Ch 8. The Centrifugal Pathways(a) 강현덕

Contents  A. Introduction  B. The Olivocochlear Bundle 1. Anatomy 2. Neurotransmitters 3. Physiology  (a) Effect on ascending system  (b) What normally activates the olivocochlear bundle?  (c) Functional significance of the olivocochlear bundle

A. Introduction  The centrifugal auditory pathways Higher stages  lower stage Example  The olivococlear bundle –From Superior olivary complex to the hair cells of the coclear

B. The Olivocochlear Bundle  1. Anatomy The cochlear receives a centrifugal, “efferent”, innervation from the superior olivary complex.

B. The Olivocochlear Bundle MOC neurons  OHCs LOC neurons  IHCs

B. The Olivocochlear Bundle cat- in brainstem  1800 fibres in all, 1200 were uncrossed Project to the nerve fibers below the IHCs  LOC efferent Project to the nerve fibers below the OHCs  MOC efferent

B. The Olivocochlear Bundle  The origin of the cells, Group 1 Group 1 : lateral group  Tightly clustered around the LSO ( LSO:lateral superior olivary nucleus)  Project to IHCs  The cells have relatively small bodies  Almost entirely on the ipsilateral side

B. The Olivocochlear Bundle  The origin of the cells, Group 2 Proportion don’t add up 100%, because other fibres add a few percent

B. The Olivocochlear Bundle  2. Neurotransmitters Acetylcholine is the main transmitter of the olivococlear bundle Another neurotransmitters  Enkephalins, dynorphins  Gamma – amino – butyric(GABA)

B. The Olivocochlear Bundle  3. Physioligy (a) Effect on the ascending system  (i) Mechanism of action on hair cell membrances –Stimulation of the centrifugal –Supply to the hair cells ‘hyperpolarization’ –Opens the K+ channels and does not open Cl- channel(turtle) –Opens the Cl- channels and does not open K+ channel(cat) –So, ‘stimulation of the centrifugal’ opens the K+ channels or Cl- channel

B. The Olivocochlear Bundle  (ii) Effect on the afferent fibres

B. The Olivocochlear Bundle Medial group of cells sends its axons to OHCs. These axons, which make up most of the COCB, can be stimulated electrically where they cross the floor of the 4 th ventricle  - reduce the N1 potential - after 15 ms, inhibitory effect appeared, and increasing over a further 50ms - Increase the magnitude of the cochlear microphonic by a few dB

B. The Olivocochlear Bundle It is now known that stimulation of the crossed olivocochlear bundle (i.e. mainly fibers from the large-celled or medial group, running to the outer hair cells) can indeed affect the responses of the IHCs, and hence the auditory nerve fibers to which they are connected.

B. The Olivocochlear Bundle 14 COCB Stimulation  Broaden the tuning curve of auditory nerve fibers

B. The Olivocochlear Bundle COCB stimulation  Also raise the threshold of IHCs

B. The Olivocochlear Bundle COCB stimulation shifts the rate-intensity functions of single auditory nerve fibers Decrease of firing rate cause a N1 potential reduction (the effects were greatest at low intensities)

B. The Olivocochlear Bundle (b) What normally activates the olivocochlear bundle? The fibers of the olivocochlear bundle are responsive to sound The fibers have thresholds within 15dB, and have a sharp tuning curves of the most sensitive afferent nerve fibers Just before the fibers enter the organ of Corti, it is possible to relate fiber properties to the site of termination along the cochlear duct The fibers are tuned to the frequency of the region they themselves influence

B. The Olivocochlear Bundle  Stimulation of one ear reduced the response of auditory nerve fibres running from the other ear

B. The Olivocochlear Bundle (c) Functional significance of the olivocochlear bundle  (i) Improving the detection of signals in noise –Stimulation of the olivocochlear bundle could reduce masking in single auditory nerve fibres

B. The Olivocochlear Bundle  (ii) Protection from noise damage When input tones bigger than 100dB, active process is no longer making a contribution for protection So, it might be unwise to protect only one ear in a noisy environment

B. The Olivocochlear Bundle  (iii) Controlling the mechanical state of the coclear It is possible that the amplification could be critically affected by small degrees of bias in the system !!! The olivocochlear bundle acts to keep the cochlea in its optimal state - affect the mechanical properties of the hair cells - changing the stiffness of the stereocilia - introduce a bias in their displacement by actively deflecting the sterocilia This requires cochlea to detect a static pressure difference across the partition

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