Figure 1. Pathways for activation of medial (MOC) and lateral olivocochlear (LOC) efferent fibers to the right cochlea. Red and blue lines illustrate the pathways for activation of the contralateral and ipsilateral MOC reflexes, respectively. Green lines illustrate the pathways for activation of the LOC reflex. The thickness of each line roughly illustrates the density of innervation. Abbreviations: LSO, lateral superior olive; MNTB, medial nucleus of the trapezoid body; PVCN, postero-ventral cochlear nucleus; SOC, superior olivary complex; VNTB, ventral nucleus of the trapezoid body; CF, characteristic frequency.
https://www.frontiersin.org/journals/neurology/articles/10.3389/fneur.2018.00197/full
https://www.sciencedirect.com/topics/medicine-and-dentistry/olivocochlear-system
Hair cell receptors within the auditory, vestibular, and lateral line systems receive an abundant efferent innervation from the central nervous system. This characteristic sets them apart from receptors in other sensory modalities. The auditory receptors receive innervation from the olivocochlear (OC) system, named because of its origin in the brain stem’s superior olivary complex and termination in the cochlea (inner ear) (Figure 1). Two separate systems of OC neurons exist. Lateral olivocochlear (LOC) neurons originate in and near the lateral superior olive and project to auditory-nerve dendrites beneath the inner hair cells. Medial olivocochlear (MOC) neurons originate in the more medial parts of the complex and project to outer hair cells (Figure 1). LOC neurons are located predominantly on the same side of the brain as the cochlea that they innervate, whereas MOC neurons are distributed bilaterally
Pictures
https://images.app.goo.gl/jASADXctVpAe3Fss6
”The Olivocochlear System: It has been known for more than 50 years that the olivocochlear bundle (Figs. 2, 29 and 32) provides the organ of Corti with efferent innervation (Rasmussen, 1946, 1953). It should be noted that the auditory-vestibular system appears unique among sensory systems because hair cell receptors receive direct projections from the brain (Robertson, 2009). Two systems of olivocochlear neurons are recognized, the medial (MOC) and lateral (LOC) systems (Figs. 29 and 32; White and Warr, 1983; Warr et al., 1986, 1997; Spangler and Warr, 1991; Warr, 1992; Vetter et al., 1991; Vetter and Mugnaini, 1992; Cantos et al., 2000). As their names imply, their respective cell bodies are located in medial and lateral regions of the SOC.
The MOC neurons innervate the OHCs (Figs. 2, 29 and 32). About 55% of them originate on the contralateral side and the reminder on the ipsilateral side (rat: Robertson et al., 1989; Horváth et al., 2000). The projection is made up of thick myelinated axons that terminate directly on the OHCs (White and Warr, 1983; Warr et al., 1997; Horváth et al., 2000). MOC neurons (Fig. 32) generally have large cell bodies that are stellate- or triangular-shaped, and they possess tapering dendritic arbors that radiate and branch profusely (Osen et al., 1984; Vetter and Mugnaini, 1992). They are located ventral to the MSO, lateral to MTz, and in the VTz (Fig. 29; Vetter and Mugnaini, 1992) and constitute a neurochemically homogeneous population of cholinergic cells (Osen et al., 1984; Vetter et al., 1991). They may receive descending input from the ipsilateral IC (see above, Vetter et al., 1993) and ascending input bilaterally from the VCP and also possibly from the globular bushy cells in the caudal VC, as White (1986) found terminal boutons similar to the calyces of Held terminating upon MOC cells. Collaterals of the axons of MOC neurons innervate VC neurons (Fig. 29; Hováth et al., 2000). The projections to the cochlea innervate clusters of OHC spanning as much as an octave of cochlear length, but the terminal axonal arbor is often centered basal to the corresponding location of radial afferent fibers of similar best frequency. MOC neurons are sharply tuned and the MOC efferent innervation is tonotopically organized (reviewed in Warr, 1992; Guinan, 1996; Helfert and Aschoff, 1997).
LOC neurons innervate the type I auditory nerve fibers that receive input from the IHCs. They originate predominantly on the ipsilateral side of the brain (Figs. 29 and 32; rat: White and Warr, 1983; Warr et al., 1997; Horváth et al., 2000; cat: Arnesen and Osen, 1984; Liberman and Brown, 1986; mouse: Brown, 1987, Wilson et al., 1991; Brown and Levine, 2008; guinea pig: Brown, 1993). In rats, two distinct types of neurons form the LOC, intrinsic and shell neurons (Fig. 32; Vetter and Mugnaini, 1992; Warr et al., 1997). All intrinsic neurons (Figs. 29 and 32) are small cells confined within the limits of the ipsilateral LSO. They possess a disc-shaped dendritic arbor, similar to that of the LSO principal neurons, with thin untapered dendrites and constitute about 85% of all LOC neurons (Vetter and Mugnaini, 1992; Warr et al., 1997; Horváth et al., 2000).
The intrinsic neurons probably receive the same input as the principal cells in the LSO. About 50% of them are GABAergic and the remaining 50% that are cholinergic, co-localize calcitonin gene-related peptide (Vetter et al., 1991). Their axons are thin and unmyelinated (being 0.77 μm thick in the organ of Corti), travel relatively short distances in the inner spiral bundle and terminate forming discrete (less than 0.2 octave in extent) terminal arborizations with a focal, tonotopic organization (Warr et al., 1997). They do not innervate the VC (Horváth et al., 2000). Most shell neurons (Figs. 29 and 32; about 95%) are located in areas surrounding the ipsilateral LSO. They are large multipolar cells with thick and tapered dendrites and constitute about 15% of the LOC cells (Vetter and Mugnaini, 1992; Warr et al., 1997). Little is known about the source of the inputs they receive. Only one-third of these shell neurons is GABAergic, while the other two-thirds are cholinergic but do not co-localize calcitonin gene-related peptide (Vetter et al., 1991). Their axons are thin and unmyelinated (being 0.37 μm thick in the organ of Corti), travel long distances (greater than 1 mm) in the inner spiral bundle and terminate forming diffuse (greater than 1 octave in extent) terminal arborizations; they are probably not tonotopic (Warr et al., 1997). In contrast to the intrinsic neurons, but similar to MOC, they innervate the VC (Figs. 2, 29; Horváth et al., 2000).
Physiological studies in several species have shown that MOC neurons are sharply tuned and have a wide dynamic range. Most of them respond to binaural stimuli. Two-thirds to contralateral stimuli and one-third to ipsilateral stimuli (Liberman, 1988; reviewed in Guinan, 1996). Stimulation of MOC neurons is thought to raise the threshold of the primary afferent fibers through the modulation of the biomechanical properties of the cochlear duct. MOC neurons may enhance transduction or signal detection through an unmasking effect, thus regulating the slow motility of the OHCs and thereby the stiffness of the basilar membrane (reviewed in Helfert and Aschoff, 1997; Eggermont, 2001). Previous studies have also demonstrated that the MOC can somewhat reduce the temporary threshold shift that occurs as an early manifestation of the damaging effects of loud sounds (guinea pig: Roberston and Johnstone, 1980; Cody, 1992; reviewed in Guinan, 1996), protecting the cochlea from acoustic injury (Taranda et al., 2009). Moreover, after cochlear dysfunction, MOC neurons seem to be a major source of synaptic reorganization in the VC that could possibly entail compensatory activation of the affected ascending auditory pathway (Kraus and Illing, 2004). The functional role of the LOC is still uncertain, but a recent study in mice has shown that lateral olivocochlear feedback maintains the binaural balance in neural excitability required for accurate localization of sounds in space (Darrow et al., 2006). In addition, several immunocytochemical studies have shown that these neurons contain neuroactive substances such as dopamine, serotonin and opioid peptides (e.g., Safieddine and Eybalin, 1992; Eybalin, 1993; Safieddine, et al., 1997; Inoue et al., 2006), which speaks in favor of a modulatory effect on the afferent fibers that innervate the IHCs. Rajan (2000) has shown in cats that even moderate background noise can significantly exacerbate the cochlear temporary threshold shift induced by loud tones, but that this is prevented when there is additional activation of the LOC. Thus, in background noise there is a conjoint activation of the MOC and LOC to powerfully protect (by almost 30 dB) the cochlea from loud sounds that would otherwise be exacerbated by the noise (Rajan, 2000). Clearly, the olivocochlear system plays a critical role in maintaining the normal operations of the cochlea. The presence of efferent connection with feedback loops grateful may introduce non-linear dynamics (Eggermont, 2001) into the auditory system.
https://www.sciencedirect.com/topics/medicine-and-dentistry/olivocochlear-system
https://images.app.goo.gl/jASADXctVpAe3Fss6
https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0378595521000411&psig=AOvVaw0p6I4cGGBIIa3liN6LMTzE&ust=1727786453480000&source=images&cd=vfe&opi=89978449&ved=0CBEQjRxqFwoTCLDNnojZ6ogDFQAAAAAdAAAAABAE