Chemical Senses

Function of olfaction and gustation

• complementary senses that let us know whether a substance should be savored or avoided

Chemoreceptors

• are used by olfaction and gustation

• 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

Olfactory epithelium of olfactory receptors

• where olfactory receptors are located

• organ of smell

• Located in in roof of nasal cavity

• Covers superior nasal conchae

• Contains olfactory sensory neurons

  • Bipolar neurons with radiating olfactory cilia

• Has supporting cells to surround and cushion olfactory receptor cells

• Has olfactory stem cells that lie at base of epithelium

Olfactory neurons of olfactory receptors

• 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)

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

• CN I is the filaments of olfactory nerve

2 receptors olfactory epithelium

1. olfactory nerve

2. mitral cell (output cell)

Olfactory epithelium and odorant receptors

How many ‘smell’ genes are active in the nose

400

Function of ‘smell’ genes in human nose

• Each encodes a unique receptor protein

• Protein responds to one or more odors

• Each odor binds to several different receptors

• Each receptor cell has one type of receptor protein

Odor coding

• 1 olfactory neuron can express only 1 receptor

• 1 receptor can detect many different molecule(odorant)

• 1 molecule can be detected by multiple receptors

How are olfactory neurons grouped

• neurons with same receptor are confined to 1 zone but are scattered in that zone

• Neurons with different receptors are interspersed

not completely true in all cases

How are we able sense smell

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

• Single odorant activates multiple glomeruli with input from different receptors

• Each glomerulus is dedicated to one type of receptor signaling

How are we able to 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 threshold, AP is conducted

Olfactory transduction(translation into neuronal signal) process

recall 5 steps of G-protein coupling

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 neurons with same receptor type converge on given type of glomerulus

• Mitral cells amplify, refine, and relay signals

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

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

  • Emotional responses to odor are elicited

Piriform cortex

Responsible for conscious perception of olfactory information

Olfactory information can either go to the piriform cortex and synapse at thalamus or just synapse only at this area

Anosmias

• olfactory defect; most result from

  • Head injuries that tear olfactory nerves

  • Aftereffects of nasal cavity inflammation

  • Neurological disorders, such as Parkinson’s disease

    • usually an early sign of Parkinson’s

irreversible

reversible one can come from a cold

Phantosmia

• olfactory hallucination

  • smelling something that is absent

• Usually caused by temporal lobe epilepsy that involves olfactory cortex

• Some people have olfactory auras prior to epileptic seizures

  • misfiring of the neurons causes faulty signaling to olfactory cortex when it did not actually come from the olfactory epithelium

Where and how taste sensed

Chemoreceptors in taste buds detect gustatory signals

Taste buds

• sensory organs for taste

• Most of 10,000 taste buds are located on tongue in papillae, peglike projections of tongue mucosa

3 papillae that have tastebuds

• Fungiform papillae: tops of these mushroom-shaped structures house most taste buds; scattered across tongue

• Foliate papillae: on side walls of tongue

• Vallate papillae: largest taste buds with 8–12 forming “V” at back of tongue

Parts of a taste bud

• Gustatory epithelial cells

• Basal epithelium cells

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

not neuronal tissue unlike olfactory, photorecption,

Basal epithelial cells

• dynamic stem cells that divide every 7–10 days

• constantly renews (blue region

5 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: alkaloids such as quinine and nicotine, caffeine, and nonalkaloids such as aspirin

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

all detected by different receptors

Purpose of taste ‘likes’ and ‘dislikes’

• 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

How are we able to taste

• To be able to taste a chemical, it must:

  • Be dissolved in saliva

  • Diffuse into taste pore

  • Contact gustatory hairs

• Then requires taste receptors

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 minutes

How is salty taste transduced

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

How is sour taste transduced

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

How are sweet, bitter, and umami transduced

• 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 depolarization and release of neurotransmitter ATP

salivary glands

• 3 main pairs

• Saliva acts as a solvent

• Facilitates clearance of taste particles

• no saliva= no ability to taste

What other senses are involved in taste perception?

• 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

How are gustatory signals transmitted to the brain?

3 main cranial nerve pairs that 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 epiglottis 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

• Cortex: detect taste

• Hypothalamus, limbic system: trigger emotional valance

• Solitary nucleus: trigger Visceral reflexes

Gustatory reflex