Action Potential In The

In humans (and other mammals) the ear has three major components: the outer, middle, and inner ears (Fig. 23.1A). The outer ear consists of a cartilaginous flange, the pinna, incorporating a resonant cavity that connects to the ear canal and finally to the tympanic membrane. It performs an initial filtering of the sound waves, increasing the sound pressure gain at the tympanic membrane in the 2 to 7 kHz region. It also aids sound localization. Bats, for instance, have highly developed pinnae, with a high degree of directional selectivity. Although less efficient in humans, the outer ear accounts for our ability to distinguish whether sounds come from above or below, in front or behind.

The function of the middle ear is to transmit the sound vibrations from the tympanic membrane to the cochlea. Because of the much higher impedance of the cochlear fluid, the middle ear must also function as an impedance-matching device, focusing the energy of the tympanic membrane on the oval window of the cochlea. If not for impedance matching, much of the energy of the sound waves in air would be reflected by the cochlear fluid. This impedance matching is carried out by the ossicles, three small bones, the malleus, incus, and stapes, that connect the tympanic membrane to the oval window. The tympanic membrane has a much higher surface area than the oval window, and the ossicles act as levers that increase the force at the expense of velocity, resulting in the required concentration of energy at the oval window.

Most of the events central to hearing occur in the inner ear, in particular the cochlea. The vestibular apparatus (the semicircular canals and the otolith organs) are also in the inner ear, but their principal function is the detection of movement and acceleration, not sound. The cochlea is a tube, about 35 mm long, divided longitudinally into three compartments and twisted into a spiral (Fig. 23.1B). The three compartments are the scala vestibuli, the scala tympani, and the scala media, and they wind around the spiral

Scala Media

Figure 23.1 Location and structure of the cochlea. A: Location of the cochlea in relation to the middle ear, the tympanic membrane, and the outer ear. B: Diagram of the cochlea at increased magnification, showing its spiral structure and the relative positions of the two larger internal compartments, the scala vestibuli and the scala tympani. (Berne and Levy, 1993, Fig. 10-6.)

Figure 23.1 Location and structure of the cochlea. A: Location of the cochlea in relation to the middle ear, the tympanic membrane, and the outer ear. B: Diagram of the cochlea at increased magnification, showing its spiral structure and the relative positions of the two larger internal compartments, the scala vestibuli and the scala tympani. (Berne and Levy, 1993, Fig. 10-6.)

Corti Diagram
Figure 23.2 Location and structure of the cochlea, continued. A: Cross-section of the cochlea in the plane indicated by the inset in Fig. 23.1B. B: Enlarged view of the organ of Corti, including the basilar membrane, the tectorial membrane, and the hair cells. (Berne and Levy, 1993, Fig. 10-6.)

together, preserving their spatial orientation. Reissner's membrane separates the scala vestibuli from the scala media, which in turn is separated from the scala tympani by the spiral lamina and the basilar membrane (Fig. 23.2A). The scala vestibuli and the scala tympani are filled with perilymph, a fluid similar to extracellular fluid, while the scala media is filled with endolymph, a fluid with a high K+ concentration and a low Na+ concentration. Sound waves transmitted through the middle ear are focused by the stapes onto the oval window, an opening into the scala vestibuli. The resultant waves in the perilymph travel along the length of the scala vestibuli, creating complementary waves in the basilar membrane and the scala tympani. At the end of the cochlea, an opening between the scala vestibuli and the scala tympani, the helicotrema, equalizes the local pressure in the two compartments. Because the perilymph is essentially incompressible, it is necessary for the scala tympani also to have an opening analogous to the oval window; otherwise, conservation of mass would preclude movement of the stapes. The opening in the scala tympani is called the round window. Inward motion of the stapes at the oval window is compensated for by the corresponding outward motion of fluid at the round window.

Transduction of sound into electrical impulses is carried out by the organ of Corti (Fig. 23.2B), which sits on top of the basilar membrane. Hair cells in the organ of Corti have hairs projecting out the top, and these hairs are attached to a flap called the tectorial membrane that sits over the organ of Corti. Waves in the basilar membrane create a shear force on these hairs, which in turn causes a change in the membrane potential of the hair cell. This is transmitted to nerve cells, and from there to the brain.

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Responses

  • jakub dickson
    What structure connects the scala vestibuli with the scala tympani?
    8 years ago
  • fnan
    How organ of corti transduces vibrations into action potential?
    6 years ago

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