page_title: Olivocochlear system
section_title: Acoustically evoked responses of the MOCS
hierarchical_section_title: Olivocochlear system / Physiology of the olivocochlear system / Acoustically evoked responses of the MOCS
caption_attribution_description: English: The basic MOC acoustic reflex. The auditory nerve responds to sound, sending a signal to the cochlear nucleus. Afferent nerve fibres cross the midline from the cochlear nucleus to the cell bodies of the MOCS (located near the MSOC), whose efferent fibres project back to the cochlea (red). In most mammals, the majority of the reflex is ipsilateral (shown as a thicker line), effectuated by the crossed MOCS.
context_page_description: The olivocochlear system is a component of the auditory system involved with the descending control of the cochlea. Its nerve fibres, the olivocochlear bundle, form part of the vestibulocochlear nerve, and project from the superior olivary complex in the brainstem to the cochlea.
context_section_description: Electrical stimulation in the brainstem can result in (i) the entire MOCS being stimulated, (ii) a discharge rate (up to 400 sec-1) much higher than is normally evoked by sound (up to 60 sec-1), and (iii) electrical stimulation of neurons other than MOCS fibres. Therefore, electrical stimulation of the MOCS may not give an accurate indication of its biological function, nor the natural magnitude of its effect.
  The MOCS’ response to sound is mediated through the MOC acoustic reflex pathway (see inset), which had been previously investigated using anterograde and retrograde labelling techniques (Aschoff et al., 1988; Robertson and Winter, 1988). Acoustic stimulation of the inner hair cells sends a neural signal to the posteroventral cochlear nucleus (PVCN), and the axons of the neurons from the PVCN cross the brainstem to innervate the contralateral MOC neurons. In most mammals, the MOC neurons predominantly project to the contralateral side (forming the ipsilateral reflex), with the remainder projecting to the ipsilateral side (forming the contralateral reflex).
  The strength of the reflex is weakest for pure tones, and becomes stronger as the bandwidth of the sound is increased (Berlin et al., 1993), hence the maximum MOCS response is observed for broadband noise (Guinan et al., 2003). Researchers have measured the effects of stimulating the MOCS with sound. In cats, Liberman (1989) showed that contralateral sound (resulting in MOCS stimulation) reduced the N1 potential, a suppression which was eliminated upon transection of the olivocochlear bundle (OCB). In humans, the largest amount of evidence for the action of efferents has come from the suppression of otoacoustic emissions (OAEs) following acoustic stimulation.
  Using acoustic stimuli to activate the MOC reflex pathway, recordings have been made from single efferent fibres in guinea pigs and cats.  Both studies confirmed that MOC neurons are sharply tuned to frequency, as previously suggested by Cody and Johnstone (1982), and Robertson (1984). They also showed that the firing rate of MOC neurons increased as the intensity of sound increased from 0 to 100 dB SPL, and have comparable thresholds (within ~15 dB) to afferent neurons. Furthermore, both studies showed that most MOC neurons responded to sound presented in the ipsilateral ear, consistent with the majority of mammalian MOC neurons being contralaterally located. No recordings have been made from MOC fibres in humans. because invasive in vivo experiments are not possible. In other primate species however, it has been shown that about 50-60% of MOC fibres are crossed (Bodian and Gucer, 1980; Thompson and Thompson, 1986).
caption: The basic MOC acoustic reflex. The auditory nerve responds to sound, sending a signal to the cochlear nucleus. Afferent nerve fibres cross the midline from the cochlear nucleus to the cell bodies of the MOCS (located near the MSOC), whose efferent fibres project back to the cochlea (red). In most mammals, the majority of the reflex is ipsilateral (shown as a thicker line), effectuated by the crossed MOCS.
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