hearing & vestibular systems

when an object vibrates, molecules in the surrounding air alternately condense (push together) and rarefy (pull apart). these fluctuations in air pressure give rise to a sound wave. sound waves travel away at approximately 700 miles per hour.

when the length of the sound wave is between 17 millimeters and 17 meters long. these sound waves are generated when physical objects vibrate between 20 and 20 000 times per second.

the amplitude or intensity of the differences in air pressure between the compressed and rarified air. This dimension determines how far the sound wave will travel.

the frequency of the changes in air pressure. It is measured in hertz (Hz, cycles per second). Each frequency has a corresponding wavelength.

the complexity of the sound wave. Our brains analyze sound wave complexity to identify the source of the sound (e.g., whether it is a piano or trumpet playing middle C). The timbre of a sound refers to the specific mixture of frequencies (fundamental frequency plus overtones) that different instruments emit when the same note is played. It is the complexity of the sound wave. We analyze the timbre of a sound and how the timbre changes over time to identify which instrument made the sound.

false, non-repeating variations in air pressure are perceived as noise, not identifiable notes.

ossicles: malleus, incus and stapes.

where the membrane is thick and narrow, closest to the oval window

• sound is funneled through the pinna (the outer ear) • at the end of ear canal (still outer ear), sounds cause the tympactic membrane (eardrum) to vibrate. these vibrations are transferred to the middle ear. • vibrations of the tympatic membrane cause the ossicles to vibrate, which in turn cause the membrane behind the oval window to vibrate. • vibrations of the oval window are transmitted to the fluid-filled cochlea (inner ear), which is a long coiled tube-like structure that contains sensory neurons.

where the membrane is thin and wide, closer to the end.

hair cells, because they have hair-like extensions called cilia

it consists of the basilar membrane on the bottom, the tectorial membrane on the top and the auditory hair cells in the middle.

sound waves cause the basilar membrane to move relative to the tectorial membrane, which causes hair cell cilia to stretch and bend. these movements physically pull open ion channels, which changes the membrane potential of the hair cells.

inner hair cells and outer hair cells

the cilia of outer hair cells physically attach to the tectorial membrane, whereas the cilia of the inner hair cells sway freely back and forth with the movement of the surrounding liquid. there are 3 times more outer hair cells than inner hair cells, but only the inner hair cells transmit auditory information to the brain. outer hair cells act like muscles to adjust the tension of the tectorial membrane. in this manner, outer hair cells control the sensitivity of the inner hair cells to specific frequencies of sound (different notes)

completely deaf

can hear but not very well.

the cilia of hair cells are connected to each other by tip links, elastic filaments that attach the tip of one cilium to the side of and adjacent cilium.

the point of attachment of the tip links each insertional plaque has a single ion channel in it that opens and closes according to the amount of stretch exerted by the tip link

loud noises can break the tip links that interconnect each cilia. hair cells cannot transmit auditory information without these tip links. tip links grow back within a few hours. tip link breakage generally corresponds to temporary loss (after a loud bang or concert) tip breakage is probably a protective measure, because too much glutamate release onto the cochlear nerve causes permanent cell death 20% of 20-year-olds seem to have some noise-induced hearing loss, presumably due to cochlear nerve damage.

the location of the most active hair cell along the length of the cochlea indicates the fundamental frequency (pitch) we perceive. the main principle of auditory perception is that different frequencies of sound activate hair cells at different spots along the basilar membrane.

true

the rate of neurotransmitter release from the most active hair cell indicates the loudness of the fundamental frequency.

The loss of outer hair cells make people insensitive to soft sounds, and it also makes it hard to identify the fundamental frequency of the sound.