Hearing And Balance

Hearing is the transduction of vibratory energy into electrical energy, which in some species is decoded in the brain into information about the pitch and magnitude of the vibration. The stimuli can come through the air for terrestrial species or through the water for aquatic species or can come from direct contact with solid objects. Because animals with hearing have bilateral symmetry, they can typically also sense the direction from which the energy comes, relative to their midline. Hearing is mainly a feature of vertebrates and arthropods, although there are suggestions that some nonarthropod invertebrates can also hear (Budelmann 1992).

Here we use the single term "hearing," but there is no one way to hear: different organisms use different aspects of sound in their own ways. Some simply need to respond to the presence of sound or, say, large, sudden sounds. Others respond to very specific patterns, frequencies, or even periodicities of sounds, such as the diverse ways that vertebrates and invertebrates respond to mating, territorial, or other such sounds from conspecifics. Organisms may need to recognize specific predator sounds. Or they may need to recognize sounds such as that of fire, moving water, and the like. Finally, of course, is the highly elaborate processing that humans do to interpret language.

Each of these uses of sound detection involves not just different levels of integration of sound impulses but different ways to detect sound. One cannot interpret elements of sound that one cannot detect and separate into its relevant elements. Nature has evolved particular mechanisms, especially perhaps in vertebrates, for discriminating sound. The variation in sound frequencies audible to a number of hearing organisms is shown in Figure 12-2.

hearing in vertebrates The Mammalian Model

There is great diversity in the shape of the ear in vertebrates, but the mechanism by which it transduces vibration into sound is similar across species (with similar mechanisms in some invertebrates). The classic system is that of mammals, and it illustrates the various modes of transmission. The outer ear collects the airborne sound waves and transfers them to the middle ear by vibrating the tympanic membrane stretched across the inner end of the ear canal; the middle ear contains three small, linked bones, the ear ossicles, that transfer the mechanical vibrations from the tympanic membrane to a thin, pliable membrane on the cochlea called the oval window, which communicates with the fluid-filled inner ear. Vibrations of the oval window are transmitted in the fluid and detected by ciliated hair cells in the spiral-shaped cochlea. Movements in the cilia are transduced into electrical energy and transmitted to the auditory portion of the brain via the acoustic nerve, which connects at the anchored end of the hair cells. The balance system, or labyrinth, is part of the same fluid-filled system and will be described below.

In all hearing vertebrates the ubiquitous and central player in this mechanosen-sory pathway, the sensory cell itself, is the ciliated hair cell. This cell type is ancient

Owl Canary Tree frog -

Bullfrog Tuna _



Beluga whale _

Mouse Rat _



Human Dog

Bush cricket Cricket

kHz .02 .05 .1 .2 .5 1.0 2.0 5.0 10.0 20.0 50.0 100.0

Hz 20 50 100 200 500 1000 2000 5000 10000 20000 50000 100000

Figure 12-2. Sound frequencies (in Hz) audible to various species.

and found diversely in the living world. Cilia are tiny hairlike appendages, consisting of microtubules containing a protein called tubulin, that protrude from the surface of the cell (Alberts 1994). Ciliated cells are found in most animal species and some lower plants. Many protozoa are ciliated as well; they use the cilia to find food by washing fluid over the cell or for locomotion. Ciliated cells line the respiratory tract of many animals to move contaminants up and out of the lungs, for example, or they move eggs through the oviduct. Ciliated cells are integral to at least three sensory systems; taste buds, odorant receptors, and auditory hair cells and similar structures are used in photoreceptors.

In the hair cells of the cochlea and semicircular canals (also called the labyrinth) of higher vertebrates, groups of 50 or more cilia form bundles at the top of each cell and project into the fluid in the basilar membrane of the cochlea, which divides the cochlea lengthwise into an upper and a lower chamber, and in a gelatinous material, the cupula, in the bulging crista ampullaris at one end of each semicircular canal (see Figure 12-3). The cilia increase in length from one side of the bundle to the other. (See Figure 12-4 for a photograph of hair cells taken with a scanning electron microscope.) Deflection of these thin hairs by the fluid into which they protrude sends the signal to the sensory cells that connect from the basal end of the hair cell to the auditory regions of the brain. In this mechanically gated signal trans-duction system, if the cilia are deflected in the direction of the longest cilium, the action potential of the cell decreases, depolarizing it, and this induces the cell to release excitatory neurotransmitter at the synapse between the hair cell and the sensory neuron. Deflection away from the longest cilium, in contrast, results in hyperpolarization of the cell and decrease in the release of neurotransmitter.

Figure 12-3. Structure of human cochlea and hair cells. Modified after (Matthews 2001).

The ear is one of the most intricately patterned structures in a vertebrate body. However, its development does not involve new principles or basically any new genes. It is beyond our scope to present ear development, but each region shown in Figure 12-3 develops through the agency of a set of signaling, transcription, and cytostructural factors (Kiernan et al. 2002; Petit 1996; Petit et al. 2001).

TFs, SFs, and signal receptor genes involved in the hind-brain region of the developing nervous system are involved in early otic patterning. A lateral otic placode invaginates to form an otic cup which closes to become the otocyst. The semicircular canals and cochlea derive from outpocketing from the otocyst. Genes from ectoderm and mesoderm are involved. Several genes related to the semicircular canals have been identified, but less is known about cochlea-specific developmental mechanisms. The hair and supporting cells become arrayed along the cochlea, probably by an activation and lateral inhibition system such as has been seen earlier, including that of the Delta/Notch system.

Figure 12-4. Scanning electron micrograph of hair cells. Reprinted with kind permission of Julian Thorpe and Guy Richardson, University of Sussex; see www.biols.susx. ac.uk/Home/Julian_Thorpe/coch8.htm for a series of SEM photos of hair cells at increasing magnification.

Figure 12-4. Scanning electron micrograph of hair cells. Reprinted with kind permission of Julian Thorpe and Guy Richardson, University of Sussex; see www.biols.susx. ac.uk/Home/Julian_Thorpe/coch8.htm for a series of SEM photos of hair cells at increasing magnification.

Although hair cells are not true neurons, sound detection is nearly instantaneous, as is signal transduction in all mechanically gated systems. Indeed, it is much quicker than photoreception, because the ion channel gating involved is a purely mechanical process. Each cilium in a bundle is connected to an ion channel on its neighboring cilium by a tip link, which is like a tiny spring (see Figure 12-3). When the bundle of cilia is deflected in the direction of the longest cilium, the tip link on the longer cilia pulls open the gate on the ion channel of the adjacent, shorter cilium. When the bundle is deflected in the opposite direction, toward the shortest cilia, the tip link relaxes, closing the ion channel of the neighboring cilium. The signal is thus transmitted through the ion channel to adjacent sensory neurons, located within the spiral ganglion of the cochlea. An axon extends from each spiral ganglion cell to the CNS via the auditory nerve. With the nerves that come from the vestibular apparatus of the inner ear, the semicircular canals, these axons make up cranial nerve VIII.

Frequency detection is graded along the cochlea, with the hair cells at the base nearest to the oval window responding to the highest frequencies and those at the apical end responding to the lowest frequencies. The nerve fibers that synapse with the hair cells at specific locations are "tuned" to specific frequencies. There are two competing theories as to how this happens. The first is the "place theory," which proposes that frequency is determined by the physical properties of the basilar membrane, in which the hair cells are embedded; the membrane is rigid and narrow at the basal end and wider and more flexible at the apex. The ratio of stiffness to mass determines how far sound waves will travel down the cochlea, and thus which hair cells will be excited and send their signal to the brain. A different, "temporal" or

"frequency theory" is based on the idea that sound is periodic in nature; the auditory nerves fire at a rate determined by the periodicity of sound waves and the rate at which the basilar membrane vibrates, and the brain determines tone by the rate at which auditory nerves discharge their signal. Neither theory alone adequately explains all aspects of sound perception, so more research on this question is needed before we have a sufficient understanding.

Large animals (including humans) and miniscule insects detect the directionality of sound in two ways. The body itself diffracts sound waves, and the brain can detect and interpret the interaural difference in sound pressure caused by this diffraction as directionality. Second, the brain can also interpret the difference in the time of arrival of the sound at each of the ears.

In terms of genetics, the known genes involved in sending hearing and balance information to the brain are similar to those used in other membrane potential systems. Most of what we know comes from human or mouse mutations. Two classes of genetic effects are observed. One class involves syndromic hearing loss, which means that hearing is lost in association with other craniofacial developmental anomalies. In nonsyndromic hearing loss, the latter anomalies do not occur and there may be no other phenotypic effects. Over 75 genes or chromosomal regions have been identified in association with nonsyndromic hearing loss (Van Camp and Smith 2003), and this is probably an incomplete list of the many pathways that are actually involved and hence mutable.

As with so many structures, catalogs of expressed genes are being assembled by many investigators, and the lists include members of most types of genes. Perhaps the most general characteristic is that mutations in genes specifically expressed in normal cells or structures of the mature inner ear, or expressed during its development, lead to defects generally consistent with their expression pattern. The genes affect hair cell structure, cytoarchitecture, mechano- and neurotransmission, transcription factors, and genes for gap junction communication between cells, among others (see, e.g., Petit 1996; Petit, Levilliers et al. 2001); tabulated in (Van Camp and Smith 2003). Some of these genes are specific to hearing apparatus, although most if not all are members of larger gene families and some have additional sites of expression. For example, one cell-junction gene, Connexin26, is frequently found to affect nonsyndromic hearing loss. Many relevant genes have been discovered in surveys of cDNA (expressed genes) from embryonic ear tissue. Although mouse homologs can typically be identified, as so often happens, when comparison has been possible between human disease and effects of mutation in the mouse, the two do not always correspond closely.

The complex transmission of mechanical signal from outer to middle to inner ear might seem rather more elaborate than necessary. Indeed, some hearing is transmitted directly to the cochlear fluid through vibrations induced by sound in the skull, and other animals have more direct transduction (see below). As with so many traits, hearing evolved its elaborations in connection with a diverse set of needs (chance, developmental or genetic connectedness, or response to selection) in a way that had to be consistent with other aspects of craniofacial development.

The sensory organs that allow vertebrates to maintain their sense of balance and equilibrium are also located in the ear, next to the hearing apparatus. (See Figure 12-5 for a comparison of the anatomy of the inner ears of fish, amphibians, birds and mammals.) These are the three mutually perpendicular semicircular canals that comprise the labyrinth and detect and convey rotational movement and the two otolith

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