0. Sensory System in Brief

Sensory System such as the visual, auditory and olfactory systems provide the CNS with information about the environment (external information)

The CNS then processes and integrates the information and transforms it into motor commands that generate appropriate behaviour

The immediate function of sensory systems is to transduce physical energy into bioelectrical signals that CNS understands

1. Peripheral Auditory Mechanisms

Nature of Sound

Sound is a mechanical energy transmitted by longitudinal radiations of the medium and shows periodic pattern of compression and rarefaction of air molecules

Sound wave propagates at $340ms^{-1}$ through air

The difference between the pressure of molecules in zones of compression and rarefaction determines the wave’s amplitude

The frequency of vibration of the sound source determines the pitch

<aside> 💡 -aural 耳的 Interaural 耳间的 Binaural 双耳的

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Anatomy of the Ear

Sound Wave Entering External Auditory Meatus

• Auricle collects and funnel sound waves into External Auditory Meatus
• Sound reaches Tympanic Membrane
  • Stretched across the canal, separating outer and middle ears
  • Bows inwards during compression
  • Returns to rest at rarefaction
  • Vibrates at the same frequency as the sound wave
  • Distance moved depends on the pressure hence measures amplitude
• Middle Ear Cavity is an air filled cavity in the temporal lobe bone of the skull
  • Exposed to atmospheric air pressure through the Auditory (Eustachian) Tube which connects middle air to pharynx

Transmission of Sound Energy from Tympanic Membrane through Middle Ear to the Inner Ear

• The Inner Ear is called Cochlea
  • Fluid-filled, spiral-shaped passage in the temporal bone
• Liquid is more difficult to move than air so sound pressure must be amplified: achieved by Ossicles:
  • Malleus, Incus and Stapes
  • Acts as piston and couple the Tympanic Membrane vibration to the Oval Window
  • The Oval Window is much smaller than the Tympanic Membrane so the pressure is increased 15-20 times

Cochlea

Anatomy

• Consists of 3 liquid-filled compartment termed Scalae
  • Scala Vestibuli - the farthest from the base and has the Oval Window at the basal end
  • Scala Typmani - at the cochlear base and has the Round Window at the basal end
  • Cochlear Duct - separates the two chambers and contains Scala Media
• The side of the Cochlear Duct in contact with Scala Vestibuli is formed by Reissner’s Membrane
• The side of the Cochlear Duct in contact with Scala Tympani is formed by Basilar Membrane
  • Upon which sits the Organ of Corti
• Spiral Ganglion hosts cell bodies of neurons innervating receptor cells and hair cells

Function

• Pressure is increased by Scala Vestibuli induced by Stapes pushing against the Oval Window
• Pressure is released by Round Window connected to Scala Typmani
• Organ of Corti is where the sensory transduction takes place

Basilar Membrane - Frequency Sorting

• Stapes push on Oval Window as response to Tympanic Membrane movement
• Air vibration transmitted into fluid movement within the Scala Vestibuli and Scala Tympani which are connected through Helicotrema
• The region of maximal displacement of the Basilar Membrane depends on the frequency of the sound source
  • At the Distal End (Apex), the membrane is broad and flaccid
    • more sensitive to low frequency oscillations
  • At the Proximal End (near Tympanum), the membrane is stiffer
    • more sensitive to high frequency oscillations
  • Continuous transition is logarithmic
• There is a Topology associated with the frequency analysis of sound
  • Tonotopic Map, where neighbouring values in stimulus space (sound frequency) are encoded by neighbouring sensory units (hair cells) in the nervous system

<aside> 💡 Distal - 远端 Proximal - 近端

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Organ of Corti

Transduction of Physical Energy into Neuronal Signals

• Organ of Corti is the receptor organ of the Inner Ear
  • contains approx. 16000 hair cells innervated by approx. 30000 afferent nerve fibres which carry info to the brain
• Hair cells are tonotopically organised:
  • At any position along the Basilar Membrane, the hair cells are most sensitive to a particular frequency
  • Frequencies are logarithmically mapped in ascending order from Cochlear Duct’s Distal End to Proximal End