No brain no gain (NBE-E4210, week 7)

This week we were introduced to human auditory system. The background information on the human hearing capability was good way to start the lecture. It was interesting to know that human have the capability to hear between 20 Hz and 20 kHz. The structure of ear was familiar to us, but it was good recap and supported this week’s topic well. The organ of Corti was new topic for us, where auditoory hair cells are located. In fact, we had previously learned the parts of ears in Finnish, so it was good to learn them in English too.

Ossicles are little bones located in the middle ear that amplify the signal of sound. The pressure change caused by the signal is then transferred into cochlea. The ear has a attenuation reflex to loud sound to prevent the loud sound entering the inner ear. The small muscles attache to ossicles contract in the presence of loud noise.

We learned that different part of basilar membrane located in the cochlea responds to different sounds, resulting in different locations of vibrations. For example, apex responds to low frequency sound and base responds to high frequency sound. As the basilar membrane vibrates, the stereocilia that is connected to tectorial membrane start to bend. The direction of bending affects the potassium influx.

The movement of hair cells to one direction opens the K+ channels, resulting in depolarization and opening the Ca2+ channels. The opening of Ca2+ channels causes the glutamate release onto spiral ganglion neuron. In contrast, the bending in the other direction closes the channels, resulting in hyperpolarization. There are many types of sound-frequency coding mechanisms, but the one which we really paid our attention on was phase-locking of spiking at lower sound frequencies, which is a parallel coding mechanism. It supports the basilar membrane based coding, which lack the ability to discriminate very low frequency.

All in all, we are fascinated of the human auditory system, especially about how complicated it is. We deepened our knowledge on how basilar membrane and hair cells can transform sounds into neural impulses and sound localization of the brain. In the end we were shown an interesting video of ventriloquism, where the person was speaking, but it seemed like the doll was talking.