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Chemical Senses:GustationChemical Senses:GustationBackground Background

Mechanism Mechanism by which we canby which we can detect chemicals detect chemicalsin both the internal and external environmentin both the internal and external environment2. Taste and olfaction are the most familiar 2. Taste and olfaction are the most familiar chemical senseschemical senses3. Many types of chemically sensitive cells3. Many types of chemically sensitive cellsa. Chemoreceptorsa. Chemoreceptorsi. Distributed throughout the bodyi. Distributed throughout the bodyii. Report subconsciously and consciously ii. Report subconsciously and consciously about our internal stateabout our internal state

TypesTypes1.1.Chemoreceptors in skin and mucousChemoreceptors in skin and mucousmembranesmembranes warn us about irritatingwarn us about irritating chemicals chemicals2. Nerve endings in the digestive organs 2. Nerve endings in the digestive organs detect many types of ingested substancesdetect many types of ingested substancesa. Viral agents may release chemicals intoa. Viral agents may release chemicals intothe GI tract that cause discomfort, activatethe GI tract that cause discomfort, activatevomiting reflexes, etc.vomiting reflexes, etc.

3. Chemical receptors in the arteries in the3. Chemical receptors in the arteries in theneck measure neck measure CO2 & O2CO2 & O2 levels in the blood levels in the blood4. Sensory endings in the muscles 4. Sensory endings in the muscles respond to acidityrespond to aciditya. Burning sensation experienced during a. Burning sensation experienced during anaerobic exercise results from lactic acidanaerobic exercise results from lactic acidformationformation

Gustation and Olfaction have similar tasksGustation and Olfaction have similar tasks1. Detection of environmental chemicals1. Detection of environmental chemicals2. Both are required to perceive flavor2. Both are required to perceive flavor3. Both have strong and direct connections3. Both have strong and direct connectionsto our most basic needsto our most basic needsa. Thirst, hunger, emotion, sex, and certain a. Thirst, hunger, emotion, sex, and certain forms of memoryforms of memory

4. Systems are separate and different and4. Systems are separate and different andonlyonly merge merge at higherat higher levels oflevels of cortical function cortical functiona. Have different chemoreceptorsa. Have different chemoreceptorsb. Use different transduction pathwaysb. Use different transduction pathwaysc. Have separate connections to the brainc. Have separate connections to the braind. Have different effects on behaviord. Have different effects on behavior

Basic categories Basic categories

1. Salty1. Salty2. Sour2. Sour3. Sweet3. Sweet4. Bitter4. Bitter

Complex flavorsComplex flavors

1. Each food activates a different combination 1. Each food activates a different combination of basic tastesof basic tastes2.Most foods have distinctive flavor as a 2.Most foods have distinctive flavor as a result of their taste and smell occurring result of their taste and smell occurring simultaneouslysimultaneously3. Other sensory modalities may contribute to 3. Other sensory modalities may contribute to a unique food-tasting experiencea unique food-tasting experiencea. Texture, temperature, pain sensitivity (some a. Texture, temperature, pain sensitivity (some hot and spicy flavors are actually a pain hot and spicy flavors are actually a pain response)response)

Organs of tasteOrgans of taste

1. Tongue 1. Tongue a. Primary organa. Primary organ2. Pharynx, palate and epiglottis have some 2. Pharynx, palate and epiglottis have some sensitivitysensitivity3. Nasal passages are located so that odors 3. Nasal passages are located so that odors can enter through the nose or pharynx and can enter through the nose or pharynx and contribute to the perception of flavorcontribute to the perception of flavor

Anatomy of the tongueAnatomy of the tongue

1. Basic tastes1. Basic tastesa. Bitter across the backa. Bitter across the backb. Sour on side closest to the backb. Sour on side closest to the backc. Salty on side more rostral than sourc. Salty on side more rostral than sourd. Sweet across frontd. Sweet across front

2. Taste distribution2. Taste distributiona. Most of the tongue is receptive to a. Most of the tongue is receptive to all basic tastesall basic tastes i. Regions are most sensitive to a i. Regions are most sensitive to a given tastegiven taste

3. Papillae 3. Papillae a. Small projections a. Small projections b. Each papillae has one to several b. Each papillae has one to several hundred taste budshundred taste buds4. Each taste bud has 50-150 taste cells4. Each taste bud has 50-150 taste cells5. Taste cells are only 1% of the tongue 5. Taste cells are only 1% of the tongue epitheliumepithelium

Taste receptor cellsTaste receptor cells

Not neuronsNot neuronsa. Form synapses with the endings a. Form synapses with the endings of gustatory afferent axons near the of gustatory afferent axons near the bottom of the taste budbottom of the taste bud

Gustatory TransductionGustatory Transduction

Basic processBasic process1. When taste receptor is activated by the 1. When taste receptor is activated by the appropriate chemical, its membrane potential appropriate chemical, its membrane potential changes changes a. Receptor potentiala. Receptor potential2. Depolarizing receptor potential cause Ca++ 2. Depolarizing receptor potential cause Ca++ to enter the cytoplasmto enter the cytoplasma. Triggers the release of NTa. Triggers the release of NT

3. Taste stimuli may:3. Taste stimuli may:a. Pass directly through an ion channel (salt a. Pass directly through an ion channel (salt and sour)and sour)b. Bind to and block ion channels (sour and b. Bind to and block ion channels (sour and bitter)bitter)c. Bind to and open ion channels (some sweet c. Bind to and open ion channels (some sweet amino acids)amino acids)d. Bind to membrane receptors that activate d. Bind to membrane receptors that activate 2nd messenger systems that in turn open or 2nd messenger systems that in turn open or close ion channels (sweet and bitter)close ion channels (sweet and bitter)

Salt Salt 1. Na+ flows down a concentration gradient 1. Na+ flows down a concentration gradient into taste receptor cell (most salts are Na+ into taste receptor cell (most salts are Na+ salts--NaCl)salts--NaCl)2. Na+ increase within the cell depolarizes 2. Na+ increase within the cell depolarizes membrane and opens a voltage dependent membrane and opens a voltage dependent Ca++ channelCa++ channel3. Ca++ increase causes the release of NT3. Ca++ increase causes the release of NT

SourSour1. Foods that are sour have high acidity 1. Foods that are sour have high acidity a. Acids (HCl) when dissolved in water a. Acids (HCl) when dissolved in water generate H+ ionsgenerate H+ ions2. H+ ions pass through the same channel 2. H+ ions pass through the same channel that Na+ does (how do we discriminate that Na+ does (how do we discriminate between salt and sour then?)between salt and sour then?)

3. H+ also blocks a K+ channel3. H+ also blocks a K+ channel4. Net movement of + into the cell depolarizes 4. Net movement of + into the cell depolarizes the taste cell (opens a Ca++ channel and the taste cell (opens a Ca++ channel and causes NT release)causes NT release)

SweetnessSweetness1. Molecules that are sweet bind to specific 1. Molecules that are sweet bind to specific receptor sites and activate a cascade of 2nd receptor sites and activate a cascade of 2nd messengers in certain taste cellsmessengers in certain taste cells2. Molecules bind receptor2. Molecules bind receptor

3. G-protein activates an effector enzyme-3. G-protein activates an effector enzyme-adenylate cyclase (cAMP produced)adenylate cyclase (cAMP produced)4. cAMP causes a K+ channel to be blocked4. cAMP causes a K+ channel to be blocked5. Cell depolarizes5. Cell depolarizes6. Ca++ channel opens and Ca++ in6. Ca++ channel opens and Ca++ in7. NT released7. NT released

Bitter Bitter 1. Chemicals in the environment that are 1. Chemicals in the environment that are deleterious often have a bitter flavordeleterious often have a bitter flavora. Senses have evolved primarily to protect a. Senses have evolved primarily to protect and preserveand preserveb. Ability to detect bitter has two separate b. Ability to detect bitter has two separate mechanismsmechanismsi. May result from this evolutionary pressurei. May result from this evolutionary pressure

2. System I2. System Ia. Bitter tastants can directly block a K+ a. Bitter tastants can directly block a K+ channel (same transduction mechanisms as channel (same transduction mechanisms as acids) acids) b. Cell depolarizesb. Cell depolarizesc. Ca++ channel is opened and Ca++ inc. Ca++ channel is opened and Ca++ ind. NT releasedd. NT released

3. System II3. System IIa. Bitter tastant binds bitter receptora. Bitter tastant binds bitter receptorb. G-protein activates an effector enzyme-b. G-protein activates an effector enzyme-phospholipase Cphospholipase Cc. Ca++ is released from intracellular storagec. Ca++ is released from intracellular storaged. Ca++ increase causes NT released. Ca++ increase causes NT release

Taste Neural PathwayTaste Neural Pathway

CircuitCircuit1. NT release from taste cells causes an AP in 1. NT release from taste cells causes an AP in the gustatory afferent axonthe gustatory afferent axon2. Three cranial nerves (VII, IX and X) 2. Three cranial nerves (VII, IX and X) innervate taste buds & carry taste information innervate taste buds & carry taste information from tongue, palate, epiglottis & esophagus from tongue, palate, epiglottis & esophagus a. Efferent target of this information is a. Efferent target of this information is gustatory nucleus in the medullagustatory nucleus in the medulla

3. Information is relayed to the thalamus 3. Information is relayed to the thalamus (VPM--ventral posterior medial nucleus)(VPM--ventral posterior medial nucleus)4. Information then goes to the primary 4. Information then goes to the primary gustatory cortex (parietal lobe)gustatory cortex (parietal lobe)