Chemical Senses

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30 Terms

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Myopia

  • Nearsightedness

  • Can’t see clearly far away objects

  • Usually develops in childhood

  • Problems with the shape of the eyeball, cornea or lens

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Hyperopia

  • Farsightedness

  • Can’t see objects up close

  • lifelong

  • problems with the shape of the eyeball cornea or lens

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Presbyopia

  • Farsightedness

  • Can’t see objects up close

  • Usually develops after the age of 40

  • Reduction in ciliary muscle function, less elasticity in the lens

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The chemical sense: smell and taste

  • Smell (olfaction) and taste (gustation): complementary sense that let us know whether a substance should be savored or avoided

  • Chemoreceptors are used by these systems

  • Chemicals must be dissolved in aqueous solution to be picked up by chemoreceptors

    • Smell receptors are excited by chemicals dissolved in nasal fluids

    • Taste receptors respond to chemicals dissolved in saliva

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Olfactory Receptors

Olfactory epithelium: organ of smell

  • Located in roof of nasal cavity

  • Covers superior nasal conchae

  • Contains olfactory sensory neurons

    • Bipolar neurons with radiating olfactory cilia

  • Supporting cells surround and cushion olfactory receptor cells

  • Olfactory stem cells lie at base of epithelium

  • Olfactory neurons are unusual bipolar neurons

    • Thin apical dendrites terminate in knob

    • long, largely nonmotile cilia, olfactory cilia, radiate from knob

      • Covered by mucus (solvent for odorants)

  • Bundles of nonmyelinated axons of olfactory receptor cells gather in fascicles that make up filaments of olfactory nerve (cranial nerve I)

  • Olfactory neurons, unlike other neurons, have stem cells that give rise to new neurons every 30-60 days

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Specificity of Olfactory Receptors

  • Smells may contain 100s of different odorants

  • Humans have ~400 “smell” genes active in nose

  • Each encodes a unique receptor protein

    • Protein responds to one or more odors

  • Each odour binds to several different receptors

  • Each receptor cell has one type of receptor protein

  • The genes encoding the receptor molecules were isolated and identified

  • Every single olfactory receptor cell expresses one and only one gene of all the genes that code for olfactory receptor molecules

  • Neurons with same receptor are confined to one zone, but scattered in that zone

  • Nuerons with different receptors are interspersed

  • Axons of sensory neurons with the same oforant receptor type converge in 2 glomeruli

  • Single odorant activates multiple glomeruli with input from different receptors

  • Each glomerulus id dedicated to one type of receptor

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The smell test

The relatively small size of the human olfactory bulb - compared to other animals - has long been cited as a reason for human’s “inferior” scent of smell. New research is starting to overturn this notion.

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Physiology of Smell

In order to smell substance, it must be volatile

  • Must be in gaseous state

  • Odorant must also be able to dissolve in olfactory epithelium fluid

Activation of olfactory sensory neurons

  • Dissolved odorants bind to receptor proteins in olfactory cilium membranes

  • Open cation channels, generating receptor potential

  • At the threshold, AP is conducted to first relay station in the olfactory bulb

Smell Transduction

  • Odorant binds to receptor, activating a G protein

    • Referred to as G(olf)

  • G protein activation causes cAMP (second messenger) synthesis

  • cAMP opens Na+ and Ca2+ channels

  • Na+ influx cause depolarization and impulse transmission

  • Ca2+ influx causes decreased response to a sustained stimulus, referred to as olfactory adaptation

    • People can’t smell a certain odor after being exposed to it for a while

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Olfactory Transduction Process

  1. Odorant binds to its receptor

  2. Receptor activates G protein (Golf)

  3. G protein activates adenylate cyclase

  4. Adenylate cyclase converts ATP and cAMP

  5. cAMP opens a cation channel, allowing Na+ and Ca2+ influx and causing depolarization

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The Olfactory Pathway

  • Filaments of olfactory nerves synapse with mitral cells located in overlying olfactory bulb

    • Mitral cells are second-order neurons that form olfactory tract

  • Synapse occurs in structures called glomeruli

  • Axons from neirons with same receptor type converge on given on given type of glomerulus

  • Mitral cells amplify, refine, and relay signals

  • The olfactory cortex is defined as those brain regions that receive the mitral and tufted cell projections from the olfactory bulb

  • Impulses from activated mitral cells travel via olfacotry tracts to piriform lobe of olfactory cortex

  • Some information sent to hypothalamus, amygdala, and other regions of limbic system

    • Emotional responses to odour are elicited

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Anosmias

Olfactory disorders; most result from

  • Head injuries that tear olfactory nerves

  • Aftereffects of nasal cavity inflammation

  • Neurological disorders, such as Parkinson’s disease

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Olfactory Hallucinations (phantosmia)

  • Usually caused by temporal lobe epilepsy that involves olfactory cortex

  • Some people have olfactory auras prior to epileptic seizures

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Where and how is taste sensed?

Gustatory signals → Detected by chemoreceptors in the taste buds

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Taste Buds

sensory organs for taste. Most of 10, 000 taste buds are located on tongue in papillae, peglike projections of tongue mucosa

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Fungiform Papillae

Tops of these mushroom-shaped structures house most taste buds; scattered across tongue

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Foliate papillae

on side walls of tongue

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Vallate Papillae

Largest taste buds with 8-12 forming “V” at back of tongue

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Gustatory Epithelial Cells

Taste receptor cells have microvilli called gustatory hairs that project into taste pores, bathed in saliva

  • Sensory dendrites coiled around gustatory epithelial cells send taste signals to brain

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Basal Epithelial Cells

Dynamic stem cells that divide every 7-10 days

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Basic Taste Sensations

There are five basic taste sensations

  1. Sweet - sugars, saccharin, alcohol, some amino acids, some lead salts

  2. Sour - hydrogen ions in solution

  3. Salty - metal ions (inorganic salts); sodium chloride tastes saltiest

  4. Bitter - alkaloid such as quinine and nicotine, caffeine, and non-alkaloids such as aspirin

  5. Umami - amino acids glutamate and aspartate; example: beef (meat) or cheese taste, and monosodium glutamate

  • Taste likes/dislikes have homeostatic value

  • Guide intake of beneficial and potentially harmful substances

  • Dislike for sourness and bitterness is a protective way of warning us if something is spoiled or poisonous

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To be able to taste a chemical, it must:

  • Be dissolved in saliva

  • Diffuse into taste pore

  • Contact gustatory hairs

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Activation of taste receptors

  • Binding of food chemical (tastant) depolarizes cell membrane of gustatory epithelial cell membrane, causing release of neurotransmitter

  • Neurotransmitter binds to dendrite of sensory neuron and initiates a generator potential that leads to action potentials

  • Different gustatory cells have different thresholds for activation

    • Bitter receptors are most sensitive

  • All adapt in 3-5 seconds, with complete adaptation in 1-5 mintues

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Taste Transduction

Gustatory epithelial cell depolarization caused by:

  • Salty taste is due to Na+ influx that directly causes depolarization

  • Sour taste is due to H+ acting intracellularly by opening channels that allow other cations to enter

  • Unique receptors for sweet, bitter, and umami, but all are coupled to G protein Gustducin

    • Activation causes release of stored Ca2+ that opens cation channels, causing depolarizationa nd release of neurotransmitter ATP

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The Salivary Glands

  • 3 Main Pairs

  • Saliva acts as a solvent

  • faciliates clearance of taste particles

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What other sense are involved in taste perception?

  • Gustatory - from taste buds (gustatory Pathway VII. IX.)

  • Touch - mechanoreceptors in the oral cavity (Somatosensory Pathway) ( V. IX.)

  • Temperature - nerve endings in the oral cavity (Somatosensory Pathway) (V. IX.)

  • Olfactory - from the olfactory epithelium (olfactory Pathway (I.)

  • Taste is 80% smell

  • If nose is blocked, foods taste bland

  • Mouth also contains thermoreceptors, mechanoreceptors and nociceptors

    • Temperature and texture enhance or detract from taste

    • Spicy hot foods can excite pain receptors in mouth, which some people experience as pleasure

    • Example: hot chili peppers

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How are gustatory signals transmitted to the brain

Taste buds sen gustatory signals via:

  • Vagus Nerve

  • Glossopharyngeal Nerve (Majority of gustatory signals)

  • Facial Nerve

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Gustatory Pathway

Two main cranial nerve pairs carry taste impulses from tongue to brain:

  • Facial Nerve (VII) carries impulses from anterior two-thirds of tongue

  • Glossopharyngeal (IX) carries impulses from posterior one-third and pharynx

  • Vagus nerve transmits (X) from epiglostis and lower pharynx

  • Fibers synapse in the solitary nucleus of the medulla, then travel to thalamus, and then to gustatory cortex in the insula

  • Hypothalamus and limbic system are involved; allow us to determine appreciation of taste

Taste perception = Thalamus Cortex

Appetite, Satiety and Emotional Valance = Lateral hypothalamus and Amygdala

Important roles of taste involve:

  • Triggering reflexes involved in digestion, such as: increased secretion of saliva into mouth

  • Increased secretion of gastric juice into stomach

May initiate protective reactions, such as:

  • Gagging

  • Reflexive vomiting

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Cordination

  • Gustatory - from taste buds

  • Touch - mechanoreceptors in the oral cavity

  • Temperature - nerve endings ini the oral cavity

  • Olfactory - from the olfactory epithlium

Primary cortices of sense → Orbitofrontal Cortex → Limbic system, Amygdala

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Gustatory Reflex

Taste buds → Sensory input → Solitary nuclei (medulla) → salvatory nuclei (pons) → Salivary Glands

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Disgeusia

  • Taste disorders are less common than disorders of smell, mostly because taste receptors are served by three different nerves

  • Not likely that all three nerves would be damaged at same time

  • Distorted or complete loss of taste

Causes of taste disorders include:

  • Upper respiratory tract infections

  • Head injuries

  • Chemicals or medications

  • Head and neck radiation for cancer treatment

  • Zinc supplements may help some cases of radiation-induced taste disorders