CNS 8 - Smell and Taste

Chemoreception

Ability to detect chemicals in the environment

Smell and taste are forms of…

chemoreception

Olfactory receptors

specialized proteins in your nose that detect odorants

olfactory receptors are in the…

olfactory epithelium

Olfactory Epithelium

lies at the top of the nasal cavity

• covering ~3 cm^2 in each of the 2 sides.

• contains ~10 million receptor cells in total

• pigmented — richness of its color correlates with olfactory sensitivity

The receptor neurons are…

ciliated neurons

• Each cell's single dendrite extends into the olfactory epithelium, where it branches to form nonmotile cilia that increase the surface area for a greater chance of catching odorant molecules.

• Each receptor cell has (many copies of) one type of odorant receptor molecule on its membrane. We have ~400 kinds of receptor cell, i.e. ~ 400 “primary odors”

Olfactory receptor cells have GPCR molecules in their membranes

odorant molecule binds its receptor

↓

activates a G protein, Golf

↓

increases the local concentration of cAMP

↓

cAMP-gated cation channels open — depolarizing the receptor neurons and triggering an AP that travels along the cell’s axon to the olfactory bulb

The receptor cells are sensitive

some can detect a single molecule of their preferred chemical, though ~40 cells must react before we experience a smell.

Olfactory receptor cells have unusual properties

• pinocytotic

• short-lived, degenerating after a month or 2, to be replaced by new ones from below.

• send their axons into the brain through tiny holes in the cribriform plate — the bone at the base of the cranial cavity

pinocytotic

continually sipping in fluid

The receptor cells project to the…

olfactory bulb

Olfactory Bulb

extension of the cerebrum, and lies on the under- side of the frontal lobes

• first processing center for smell (olfaction)

projection from the receptors to the bulb

the olfactory nerve or cranial nerve I (1)

Many receptor cells converge on each bulb neuron

As with rods converging on ganglion cells, this arrangement en- hances sensitivity but discards spatial information

The bulb projects directly to olfactory cortex…

bypassing thalamus

• neurons in a olfactory bulb project directly to olfactory cortex

Olfactory cortex

the brain's smell-processing center, located on the frontal and temporal lobes

The bulb also projects to the…

limbic system

limbic system

old group of brain regions concerned with motivation and emotion

• amygdala and hippocampus?

Olfaction adapts…

slowly but completely

• Sewer workers don’t notice anything objectionable, and people are often unaware of their own body odors

• Food evaluators take steps to avoid adapting

Rodents and maybe humans have…

pheromones

Pheromones

chemicals released by an animal into the environment which affect the physiology or behavior of other members of its species

vomeronasal organ (VNO)

Rodents have this olfactory structure in the nasal cavity called the vomeronasal organ (VNO), which is involved in their behavioral responses to sex pheromones

In humans the VNO…

isappears during fetal development, but we do respond to airborne chemical signals

Main taste receptor cells are clustered in taste buds

• We have ~5000 taste buds, mainly on the top of the tongue but also on the soft palate, epiglottis and upper esophagus.

• Babies have 10,000

• A taste bud lives only ~10 days.

Taste bud

• Each bud contains ~100 receptor cells, which are epithelial cells (not neurons) arranged like petals.

• They contact the oral cavity through a small opening, the taste pore

• contains at least 5 kinds of receptor cell

The taste pore

small opening where taste buds contact the oral cavity

Sweet and umami receptor cells

detect sugar (energy) and the amino acid glutamate (indicating protein), respectively

Bitter receptor cells

detect poison

Salty and sour receptor cells

detect Na+ and H+— 2 important ions

There are receptor cells of all 5 kinds all over the top of the tongue

it is not true that sweetness is sensed only by the tip of the tongue

• BUT different areas of the tongue do vary slightly in their thresholds for different flavors

Taste receptor cells are grouped into 3 types

• Type I cells

• Type II cells

• Type III cells

Type I cells

may sense salt

Type II cells

sense sweet, bitter, and umami — Release ATP, which acts on neurons and type IIIs

Type III cells

sense sour

• form synapses with sensory neurons, activating them with serotonin

Different kinds of cell employ different membrane proteins

Gustducin

a G protein found in taste receptor cells for sweet, umami, and bitter tastes

• activates signal path- ways, increasing intracellular [Ca2+] and triggering release of ATP.

Detection of salt and sour involves ion channels which are…

not linked with G proteins — not GPCRs

What determines our overall experience of food beyond just taste?

It depends on smell, temperature, pain, texture, crunch, appearance, and cognition.

TRP channels

nerve endings in the walls of the mouth that are sensitive to temperature and chemicals

TRPV1 vanilloid receptors

respond to heat and to capsaicin in chilies

TRPM8 channels

respond to cold and to menthol.

Chemoreceptors in our stomach and intestines monitor their contents;

some of these receptors resemble ones on the tongue, e.g. for sweet and umami.

Taste signals take several paths to the brain

Receptor cells in the taste buds excite fibers of the…

cranial nerves VII, IX, and X (7, 9, and 10)

• These pathways synapse in medulla and thalamus en route to the cortex.

Cranial Nerve VII (7)

facial nerve

Cranial Nerve IX (9)

glossopharyngeal nerve

Cranial Nerve X (10)

vagus nerve

cranial nerve V (5) - the trigeminal

excited by TRP receptors in the walls of the mouth