07 - Auditory, Olfactory, Taste

external ear

concentrates sounds

• shape filters spectrum of sounds we hear

• ridges filter frequencies → enables elevation localization of sound source (above vs below)

  • blocking pinna (ear shape) → poor elevation detection

middle ear

converts sound waves in air into mechanical vibrations along ossicles

• collects and concentrates force from sound onto ossicles

• amplifies sound into cochlea

• protection from environment

cochlea

transduction of auditory frequency in basilar membrane of cochlea

• pitch = frequency

• sound creates traveling wave, which peaks at frequency-specific location

  • high frequency → detected at base

  • low frequency → detected at apex

organ of Corti

structure in cochlea of inner ear that produces nerve impulses in response to sound vibrations

• inner hair cell → sends information to brain along afferent fibers

• outer hair cell → amplifies sound; receives signals from brain through efferent fibers

  • contains prestin motor protein to amplify sensitivity of sound

  • changes length in response to stimulation

  • brain can modulate outer hair cells to adjust sensitivity

noise-induced hearing loss

chronic exposure to one frequency

• exposure to intense sound at one frequency

• damages hair cells only at that location in cochlea

hair cell transduction

traveling waves cause shearing forces on hair cells

• stereocilia deflection

• ion channel opening

• NT release onto auditory nerve

taste

helps animals seek necessary foods and reject harmful foods; dependent on concentration and context

• sweet (energy) → sugars, alcohols, glycols, aldehydes

• bitter (potential toxins) → long chain organic substances, alkaloids

• sour (acidity) → pH

• salty (electrolytes) → salts

• umami (amino acids) → L-glutamate

taste bud anatomy

taste buds sit in invaginations of papillae

• taste receptor cells synapse onto afferent nerves

• microvilli contact oral cavity

• basal cell located at base for cell regeneration

taste transduction

• salty, sour → ionotropic

  • direct ion entry causes depolarization

  • leads to Ca²⁺ influx for NT release

  • increase in afferent nerve firing

• sweet, bitter, umami → metabotropic

  • G-protein coupled receptor that uses second messengers

  • causes Ca²⁺ release in cell for NT release

  • bypasses depolarization pathway

TRP channels

temperature receptors

• capsaicin → activates heat receptors

• menthol → activates cold receptors

tongue regions

different amounts of taste sensitivities in different papillae

• bitter → back of tongue

• salty, sweet → front of tongue

• sour → sides of tongue

• umami → widespread throughout tongue

sensory innervation of tongue

• bitter → CNIX (glossopharyngeal)

  • posterior 1/3 of tongue

  • triggers gag reflex as protective function

• other tastes → CNVII (facial)

  • anterior 2/3 of tongue

gustatory cortex

taste represented at bottom of brain

• different parts of brain sensitive to different kinds of tastes

olfactory receptor cells

located in nasal epithelium

• cilia interface with nasal cavity

• axons pass through cribriform plate

  • cribriform plate → bony border between nasal cavity and brain

• synapse in olfactory bulb at glomeruli

• glomeruli integrates information from different olfactory receptor cells

  • same receptor → same glomerulus

  • each odor activates pattern of glomeruli

  • smell = pattern recognition

olfactory transduction

all olfactory receptors are metabotropic (GPCR)

• no direct ion channels

• use second messengers for Ca²⁺ influx

odorant receptor protein

receptors located on cilia of olfactory receptor cells

• each receptor is structurally similar but has small structural differences that changes what molecules they are sensitive to

  • conserved vs variable amino acids

  • mutations further change sensitivities

• each olfactory receptor neuron expresses one type of odorant receptor

  • specific to one class or type of odor, but sensitive to a variety of compounds

central olfactory pathways

olfactory signals do not pass through thalamus

• medial olfactory area → projects to limbic system

  • senses can directly affect emotion

• lateral olfactory area → projects widely to other cortical areas and hippocampus

  • senses can trigger memories

orbitofrontal cortex

region of brain that receives taste and smell

• experience of taste influenced by sense of smell

• creates flavor