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