Hearing

• defects

  
  

  What is tinnitus caused by?

  Spontaneous activity in hair cells, spiral ganglion cells, or neurones of the CNS.

  
  

  -> It can indicate damage to hair cells (e.g. after excessive noise), but is often of unknown aetiology, and is difficult to treat
• defects

  
  

  What is tinnitus?

  Constant sensation of tone e.g. high pitch, low pitch, buzzing etc. which can persist for many years
• defects

  
  

  What can cause central deafness?

  - Vascular accident

  - Trauma

  - MS

  - Infection

  - Tumour

  - Neonatal distress
• defects

  
  

  How do cochlear implants work?


  Electrodes are inserted into the cochlea to electrically stimulate the appropriate auditory nerve fibres
• defects

  
  

  What can cause sensorineural deafness?

  Problems with cochlea:

  Infection, trauma, noise, age, ototoxic drugs, genetic defects (myosins, gap junction mutations, etc.), tumours
• defects

  
  

  What can cause conductive deafness?

  - Earwax

  - Damage to eardrum

  - Otosclerosis of middle ear

  - Trauma

  - Middle ear infections

  - Genetic defects
• defects

  
  

  What are the different types of deafness?

  1. Conductive deafness

  (blockage/problem with outer or middle ear)

  
  

  2. Sensorineural deafness

  (blockage/problem with inner ear or cochlea)

  
  

  3. Central deafness

  (damage higher up in the brain in the auditory pathway)
• central connection

  
  

  what nucleus is the source of efferent fibres?

  the superior olivary nucleus is a source of efferent fibres that project back to the cochlear hair cells

  - feedback inhibition to refine sound perception
• central connection

  
  

  What controls auditory reflexes e.g. turning head towards sound?

  Tectospinal connections from inferior colliculus (via superior colliculus)
• central connection

  
  

  What does bilateral connectivity allow?


  Distance and directional sensitivity
• central connection

  
  

  What is associated with the auditory cortex on the left hand side of brain?

  Wernicke's speech sensory area & Broca's speech motor area
• central connection

  
  

  Where do tonotopic frequency maps exist?

  - Cochlear nuclei

  - Superior olivary nucleus

  - Inferior colliculus

  - Ventral division of medial geniculate body

  - Some parts of auditory cortex
• central connection

  
  

  How is tonotropic organisation preserved in the cochlear nucleus?

  Low freq--> ventrolateral parts of both nuclei

  
  

  High freq-->dorsal parts of both nuclei
• central connection

  
  

  What are the 2 main divisions of the cochlear nucleus?

  Dorsal cochlear nucleus

  Ventral cochlear nucleus
• what two nerves come together to form the vestibulocochlear nerve (cranial nerve eight (CN VIII))?

  vestibular and cochlear nerves
• What is the spiral ganglion?

  Cell bodies of the auditory (primary afferent) nerve fibres group together outside of the cochlea
• amplitude coding

  
  

  Comment on the anatomical arrangement of auditory (primary afferent) nerve fibres.

  It follows that of the BM, preserving tonotopy
• amplitude coding

  
  

  How do voltage-gated Ca channels relate to amplitude coding?

  the channels sense changes in membrane voltage & adjust rate of glutamate release from inner hair cells so afferent AP firing rate reflects amplitude of BM deflection

  i.e. AP firing rate codes for amplitude of sound (loudness)
• amplitude coding

  
  

  explain what happens to the inner hair cells


  (each hair cell is innervated by between 10-20 primary afferent fibres)

  the afferent fibres are bipolar neurones with two axons

  1. peripheral- innervates hair cell

  2. central- goes to brain stem

  
  

  
  

  ----> The sound pressure waves make the BM vibrate, which vibrates the hair cells and bends the stereocilia. Bending causes depolarisation and so voltage calcium ion channels sense these changes and cause the release of neurotransmitter (glutamate). This causes the afferent fibres to fire an AP.
• amplitude coding

  
  

  what is amplitude coding?

  Amplitude is coded through the number of nerves activated and the rate of their signals.

  
  

  -> it is essentially the measure of energy in sound waves-

  the greater the amplitude, the louder the sound.
• since the outer hair cells are effectively vibrating, what do they emit?


  Sound pressure waves- otoacoustic emissions.

  
  

  These emissions propagate in reverse, back through middle ear to move tympanic membranes (useful to clinicians as hearing tests, esp infants)
• how do the outer hair cells amplify weak signals (quiet sounds)?

  as they vibrate themselves, they amplify the vibrations of the BM and so amplify weak signals (quiet sounds)

  
  

  -> if an individual does not have the outer hair cells, it can lead to significant deafness
• What is the response from outer hair cells dependent on?

  Voltage:

  
  

  Depolarisation--> contraction

  Hyperpolarisation--> elongation

  
  

  (effectively they are vibrating themselves- this amplifies the vibrations of the BM)
• what is the role of the outer hair cells?

  They amplify BM motion & enhance frequency sensitivity.

  
  

  They do this as they respond to electrical stimulation by changing their length- 'electromotility'- due to special motor protein, prestin.
• frequency coding

  
  

  What is tonotopy/place code?

  position on the basilar membrane will tell you the frequency of the sound. (so the position of the hair cells can code for the frequency of the sound)

  
  

  Peak amplitude of wave depends on frequency of sound:

  - at low frequency, peak amp is near apex

  - at high frequency, peak amp is near base

  
• frequency coding

  
  

  if the sound has high frequency which pathway through the conchae does it take?


  the pressure waves travels through the shortcut only remaining at the stiff end.

  -> basilar membrane vibrations will be maximal at the basal/stiff end

  
• frequency coding

  
  

  if the sound has low frequency which pathway through the conchae does it take?

  through the whole conchae

  -> basilar membrane vibrations will be maximal at the apex/flexible end

  
• frequency coding

  
  

  describe the easiest pathway of pressure through the conchae


  the best pathway for the pressure waves is the path of least resistance (so through the whole conchae). However, this means the pressure waves will have to vibrate through the long column of fluid.

  
  

  at low frequency, it is not so difficult.

  at high frequency, it is difficult and requires greater energy.

  
• frequency coding

  
  

  How does the structure of the basilar membrane change?

  It's narrow & stiff at the base, near the oval window and becomes wide & flexible near the apex

  
• frequency coding

  
  

  where does frequency coding happen?

  at the inner hair cells
• frequency coding

  
  

  what is frequency coding?


  (frequency is coded by the activity level of a sensory neuron). so, in this case, a hair cell would fire action potentials related to the frequency of the sound wave.
• What would it look like if you were to record the voltage of the inner hair cells in the organ of Corti as they're vibrating?

  - when the stereocilia are bent towards the axis of polarity it causes hair cell depolarisation (more positive)

  
  

  - when the stereocilia are bent away from the axis of polarity it causes hair cell hyperpolarisation (more negative)

  
  

  ----> this produces oscillations

  THE VIBRATION IS TRANDUCED INTO AN ELECTRICAL SIGNAL

  
• what happens when the basilar membrane vibrates?

  the hair cells on the basilar membrane vibrate and the stereocilia bends backwards and forwards.

  (their tips are restricted by the tectorial membrane)

  
• Where do the hair cells stereocilia protrude in the organ of Corti?

  into the Scala media and the tectorial membrane
• where do the hair cells sit in the organ of Corti?

  on the basilar membrane
• how many rows of hair cells does the organ of Corti have?

  - 1 row of inner hair cells

  - 3 rows of outer hair cels

  
• What is the organ of Corti?

  In the middle scala of the ear, the actual hearing apparatus (it contains hair cells).

  - transduces the information about the pressure wave sand converts it into an electrical signal

  
• when does the basilar membrane vibrate?

  as the pressure wave is passing through the scala media
• What is the basilar membrane?


  A membrane in the cochlea that bears the organ of Corti.

  - it is sitting on the base of the scala media

  
• after the sound waves enter the oval window, where are they?

  in the inner ear. It causes the fluid molecules in the perilymph of the scala vestibuli to vibrate.

  
  

  -> This pressure wave travels through the scala vestibuli, scala media, and scala tympani and out the round window.