The auditory system

1. The auditory system Structure and function

2. Objective • Recap – outer, middle and inner ear to BM response • To continue the hearing process from the inner ear to the brain • How the physical properties of sound, such as frequency and amplitude, are represented in the auditory system

4. Organ of Corti • The organ of corti is the hearing sense organ and lies on the BM • It consists of supporting cells and hair cells • 2 groups of hair cells: inner and outer hair cells • Protruding from each hair cell are hairs called stereocilia • The tectorial membrane lies above the stereocilia –shearing motion between BM and tectorial membrane – causes stereocilia to be displaced

7. Inner hair cells • Function is to convert BM mechanical movement into neural activity – achieved in the following way (Moore, 2004): • The stereocilia are joined by fine links called ‘tip links’. • Deflection of the stereocilia leads to the opening of “transduction channels”. • Flow of potassium ions into the hair cell – voltage difference between the inside and outside of the hair cell.

8. Inner hair cells • Causes the release of neurotransmitter and the initiation of action potentials in the neurons of the audtiory nerve. • Action potential – ‘firing’ of a neuron. Propagation is in one direction only down the length of the axon • Most of the afferent neurons make contact with the inner hair cells • Possibly all information about the input sound is conveyed via the inner hair cells

9. Outer hair cells • Have a role in achieving high sensitivity and sharp tuning • Most of the efferent neurons synapse directly with the outer hair cells • Efferent neurons carry information from higher auditory system to cochlea • Afferent neurons carry information from the cochlea to the higher auditory system

10. Auditory nerve • Consists of vestibular and cochlear nerve • Cochlear nerve – the axon fibres of neurons whose cell bodies are in the spiral ganglion of the cochlea • dendrites of these neurons synapse with the hair cells – dendrites information receivers

11. Afferent auditory nerve • Transmits hearing information from cochlea to central nervous system • Important findings from recording impulses in single auditory nerve fibres. • Spontaneous firing, frequency selectivity of fibres, phase locking

12. Spontaneous firing rates • Firing in the absence of sound stimulation • Fibres divided into three groups based on their spontaneous firing rates • High, medium and low spontaneous rate fibres • High spontaneous rate fibres generally have lower thresholds and smaller dynamic ranges than medium or low spontaneous rate fibres

13. Frequency selectivity • Fibres show frequency selectivity – as fibres responding to activity at restricted regions of the BM • Characteristic frequency (CF) – frequency at which the threshold of a fibre is lowest • Fibres with high CF found in periphery of auditory nerve – orderly decrease in CF towards centre of the auditory nerve • Tonotopic organisation – place representation of frequency on BM maintained in auditory nerve

14. Phase locking • The firing of a neuron at one distinct point (phase) in the period of a sound wave. • temporal regularity in the firing pattern of a neuron in response to a periodic stimulus • upper limit: 4-5 kHz • May not fire on every cycle, but will occur at roughly the same phase of the waveform • Time intervals between spikes are approximately integer multiples of the period of the waveform • an indicator of the period of the waveform

16. The brainstem and auditory cortex • Binaural perceptions, such as our ability to localise sounds depends on the interaction of information from both ears • The information from both auditory nerves is first combined in the brainstem • Highly complex system, many of the neural pathways within and between the nuclei have yet to be investigated • Brainstem nuclei: cochlear nucleus, superior olivary nucleus, inferior colliculus, medial geniculate body.

18. Tonotopic organisation of frequency is preserved – neurons aligned respective to the frequencies to which they are most sensitive • The responses of neurons at higher levels in the auditory system – not as well studied as responses in the auditory nerve • Some neurons in the auditory cortex only respond to complex stimuli, or to stimuli with time varying characteristics

19. Cochlear nucleus • The auditory nerve carries signals from the cochlea to the cochlear nucleus in the brainstem. • First brainstem nucleus at which afferent auditory nerve fibres synapse • Monaural – input from same side ear only • 3 parts – anteroventral, posteroventral, dorsal • Not clear what its function in auditory processing is. – tonotopic organisation of frequency

20. Superior olivary complex • Binaural - Here the inputs from both ears converge • Important for sound localisation • Use of timing and intensity differences

21. Inferior colliculus • Receives inputs from the olivary complex and the cochlear nucleus • Some cells are monaural, some are binaural • Much (but not all) of the input to the inferior colliculus comes from the opposite ear • May function in sound localisation, and in combining information from lower areas in the brainstem

22. Medial geniculate nucleus • Last stop in the auditory pathway before the cerebral cortex. • 2-way information flow between medial geniculate and the auditory cortex • Feedback from the brain is tightly integrated with sensory information flowing up to the brain

23. Auditory cortex • Primary auditory cortex (A1) – the first area within the temporal lobes of the brain responsible for processing acoustic information • Each cochlea has input to each auditory cortex • Tonotopic organisation of frequency is maintained in the A1 • 2-way information between cortex and brainstem

24. No response to steady, unchanging tones – pure tones • Some cortical neurons respond to tones increasing or decreasing in frequency • Some respond to amplitude variations

25. Next: frequency resolution (selectivity) and pitch perception, reading material