Chemical Senses

Learning Objectives:

  • Describe what taste sensation is and the mechanisms involved

  • Describe smell transduction and the mechanisms involved

  • Describe what auditory transduction is and the mechanisms involved

 

Special senses: Taste, Smell, Audition

Taste transduction

Taste:

  • Evoked by chemical substances called "tastants" acting on the tongue.

  • Tastants must be water-soluble to mix with saliva and reach receptors.

  • There are 5 defined tastes:

    • Salt (NaCl)

    • Sweet (sugar)

    • Umami (savory, meatiness)

    • Sour (acidic)

    • Bitter

  • All have specific receptors in taste receptor cells in the tongue

  • Other sensations include fat and heat from substances like chili and pepper, and kokumi (mouth feel, a flavour) is a recently identified ‘taste’ described as “fullness” or “body”

    • May be properly defined as a flavour rather than a taste

 

Taste structures: papillae, taste buds, central pathways

Papillae: Specialized structures on the tongue containing taste buds (three kinds on different areas of tongue).

  • Tongue is innervated by the cranial nerves

Taste Buds: Composed of taste receptor cells (TRCs), found if you zoom into a papillae

  • Each receptor cell has a receptor that is coupled to a G protein, causing the influx of sodium and calcium ions which may trigger the release of neurotransmitters which signal to afferent sensory fibres which travels to the CNS

 

Taste bud and taste cells (TRCs)

Different TRCs (taste receptor cells) express different types of receptors

  • Transduction can be:

    • Type I (TRC1): simple, support function similar to glial cells (express enzymes and transporters that remove neurotransmitters), low NaCl sensing.

    • Type II (TRC2): Respond to sweet, umami, bitter via G-protein coupled receptors.

      • When a ligand binds to the G-protein coupled receptor, it triggers increased calcium in the cell, which triggers sodium influx which allows the release of ATP a neurotransmitter, causing an action potential in the afferent nerve fibres

    • Type III (TRC3): Respond to sour via proton selective channel, otopetrin-1.

 

GPCR taste receptor pathways – TRC2

  • Transmitter likely to be ATP acting via ionotropic, heteromeric P2X2/P2X3 receptors

  • Release mechanism of ATP is not via conventional vesicle exocytosis

  • The combined action of increased Ca2+ and membrane depolarization (through sodium influx)activates the complex of calcium homeostasis modulator 1 and 3 (CALHM1/3) channel and pannexin1 channels, thus resulting in the release of the neurotransmitter ATP onto the nerve terminal

    • Semaphorin 7A (Sema 7A) and 3A (Sema 3A) are in close contact with sweet and bitter receptors as they fine-tune sweet and bitter ganglion neurons

    • VFD – venus flytrap domain; CRD – cysteine rich domain form part of the extracellular domain (ECD)

 

From taste cell to gustatory afferent

  • Depolarisation of taste cells release ATP which depolarizes gustatory afferent terminals

    • Via heterotrimeric P2X2/P2X3 purinoceptors

Others involved as modulators include:

  • 5-hydroxytryptamine (5-HT, serotonin)

  • GABA

  • Acetylcholine

  • Noradrenaline (norepinephrine)

  • Glutamate (from gustatory afferent terminals)

 

Summary

  • Tastants are water soluble and there are 5 defined tastes

  • All have specific receptors in the tongue

  • There are 3 different types of taste receptor cells (TRCs) with different transduction mechanisms

  • There are several signals that lead from the taste cell to the gustatory afferent

 

Smell Transduction

Olfaction Overview

  • Olfaction and taste contribute to flavor

  • Food/drink in the mouth can activate taste (gustatory) afferents and olfactory afferents

    • Volatile odorants diffuse into the nasal cavity activating olfactory receptors.

 

Olfactory Epithelium

Located in the roof of the nasal cavity; contains olfactory receptor neurons that turn over continuously (and have receptor cilia in the nasal mucus)

  • Olfactory receptor cells send axons through the cribiform plate to the olfactory bulb.

 

Olfactory Transduction

  • Humans have around 350 olfactory receptors, but can distinguish over 10,000 odours due to a combinatorial code.

    • olfactory receptors detect more than one odorant and each odorant detected by more than one receptor

  • Each receptor has its own preferred odorants

  • All receptors activate Golf to activate adenylate cyclase

    • Cyclic AMP activates cAMP dependent cation channel to depolarize membrane (through sodium and calcium channels) and produce a receptor potential

 

Specificity of olfactory receptor neurons

  • Individual neurons respond to more than one odorant

  • Identification of odorants depends on convergence within pathway

  • Processing involves identity of each olfactory neuron responding

  • Olfactory neuron precursors look like epithelial cells

 

Role of olfactory bulb

  • Very primitive part of brain

  • Processes olfactory signals; individual glomeruli encode one specific odor.

  • Second order olfactory neurons have branching dendritic trees that form glomeruli with terminals of olfactory receptor cells

  • Individual glomeruli encode only one odour

  • Granule cell neurons act as tuning interneurons

Note: adult neurogenesis (creation of new neurons) in the subventricular zone adds new neurons to the olfactory bulb via migration to tune the signal

 

Flavour, a sensation with many transductions

The sensory experience of food and drink:

  • Dominated by smell and taste (but includes texture, appearance, temperature, pain (chilli), fat)

  • Taste: 5 defined tastes

    • Mouth also detects texture (cutaneous mechanoreceptors), fat (free fatty acid receptors, heat, both temperature and chemicals that excite thermal nociceptors (capsaicin activate TRPV1 channels)

  • Smell – humans detect more than 10,000 different odors

 

Summary

  • Both olfaction and taste contribute to flavour as food/drink in mouth can activate both gustatory and olfactory afferents

  • Olfactory receptor cells are neurons and they turn over continuously

  • These neurons send axons through the cribiform plate to the olfactory bulb

  • There are up to 2000 (only about 350 in humans) different olfactory receptor proteins and each as a preferred odorant

  • All receptors activate Golf to activate adenylate cyclase

  • Identification of odorants depends on convergence within a pathway (think about glomeruli)

  • Second order olfactory neurons have branching dendritic trees that from glomeruli with terminals of olfactory receptor cells and individual glomeruli encode only one odour.

  • Granule cell neurons act as tuning interneurons

 

Auditory Transduction

Audition Overview

  • Sound is detected by hair cells in the cochlea, ear is also involved in balance.

  • Involve detection of movement via hair cells in the cochlea or semicircular canals

  • Hair cells are modified epithelial cells, not neurons.

  • Movement is heavily modified by peripheral structures

 

Inside the inner ear – basilar membrane

  • Vibration enters cochlea via the oval window (connects the stapes and the cochlea)

  • Fluid fills the cochlea

  • Round window is pressure relief

  • The organ of Corti sits on the basilar membrane containing hair cells

  • Basilar membrane is tuned to pass high frequency vibrations close to oval window, low frequency vibrations closer to helicotrema

    • Cilia length changes along the basilar membrane to allow this to occur

 

Cross section of the cochlea

Three chambers:

  • Scala vestibule connects to the oval window

    • Vibration enters

  • Scala media were vibration passes through

  • Scala tympani connects to round window

    • Vibration leaves

Other structures:

  • Tectorial membrane (attached to hair cells) and basilar membrane

  • Spinal limbus is bony anchor

  • Inner (less abundant) and outer hair cells (more abundant)

    • Which are innervated

 

Sensory transduction in auditory and vestibular hair cells

  • Cilia of hair cells are connected by tip links

  • Vibration moves cilia opening stretch sensitive K+ channels in the tips of the cilia

  • K+ concentration in scala media is very high, opening K+ channels leads to depolarization

    • Movement in the opposite direction hyperpolarises

  • Depolarisation leads to glutamate release from basal membrane onto afferent terminals (which go to brain) that are also depolarised

 

Extra features

  • Inner hair cells receive terminals from several auditory afferents with different thresholds

  • Individual auditory afferents can contact several outer hair cells

  • Outer hair cells receive efferent input which “tunes” the local stiffness of the transduction system, thereby sharpening the frequency response of the neighbouring inner hair cells

 

Summary

  • Detectors in the ear are not neurons but modified epithelial cells

  • Sensory transduction in auditory and vestibular hair cells are connected by tip links and vibration moves cilia, opening stretch sensitive K channels.

  • Depolarisation leads to glutamate release from basal membrane onto afferent terminals that are also depolarised

  • Inner hair cells receive terminals from several auditory afferents with different thresholds

  • Outer hair cells receive efferent input which ‘tunes’ the local stiffness of the transduction system, hence sharpening the frequency response of neighbouring inner hair cells.