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Auditory and Vestibular Sensation 463 The BK channels in hair cells of frogs,39 turtles,40 and chickens41,42 are encoded by a gene whose mRNA is subject to alternative splicing of its composite exons. Some differentially spliced isoforms of the channel are kinetically distinct.43 Additional variability may be provided by an accessory β-subunit that combines with the channel and slows its gating kinetics.44 Although BK channels also are expressed in the mammalian cochlea, there is little alternative splicing,45 and no evidence for electrical resonance in inner or outer hair cells. Nonetheless, as in birds,46 BK channels appear in rodent cochlear hair cells near the onset of hearing47 and are more numerous in higher-frequency hair cells.48,49 The expression of mRNA significantly precedes that of functional channels, and developmental and tonotopic expression patterns appear to be regulated at the level of membrane localization of protein.50,51 The Auditory Pathway: Transmission between Hair Cells and Eighth Nerve Fibers Depolarizing and hyperpolarizing receptor potentials alter the open probability of voltagegated calcium channels in the hair cell’s basolateral membrane. Calcium entry in turn alters the rate of release of neurotransmitter (glutamate) onto the terminal of a postsynaptic afferent neuron. Hair cells, like retinal photoreceptors and bipolar cells, employ so-called ribbon synapses for tonic transmitter release52 (Figure 22.11; see also Chapters 13 and 20). Even in the absence of a stimulus, the membrane potential of the hair cell is positive to the threshold for gating of voltage-activated calcium channels; consequently, release of glutamate is ongoing and excites the afferent fibers, giving rise to spontaneous action potentials. At frequencies below ~5 kHz, the sinusoidal receptor potential in cochlear hair cells alternately increases and decreases the rate of transmitter release, producing phaselocking in the postsynaptic firing pattern. At frequencies greater than 5 kHz, the hair cell’s membrane time constant prevents rapid changes in membrane potential and the resulting afferent activity simply increases above the spontaneous rate for the duration of the tone, without cycle-by-cycle phase locking. A type I afferent neuron in the mammalian cochlea has a single dendrite that is postsynaptic to a single ribbon in a single inner hair cell.53 Afferent action potential activity often exceeds 100 Hz, requiring that the ribbon synapse have an impressive capacity to marshal and release vesicles.54–56 Given that each inner hair cell ribbon tethers only 100–200 vesicles,57 it is still mysterious how this occurs. A further surprise is that spontaneous (A) 37 Art, J. J., and Fettiplace, R. 1987. J. Physiol. 385: 207–242. 38 Art, J. J., Wu, Y. C., and Fettiplace, R. 1995. J. Gen. Physiol. 105: 49–72. 39 Rosenblatt, K. P. et al. 1997. Neuron 19: 1061–1075. 40 Jones, E. M., Laus, C., and Fettiplace, R. 1998. Proc. R. Soc. Lond., B, Biol. Sci. 265: 685–692. 41 Jiang, G. J. et al. 1997. Proc. R. Soc. Lond., B, Biol. Sci. 264: 731–737. 42 Navaratnam, D. S. et al. 1997. Neuron 19: 1077–1085. 43 Jones, E. M., Gray-Keller, M., and Fettiplace, R. 1999. J. Physiol. 518: 653–665. 44 Ramanathan, K. et al. 1999. Science 283: 215–217. 45 Langer, P., Grunder, S., and Rusch, A. 2003. J. Comp. Neurol. 455: 198–209. 46 Li, Y. et al. 2009. BMC Dev. Biol. 9: 67. 47 Kros, C. J. 2007. Hear. Res. 227: 3–10. 48 Engel, J. et al. 2006. Neuroscience 143: 837–849. 49 Wersinger, E. et al. 2010. PLoS One. 50 Kim, J. M. et al. 2010. J. Comp. Neurol. 518: 2554–2569. 51 Sokolowski, B. et al. 2009. Biochem. Biophys. Res. Commun. 387: 671–675. 52 Matthews G., and Fuchs, P. 2010. Nat. Rev. Neurosci. 11: 812–822. 53 Liberman, M. C. 1982. Science 216: 1239–1241. 54 Frank, T. et al. 2009. Proc. Natl. Acad. Sci USA 106: 4483–4488. 55 Griesinger, C. B., Richards, C. D., and Ashmore, J. F. 2005. Nature 435: 212–215. 56 Rutherford, M. A., and Roberts, W. M. 2006. Proc. Natl. Acad. Sci USA 103: 2898– 2903. 57 Sobkowicz, H. M. et al. 1982. J. Neurosci. 2: 942–957. (B) Displacement Period Amplitude FIGURE 22.11 Hair Cell to Afferent Signaling (A) Type I spiral ganglion neurons are postsynaptic to single ribbon synapses of a single inner hair cell. The ribbon synapse has an electrondense core to which are tethered ~100 synaptic vesicles. Sinusoidal stimulation of the hair cell gives rise to phase-locked activity in afferent neurons. (B) Individual afferent fibers contacting a single inner hair cell can have different firing rates, both spontaneous and evoked. ©2011 Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured or disseminated in any form without express written permission from the publisher. 464 Chapter 22 transmitter release from ribbon synapses appears to be multivesicular; that is, it can be composed of several vesicles released simultaneously.58 It is likely that ribbon function may involve unique proteins differing from those serving release at other chemical synapses.59,60 58 Glowatzki, E., and Fuchs, P. A. 2002. Nat. Neurosci. 5: 147–154. 59 Safieddine, S., and Wenthold, R. J. 1999. Eur. J. Neurosci. 11: 803–812. 60 Roux, I. et al. 2006. Cell 127: 277–289. 61 Young, E. D., and Sachs, M. B. 1979. J. Acoust. Soc. Am. 66: 1381–1403. 62 Meyer, A. C. et al. 2009. Nat. Neurosci. 12: 444–453. 63 Grant, L., Yi, E., and Glowatzki, E. 2010. J. Neurosci. 30: 4210–4220. 64 Palmer, A. R., and Russell, I. J. 1986. Hear. Res. 24: 1–15. 65 Koppl, C. 1997. J. Neurosci. 17: 3312–3321. 66 Fekete, D. M. et al. 1984. J. Comp. Neurol. 229: 432–450. 67 Ramachandran, R., Davis, K. A., and May, B. J. 1999. J. Neurophysiol. 82: 152–163. 68 Sadagopan, S., and Wang, X. 2008. J. Neurosci. 28: 3415–3426. Stimulus Coding by Primary Afferent Neurons Neuronal signaling in the auditory pathway begins with the spiral ganglion neurons that receive transmitter released from hair cells and send their central axons to the cochlear nucleus of the brainstem. Many decades of single fiber recordings have catalogued the acoustic responses of these primary afferents.61 Each spiral ganglion neuron responds selectively to the frequency of sound that is optimal for the inner hair cell to which it is attached. Each inner hair cell is the sole presynaptic partner of a group of type I afferent neurons, numbering from 10 to 30, depending on location in the cochlea (see Figure 22.11). Both acoustic threshold and spontaneous firing rate vary among this pool of afferents53 helping to extend the dynamic range of the cochlea. Presumably, individual ribbon synapses of an inner hair cell can have different release properties.62,63 The selective innervation of inner hair cells on the mechanically tuned basilar membrane produces an array of 10,000 or so afferent neurons that serve as frequency-labeled lines—the first stage of the tonotopically organized auditory pathway. In addition to this pitch-is-place mechanism, phase-locked firing of afferent action potentials to acoustic sinusoids can be used to encode frequency up to about 3 kHz in the guinea pig64 and up to 10 kHz in the barn owl.65 Brainstem and Thalamus Cerebral cortex Auditory cortex Medial geniculate nucleus Thalamus Inferior colliculus Midbrain The main auditory pathways are illustrated schematically in Figure 22.12. Auditory fibers of the eighth nerve travel centrally and send branches to both the dorsal and ventral cochlear nuclei.66 Second-order axons ascend in the contralateral lateral lemniscus to innervate cells in the inferior colliculus (the nucleus of the lateral lemniscus is a synaptic way station for some of these fibers). Neurons in the ventral cochlear nucleus also provide collateral branches to both the ipsilateral and contralateral superior olivary nuclei. Third-order cells in the olivary nuclei, in turn, send ascending fibers to the inferior colliculus. The ascending pathway continues through the medial geniculate nucleus of the thalamus to the auditory region on the transverse surface of the temporal lobe of the cerebral cortex. Each level in the auditory pathway is tonotopically mapped. However, as one ascends higher in the auditory system, individual cells have more complex response properties than the simple V-shaped tuning curves of cochlear afferent neurons. For example, some cells in the inferior colliculus67 and auditory cortex68 are only excited near threshold at their characteristic frequency, and louder tones are inhibitory. Sound Localization (Dorsal) Cochlear nuclei (Ventral) Medulla Auditory nerve Superior olivary nuclei Cochlea The impressive sensitivity and frequency selectivity of the auditory system may have evolved to improve the animal’s ability to locate a sound in space. The advantages of doing so are obvious; long-range signals emitted as sound waves can reveal a distant predator or prey in FIGURE 22.12 Auditory Pathways Central auditory pathways are shown schematically on transverse sections of the medulla, midbrain, and thalamus, as well as on a coronal section of the cerebral cortex. Auditory nerve fibers end in the dorsal and ventral cochlear nuclei. Second-order fibers ascend to the contralateral inferior colliculus; those from the ventral cochlear nucleus also supply collaterals bilaterally to the superior olivary nuclei. Further bilateral interaction occurs at the level of the inferior colliculus. Neurons of the inferior colliculus project to the medial geniculate nucleus of the thalamus, which in turn projects to the auditory cortex. (After Berne and Levy, 1988.) ©2011 Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured or disseminated in any form without express written permission from the publisher.