The Centrifugal Pathways

  1. Centrifugal (efferent) pathways parallel the centripetal (afferent) auditory pathways along the length of the system, forming a system which runs from the cortex to the hair cells. In many stages of the auditory pathways, the centrifugal fibers run adjacent to but not actually within, the tracts and nuclei principally associated with the ascending system.
  2. The cochlea is innervated by the olivocochlear bundle, which arises bilaterally in the superior olivary complex. The medial olivocochlear bundle arises medially in the superior olivary complex and projects to the outer hair cells predominantly on the contralateral side. The cells of origin have relatively large bodies and give rise to relatively large, myelinated axons. The lateral olivocochlear bundle arises laterally in the superior olivary complex and projects to the dendrites of the auditory nerve fibers near the inner hair cells, predominantly on the ipsilateral side. The cells of origin have relatively small bodies and give rise to relatively small unmyelinated axons. Both the MOC and LOC use acetylcholine as a neurotransmitter. In addition, they use neurotransmitters or neuromodulators such as GABA, enkephalins, dynorphins and CGRP (calcitonin gene related peptide) with a subset of LOC fibers using dopamine instead of acetylcholine as a neurotransmitter.
  3. The MOC reduces the active amplification of the traveling wave in the cochlea by the outer hair cells. It does this by opening Ca2+ channels in the membrane, which are then likely to open Ca2+ activated K+ channels (SK2 channels) in the membrane. There are effects over two different time scales in hair cells and both may affect mechanical amplification.  Fast effects (tens of milliseconds) are thought to occur because opening the K+ channels (1) partially short circuits the membrane (so that transducer currents do not produce such large voltage changes, reducing active amplification) and (2) moves the active amplifier and possibly the mechanotransducer channels away from their optimal operating points, also reducing active amplification. Slow effects (over seconds) are thought to occur because the outer hair cells become less stiff, likely to be a result of Ca2+ dependent second messenger cascade acting on the cytoskeleton and possibly the motor proteins , also affecting active amplification. The result is that the amplitude of the traveling wave in the cochlea is reduced, especially in the region of the sensitive, sharply tuned peak. Inner hair cell responses and auditory nerve fiber responses are also changed.
  4. The functions of the LOC are uncertain; it is likely to modulate the neural excitability of the afferent nerve fibers.
  5. Nerve fibers of the olivocochlear bundle can be driven by sound and respond, to sounds of the best frequencies of the areas that they themselves innervate. The fibers therefore are able to make closed-frequency specific feedback loops. They can also be driven by descending fibers from the more central auditory nuclei, also in a frequency specific way. 
  6. The olivocochlear bundle is likely to (1) help protect the cochlea from acoustic trauma, both by reducing the traveling wave in the cochlea and by direct effects of the LOC on the afferent nerve fibers, (2) enhance the response to narrow band signals in wideband masking noise and (3) may modify the auditory input in attention.
  7. The cochlear nucleus receives a centrifugal innervation, including branches of the olivocochlear bundle, together with other centrifugal fibers from the superior olive, the inferior colliculus and from several other brainstem areas. The neurotransmitters include acetylcholine, GABA, glycine and noradrenaline. The cholinergic innervation of the nucleus may assist in the detection of signals in wide-band masking noise. The centrifugal pathways also can modulate temporal integration in the nucleus and affect delayed masking.
  8. The auditory cortex is a rich source of centrifugal fibers running to the inferior colliculus, to all parts of the nucleus though mainly to the non-specific (extra-lemniscal) divisions of the colliculus. In the inferior colliculus, the corticofugal fibers can affect frequency-specific processing, temporal processing and the representation of sound location. It can also enhance sounds with certain temporal structures at the expense of other sounds. 
  9. The auditory cortex also sends many centrifugual connections to the medial geniculate body. The centrifugal and centripetal fibers make closed feedback loops suggesting that within each functional division , the thalamus and cortex work as a closely integrated unit. The auditory cortex enhances the responses and sharpens the frequency selectivity of the cells that project to the area being stimulated. The centrifugal input is also able to enhance stimulus-specific adaptation (SSA) in the medial geniculate and therefore also helps prime the system to respond to novel stimuli. Electrical stimulation of the auditory cortex is also able to activate the olivocochlear bundle, forming a mechanism by which the cortex could also modify the auditory input as early as the cochlea. 
  10. The general conclusion is that the centrifugal pathways allow the higher levels of the auditory system to modify sensory processing at lower levels. They enhance the responses of the system and allow it to devote large numbers of neurons to stimuli that are of particular current significance for the animal.