Physioloy of hearing Flashcards
List the functions of hearing
Alerting to dangers
Localising objects
Recognition
Communication via speech
At what speed do sound waves travel at?
340 m/s
What are the two attributes of sound?
Frequency
Intensity
What is the frequency range of ideal human hearing:
approximately 20-20,000 Hz
How does the intensity range of human hearing vary?
Intensity range of human hearing varies over 14 orders of magnitude
Intensities >90 dB can lead to permanent hearing damage
What constitutes the peripheral auditory system?
Outer ear, middle ear, cochlea and auditory nerve (part of the VIIIth cranial nerve)
What is the middle ear?
An impedance matching device: increases pressure ~45x, by the ratio of tympanic membrane and oval window areas, and to a lesser extent by the lever action of the middle ear ossicles
What is the function of the middle ear?
Prevents sound from being reflected back from the fluid-filled cochlea
What is ottis media?
Infection or inflammation of middle ear Usually self-limiting Common in children Often from upper respiratory tract infection Secretory form with effusion “Glue ear”
What treatment is available for ottis media and glue ear?
If chronic causes a conductive hearing loss
May need draining
Grommets
What is otosclerosis?
Fusion of stapes with oval window
Maybe why Beethoven went deaf
Can be fixed by surgery
Describe the cochlea
Cochlea is a long, coiled, fluid filled tube
Different parts of tube are tuned to different frequencies
What is the basal end tuned to?
high-frequency sound
What is the apical end tuned to?
low-frequency sounds
What is the Scala vestibuli (SV) connected to ?
connected to oval window
What is the Scala media (SM)?
Is a separate chamber
What is the Scala tympani (ST)?
Connected to round window
How do the scala vestibuli and scala tympani communicate?
SV and ST communicate via helicotrema at apex of cochlea
What do the scala vestibuli and scala tympani contain?
Scala vestibuli and scala tympani contain perilymph, a normal extracellular fluid with high Na+ and low K+
What does the scala media contain?
Scala media contains endolymph, an unusual extracellular fluid rich in K+ and low in Na+ (produced by stria vascularis), and an electrical potential of about +80 mV
What is the function of the organ of corti?
Detects the sound induced motions of the basilar membrane
What does the organ of corti contain?
Inner and outer sensory hair cells
What is the apical membrane of hair cells bathed in?
Endolymph
What is the basolateral membrane of hair cells bathed in?
Perilymph
What are the outer hair cells innervated by?
Efferent nerve fibres
Are the cochlea hair cells regenerated?
No
Describe mechanotransduction of hair cells
Deflection of the hair bundle opens non-selective cation channels, the mechano-electrical transducer (MET) channels, at the lower end of the tip links, between neigbouring stereocilia (‘hairs’)
K+, the major cation in endolymph enters and depolarises the hair cell, driven by its electro-(chemical) gradient [the electrical gradients is +120 to +140 mV; there is little or no 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 sensori-motor cells
Describe the electromobility of outer hair cells
Outer hair cells amplify basilar membrane motion
Depolarise – shorten; Hyperpolarise – lengthen
Prestin, a modified anion exchanger in the basolateral membrane, is the OHC motor
Describe the afferent innervation of the cochlea
Neurons in cochlear (spiral) ganglion innervate hair cells and project axons to the brain via the auditory branch of the VIIIth nerve
Each inner hair cell is innervated by axons from 10-20 Type I spiral neurons that signal the reception of sound over a wide range of intensities to the brain
Outer hair cells are innervated by Type II spiral neurons that signal the reception of painfully loud sound that causes cochlear damage to the brain
Describe the efferent innervation of the cochlea
Efferent fibres from the medial olive innervate the outer hair cells directly
Efferent fibres from the lateral olive synapse on the Type I afferent fibres
Activation of efferent system modifies the sensitivity of the cochlea
List the causes of sensorineural hearing loss
Noise
Ageing
Ototoxic drugs
Genetic mutations
Name some targets of deafness genes in the cochlea
Tight junctions Gap junctions Afferent synapse Transduction complex Stria vascularis Tectorial membrane
What are cochlear implants?
surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf
Expensive: limited to the Western world
Results often good enough to recognise and comprehend speech
Describe the central auditory system
Tonotopically organised
Describe the cochlear nucleus
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
Describe the superior olivary complex
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)
Describe the inferior colliculus
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
Where is the primary auditory cortex located?
Primary auditory cortex is located on upper surface of temporal lobe
Lesions in auditory cortex cause defects in:
sound localisation, discrimination of temporal pattern, intelligibility of speech
Lesions in Broca’s (motor aphasia) and Wernicke’s (sensory aphasia) areas
Impair the production and comprehension of speech
