Hearing

The auditory system analyzes sounds according to:

  • Frequency

  • Amplitude



Sound is measured in dB: dB SPL = 20 log10(Po/Pr)



 Sound

  • Intensity and frequency + time

  • Complex sound: has the same pitch of 500 hertz, mixture of A,B,C,D



Acoustics: 

  • Period: how much time between each wave



hearing 

  • Some drugs kill high frequency hearing




SPL: sound pressure level

Sd SPL: the level of sound intensity or pressure




Outer ear: see it + ear canal, ends at tympanic membrane

  • Pinna or auricle

    • Functions: Funnel sound into the ear canal

      • Amplification

      • Protection

      • Sound localization (in the vertical plane)

        • Change the sound change the spectrum, brain hears sound of different qualities and the pina provides this information to the brain

          • Spectral shape Pu

    • Structure: 

      • Lobe

      • Tragus: tip of ear

      • Cunghuck (??)

      • Helix

      • Anti helix

      • Anti Tragus

  • External Auditory Meatus (EAM): canal to tympanic membrane (TM)

Middle ear: 

Functions: impedance match (overcome air-water transmission problem), equalize air pressure

  • Tympanic membrane (border between outer and middle ear)

  • Ossicle (bones): malleus, incus, stapes (smallest)

    • Conducts mechanical sound energy from the ™ to the inner ear

  • Muscles: tensor tympani, stapedius

    • When two muscles contract, make sound transmission more difficult, protect us when there is a very loud sound

    • Acoustic reflex, make sound transmission more difficult to protect our hearing

      • Facial never (CN 7) controls stapedius muscle

      • CN5 innervates the tensor tympani



  • Pharyngotoymanic tube: 

    • Tube in the posterior- inferior portion of the ME space leads to the nasopharynx

      • Maintains equal pressure between the ME and the atmosphere

      • Normally closed

      • Opens periodically

      • Hold nose and blow to open constriction

      • Negative pressure causes: ____, shown to cause speech language issues

  • Functions: there to allow us to hear

    • Sound is vibration in the ear, the inner ear is underwater, areas amplification (™ is big) and a little oval window called the footplate (same size/area of stapes

      • 20:1 amplification

      • Little weight can list up a heavy weight

        • Both processes called impedance match (overcomes air-water transmission problems)

        • Protects against acoustic trauma 

  • Why do children get more ear infections

    • Middle ear is flatter, wider, and shorter

    • While adults are longer and so it takes a while for bacteria to find its way in

    • Different diseases: easier to treat when it is fluid + more clear

  • 30 dB hearing loss from air to water

    • Moderate hearing loss is loss within this range

  • Oval window: entry to inner ear

  • Eustachian tube: path to nasopharynx, tiny opening allowing air to travel back and forth (providing fresh air), take waste out including fluid (problem with tube results in ___ medior, issue in young children or babies)

  • Tympanic cavity: majority of it is filed with air

Inner ear:__, nerve carries information to the brain

  • Where sense of hearing happens, sense of motion and position (everything up until this point has just been vibrations)

    • Semicircular canals

    • Vestibule 

  • Cochlea: 

    • Snail shaped (2 ¾ turn) 

      • Fluid filled space

      • 35 mm in length (if unrolled)

    • 3 chambers inside: scalli

      • Scala vestibuli →  (filled with perilymph)

      • Cochlear duct (scalamedia) → (filled with perilymph)

      • Scala tympani→ (filled with endolymph)

    • Organ of corti (most important sensory structure for hearing)

      • Sits on the basilar membrane

      • 2 types of sensory cells (N= 15,000)

        • Inner hair cells

          • 90% info goes to

          • 1 row

        • Outer hair cells 

          • 10% info goes to

          • 3 rows

        • Supporting cells

          • Forms the structures allows hair cells to sit on top

      • Sterior sellair (hair bundles)

      •  Genatinus membrane

      • Tectorial membrane covers atop

      • Shearing force: ___ path

HIGH FREQUENCY WAVES AT START, LOW FREQUENCY WAVES IN MIDDLE



Basilar membrane displacement

  • Base

  • Middle of the turn

  • Apex



Coding of frequency and intensity

  • Frequency

    • Place theory: hair cells at different locations along basilar membrane respond to different frequencies

    • Temporal theory: relative timing of impulses in individual neurons conveys frequency information

      • Action potentials convey frequency information

  • Amplitude

    • Displacement of basilar membrane is converted to neural firing rate




Transduction - general of hearing in cochlea

  • Stapes moves against oval window; causes cochlear fluid to vibrate

  • Basilar membrane vibrates in response

    • Thin, stiff base, responds to higher frequencies

    • Wide, flaccid apex responds to lower frequencies

  • Movement of basilar membrane displaces organs of Corti toward tectorial membrane

  • Stereocilia of hair cells shear against tectorial membrane,..., and neurotransmitters release

  • Auditory nerve endings synapse with hair cells and pick up the neurotransmitters. Nerve impulses are generated and conducted to the brain.



Apex = low frequency 



Transduction:

  • Changing to mechanical vibrations to electricity, uses electrical signals for the what the brain hears: 



CNS
- MGN/MGB: mediate gen → auditory cortex: in the temporal lobe

  • 4 stops, what they are + where they are 

  • (hearing is not happening until it get to the sensory cortex, we hear with our brains

  • SOC: 

  • DRAW OUT PATHWAY



Transverse temporal gyri: Heschl’s gyri



Central auditory system:

  • Anatomy

    • From auditory division 8th nerve to auditory cortex

    • Multiple stops in the brainstem

    • Information travels up bilaterally

  • Physiology

    • Processing of acoustic input

      • ‘Speech perception’

    • Binaural processing

      • Localization

      • Summation

      • Segregation of speech from noise



Hearing loss:

  • 360 million people world-wide have disabling hearing loss. 

  • In young children, language learning is difficult because normal auditory input is missing

  • A majority of hearing disorders stem from hair cell damage

  • People with hearing loss can benefit from hearing devices such as hearing aids, assistive devices, and cochlear implants, and from captioning, sign language training, education and social support.



What is the units of frequency of sound?

  • Frequency: 500 hertz,

  • Intensity: Dbspl (or dB), sound pressure levels

What are the functions of the outer ear?

  • Protection

  • Amplification

  • Sound localization (in the horrizonal plane?, two ears)

What are the functions of the middle ear?

  • Impedance match

What are three tiny bones that make up the ossicular chain?

  • Mdious sinus and stapes

The middle ear provides approximately 30 dB gain to sound

Know the anatomic terms:

  • Cochlea, basilar membrane, tectorial membrane, organ of corti, hair cells, stereocilia, synaps, auditory nerve, brainstem, auditory cortex, (three scalli + fluid in the space)

Explain how we hear (transduction)

  •  Without hair cells we have hearing loss

Advanced topics: how do we hear sounds of different frequencies

  • Two theories: place theory and ___ theory

    • Details of two theories