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The auditory nerve, central pathways and binaural hearing
The auditory nerve codes level by firing rate and the number of fibres, and frequency by place and phase locking; from the cochlear nucleus the message climbs through the superior olive — the first station that compares the two ears — to the cortex in Heschl's gyrus, and an efferent system feeds control back to the outer hair cells.
Why it matters in the exam
The bank's nerve questions: the auditory nerve has about 30,000 afferent fibres; about 90–95% are type I and reach the inner cells (each inner cell receives many fibres), and the rest are type II to the outer cells. Fibres from one inner cell share a single characteristic frequency and differ in spontaneous rate and threshold. A fibre's spike is all or none, so level is coded by rate and by recruiting more fibres; with a sustained tone there is adaptation — a peak at onset, then a decline. Phase locking is strong up to about 1 kHz, weakens above about 2 kHz and is gone around 4–5 kHz, so an 8 kHz tone is coded by place alone. Noise that caused only a temporary shift can destroy synapses (synaptopathy): a normal audiogram, trouble in noise and a reduced wave I.
The pathway questions: cochlear nucleus → superior olivary complex (MSO codes time differences at low frequencies, LSO level differences at high ones) → lateral lemniscus → inferior colliculus → medial geniculate body of the thalamus → primary auditory cortex in Heschl's gyrus in the temporal lobe. ABR wave V appears at about 6 ms, and the cortex responds at about 10–20 ms. The MOC system runs from the superior olive to the outer cells and lowers their gain; noise in the opposite ear reduces the TEOAE by about 1–2 dB, which is how the system is tested. By the duplex theory, low frequencies are localised by time differences and high ones by the level differences that head shadow creates (above about 1500 Hz). With two ears, threshold improves by about 3 dB (binaural summation) and loudness above threshold by up to about 6 dB; binaural fusion joins complementary parts from each ear into one percept.
Key points
- About 30,000 fibres; type I to inner cells.
- Level — rate and recruitment; frequency — place and phase locking up to about 4–5 kHz.
- Superior olive: MSO time, LSO level.
- Path: cochlea, olive, lemniscus, colliculus, MGB, Heschl.
- MOC lowers gain; TEOAE suppression of about 1–2 dB.
Summary table
| Station or effect | What it does | The fact to remember |
|---|---|---|
| Auditory nerve | About 30,000 fibres; type I to inner cells, type II to outer cells | Fibres from one inner cell — one characteristic frequency |
| Level and frequency coding | Level — firing rate and recruitment; frequency — place and phase locking | Phase locking almost gone above 4–5 kHz |
| Superior olive | The first binaural station | MSO — time differences; LSO — level differences |
| Inferior colliculus and thalamus | Obligatory midbrain relay; the MGB passes to cortex | Wave V arises around the lemniscus and colliculus |
| Auditory cortex | Heschl's gyrus, upper temporal lobe | Reached in about 10–20 ms |
| MOC efferents | Lower outer-cell gain (acetylcholine) | TEOAE suppression with contralateral noise; helps hearing in noise |
| Two-ear hearing | Head shadow, summation, fusion and release from noise | Summation about 3 dB at threshold; elevation from the pinna |
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Sources
- Pickles, An Introduction to the Physiology of Hearing, 4th edition (2012) — The auditory nerve; brainstem nuclei; the auditory cortex
- Kujawa & Liberman, Journal of Neuroscience, 2009 — Adding insult to injury: cochlear nerve degeneration after temporary noise-induced hearing loss
- Guinan, Ear and Hearing, 2006 — Olivocochlear efferents: anatomy, physiology, function, and the measurement of efferent effects in humans
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.