Basilar membrane

The computer model allows use to predict what is going in the real organ of Corti (Figure 9.6). However, most experimental data currently relates only to the motion of the basilar membrane. By comparing the model response under different experimental conditions (Figure 9.7), we can get valuable insight into how the cochlear amplifier operates. The... 

A transverse cross section through the cochlea is shown in Fig. 6.2. Two fluid-filled spaces, the scala vestibuli and the scala tympani, are separated by the cochlear partition. The cochlear partition is bounded on the top by Reissner s membrane and on the bottom by the basilar membrane, which in turn forms part of the organ of Corti. A more detailed view of the organ of Corti (after Rasmussen [Rasmussen, 1943]) is presented in Fig. 6.3. 

The tectorial membrane rests at the top of the organ of Corti, and the basilar membrane forms the base. Two types of hair cells are found along the basilar membrane. There are three rows of outer hair cells and one row of inner hair cells. The outer hair cells form part of the mechanical system of the cochlear partition, while the inner hair cells provide transduction from mechanical motion into neural firing patterns. There are about 30,000 nerve fibers in the human ear. The vast majority are afferent fibers that conduct the inner hair cell neural pulses towards the brain approximately 20 fibers are connected to each of the 1,500 inner hair cells. Approximately 1,800 efferent fibers conduct neural pulses from the brain to the outer hair cells [Pickles, 1988],... 

Rhode, 1971] Rhode, W. (1971). Observations of the vibration of the basilar membrane in squirrel monkeys using the Mossbauer technique. J. Acoust. Soc. Am., 49 1218-1231. 

Outer hair cells have a special function within the cochlea. They are shaped cylindrically, like a can, and have stereocilia at the top of the cell (Figure 13), and a nucleus at the bottom. When the stereocilia are bent in response to a sound wave, an electromotile response occurs. This means the cell changes in length. Therefore, with every sound wave, the cell shortens and then elongates. This pushes against the tectorial membrane, selectively amplifying the vibration of the basilar membrane. This allows us to hear very quiet sounds. 

When a sound pressure wave impinges on the ear, it is amplified by the external auditory meatus and causes the tympanic membrane to vibrate in a characteristic manner. This vibration is transformed by the auditory ossicles of the middle ear into movements of the stapedial footplate. These movements create pressure waves in the fluids of the inner ear which displace the basilar membrane of the cochlear duct and cause the hair cells located on the top of the basilar membrane to generate electrical potentials. This potential elicits impulses in the auditory nerve. After the auditory nerve, the nerve impulses are transmitted through the cochlear nuclei, the trapezoid body, the... 

Fig. 1 The organization of the human inner ear. The human inner ear is embedded into the temporal bone and contains the sensory epithelia of the cochlea and the vestibular system. In the organ of Corti, IHCs are responsible for afferent signal propagation and OHCs actively amplify sound-induced basilar membrane motion. Hair cells in the otolith organs (utricle and saccule) and the ampullae of the semicircular canals detect linear and rotational movement of the head, respectively, and the overall position of the head. Tags colored in red denote the position of the sensory epithelia red line, organ of Corti open circles, otolith organs closed circles, ampullae. Red arrows illustrate the direction of mechanical movement...

Sellick PM, Patuzzi R, Johnstone BM (1982) Measurement of basilar membrane motion in the guinea pig using the Mossbauer technique. J Acoust Soc Am 72( 1 ) 131-141... 

The snail-shaped cochlea, located in the temporal bone of the skull, contains a bony labyrinth and a membranous labyrinth. The bony labyrinth consists of the otic capsule (the external shell) and the modiolus (the internal axis). The membranous labyrinth, coiled inside the bony labyrinth, consists of three adjacent tubes the scala vestibuli, the scala media, and the scala tympani (O Figure 4-1). The scala vestibuli and the scala media are separated by Reissner s membrane the scala media and the scala tympani are separated by the basilar membrane and part of the osseous spiral lamina. The scala vestibuli and the scala tympani are filled with perilymph, a fluid whose ionic composition is similar to that of cerebrospinal fluid. The fluid sealed inside the scala media, the endolymph, contains a high concentration of potassium. 

Early studies of human cadavers led Von Bekesy to propose that the cochlea works as a wide-range frequency resonator (reviewed by Dallos, 1992 Robles and Ruggero, 2001) (O Figure 4-2). A stiffness gradient from the basal to apical basilar membrane defines the frequency resonance map. The basal portion of the basilar membrane is thicker, shorter, and stiffer than the apical portion. Therefore, the basal portion of... 

Johnstone BM, Patuzzi R, Yates GK. 1986. Basilar membrane measurements and the travelling wave. Hear Res 22 147-153. 

Robles L, Ruggero MA, Rich NC. 1991. Two-tone distortion in the basilar membrane ofthe cochlea. Nature 349 413-414. 

The sub-basilar tympanic cells that line the basilar membrane are atypical in that they show a strong glutamate immunoreactivity that even exceeds that of the hair cells (Fig. 1 A,C). The functional significance of this finding is not clear but it underscores the fact that a high glutamate level cannot always be equated with a transmitter pool (see Chapter 1). 

Figure 8.2 is a cross-sechon of the human ear. Sound is collected and funneled into the ear canal by the outer ear. At the end of the ear canal, the sound impinges upon a membrane called the ear drum. The bones of the middle ear convey the ear drum s vibration to the inner ear. The inner ear consists of a fluid filled Basilar membrane that has tiny hair cells on the inside. The hair cells sense the vibrahon conveyed to the Basilar membrane and convert this into electrical signals that are then conveyed to the brain. 

Human hearing arises from airborne waves alternating 50 to 20,000 times a second about the mean atmospheric pressure. These pressure variations induce vibrations of the tympanic membrane, movement of the middle-ear ossicles connected to it, and subsequent displacements of the fluids and tissues of the cochlea in the inner ear. Biomechanical processes in the cochlea analyze sounds to frequency-mapped vibrations along the basilar membrane, and approximately 3,500 inner hair cells modulate transmitter release and spike generation in 30,000 spiral ganghon cells whose proximal processes make up the auditory nerve. This neural activity enters the central auditory system and reflects sound patterns as temporal and spatial spike patterns. The nerve branches and synapses extensively in the cochlear nuclei, the first of the central auditory nuclei. Subsequent brainstem nuclei pass auditory information to the medial geniculate and auditory cortex (AC) of the thalamocortical system. 

Johnstone B.M. and Boyle A.J.F (1967). Basilar membrane vibration examined with the Mossbauer technique. Science 158 390-391. 

FIGURE 63.1 Finite element calculation for the deformation of the cochlear partition due to pressure on the basilar membrane (BM). Outer hair cell (OHC) stereocilia are sheared by the motion of the pillars of Corti and reticular lamina relative to the tectorial membrane (TM). The basilar membrane is supported on the left by the bony shelf and on the right by the spiral ligament. The inner hair cells (IHC) are the primary receptors, each with about 20 afferent synapses. The inner sulcus (1C) is a fluid region in contact with the cilia of the inner hair cells. 

With a few exceptions of specialization, the dimensions of all the components in the cross section of the mammalian cochlea change smoothly and slowly along the length, in a manner consistent with high stiffness at the base, or input end, and low stiffness at the apical end. For example, in the cat the basilar membrane width increases from 0.1 to 0.4 mm while the thickness decreases from 13 to 5 fim. The density of transverse fibers decreases more than the thickness, from about 6000 fibers per /xm at the base to 500 per jjim at the apex [Cabezudo, 1978]. 

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