Sound And The Structure Of The Auditory System Notes

Lecture 35, 38 & 40

Sound and Structure of the Auditory System

• Properties and perception of sound

  • Amplitude or ‘loudness’ (dB)

  • Frequency or ‘pitch’ (Hz)

• Conductive pathway for sound

  • Conductive- outer ear

    • External auditory canal

    • Tympanic membrane (eardrum)

  • Middle ear

    • Malleus; Incus; Stapes (in the oval window)

    • Round window (covered by secondary tympanic membrane)

  • Sensory-Inner ear

    • Semicircular canal (vestibular branch of VIII)

    • Cochlea (cochlea branch of VIII)

    • Auditory tube

• The ear- structure and function

  • Middle ear function- impedance matching

    • More energy required to move fluid than air

    • Ossicles have lever action

    • Eardrum to round window surface area= 20:1 in humans

    • Results in 20x pressure, enough to move fluid

    • Without impedance matching only 0.1% energy transfer

  • Middle ear- ossicular reflex

    • Protective reflex against loud sounds (>70dB)

    • Stiffens lever reducing energy conduction (done by tensor tympani muscle & stapes muscle)

    • No role above 1 or 2kHz in man

    • Helps discrimination where lots of low freq. Noise

    • Ineffective for impulse noise (50-100ms delay)

  • Inner ear

    • 1, 7 & 8= Vestibular system (semi-circular canals)

    • 10= Oval window moved by ossicles

    • 11= Round window

    • 13= Cochlea

    • 5= Basilar membrane inside cochlea

  • Cochlea

    • Spiral structure

    • Fluid filled, compartmentalised

• Cochlear fluids

  • Perilymph

    • Resembles CSF

    • Bathes cell bodies of organ of Corti

    • K 7mM; Na 140mM; 0mV potential

  • Endolymph

    • Resembles ICF

    • Sealed in tight compartment

    • Bathes surface of organ of Conti

    • Maintained by stria vascularis

    • K 145mM; Na 1mM; +80mV potential

• In mammals, organ of Corti=organ of hearing

  • Located on flexible basilar membrane

  • Inner & outer hair cells- sensory

  • Spiral ganglion nerve cells

  • Associated with supporting and non-sensory cells

  • Hair cells held rigidly

• The specialist hair cells

  • Hair cells- sound to nerve impulse

    • Inner (IHCs- 3,500)) and Outer (OHCs- 12,000) hair cells

    • Afferent & efferent connections

    • Sensitive to damage and disease (not replaced in mammals)

    • Mechano-electrical transduction

  • The sensory hair cells

• Stereocilia (hair bundle)

  • Mechanosensing organelles ofhair cells

  • Respond to fluid motion for various functions (hearing and balance)

  • Turn the fluid pressure and other mechanical stimuli into electric stimuli via the many microvilli that make up stereocilia rods

  • Lined up in theOrgan of Cortiwithin thecochleaof the inner ear

  • Transform the mechanical energy of sound waves into electrical signals for hair cells, leads to an excitation of theauditory nerve

  • Cytoplasm with embedded bundles of cross-linkedactinfilaments

    • Actin anchor to terminal web & top of cell membrane

  • Myosin, fimbrins and actin

• The basilar membrane

  • Maximum displacement of the basilar membrane occurs at different positions depending on frequency of sound

  • Movement of stapes displaces fluid in scala vestibuli

    • Travelling wave

    • Displaces basilar membrane

• Stimulus transduction

  • Bundle stimulation opens ion channels

    • Mechanism

      • Sound wave moves hair cells

      • Tip links-gated springs are attached to channels of adjacent hair cells, and pulls them open

      • Mechanically-gated transducer channels open

      • K ions rush in from the Endolymph, then Ca rushes in too, making the hair cell more positive

    • Movements are tiny

      • Saturation at 20nm

      • Perceptible sound at 0.3nm

  • Hair cell transduction

    • Hair cells move, channels open

    • K & Ca rush into hair cell from Endolymph (+80mV)=Depolarisation (-55mV)

    • This depolarisation causes Ca to rush in leading to NT release into nerves

    • Hyperpolarisation then takes place in the cell

  • Hair cells- ion channels and membrane proteins

    • Inner hair cells

      • Potential= -65 to -70mV

      • Most afferent innervation

      • Channels

        • Out= IK,s; IK,f (Ca)

        • In= L-type CA; I_T

    • Outer hair cells

      • Potential=-75mV

      • Mostly efferent innervation

      • No/less Ca channels=less transmitter release

      • Prestin; AChR

      • Channels

        • In= I_T

        • Out= IK(Ca); IKn

  • Hair cell innervation and function

    • OHC

      • Type II afferent (5%)

      • 1:50 neurone to OHCs

      • Thin, unmyelinated

    • IHC

      • Type I afferent (90%)

      • 1:1 neurone to IHCs

      • Each IHC many neurones

      • Thick, myelinated

  • Differences in hair cell innervation

    • IHC

      • Mainly afferent nerve connections via SGN

      • Efferent connections are postsynaptic

      • IHC= sound encoders

    • OHC

      • Mainly efferent connections from brain stem

Neural encoding in the auditory system

• Encoding frequency

  • Stimulus, I_T, receptor potential and firing

    • Rest= some I_T

    • +ve stimulus= increased I_T

    • -ve stimulus= decreased I_T

  • Encoding frequency

    • Low frequencies

      • Sound wave come along

      • Causes bundle movement, back and forth (decreased/increased I_T)

      • Receptor potential voltage (mV) goes up and down as this happens

      • Auditory nerve impulses with change in potential= ‘PHASE-LOCKED’

      • Low frequency sounds can be encoded by phase-locking up to ~1kHz

        • Receptor potential and nerve can follow stimulus

    • High frequencies

      • Receptor potential voltage cannot keep oscillating as frequency too high, so stays up

      • Auditory nerve impulses non ‘phase-locked’ (can’t keep up)

• Tonotopy

  • General

    • From basilar membrane

    • Spatial arrangement of sound frequency

    • Sound waves propagate along basilar membrane

    • Membrane has regional variations in fibrous structure

    • Maximal displacement on frequency of stimulus

    • Base= Thin, narrow & stiff (high frequency); Apex= Wide & floppy (low)

  • From basilar membrane to brain

    • Tonotopic map maintained at all relay positions on way to auditory cortex

    • Stimulation of hair cells dependent on membrane displacement

    • Tonotopic map gives spectral analysis of sounds

    • Isolated bands of cortex responding to different frequencies

    • Spatial maps=common feature of sensory systems

• Cochlear amplifier

  • OHCs are active

    • Depolarisation activates PRESTIN (motor protein) fast

    • Rigidly held cells contract, amplifying basilar membrane movement

      • When they shorten=basilar membrane narrower and stiffer= amplified movement

    • Gain modulated by efferent system...

Buy the full version of these notes or essay plans and more in our Neurology Notes.