Sound is the result of vibrating air molecules that produce pressure waves. When air pressure is plotted as a function of the distance from the sound-producing object, the result is a sinusoidal wave. The amplitude of this wave denotes how loud the sound is (measured in decibels), and the frequency denotes the sound’s pitch (measured in Hertz).
When the pressure waves reach the ear, they are funneled from the external ear (the parts of the ear visible from the outside) into the auditory canal. The vibrating air molecules beat on the tympanic membrane, more commonly referred to as the ear drum. Vibrations of the tympanic membrane, trigger the movement of three tiny bones: the malleus, incus, and stapes. These bones contact the oval window, the place where the acoustic information reaches the cochlea.
The signal that reaches the cochlea is greatly amplified because the oval window is much smaller than the tympanic membrane and because the three bones engage in lever actions that provide a mechanical advantage.
Anatomy of the Cochlea
The cochlea is a small, coiled structure that resembles a snail — in fact, the name cochlea is derived from the Latin word for snail. If it were unrolled, the cochlea would appear to be a single tube roughly 35 mm in length. It is in this structure that sound is transduced as a neural signal, but the cochlea also functions as a frequency analyzer, determining the pitch of sounds that enter it.
Within the cochlea are three fluid-filled chambers: the scala vestibuli, scala media, and scala tympani. Within the scala media is a structure called the organ of corti. This structure has two membranes: the tectorial and basilar membranes. These membranes are connected via hair cell neurons, the sensory receptors of the auditory system. The hair cell neurons, despite their name, do not actually have hair. Rather, they feature stereocilia that protrude from the cells into the scala media. See the diagrams at the bottom of this page for a look at the cochlea's anatomy.
How the Organ of Corti Detects Sound Waves
When the sound waves are transferred from the tympanic membrane to the oval and round windows of the cochlea, a traveling wave of the same frequency is sent down the cochlea along the basilar membrane, from the base to the apex. The cochlea is structured such that different places along the basilar membrane respond to different frequencies of sound. Cells at the base of the membrane respond to high frequencies, and cells at the apex respond to low frequencies.
The vertical movement of the basilar membrane that accompanies acoustic stimulation causes a shearing force that rubs the tectorial membrane across the tops of the hair cell neurons. This bends the stereocilia, resulting in the transduction of the acoustic information into an electrical signal sent to the brain.
Sensory Transduction by Hair Cell Neurons
The scala media is filled with a fluid called endolymph. This fluid is rich in positively charged potassium ions. Neurons operate by selectively allowing ions to permeate their membranes, a process that changes the electrical potential of the cell. In the case of hair cells, the movement of potassium ions causes the change in electrical potential.
The movement of the stereocilia in response to a traveling wave open protein channels that allow the potassium ions in the endolymph to move inside the cell. This causes the cell to become more positive. Once the cell is suitably charged, other channels open that allow calcium ions inside. The calcium ions trigger the release of neurotransmitters onto the auditory nerve that ultimately sends the message to the brain.
Inner Hair Cells and Outer Hair Cells
There are two types of hair cell neurons in the organ of corti: inner hair cells and outer hair cells. The inner hair cells are arranged in a single row and perform the sensory transduction process described above. Ninety-five percent of the fibers in the auditory nerve come from these cells.
The outer hair cells, organized in three rows, play a different role in ear-brain communication. After the inner hair cells relay the acoustic information to the brain, the brain processes the message and sends a signal to the outer hair cells via efferent neurons. The outer hair cells then adjust to tune the tectorial membrane. The movement of outer hair cells give rise the phenomenon of otoacoustical emissions, sounds that emanate from the ear itself.
The Primary Auditory Cortex and Brain Regions that Process Auditory Stimuli
The information from the ears are processed primarily in the temporal lobe. The primary auditory cortex is laid out to correspond with the tonotopic map of the cochlea. That is, different areas within the primary auditory cortex process specific sound frequencies. A disproportionately large space in this cortex is devoted to sounds of roughly 2,000 Hz, the frequency of typical human speech.
There also exists a secondary auditory cortex, which contains neural networks important for communication (e.g. Wernicke’s area, which produces speech).
References
Purves, D., Augustine, G. J., Fitzpatrick, D., Hall, W., LaMantia, A., McNamara, J. O., & White, L. E. (Eds.). (2008).Neuroscience (4th ed.). Sunderland, MA: Sinauer Associates.
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