![]() Cortical feedback would initiate conformational changes to the outer hair cells to decrease movement within the cochlea (i.e., dampen the noise). One example would be when an individual goes to a loud concert. Because the outer hair cells receive input from cortex, the cortex can start these changes to protect the health of hair cells in the presence of loud environments. This form of stiffening can dampen the excitation of hair cells and thus alter what sound transmits through the auditory system. These neurons are special in that they can contract the length of their cell body which alters the stiffness of the basilar membrane. Outer hair cells synapse on only 10% of the spiral ganglion neurons. Thus, the inner hair cells facilitate a majority of auditory processing. ![]() Most (90%) of auditory nerve fibers receive their input from the inner hair cells. These axons make up the auditory nerve (Figure 1B). Neurons within the spinal ganglion have peripheral axons that synapse at the base of the hair cell soma. Unlike other cells within the brain, hair cells within the Organ of Corti of the cochlea do not have axons. This arrangement of cells is called a tonotopic gradient. In this way graded flexibility allows hair cells within the cochlea to respond to a specific range of frequencies from high at the base to low at the apex of the cochlea. This shift in flexibility and altered anatomy influences how the basilar and tectorial membranes move and cause the hair cells to respond to lower frequencies. As you move toward the apex of the cochlea, there is more flexibility within the cochlea and the stereocilia length is more than twice as long as hair cells at the base. Therefore, cells near the oval window (base of the cochlea) respond to high frequencies. The anatomy of the region close to the oval window is stiffer and hair cell stereocilia shorter. How the tectorial and basilar membranes move changes depending on the location within the cochlea. Depending on the direction of the shift, the movement will mechanically open or closes potassium channels to facilitate activation or deactivation of the cell. This shift bends the stereocilia with respect to the cell body of the hair cells. At the apex of each cell, stereocilia connect to a second membrane (tectorial membrane) within the scala media (Figure 1B).Īs the scala vestibule and scala tympani oscillate, the basilar membrane shifts with the tectorial membrane. The base of these cells is embedded within the basilar membrane. It houses mechanical receptor cells: 3 rows of outer hair cells and one row of inner hair cells. The Organ of Corti resides on the basilar membrane inside the scala media. Waves from these regions press against and transmit wave energy to the scala media through the basilar membrane (within the floor of the scala media). Oscillation of the oval window induce waves through the scala tympani and then scala vestibule of the cochlea. Between these fluid-filled areas is the scala media (Figure 1B). It is continuous with the scala vestibule (lining the inner portion of the cochlea) at the helicotrema. The scala tympani lies within the outer portion of the cochlea. In cross-section, each aspect of the cochlea has 3 sections: the scala tympani, scala vestibule, and scala media (Figure 2). The cochlea is a fluid-filled (perilymph) structure that spirals 2 ½ turns around a central pillar (modiolus). This conversion occurs within the cochlea of the inner ear. Within the cochlea, mechanical energy converts to electrical energy by auditory receptor cells (hair cells). The 3 middle ear bones amplify this energy and transfer it into the cochlea. Movement of the tympanic membrane initiates vibration of 3 small bones within the middle ear: the malleus, incus, and stapes which transfer the vibration to the inner ear at the oval (vestibular) window (Figure 1A). Contact between the eardrum and environmental pressure waves causes movement of the membrane. Sound waves reach the outer ear and travel down the external acoustic meatus to reach the eardrum (tympanic membrane). Humans typically hear within a frequency range of 20-20,000 Hz. We measure frequency in Hertz (Hz cycles of compression and rarefaction per second). The number of periods of compression and rarefaction within a specified amount of time is the frequency of a specific sound. This causes increases and decreases in pressure (i.e., alternating compression and rarefaction) of air within the environment. Energy waves travel through a medium by moving molecules. Peripheral Auditory System: How sound reaches the brain.
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