Chapter 8: The Chemical Senses

Three Main Cell Types of Olfactory Epithelium

1. Olfactory Receptor Cells.. 2. Supporting Cells.. 3. Basal Cells

Olfactory Maps

A *sensory map* is an orderly arrangement of neurons that correlates with certain features of the environment. We have seen that the axons of each receptor cell type synapses upon particular glomeruli in the olfactory bulbs. Such an arrangement yields a sensory map in which neurons in a specific place in the bulb respond to particular odors

Action Potential Firing Rates of Four Different Primary Gustatory Nerve Axons in a Rat

A receptor cell's response to a chemical depend on the particular transduction mechanisms present in each cell

Taste Pore

A small opening on the surface of the tongue where the taste cell is exposed to the contents of the mouth

Olfactory Bulb

After leaving the epithelium, small clusters of the axons penetrate a thin sheet of bone called the *cribriform plate*, then course into the *olfactory bulb*

Bitterness

Bitter substances are detected by the 25 or so different types of T2R receptors in humans. Bitter receptors are poison detectors, and because we have so many, we can detect a vast array of different poisonous substances.

Bitter Substances

Bitter substances range from simple ions like K^+(KCl is bitter and salty) and Mg^2+ to complex organic molecules such as quinine and caffeine. Many bitter organic compounds can be tasted even at very low concentration. Poisons are often bitter

Transduction Mechanisms for Bitter, Sweet, and Umami Tastants

Bitter, sweet, and umami receptors all seem to use the same second messenger pathway to carry their signals to different afferent axons. Tastants bind directly to G-protein-coupled membrane receptors, which activate phospholipase C, which increases the synthesis of IP3. IP3 triggers the release of Ca^2+ from internal storage sites, and Ca^2+ opens a taste-specific ion channel, leading to depolarization and transmitter release. The main transmitter is ATP, which released from the taste cells by diffusing through ATP-permeable channels

Odorants

Chemical stimuli in the air which dissolve in the mucus layer before they reach receptor cells

Sweet and Bitter Taste Receptors

Chemicals binding to the T1R2 + T1R3 receptors activate exactly the same second messenger system that the bitter receptors activate, but bitter receptor proteins and sweet receptor proteins are expressed in different taste cells that are connected to different gustatory axons

Pheromones

Chemicals released by the body which are important signals for reproductive behaviors, and the may also be used to mark territories, identify individuals, and indicate aggression or submission

Adaption

Decreased response despite the continuing presence of a stimulus is called *adaption*, and we will see that it is a common feature of receptors in all the senses

Distinguishing Flavors

Each food activates a different combination of basic tastes, helping make it unique. Most foods also have a distinctive flavor as a result of their combined taste and smell occurring simultaneously. Other sensory modalities contribute as well, such as temperature, texture, and pain

Taste Buds

Each taste bud has 50-150 *taste receptor cells*, or taste cells, arranged within the bud like the sections of an orange. A person typcally has 2000-5000 taste buds

Sourness

Foods taste sour because of their high acidity. Acids dissolve in water and generate hydrogen ions(H^+), which are the causative agents of acidity and sourness. It is likely that H^+ can bind to and block special K^+-selective channels. When K^+ permeability of a membrane is decreased, it depolarizes, releasing neurotransmitters.

Saltiness: High Concentrations

High salt levels activate bitter and sour taste cells, which normally triggers avoidance behaviors. How this happens is still a mystery

Odorant Receptor Genes

Humans have about 350 odorant receptor genes that code for function receptor proteins. These genes comprise about 3-5% of the entire mammalian genome and each has a unique structure, allowing the receptor proteins encoded by these genes to bind different odorants

Olfactory Receptor Cell During Stimulation

If stimulation of olfactory receptor cells results in a receptor potential that is large enough, the potential will exceed the threshold for action potentials in the cell body, and spikes will propagate out along the axon into the central nervous system(CNS)

Distinguishing Tastes

In the case of taste, receptor cells are sensitive to a small number of taste types, often only one; gustatory axons and the neurons they activate in the brain tend to response more broadly - for example, strongly to bitter, moderately to sour and salt, and not at all to sweet. Only with a large population of taste cells, with different response patterns, can the brain distinguish between specific alternative tastes

Specific Mapping of Olfactory Receptor Neurons onto Glomeruli

It seems that each glomerulus receives input from only receptor cells of one particular type. This means that the array of glomeruli with a bulb is a very orderly map of the receptor genes expressed in the olfactory epithelium, and, by implication, a map of odor information

Sweet Substances

Many substances are sweet, from familiar sugars like fructose(honey and honey) and sucrolose(white table sugar) to certain proteins(monellin) to artificial sweeteners such as saccharin and aspartame(made from 2 amino acids)

Chemoreceptors

Many types of chemically sensitive cells, called *chemoreceptors*, are distributed throughout the body on the skin, intestines, blood vessels, and muscles

Umami (Amino Acids)

Most amino acids taste good, although some taste bitter. The transduction process for umami is identical to that for sweetness, with one exception. The umami receptor, like the sweet receptor, is composed of members of the T1R protein family, but in this case, it is T1R1 + T1R3

Taste Responsiveness of Taste Cells and Gustatory Axons

Most taste receptor cells respond primarily or even exclusively to just one of the five basic tastes. Several gustatory axons are influenced by several of the basic tastes, but each has a different bias

2nd Part of Summary of Central Taste Pathway

Neurons of the gustatory nucleus synapse on a subset of small neurons in the *ventral posterior medial (VPM) nucleus*, a portion of the thalamus that deals with sensory information from the head

Odorants Producing Smell

Odorants dissolved in the mucus bind to the surface of the cilia and activate the transduction process

Structure of Olfactory Receptor Neurons

Olfactory receptor neurons have a single, thin dendrite that ends with a small knob at the surface of the epithelium. Waving from the knob, within the mucus layer, are several long, thin cilia.

The Location and Structure of an Olfactory Bulb

Olfactory receptor neurons send axons into the two olfactory bulbs. The input layer of each bulb in mice contains about 2000 spherical structures called *glomeruli*, each about 50-200 um in diameter.

Olfactory Transduction

Olfactory receptors probably use only one signaling system for transduction. The olfactory pathway can be summarized as follows: Odorants--> Bind to membrane odorant receptor proteins--> Stimulate G-protein(Golf)--> Activate adenylyl cyclase-->Form cAMP--> Bind cAMP to a cyclic nucleotide-gated cation channel--> Open cation channels and allow influx of Na^+ and Ca^2+--> Open Ca^2+-activated Cl^- channels--> Cause current flow and membrane depolarization(receptor potential)

Types of Papillae

Papillae are shaped like ridges(foliate papillae), pimples (vallate papillae), or mushrooms(fungiform papillae)

Papillae

Small, taste-sensitive projections scattered about the surface of the tongue. Each papilla has from one to several hundred *taste buds*, visible only with a microscope

Basal Cells and Afferent Axons

Taste buds also have basal cells that surround the taste cells, plus a set of gustatory afferent axons

Taste Receptor Cells

Taste receptor cells are not neurons according to standard histological criteria. They do form synapses with the endings of the gustatory afferent axons near the bottom of the taste bud

Taste Stimuli

Taste stimuli, or tastants, may (1) directly pass through ion channels (salt and sour), (2) bind to and block ion channels (sour), or (3) bind to G-protein-coupled receptors in the membrane that activate second messenger systems that, in turn, open ion channels (bitter, sweet, and umami)

3rd Part of Summary of Central Taste Pathway

The VPM taste neurons then send axons to the *primary gustatory cortex*

Microvilli

The apical ends have thin extensions called *microvilli* that project into the *taste pore*

Population Coding

The brain distinguishes between flavors in a way that includes features of roughly labeled lines and *population coding*, in which the responses of a large number of broadly tuned neurons, rather than a small number of precisely tunes neurons, are used to specify properties of a particular stimulus, such as taste

Apical End

The chemically sensitive part of a taste receptor cell is its small membrane region near the surface of the tongue

1st Part of Summary of Central Taste Pathway

The cranial nerves are involved in a variety of other sensory and motor functions, but their taste axons all enter the brain stem, bundle together, and synapse within the slender *gustatory nucleus*, a part of the solitary nucleus in the medulla

The Five Basic Tastes

The five basic tastes qualities are saltiness, sourness, sweetness, bitterness, and umami(meaning "delicious" in Japanese)

Labeled Line Hypothesis

The idea that there are many specific taste receptors for many basic tastes and each type is connected by a separate set of axons to neurons in the brain that also response to only one specific taste

Ageusia

The loss of taste perception. Lesions within the VPM thalamus or the gustatory cortex - as a result of a stroke, for example - can cause ageusia

Central Taste Pathway

The main flow of taste information is from the taste buds to the primary gustatory axons, into the brain stem, up the thalamus, and to the cerebral cortex

Most Familiar Chemical Senses

The most familiar of our chemical senses: taste or *gustation*, and smell, or *olfaction*

Anosmia

The olfactory axons are fragile, and in a traumatic injury, such as a blow to the head, the forces between the cribriform plate and surrounding tissue can sever the olfactory axons. After this type of injury, the axons cannot regrow, resulting in *anosmia*, the inability to smell

Olfactory Epithelium Zones

The olfactory epithelium is organized into a few large zones, and each zone contains receptor cells that express a different subset of receptor genes

Olfactory Population Coding

The olfactory system uses the responses of a large population of receptors to encode a specific stimulus. In this image, by looking at the combination of responses from all three cells, the brain could distinguish the citrus smell unambiguously from floral, peppermint, and almond. By one recent estimate, humans can discriminate between at least one trillion different combinations of odor stimuli

Central Olfactory Pathways

The output axons of the olfactory bulbs course through the olfactory tracts and project directly to several targets. Among the most important targets are the primitive region of cerebral cortex called *olfactory cortex* and some of its neighboring structures in the temporal lobe. Conscious perceptions of smell may be mediated by a path from the *olfactory tubercle*, to the *medial dorsal nucleus* of the thalamus, and to the *orbitofrontal cortex*

Transduction

The process by which an environment stimulus causes an electrical response in a sensory receptor cell

Saltiness: Low Concentrations

The taste of salt is mostly the taste of the cation Na^+, but taste receptors use very different mechanisms to detect low and high concentration. To detect low concentrations, salt-sensitive taste cells use a special Na^+-selective channel that is common in other epithelial cells and which is blocked by the drug amiloride

Areas of Higher Sensitivity of the Tongue

The tip of the tongue is most sensitive to sweetness, the back to bitterness, and the sides to saltiness and sourness. Although, most of the tongue is sensitive to all basic tastes

T1R and T2R Taste Receptors

The transduction processes underlying bitter, sweet, and umami tastes rely on two families of related taste receptor proteins, called T1R and T2R. The various subtypes are all G-protein-coupled receptors that detect neurotransmitter and are also dimers, which are two proteins affixed to each other

Sweetness

There are many different sweet tastants, some natural and some artificial and all seem to be detected by the same taste receptor protein. Sweet receptors are all dimers of G-protein-coupled receptors. A functioning sweet receptor requires two very particular members of the T1R receptor family: T1R2 and T1R3. If either is missing or mutated, an animal may not perceive sweetness at all

Temporal Coding in the Olfactory System

There is growing evidence that the temporal patterns of spiking in olfactory neurons are essential features of olfactory coding. Oder are inherently slow stimuli, so the rapid timing of action potentials is not necessary for encoding the timing of odors. *Temporal coding*, which depends on the timing of spikes, might instead encode the quality of odors

Supporting Cells

These cells are similar to glia: among other things, they help produce mucus

Olfactory Receptor Cells

These cells are the sites of transduction. Unlike taste receptor cells, olfactory receptors are genuine neurons, with axons of their own that penetrate into the central nervous system

Basal Cells

These cells are the source of new receptor cells. Olfactory receptor cells are one of the very few types of neurons in the nervous system that are regularly replaced with a life cycle that lasts about 4-8 weeks

Three Cranial Nerves of the Central Taste Pathway

Three cranial nerves carry primary gustatory axons and bring taste information to the brain. The anterior two-thirds of the tongue and the palate send axons into a branch of cranial nerve VII, the facial nerve. The posterior tongue is innervated by a branch of cranial nerve IX, the glossopharyngeal nerve. The regions around the throat , including the glottis, epiglottis, and pharynx, send taste axons to a branch of cranial nerve X, the vagus nerve

Umami Substances

Umami is defined by the savory taste of the amino acid glutamate; monosodium glutamate, or MSG, is the familiar culinary form

Olfactory Axons

Very thin, unmeylinated axons on the opposite side of the olfactory receptor cells. Collectively, the olfactory axons constitute the olfactory nerve(cranial nerve I)

Olfactory Epithelium

We do not smell with our nose. Rather we smell with a small, thin sheet of cells high up in the nasal cavity called the *olfactory epithelium*. The olfactory epithelium has three main cell types

Organs of Taste

We taste primarily with our tongue, but other areas of the mouth, such as the palate, pharynx, and epiglottis, are also involved

Receptor Potential

When an appropriate chemical activities a taste receptor cell, its membrane potential changes, usually by depolarizing. This voltage shift is called the *receptor potential*. If the receptor potential is depolarizing and large enough, some taste receptor cells, like neurons, may fire action potentials

Threshold Concentration

When exposing a single papilla to a solution, a concentrations too low will not be tasted, but at some critical concentration, the stimulus will evoke a perception of taste; this is the *threshold concentration*

Glomeruli

With each glomerulus, the endings of about 25,000 primary olfactory axons (axons from receptor cells) converge and terminate on the dendrites of about 100 second-order olfactory neurons.

Broad Tuning of Single Olfactory Receptor Cell

Within each olfactory epithelium zone, individual receptor types are scattered randomly. Each receptor protein binds different odorants more or less readily, so its receptor cell is more or less sensitive to those odorants