COMD 4333

Auditory Processing

Auditory System Function & Energy Transformations

transform sound waves → action potentials that code frequency, intensity, and other characteristics of the sound waves

Sound Waves

1. mechanical energy (movement)

2. hydraulic energy (fluid)

3. electrical energy (nerve impulses)

4. neurochemical

   1. action potentials

   2. brain pathway

Outer Ear - Pinna, EAC, TM

Pinna: focuses sound energy into external auditory meatus

• leads to movement of tympanic membrane (ear drum)

Middle Ear

air-filled cavity

• Ossicles (malleus, incus, stapes) move in response to movement of tympanic membrane

  • Eustachian Tube: open into nasopharynx

    • equalizes pressure on both sides of tympanic membrane

• Oval Window: entry into cochlea

  • covered by Stapes Footplate

• vs. round window: other end of cochlea

Inner Ear - semicircular canals, vestibule, cochlea

bony labyrinth: bony cavities within temporal bone

• membranous labyrinth housed within the bony labyrinth

1. Semicircular Canals (vestibular)

2. Vestibule (vestibular)

3. Cochlea (auditory)

Cochlea

inner ear part, 3 cavities

1. scala vestibuli

   1. superior

   2. Resiner membrane

   3. Perilymph

2. scala media

   1. cochlear duct

   2. Basilar membrane

   3. Endolymph: HIGH K+ concentration HIGH ON K

3. scala tympani

   1. Perilymph

vestibuli, media, tympani

VMT victory my town

RB

Basilar Membrane

• base: narrow and stiff

• apex: broad and less stiff

• basilar membrane arranged by FREQUENCY

• different sound frequencies displace different regions of basilar membrane

  • base: high frequencies

  • apex: low frequencies

• traveling wave created by movement of stapes in oval window

Organ of Corti

Movement of basilar membrane displaces organ of corti

• 1 row Inner hair cells - sensory

• 3 rows Outer hair cells – sensory & motor

• Tips of hair cells close to tectorial membrane (tectorial membrane sits on top of organ of corti; organ of corti sits on top of basilar membrane)

• 4,500 IHC – each synapses with multiple spiral ganglion neurons

  • INNER HAIR CELLS SYNAPSE WITH MULTIPLE

• 20,000 OHC – multiple OHC synapse onto each spiral ganglion neuron

  • OUTER HAIR CELLS SYNAPSE ONE AT A TIME

Hair Cells

Stereocilia

• Extend from hair cells

• End in proximity to tectorial membrane

• Arranged by length

  • Kinocilium being the tallest

• stereocilia Connected by tip links

Action Potentials

• Cilia bend toward kinocilium → tip links open → influx of K+ ions

• K+ moves INTO hair cell leading to depolarization

• Opens calcium channels → lets calcium INTO hair cells

• Triggers RELEASE of neurotransmitter (glutamate)

  • Glutamate bind to receptors on Spiral Ganglion Neurons

  • cell bodies in spiral ganglion outside of the cochlea, axons create cochlear portion of CN VIII

Cilia bend toward kinocilium, tip links open, K moves in → depolarization, P move in hair cells, neurotransmitter released, it binds to spiral ganglion neuron

• Cilia bend AWAY from kinocilium → tip links close → no influx (no further response from hair cell)

Efferent Signals to Outer Hair Cells

stimulation → activate microtubules, lengthen OHC cell bodies

• electromotility - conversion of electrical energy into mechanical forces

• increase the effect of basilar membrane movement & displacement of cilia

• Enhance detection of low intensity sounds

  • Improves detection of sound in noise

  • 100-fold increase in effect of actual basilar membrane movement

Coding Frequency

Frequency = pitch

1. Place coding: Hair cell location on basilar membrane

   1. Base = high frequency; apex = low frequency

2. Characteristic frequencies: hair cells respond best (more action potentials) to specific sound frequencies

3. Tonotopic organization

Coding Intensity

intensity = loudness

1. number of action potentials

2. number of neurons sending action potentials

Coding Location

localization of sound

1. Interaural time and intensity differences

Sound wave travels around the head; reaches second ear later and a bit softer

Auditory Pathway Summary

1. Spiral ganglion axons → brainstem, synapse in dorsal & ventral cochlear nuclei

2. Cochlear nuclei → superior olivary complex (SOC)

3. SOC extend through lateral lemniscus

4. Synapse in inferior colliculus

5. Inferior colliculus → Medial Geniculate Nucleus (MGN in Thalamus)

6. MGN → Heschl’s gyrus/A1

   1. A1 = posterior superior temporal gyrus

C-SLIMA

1. Cochlear Nuclei

2. Superior Olivary Complex

3. through Lateral Lemniscus

4. Inferior Colliculus

5. Medial Geniculate Body

6. A1 / Heschl’s Gyrus

cochlear nucleus, superior olivary complex, inferior colliculus, medial geniculate body, A1

Cochlear Nuclei

• Ipsilateral, monaural processing in cochlear nuclei

• 80% cross over to contralateral SOC

• All processing is binaural beyond cochlear nuclei

IPSILATERAL PROCESSING

Superior Olivary Complex

• Receives information from both ears

• Localizes sound based on time delay and intensity difference

SOC = LOCALIZATION

Lateral Lemniscus and Inferior Colliculus

Lateral Lemniscus

• Tract between SOC and Inferior colliculus

  • Not a synapse

  • connects SOC and inferior colliculus SLIMA

Inferior Colliculus

• Some signals sent from inferior colliculus to superior colliculus for auditory-visual integration

Medial geniculate nucleus (MGN) and A1/Heschl’s Gyrus

Medial geniculate nucleus (MGN)

• In thalamus

• From ipsilateral inferior colliculus

A1 / Heschl’s gyrus

• From ipsilateral MGN

only cross over occurs after cochlear nuclei

Thalamic Nuclei called Medial geniculate nucleus (MGN)

Nuclei:

• Medial geniculate nucleus

Connections to:

• Brainstem auditory areas

• Superior temporal gyrus

• Heschl’s gyrus

Functions:

• Auditory processing

Heschl’s Gyrus

• heschl’s gyurs = Primary auditory cortex (A1)

• Dorsal surface of superior temporal gyrus

• Tonotopic organization

Auditory System Summary

1. Sound waves → Mechanical energy (movement)

   1. Vibration/pressure from sound waves moves tympanic membrane and ossicles

2. Mechanical energy → Hydraulic energy (fluid)

   1. Movement of tympanic membrane & ossicles causes movement of fluid in cochlea

3. Hydraulic energy → Electrical energy (nerve impulses)

   1. Fluid movement causes movement of hair cells in cochlea

4. Electrical energy → Action potential

   1. Hair cells release glutamate, exciting the auditory nerve (CN VIII)

● Brain Pathway

○ CN VIII → cochlear nucleus → superior olivary complex

→ lateral lemniscus → inferior colliculus → medial

geniculate body → auditory cortex (Heschl’s gyrus)

Hearing Loss

Conductive hearing loss

• Problem with conduction of sound waves in outer/middle ear

• Typically treatable

• Causes: Ear infection, ear wax, damage to ossicles

Sensorineural hearing loss

• Damage to sensory mechanism (cochlea – hair cells) or neural pathway

Causes:

• Noise exposure

• Acoustic neuroma and/or vestibular schwannoma

  • Tumor in inner ear, leading to hearing loss and/or vestibular dysfunction

• Presbycusis: Age related hearing loss

Cortical Damage

• fluent aphasia: unilateral LEFT hemisphere damage

  • Wernicke’s Area

  • impaired comprehension, jargon, empty speech

• receptive aprosodia: unilateral RIGHT hemisphere damage

  • impairments in interpreting and/or producing prosody

Broca’s = Broken Speech (trouble producing speech, non-fluent, can comprehend fine) and Wernicke’s = Word Salad (fluent but nonsensical, trouble understanding).

Cortical Deafness and Pure Word Deafness

Cortical Deafness

• Bilateral damage to Heschl’s gyrus

• Inability to hear despite intact peripheral auditory system & pathways to cortex

• Unilateral damage to Heschl’s gyrus has limited effects

a rare form of central hearing loss caused by damage to the brain's primary auditory cortex, usually from bilateral strokes or lesions in the temporal lobes. Patients appear profoundly deaf, unable to hear or distinguish sounds, yet have intact peripheral ears. It is characterized by severe auditory processing failures

Pure word deafness

• Damage to white matter between primary and association areas

• Speech sounds like noise; uninterpretable

• Preserved reading, writing, speaking; intact auditory perception of noises/environmental sounds

Vestibular System

Structures in the inner ear

• Semicircular canals

• Utricle and saccule

• Vestibular nuclei in brainstem

Projections to:

○ Superior Colliculus

○ Reticular formation

○ Cerebellum

○ Spinal Cord

○ Cortex

3 Semicircular Canals and Vestibule

3 Semicircular Canals

• At right angles (X, Y, Z)

  • Anterior

  • posterior

  • lateral

• Ampullae at base of each canal

• Responsible for rotation and angular acceleration

SC - RA

Vestibule

• otolith organs

  • Utricle

  • Saccule

• responsible for linear acceleration

V - L

Semicircular Canals

• ampulla: enlarged area at base of each semicircular canal

  • crista: sensory organ within the ampulla

    • crista contains hair cells

• cupula: gelatinous structure encasing cilia

Ampulla Crista

• movement of head causes → movement of endolymph in semicircular canals causes → movement of cilia TOWARD kinocilium

  1. movement of cilia toward kinocilium

  2. K+ channels open

  3. depolarization

  4. calcium channels open

  5. triggers release of glutamate

  6. glutamate binds to receptors on scarpa ganglion neurons (aka vestibular portion of CN VIII) creating EPSPs

Movement of cilia AWAY from kinocilium → closing of K+ channels → hyperpolarization & no signal

Vestibule Structures

Hair cells embedded in macula (sensory receptor patch found in the vestibular system (utricle and saccule) of the inner ear, crucial for detecting linear acceleration, gravity, and head tilt.)

• Cilia embedded in gelatinous structure (otolithic membrane)

• Otoconia: calcium carbonate crystals on surface of otolithic membrane

• Otolithic organs:

  • Utricle (horizontal) UH

  • Saccule (vertical) SV

Movement of head → shifting of otolithic membrane

• Enhanced by weight of otoconia

• Cilia bend toward kinocilium → open K+ channels → depolarization → open Ca2+ channels → release of neurotransmitter

• Utricle positioned horizontally

• Saccule positioned vertically

• Allows us to detect linear movements in these planes

Vestibular Nerve and Nuclei

Vestibular nerve

• Cell bodies in Scarpa’s ganglion (vestibular potion of CN VIII)

• Join with auditory nerve fibers

• Synapse in vestibular nuclei in brainstem

Vestibular Projections

from vestibular nuclei (in brainstem) to

1. superior colliculus in midbrain

   1. eye movement, head/eye position, visual fixation with head movement

2. cerebellum

   1. vestibulocerebellum

   2. coordinates movement and reflexes, maintains balance

3. spinal cord

   1. integrated connections for balance, equilibrium, proprioception

4. reticular formation

   1. visceral-autonomic activities

   2. cause of sea-sickness/motion sickness

Olfaction and Gustation Overview

• chemical senses: smell and taste

  • chemoreceptors

• closely related

  • combine to form our perception of different flavors

Olfactory System

• Olfactory Epithelium

  • Superior surface of nasal cavity

• Olfactory Neurons

  • Continually regenerated

  • 4-8 week lifecycle

• Supporting Cells

  • Secrete mucous

  • Replaced every 10 minutes

olfactory epithelium, olfactory neurons, supporting cells

Odors

Odorants: chemical compounds

• Dissolve in mucous and bind to olfactory neuron cilia

• Humans have 350 odorant receptors

  • Distinct smells identified by population coding

    • Patterns of action potentials from groups of olfactory neurons

• Combine to form over 1 trillion Specific odors

Olfactory Pathway

• 1st order neurons: in olfactory epithelium

  • odorants bind to receptors

  • triggers cascading process to depolarize cell

    • axons = olfactory nerve (CN I)

  • synapse onto glomeruli in olfactory bulb

• 2nd order neurons: in olfactory bulb

  • travel down olfactory tract

  • axons extend to olfactory cortex

olfactory epithelium → olfactory bulb down oflactory tract extend to olfactory cortex

Olfaction

• action potentials stop when odors diffuse or enzymes destroy odorants

• olfactory neurons habituate

  • stop responding after time

Olfactory Cortex

Olfactory cortex made of 3 parts:

• Piriform cortex

• perirhinal cortex

• entorhinal cortex

• Smell is the only sense that does NOT make a stop in the thalamus before reaching the sensory cortex

  • Though there are branches that connect to mediodorsal nucleus after processing

Olfactory Processing

connections to:

1. Orbitofrontal cortex

   1. Odor discrimination

   2. Impacts motivation, emotion, memory

2. Hypothalamus

   1. Appetite, eating/drinking

3. Limbic system (amygdala, cingulate gyrus)

   1. Emotion & motivation

4. Hippocampus

   1. Memory

orbitofrontal cortex, hypothalamus, limbic system, hippocampus

Olfactory Impairments

• anosmia: loss of sense of smell

• conductive anosmia: caused by blockage of transmission

• sensorineural anosmia: caused by damage to neurons/pathways involved in smell

  • TBI, AD, PD, healthy aging

• Hyposmia: reduced sensitivity to odors

• Dysosmia: alterations in olfactory perception

  • Parosmia: distorted perception

  • Phantosmia: olfactory hallucination

    • Cacosmia: perception of foul smell

Gustation: Sense of Taste

5 basic tastes

• Bitter

• sweet

• sour

• salty

• umami

• Taste perception is a combination of taste, smell, temperature, and texture

  • Influenced by level of hunger and cognitive expectations

  • Cortical responses differ based on viscosity, temperature, etc. even if taste is held constant

Taste Receptors

• papillae

  • Each houses hundreds of taste buds, which house 50-150 taste receptors

• Most papillae located on on tongue

  • Some in pharynx, palate, epiglottis

papillae, inside is taste buds, inside is taste receptors

Tastants (chemicals)

chemicals that stimulate taste receptors

• Taste cells respond to multiple tastants but are tuned to respond best to specific tastes

• Specific flavors created by patterns of action potential

Gustatory Pathway ANTERIOR INSULA

1st order neuron

• receptors (dendrites) receive signals from taste receptors

• cell bodies in CN ganglia

  • facial (CN VII): anterior 2/3 of tongue

  • Glossopharyngeal (CN IX): Posterior 1/3 of tongue

  • Vagus (CN X): Pharynx

• Axons enter medulla and synapse in gustatory nucleus

2nd order neuron:

• Cell bodies in gustatory nucleus

• Most extend to VPM

• Others → brainstem nuclei (swallowing, digestion, salivation, respiration, gagging, etc.)

3rd order neuron:

• Cell bodies in VPM go to:

  • Anterior insula (gustatory cortex/G1)

  • Inferior/orbitofrontal cortex

  • Hypothalamus

  • Limbic system

Gustatory Impairments

Taste cells regenerate every 2 weeks, but cranial nerves do not

• Ageusia: Loss of sense of taste

• Dysgeusia: reduced/altered sense of taste

• Impairments often lead to loss of appetite and weight loss

-usia