The auditory system is interesting because it essentially maintains a "tonotopic" (i.e. organized by sound frequency) map all the way from the cochlea to the region of the Temporal Lobe that contains the auditory cortex. For that reason, this discussion will start with the neural receptors and move to the sensory portions of the brain, rather working from the brain back to the ear.
Externally, ears are shaped to funnel sound into the auditory canal, and then to the tympanum (ear drum). The eardrum vibrates in accordance with the received sound much like a drum head or the diaphragm on a speaker. In fact, if you have ever listened carefully to a snare drum, you can hear the vibration of the drum head in response to sound by the way the drum head in turn vibrates and produces the characteristic rattling sound of the snare.
The Malleus, Incus and Stapes (hammer, anvil and stirrup) are small bones that then transfer the vibratory movements of the tympanic membrane to the fluid filled cochlea. Inside the cochlea is a flexible membrane, the basilar membrane that flexes and moves with frequency of vibration. The Figure at the right shows the arrangement of tympanum (lower left) and cochlea (unrolled, right). The basilar membrane of the cochlea vibrates in a standing wave corresponding to the frequency of sound detected by the ear. Thus the system to this point simply acts as a “transducer” to reproduce the sound waves from the air into the fluid and membrane environment of the cochlea.
This is the point at which the “tonotopic map” starts. The next figure, at left, shows how different frequencies are detected at different points along the basilar membrane. As in the standing wave shown above, the basilar membrane reaches maximum deflection for specific sound frequencies at different points along its length. Thus, all that is needed to encode a particular frequency is to place detectors at the appropriate points along the basilar membrane. In the Mailbag a couple of weeks ago, we looked into *how* a receptor neuron could detect sound when the neuron is *smaller* than wavelength of the sound wave to be detected. This is how it works.
Our final figure, from Gray’s Anatomy, shows the “Organ of Corti” which runs along the basilar membrane and detects the amount by which the membrane is deflected. The neural receptors are the “Hair Cells” with the base of the neuron attached to the basilar membrane, and the top of the neuron attached to the overhanging tectoral membrane. Movement of the basilar membrane compresses or stretches the hair cells, opening and closing some ion channels, thus changing the neuron’s action potential firing rate – the greater the deflection, the greater the change in firing rate.
The cochlea is organized in a manner that detects different sound wave frequencies at positions along its length. Therefore, to keep those frequencies separate in the brain, it is only necessary to keep an accurate wiring diagram for the auditory nerve (Cranial Nerve VIII) from cochlea to auditory cortex. But keeping sound frequencies straight is not the only thing the auditory system does. Differences in tone, pitch, frequency *change* are all necessary for localization and creating a three-dimensional *sound* map of the environment – but more on that in the next blog!
Tune in next time, same blog, same internet!