Physiology of Hearing

Question 1

What is the range of human hearing?

Answer 1

• Frequency range of ideal human hearing: approximately 20-20,000 Hz
• Adults progressively lose high frequencies
• Intensity range of human hearing varies over 14 orders of magnitude
• Intensities >90 dB can lead to permanent hearing damage

Question 2

What structures make up the Peripheral Auditory system?

Answer 2

• Outer ear
• Middle ear
• Cochlea
• Auditory nerve (CN-VIII)

Question 3

What is the function of the middle ear?

Answer 3

• acts an impedance matching device
• increase the pressure by ~45x by the focusing force on the tympanic membrane to the oval window which has a relatively smaller diameter
• this process relies on the mechanical advantage from lever action of the middle ear ossicles
  
  • malleus, incus, stapes
• this prevents sound from being reflected back from the fluid-filled cochlea

Question 4

What is Ottis Media (& Glue ear)

Answer 4

• Infection or inflammation of middle ear
  
  • Usually self-limiting
• Common in children
  
  • Often from upper respiratory tract infection
• Secretory form with effusion
  
  • “Glue ear”
    
    • If chronic causes a conductive hearing loss
  • May need draining
    
    • Grommets

Question 5

What is Otosclerosis?

Answer 5

• Fusion of stapes with the oval window
• cause of defences (Beethoven)
• can be fixed by surgery

Question 6

What is the anatomical function of parts of the Inner Ear?

Answer 6

• Cochlea is a long, coiled, fluid-filled tube
• Different parts of tube are tuned to different frequencies
• Basal end is tuned to high-frequency sound
• Apical end is tuned to low-frequency sounds

Question 7

Go over the cross-sectional anatomy of the cochlear duct

Answer 7

• Scala vestibuli (SV), is connected to oval window
• Scala media (SM), is a separate chamber
• Scala tympani (ST), connected to round window
• SV and ST communicate via helicotrema at apex of cochlea

Question 8

Explain the composition of the Cochlea fluids

Answer 8

• Scala vestibuli and scala tympani contain perilymph,
  
  • a normal extracellular fluid with high Na+ and low K+
• Scala media contains endolymph,
  
  • an unusual extracellular fluid rich in K+ and low in Na+ (produced by stria vascularis),
  • has an electrical potential of about +80 mV

Question 9

Describe the organisation of the Organ of Corti

Answer 9

• Detects the sound-induced motions of the basilar membrane
• Contains two types of sensory hair cells, inner hair cells and outer hair cells
• Apical membrane of hair cells is bathed in endolymph
• Basolateral membrane of hair cells is bathed in perilymph
• Inner hair cells are innervated by afferent nerve fibres
• Outer hair cells are mainly innervated by efferent nerve fibres
• Only 15,000 hair cells in each human cochlea, not regenerated after loss

Question 10

Explain mechanotransduction in hair cells

Answer 10

• Deflection of the hair bundle opens non-selective cation channels and the mechano-electrical transducer (MET) channels at the lower end of the tip links between neighbouring stereocilia (‘hairs’)
• K+, the major cation in endolymph enters and depolarises the hair cell, driven by its electro-(chemical) gradient; Ca2+ also enters and causes adaptation
• Voltage-gated Ca2+ channels open, Ca2+ triggers vesicle release
• Afferent nerve fibres (Aff NE) are activated
• Inner hair cells are sensory, outer hair cells are sensorimotor cells

Question 11

Give an overview of the electromotility of Outer Hair Cells

Answer 11

• Outer hair cells amplify basilar membrane motion
• Depolarise – shorten; Hyperpolarise – lengthen
• Prestin, a modified anion exchanger in the basolateral membrane, is the OHC motor

Question 12

Give an overview of the afferent innervation of the cochlea

• neurons
• inner/ outer hair cells

Answer 12

• Neurons in cochlear (spiral) ganglion innervate hair cells and project axons to the brain via the auditory branch of the VIIIth nerve
• Inner hair cell are innervated by axons from 10-20 Type I spiral neurons
  
  • they signal the reception of sound over a wide range of intensities to the brain
• Outer hair cells are innervated by Type II spiral neurons
  
  • they signal the reception of painfully loud sound that causes cochlear damage to the brain

Question 13

Give an overview of the efferent innervation of the cochlea

• inner/ outer hair cells

Answer 13

• Efferent fibres from the medial olive innervate the outer hair cells directly
• Efferent fibres from the lateral olive synapse on Type I afferent fibres which then synapse onto the inner hair cells
  
  • Activation of efferent system modifies the sensitivity of the cochlea

Question 14

What are the targets of deafness genes in the cochlea?

Answer 14

Question 15

What are Cochlear implants?

Answer 15

• surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf
• Expensive: limited to the Western world/ wealthy people
• Results often good enough to recognise and comprehend speech
  
  • Maximum 24 channels to substitute for 15,000 hair cells
  • Speech is reported to sound “robotic”
  • Music sounds awful

Question 16

What structures make up the central auditory system

Answer 16

• Cochlear nucleus
• Superior olivary complex
• Nuclei of the lateral lemniscus
• Inferior colliculus
• Medial geniculate body
• Primary auditory cortex

Question 17

What is the role of the Cochlear nucleus in the central auditory system?

Answer 17

• Parallel processing starts in cochlear nucleus
• Auditory nerve fibres from cochlear ganglion innervate many types of neuron
• Neurons extract information about level, onset and timing of sounds

Question 18

What is the role of the Superior Olivary Complex in the central auditory system?

Answer 18

• Two binaural cues are used to localize sounds in space
• Interaural level differences are detected in the lateral superior olive (LSO)
• Interaural time differences are detected in the medial superior olive (MSO)

Question 19

What is the role of the Inferior Colliculus in the central auditory system?

Answer 19

• Obligatory synaptic station for all afferents
• Laminar organisation in ICC, iso-frequency sheets
• Combines complex frequency and amplitude analysis of DCN
• with information on sound localization from SOC
• May encode complexity and localization of sounds
• Auditory reflex centre; reflexive orientation to stimuli

Question 20

what is the role of the Auditory Cortex

• associated lesions

Answer 20

• Primary auditory cortex is located on upper surface of temporal lobe
• Many functions including the analysis of complex sounds (eg speech) and sound localization
• Lesions in auditory cortex cause defects in: sound localisation, discrimination of temporal pattern, the intelligibility of speech
• Lesions in Broca’s (motor aphasia) and Wernicke’s (sensory aphasia) areas also impair the production and comprehension of speech