CDS 428/528 Auditory Periphery 2024-10-31 PDF

Unit 3: Auditory Periphery CDS 428/528 2024-10-31 Chris Heffner Class Business Tue, 11/05: In-Class Final Project Workday (recommended: research) Thu, 11/07: Lecture, Review Papers Due Tue, 11/12: Lecture Thu, 11/14: Demo 3 Tue, 11/19: Lecture Thu, 11/21: Lecture, P...

Unit 3: Auditory Periphery CDS 428/528 2024-10-31 Chris Heffner Class Business Tue, 11/05: In-Class Final Project Workday (recommended: research) Thu, 11/07: Lecture, Review Papers Due Tue, 11/12: Lecture Thu, 11/14: Demo 3 Tue, 11/19: Lecture Thu, 11/21: Lecture, Primary Papers Due, Final Project Office Hours Due (schedule now with me or Abbie) Tue, 11/26: Lecture Thu, 11/28: Thanksgiving (no class) Tue, 12/03: Test 3 Thu, 12/05: Travel Day and Final Project Due  Auditory System AuD SLP Knowing machinery of auditory Stroke! Aphasia is often localized system is key for audiology to temporal cortex Your world depends a lot on the Stuttering may involve different mechanisms you’re learning at connections between temporal the beginning of this unit! cortex and basal ganglia In both cases: spoken language is going to be the most common (though not only!) communication mode for your clients For the next three classes, we’re learning how sounds are sensed and perceived Acoustic Properties Frequency/pitch Intensity/loudness Duration/speed Auditory System: External Apparatus Auricle/Pinna: outer ear; “satellite dish” for ear Ear Canal: focuses sound on eardrum/tympanic membrane Tympanic Membrane: converts sound waves into motion of ossicles Auditory System: Middle Ear Ossicles (ear bones): consolidate sound transmission to smaller area, strengthening sound Malleus/hammer Incus/anvil Stapes/stirrup Help to enhance transfer of sound, esp. at higher frequencies Evolved from jawbones – one of the defining characteristics of mammals Auditory System: Cochlea Cochlea: General Organization Sound enters through the oval window Fluid in cochlea vibrates; these vibrations are picked up by hair cells Just below oval window, attached to different tube, is round window --> without this, no vibrations Cochlea itself is 3.5 cm long Cochlea: Tubes Oval window is connected to fluid in scala vestibuli Scala vestibuli reaches apex of cochlea and becomes scala tympani, which comes back down other side of cochlea Fluid filling these is called perilymph Separated by basilar membrane Cochlea: Scala Media Wedged between scala vestibuli and scala tympani is scala media Filled with endolymph Surrounded by membranes on each side: Reissner's membrane with scala vestibuli is rather simple Basilar membrane with scala tympani has complex system of organs on top Home of organ of Corti and tectorial membrane Tubes: Concentrations and Potentials Relative concentration of ions is quite different between High Na+ perilymph and endolymph Potassium: high in endolymph, low in perilymph Low Na+ Sodium: high in perilymph, low in endolymph This leads to potential difference of +80 mV between these fluids (endo > peri) High Na+ Basilar Membrane Key to understanding frequency information Waves propagate down length of cochlea, like waves on a beach Membrane is: Narrow, thick, and stiff at base = high frequencies Wide, thin, and floppy at tip = lower frequencies Membrane acts a bit like a "prism" or Fourier transform Questions? Cochlea: Organ of Corti Basilar membrane and scala media are home of organ of Corti Organ of Corti causes sensation Vibrations against basilar membrane lead hair cells to scrape against tectorial membrane These scrapings lead channels to open that lead to hearing Cochlea: Inner Hair Cells 3000-4000 per cochlea Mammals generally cannot regrow hair cells (but birds and fish can) When hair cells scrape against tectorial membrane, they open and close potassium channels Synapse with up to 10 rapidly conducting neurons Inner Hair Cell Properties Very fast and accurate Activated by ripples the size of a virus May release neurotransmitters in 10 billionths of a second Not neurons, but can release neurotransmitters Generate receptor potential changes when stimulated Transduction: Beginnings Acoustic energy pushes against the cochlea Energy → wave in cochlea Wave → vibration in organ of Corti Vibration in organ of Corti → shearing of the stereocilia against the tectorial membrane Stereocilia: Organization The actual transducing part of the hair cells are the stereocilia Located on top of hair cells Project into tectorial membrane Varied in height Connected through tip links (almost like springs) Tip links connected to mechanically- gated K+ channels Stretching → release or prevention of release Cycles of depolarization and hyperpolarization Transduction: Signals Bending the stereocilia leads to K+ channels opening within the stereocilia Due to electrical gradient, K+ wants to enter cell 145 mV electrical difference! This acts against concentration gradient of K+ Transduction: Next Steps Change in potential due to K+ leads to opening of Ca2+ channels Calcium enters cell → transmission of glutamate in proportion to stimulus strength Glutamate release → action potentials in auditory nerve Much faster than any non- electrical solution! Tonotopy Auditory Transduction Video https://www.youtube.com/watch?v=lDXVZOU_f_E Questions? Auditory Nerve: Signals Connections to brain 95% inner hair cells Each auditory nerve fiber touches just one IHC Single IHC sends signals to many auditory nerve fibers IHCs are clearly involved in auditory perception 5% outer hair cells Each auditory nerve fiber may touch 5 to 100 OHCs Likely not very involved in auditory perception Connections to cochlea: most go to outer hair cells Auditory Nerve: Organization Fibers of auditory nerve are tonotopically arranged Each fiber has "characteristic frequency response", with distribution of responses across frequencies Fibers tend to "accept" other frequencies nearby as well as their preferred frequency Path of cochlear nerve Connects with: Vestibular nerve Parts of facial nerve Labyrinthine artery Makes its way to dorsal and ventral cochlear nuclei Auditory Nerve: Characteristic Frequencies Auditory Nerve: Phase Locking Besides tonotopy, brain has other ways to pick up on frequencies One of these: phase locking Neuron fires only at certain times the sound wave hits a certain point (a peak or a valley) Allows auditory nerve to pick up on frequencies above certain levels with higher acuity Auditory Nerve: Intensity On average, nerve fibers increase firing linearly with intensity However, at high intensities, it tends to become non-linear Some cells increase radically more Some cells increase much less Cochlea: Outer Hair Cells Organ of Corti has both inner and outer hair cells; organization is usually 1:3 ratio Inner hair cells directly turn sounds to electrical signals Outer hair cells work to amplify quieter sounds, especially at higher frequencies Outer hair cells are found only in mammals Synapse with unmyelinated neurons OHCs: Inputs and Outputs Inputs MOTOR inputs from central nervous system Primarily from superior olive, which we'll learn about soon Outputs Not many that are sensory Related to otoacoustic emissions OHCs: Function Seem to be important for sound transduction Amplify resolution of low-amplitude signals Do this mostly through motion Changes in configuration or shape Expansion/contraction, like muscles OHC can push together and pull apart the membranes, which influences organ of Corti These adjustments can be fast to help with hearing OHCs: Video https://www.youtube.com/watch?v=edVHqFyuIws Questions?