Olivocochlear system

The olivocochlear system is a component of the auditory system involved with the descending control of the cochlea. Its nerve fibres, the olivocochlear bundle (OCB), form part of the vestibulocochlear nerve (VIIIth cranial nerve, also known as the auditory-vestibular nerve), and project from the superior olivary complex in the brainstem (pons) to the cochlea.

The olivocochlear bundle (OCB) originates in the superior olivary complex in the brainstem. The vestibulocochlear anastomosis carries the efferent axons into the cochlea, where they innervate the organ of Corti (OC). The OCB contains fibres projecting to both the ipsilateral and contralateral cochleae, prompting an initial division into crossed (COCB) and uncrossed (UCOCB) systems. More recently, however, the division of the OCB is based on the cell bodies’ site of origin in the brainstem relative to the medial superior olive (MSO). The medioventral periolivary (MVPO) region, also known as the ventral nucleus of the trapezoid body, a diffuse region of neurons located medial to the MSO, gives rise to the medial olivocochlear system (MOCS). The lateral superior olive (LSO), a distinct nucleus of neurons located lateral to the MSO, gives rise to the lateral olivocochlear system (LOCS). The MOCS neurons are large multipolar cells, while the LOCS are classically defined as composed of small spherical cells. This division is viewed as being more meaningful with respect to OCB physiology. In addition to these classically defined olivocochlear neurons, advances in tract tracing methods helped reveal a third class of olivocochlear neurons, termed shell neurons, which surround the LSO. Thus, LOCS class cell bodies within the LSO are referred to as intrinsic LOCS neurons, while those surrounding the LSO are referred to as shell, or extrinsic, LOCS neurons. Shell neurons are typically large, and morphologically are very similar to MOCS neurons.

The LOCS (originating from both the intrinsic and shell neurons) contains unmyelinated fibres that synapse with the dendrites of the Type I spiral ganglion cells projecting to the inner hair cells. While the intrinsic LOCS neurons tend to be small (~10 to 15 µm in diameter), and the shell OC neurons are larger (~25 µm in diameter), it is the intrinsic OC neurons that possess the larger axons (0.77 µm compared to 0.37 µm diameter for shell neurons). In contrast, the MOCS contains myelinated nerve fibres which innervate the outer hair cells directly. Although both the LOCS and MOCS contain crossed (contralateral) and uncrossed (ipsilateral) fibres, in most mammalian species the majority of LOCS fibres project to the ipsilateral cochlea, whilst the majority of the MOCS fibres project to the contralateral cochlea. The proportion of fibres in the MOCS and LOCS also varies between species, but in most cases the fibres of the LOCS are more numerous. In humans, there are an estimated (average) 1,000 LOCS fibres and 360 MOCS fibres, however the numbers vary between individuals. The MOCS gives rise to a frequency-specific innervation of the cochlea, in that MOC fibres terminate on the outer hair cells at the place in the cochlea predicted from the fibres’ characteristic frequency, and are thus tonotopically organised in the same fashion as the primary afferent neurons. The fibres of the LOCS also appear to be arranged in a tonotopic fashion. However, it is not known whether the characteristic frequencies of the LOCS fibres coincide with the characteristic frequencies of the primary afferent neurons, since attempts to selectively stimulate the fibres of the LOCS have been largely unsuccessful. Intrinsic LOCS derived axons travel only approximately 1 µm within the organ of Corti, generally spiraling apically. They give off a small tuft of synaptic boutons that is compact in its extent, often involving less than 10 IHCs. In comparison, shell neurons spiral both apically and basally, and can cover large territories within the organ of Corti. The shell axons often cover 1-2 octaves of tonotopic length. Their terminal arbor is quite sparse, however.

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