent synapse with

mitral cells

olfactory bulb

relay station where incoming olfactory information is organized and refined before being sent to higher brain regions

glomeruli

play key role in organizing scent perception

mitral cells

refine the smell signals and relay them to the brain for further processing

2 routes of fibers leaving the olfactory bulb

1. Subcortical route

2. Thalamic-cortical route

subcortical route

going primarily to regions of the limbic system, especially the lower medial sides of the temporal lobes (primary olfactory cortex)

for close coordination btwn smell and primitive memory as well as behavioral reactions associated with feeding, mating, direction

thalamic-cortical route

includes hypothalamic involvement, permits conscious perception and fine discrimination of smell.

olfactory system adapts quickly

“odor eating enzymes” in olfactory mucosa clear odiferous molecules, smells do not linger long (chemically similar to detoxification enzymes in liver)

vomeronasal organ

detects pheromones, located in nose next to vomer bone. “sexual nose”

how does core concept of evolution play a role in chemosensation?

animals have evolved to be able to detect chemical compounds in their environment through taste and smell. Evolved to detect things that will be beneficial vs. dangerous to them

how does core concept of flow down gradients play a role in chemosensation?

movement of ligand (tastants and odorants) from environment to their receptors. Molecules move from hight concentration areas to low concentration areas.