The Brainstem Auditory Pathway

Sound is detected by the inner hair cells in the cochlea, which excite the spiral ganglion cell processes so that bursts of action potentials pass into the brain along the 8th Nerve and project into the Cochlear Nucleus. Bushy cells of the anterior Ventral Cochlear Nucleus (aVCN) give a large projection which crosses the midline and forms the giant synapse or Calyx of Held in the contralateral Medial Nucleus of the Trapezoid Body (MNTB) which is mediated by the neurotransmitter glutamate. So what does the calyx of Held synapse with the MNTB do? It provides an inhibitory glycinergic projection to three key nuclei in the Superior Olivary Complex (SOC): The Medial Superior Olive (MSO); which computes interaural timing differences (ITD). The Lateral Superior Olive (LSO); which integrates an ipsilateral excitatory input with the inhibitory projection from the MNTB - to compute interaural level (or volume) difference (ILD). The MNTB also provides a powerful inhibition to the Superior Paraolivary Nucleus (SPN) which is concerned with detecting the end of a sound or gaps in sounds (see research projects). The common feature of auditory processing in this region is fast and accurate transmission of information as action potential trains. This enhances comparison of sound between both ears for the physiological functions of sound localisation in auditory space, for feature extraction in a noisy environment and for computation of temporal features, such as gap detection.

Brainstem Auditory Pathway

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