Download TSM54 - The Auditory Pathway

TSM54: THE AUDITORY PATHWAY
28/10/08
LEARNING OUTCOMES
Describe the auditory pathway and its main functions in the analysis and localisation of sound
THE AUDITORY PATHWAY
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Cell bodies of the sensory cochlear nerve fibres are found in the spiral nucleus within the cochlea
Afferent fibres first synapse in the rostral medulla at the dorsal and ventral cochlear nuclei
o Second order neurones ascend and decussate to synapse in the superior olivary nuclei
o Further neurones ascend the lateral lemniscus and converge at the inferior colliculi
o Third order neurones ascend to the thalamus to synapse in the medial geniculate bodies
o Finally, fibres terminate at the superior temporal gyrus in the primary auditory cortex
Fibres from each ear therefore relay bilaterally to the primary auditory cortex
o Information from each ear is integrated at the olivary complex
o This is crucial in the localisation of sound
SOUND LOCALISATION
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Unlike light on the retina there is no ‘point-to-point’ mapping of sound in the ears
There are two distinct aspects to sound localisation which utilise different mechanisms:
o Elevation – in the vertical plane
 Interference patterns of sounds reflected by different parts of the pinna
o Azimuth – in the horizontal plane
 Interaural onset time or phase difference for low frequency sounds
 Interaural sound level difference for high frequency sounds
Interaural sound level differences are only detected for high frequency sounds because:
o If the wavelength is greater than the diameter of the head then the sound waves diffract
o Higher frequency sounds cast a ‘sound shadow’ around the head as they do not diffract in the
same way which results in an interaural sound level difference
Describe the mechanisms for encoding sound frequency
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The human ear is capable of recognising sound frequencies between 20 Hz and 20 kHz
o High frequencies produce maximal displacement at the base of the cochlea
o Low frequencies produce maximal displacement at the apex of the cochlea
There are roughly 12,000 outer hair cells but only 3,500 inner hair cells
There are roughly 30,000 cochlear nerve fibres
o Each fibre is particularly sensitive to a characteristic frequency
o Louder sounds are more easily encoded by multiple fibres
There are two theories for how sound frequencies are represented in the CNS:
o Place code – ‘tonotopic’ spatial mapping from the cochlea to the cochlear nucleus
o Temporal code – phase locking of action potentials at low frequencies only
Outline the processing of speech
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Speech is processed ipsilaterally in the dominant hemisphere of the brain – usually the left
o Comprehension is processed in Wernicke’s area near the lateral sulcus
o Formation is processed in Broca’s area in the inferior frontal gyrus
Outline the reasons for deafness
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Sound levels are measured in decibels (dB SPL – sound pressure level)
o 0 dB is the quietest sound detectible (arbitrary reference level) – 20 μPa pressure
o 30 dB is roughly the level of a whisper
o 60 dB is the level of normal conversation
o 90 dB is the threshold for hearing damage
o Sound above 140 dB is essentially perceived only as pain
Conductive hearing loss is due to problems in the middle or external ear limiting sound conduction
o Infections are the most common cause e.g. otitis media
Sensorineural hearing loss is due to inner ear dysfunction – conduction is often normal
o This is usually due to structural defects in the cochlea, particularly of the hair cells
o Can also be caused by vestibulocochlear nerve neuropathy
Presbycusis is the progressive bilateral hearing loss associated with old age
o The ears become progressively less sensitive to higher frequencies