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Collin College

BIOL 2401

Week 10

Special Senses

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General Organization

•  Organs of the Special senses are structurally more complex than the structures that make up the general senses.

•  Special senses classification : –  Olfactory –  Gustation –  Vision –  Equilibrium –  Hearing

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Olfactory Sensation

•  The reception for the sense of smell is provided by a pair of olfactory organs.

•  Olfactory organs are located in the nasal cavity

•  The olfactory organs are made up of two layers –  Olfactory epithelium –  Lamina propria

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Olfactory Epithelium

•  Olfactory epithelium contains –  Olfactory receptors –  Supporting cells –  Basal cells

•  It covers certain areas of the ethmoid bone, that protrudes into the nasal cavity –  the inferior surface of the

cribiform plate –  Superior aspect of perpendicular

plate –  Superior nasal conchae

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Olfactory Sensation

•  Breathing in air through the nose circulates the air around within the nasal cavities

•  The nasal concha help in the swirling motion of the air

•  Molecules in the air need to be trapped into a liquid mucus phase before the receptors can be stimulated by them

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Lamina Propria

•  A Lamina propria is located deep to (under) the Olfactory epithelium and contains –  Numerous blood vessels –  Nerves –  Olfactory Glands (Bowman’s Glands)

•  Bowman’s glands provide the mucus via ducts to the surface of the epithelium such that molecules can be trapped into a liquid phase.

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Olfactory Epithelium

An interesting fact is that the olfactory receptor cells are the only neurons of the nervous system that are exposed to the external environment !

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Olfactory Epithelium

The area of the olfactory epithelium (red) in dogs is some forty times larger than in humans. Mice have about one thousand different odorant receptor types.

Dogs have up to a billion olfactory receptor cells. Humans do not have as many different receptors as mice and dog; some of the genes for these receptors have been lost during evolution. Nonetheless, there are still several millions (6-10 million) of olfactory receptor cells in our olfactory epithelium.

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Membrane Receptors and G-proteins

•  The perception of smell and taste are referred to as the chemical senses, because it depends on the presence of chemoreceptors that need to bind chemicals.

•  We discussed binding of chemicals to receptors that are directly coupled to opening (closing) of ion channels

•  These were called chemically gated ion channels

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Membrane Receptors and G-proteins

•  Other receptors exist in cell membranes that bind chemicals and activate a group of membrane proteins called G- proteins

•  When certain G-proteins are activated, they dissociate into subunits, and one of the subunits (alpha subunit) ends up activating a membrane enzyme called Adenylate Cyclase

•  Adenylate cyclase uses ATP and turns it into cyclic AMP (cAMP) and PPi

cAMP is an important intracellular messenger that can turn on many different enzymes or activate

other proteins.

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Membrane Receptors and G-proteins

Smell molecule (Odorant)

In olfactory receptor cells, cAMP opens a Na+ channel

•  Dissolved chemicals interact with the olfactory cilia and result in binding to receptors that are coupled to G-proteins.

•  This results in production of cAMP inside the cilia, which in turn opens Na+ channels. Na+ enters the cell ( and some calcium) and depolarization occurs.

•  If sufficient depolarizations occur ( graded potentials), an action potentials is triggered.

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Olfactory Receptors

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•  Voltage recordings from an olfactory receptor cell show that odorant molecules generate a receptor potential in the cilia

•  This reaches into the cell body and generates action potentials in the soma

•  Finally, action potentials propogate contionuously down the olfactory nerve axon towards the olfactory bulb)

Olfactory Receptors

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Olfactory Receptors

•  Experiments showed that the current response only occurs when ‘odorants’ are released at the cilia.

•  This indicates that the receptors for smell molecules are located in the cilia.

Microscopic picture of smell receptors with cilia

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Olfactory Receptors

•  Question : Why not a simple chemically gated channel such as in synapses. The result would be similar and not so complex ?

•  Answer: a) In synapses, the chemical (aka neurotransmitter) that binds to a specific c.g. channel is always very similar. b) Since smell molecules are very variable in nature, it would require thousands of different c.g. channels, each specific for a certain smell molecule

Smell molecule

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The odorant receptors are located in the cilia of the olfactory receptor cells in the nasal cavity. Even though our DNA has hundreds of genes for smell receptors, each olfactory receptor cell expresses only one type of odorant receptor. In addition, each receptor can detect only a limited number of odorant substances.

Olfactory Receptors

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Olfactory Perception

•  Each olfactory receptor cell expresses only a single type of receptor protein and different receptor cells are scattered all over the olfactory epithelium

•  This example shows 3 different cells and the recordings when stimulated by certain odor molecules

•  The coding of the odor is in the cellular responses to each of the cells

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The chart provides a simplified example on how the odorant receptor family in our olfactory epithelium is used in a combinatorial manner to detect odorants and encode their unique identities.

Different odorants are detected by different combinations of receptors and thus have different receptor codes. These codes are translated by the brain into diverse odour perceptions.The immense number of potential receptor combinations is the basis for our ability to distinguish and form memories of more than 10,000 different odorants.

Olfactory Perception

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Olfactory Pathways

(mitral cell) •  The axons of the olfactory neurons pass

through the holes of the cribiform plate and make synapses within the Olfactory bulb

•  Synapses are made with mitral cells in areas called glomeruli.

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•  A particular glomerulus usually receives signals from those olfactory neurons that express similar receptor proteins, and thus bind similar “odor” molecules. ( follow for example the lighter colored neurons vs the darkest colored neurons)

•  This is refererred to as convergence (many olfactory neurons synapsing with the same mitral cell) .This allows for possible inhibition of facilitation

Olfactory Pathways

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Olfactory Pathways

•  Olfactory impulses flow via olfactory tracts to the primary olfactory cortex (medial aspect of temporal lobe, containing the piriform cortex).

•  The olfactory cortex is also called the Rhinencephalon, or “nose brain.” This is the most primitive part of the cerebrum and connects directly to the limbic system (emotional system), which is why smells often directly trigger emotions as well as our deepest memories.

•  From here connections are made via thalamus with orbitofrontal cortex for odor discrimination.

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Vomero Nasal Organ (VMO)

•  Most animals have an additional sense organ, called the VomeroNasalOrgan, that picks up “pherormones”. These are chemical messengers that carry information between individuals of the same species, and hence, the VMO is sometimes referred to as the "sixth sense."

•  In animals, VMO plays an important role in social behavior and reproduction since “pherormones” are chemicals that attract the opposite sex.

VomeroNasal

Organ ?

•  The existence of the VMO is very controversial in humans. In other words, do humans release sexual molecules and can we “sniff” them out ?

•  Beware of the internet, as many companies will make you believe that VNO in humans is a fact and working extremely well.

•  Most studies agree the organ regresses during fetal development.

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Olfactory Disorders

•  Anosmia :Inability to detect odors

•  Hyposmia : Loss in perception of some odors

•  Dysosmia : Distorted identification of smell

•  Phantosmia : detecting odors that are not really present

Note : As humans , we still have the capacity to detect certain odors at extreme low concentrations. Certain odors trigger olfaction by just the presence of one molecule. Beta- mercaptane is an example of this.

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Gustation

•  Gustation provides the information for the foods and liquids we consume

•  Taste receptors are distributed •  Superior surface of the tongue •  Portions of the pharynx and

larynx

•  Taste receptors, together with specialized epithelial cells form structures called the taste buds

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Gustation

•  On the tongue, the taste buds are located in epithelial projections called the lingual papillae.

•  Human tongue has 3 different kinds of papillae •  Filiform : do not contain tastebuds but provide

friction for moving food around

•  Fungiform : contain around 5 taste buds

•  Circumvallate : contain around 100 tastebuds and are located in back of the tongue in a V- shape

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Gustation

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Gustation Tastebuds

Each tastebud contains about 50 -100 cells. There are 4 different kinds of cells. Two important ones are

•  Basal cells which appear to be stem cells

•  Gustatory cells extend taste hairs through a narrow taste pore

•  Typical gustatory cells lasts for about 10 days

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Gustation Tastebuds

•  Each taste receptor cell is connected, through an ATP-releasing synapse, to a sensory neuron leading back to the brain.

•  However, a single sensory neuron can be connected to several taste cells in each of several different taste buds.

•  The sensation of taste — like all sensations — resides in the brain

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Gustation Tastebuds

Within a tastebud, cells will respond to different substances but each in it’s own way . The example shows that Cell 1 barely responds to Quinine and HCl in contrast with Cell 2.

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Gustation Pathways

•  The Taste buds are monitored by cranial nerves : VII, IX and X. The specific branch of the Facial nerve is called the chorda tympani

•  Sensory afferent nerves synapse within the solitary nucleus of the medulla oblongata

•  Then on to the thalamus and the primary gustatory cortex, located in the insula. Connections are also made with hypothalamus, limbic system

•  Information about Texture is provided via the Trigeminal nerve •  In addition, Olfactory receptors enhance taste perception by

several orders of magnitude

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Gustation Pathways

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Gustation Discrimination

•  Primary taste sensations – Sweet, sour, salty, bitter – Receptors also exist for umami (taste for the

presence of glutamate, small peptides) and water

•  The number of taste buds declines with age

•  Taste sensitivity shows significant individual differences, some of which are inherited. We also show different sensitivities to taste, some of which are enhanced by temperature.

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Gustation Discrimination

Examples of some human taste thresholdsTaste Substance Threshold for tasting

Salty NaCl 0.01 MSour HCl 0.0009 MSweet Sucrose 0.01 MBitter Quinine 0.000008 MUmami Glutamate 0.0007 M

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Gustation Discrimination

Sensation requires dissolved chemicals to bind to the receptor hairs of the gustatory receptors

•  With salty substances (e.g., table salt, NaCl), the receptor is an ion channel that allows sodium ions (Na+) to enter directly into the cell.

•  This depolarizes it allowing calcium ions to enter triggering the release of ATP at the synapse to the attached sensory neuron and generating an action potential in it.

Salt

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Gustation Discrimination

•  In lab animals, and perhaps in humans, the hormone aldosterone increases the number of these salt receptors. This makes good biological sense:

–  The main function of aldosterone is to maintain normal sodium levels in the body.

–  An increased sensitivity to sodium in its food would help an animal suffering from sodium deficiency (often a problem for ungulates, like cattle and deer).

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Gustation Discrimination

•  Several types of receptors may be involved in detecting the protons (H+) liberated by sour substances (acids).

•  In one type, the protons block potassium channels thus interrupting the normal outflow of K+ that creates the resting potential of the cell. The resting potential of the cell is reduced, resulting in depolarization.

•  This opens v.g Calcium channels and this causes the release of neurotransmitter

Sour

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Gustation Discrimination

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• Na+ flows down a concentration gradient into the taste receptor cell (most salts are Na+ salts such as NaCl)

• Na+ increase within the cell depolarizes the membrane and opens a voltage dependent Ca++ channel

• Ca++ increase causes the release of NT.

Gustation Discrimination

• Foods that are sour have high acidity (low pH) • Acids (HCl) when dissolved in water generate H+

ions • H+ ions pass through the same channel that Na+

does • H+ also blocks a K+ channel • Net movement of + into the cell depolarizes the

taste cell a. Opens a Ca++ channel b. Causes NT release

(1) Na+

(3) Ca++

(1) H+

Ca++

K+ (2) Depolarization

(5) Exocytosis

(2) Closes K+ channel (3) Depolarization

(4) Opens v.g Ca channel (4) Opens v.g Ca channel

(6) Exocytosis

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Gustation Discrimination

•  Sweet substances (like table sugar — sucrose) bind to G-protein-coupled receptors (GPCRs) at the cell surface.

–  Each receptor contains 2 subunits designated T1-R2 and T1-R3

–  They are coupled to G proteins. –  The complex of G proteins has been named gustducin

because of its similarity in structure and action to the transducin that plays such an essential role in rod vision.

Sweet

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Gustation Discrimination

•  Activation of gustducin triggers a cascade of intracellular reactions: –  activation of adenylyl cyclase –  formation of cyclic AMP (cAMP) –  cAMP activates Protein Kinase A (PKA) –  PKA phosphorylates K+ channels , which blocks them –  That leads to depolarization of the cell. –  v.g Ca++ channels open. Calcium comes in and helps to release the NT

•  The mechanism is similar to that used by our odor receptors

•  The hormone leptin inhibits these sweet cell receptors by opening their K+ channels. This hyperpolarizes the cell making the generation of action potentials more difficult. Could leptin, which is secreted by fat cells, be a signal to cut down

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Gustation Discrimination

Sweet Taste molecule

Phosphorylates and blocks K+ channel Depolarization

Opening of v.g Calcium channel Release of N.T.

PKA

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Gustation

Bitter molecules can work via 2 different kinds of receptors

A.  Bitter taste molecules can directly block K+ channels ( similar as acids do)

B.  Bitters bind to G-protein-coupled receptors. e.g., quinine, phenyl thiocarbamide [PTC], •  These are different types of G-Protein

coupled receptors that activate a different enzyme called Phospholipase C

•  It produces a component called IP3 that now releases calcium from the Smooth Endoplasmic reticulum

•  The results is that it opens up a channel through which ATP is released

Bitter

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Gustation

Despite this — and still unexplained — a single taste cell seems to respond to certain bitter-tasting molecules in preference to others.

The sensation of taste — like all sensations — resides in the brain.

This is shown by the fact that transgenic mice that express

•  T2-R in cells that normally express T1Rs (the sweet receptors) respond to bitter substances as though they were sweet;

•  a receptor for a tasteless substance in cells that normally express T2Rs (bitter) are repelled by the tasteless compound.

So it is the activation of hard-wired neurons that determines the sensation of taste, not the molecules nor the receptors themselves.

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Recent molecular and functional data shown there is no tongue 'map' ; modalities for taste are present in all areas of the tongue and vary from person to person…. everyone experiences food the same although differently ….

Gustation

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Gustation

Sugar substitutes are becoming more prevalent. They taste sweeter and have a lower caloric/glycemic index

Sweetness Index Ex: Glucose 1

Aspartame 180 Sucralose 600 Neotame 8000 Advantame 20,000

Many of these bind to sweet receptor much strongly and thus evoke a stronger “sweet” response. Many sugar substitutes, such as saccharin and acesulfame K (also known as SunetteTM), do not provide any calories. This means that they are not metabolized as part of the normal biochemical pathways that yield energy in the form of adenosine triphosphate, or ATP.

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Gustation

So what about spicy/hot food ?

“Spicy” is not really a taste sensation. Many spicy foods contain compounds such as capsaicin. They bind a Transient Receptor Potential channel called TRPV1, which are most often calcium ion channels.

These channels evolved in order to sense pain / temperature response in the skin but also developed in the mouth and on the tongue.

Eating spicy food thus triggers the same hot/pain response, which the brain translates as spicy….

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Gustation

Research has discovered a whole family of such receptors.

Cold temperatures are sensed by a receptor channel closely related to TRPV1 (called TRPM8).

The menthol in peppermint and spearmint binds to this receptor. So minty gums trick your mind into thinking you're eating something cold.

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