Hearing

Sound waves are physical vibrations of air/water, produced by a solid object by making alternating phases of compression and dilution of the medium.

The frequency (= pitch) of a sound is determined by the number of times the object vibrates per second.

  • Frequencies below 16 Hz are called infrasounds

  • Frequencies above 20,000 Hz are called ultrasounds

The intensity of a sound corresponds to the energy it contains, and depends on the amplitude of the vibration.

The wavelength of the sound wave is determined by its velocity divided by the frequency. Sound propagates at 334 m/s in air, while it’s much faster in water, thus making the wavelengths longer.

Invertebrate ears

  • Many invertebrates detect sound by direct vibrations via the mechanoreceptors in their legs.

    • E.g. spiders use substrate waves to locate prey, and water striders use water waves caused by prey animals on the water surface.

  • Only a few groups have developed true hearing, e.g. crustaceans, spiders, and insects

    • E.g. cerci of cockroaches

    • Johnston’s organ on mosquitoes

    • Tibiae on forelegs of crickets (abdomen on grasshoppers and cicadas)

    • Ears on abdomen of moths to avoid bats

Vertebrate ears

  • Use hair cells just like in the vestibular organs

    • React to deflections of a few nanometers

Human ear

  • Outer ear

    • Pinna collects auditory stimuli

      • Funnels them into the auditory canal toward the tympanum

      • Also modifies the incoming sound, allowing directional hearing

  • Middle ear

    • Acts as an amplifier of the incoming sound

    • Sound causes vibrations of the tympanic membrane

      • Membrane vibration is transferred via the ossicles (malleus, incus, and stapes) to the oval window of the fluid-filled cochlea

  • Inner ear

    • The oval window causes movement of the fluid in the cochlea and subsequent stimulation of the hair cells

  • The organ of Corti

    • The cochlea is divided into three chambers by two membranes (basilar membrane and Reissner’s membrane

      • Attached to the basilar membrane lies the organ of Corti = the hearing organ

        • 3 rows of outer hair cells and 1 row of inner hair cells connect the basilar membrane with the tectorial membrane

        • Each hair cells has 20-100 stereocilia

          • Stand in several rows, connected by tip links

When sound causes the oval window to vibrate, a pressure wave moves along the cochlea and causes the basilar membrane to vibrate up and down. The basilar membrane and the tectorial membrane will move very slightly relative to each other, causing deflection of the stereocilia. The tip links will stetch and open membrane channels, and then subsequently relax and close membrane channels. This generates an electrical signal in the hair cells.

Frequency discrimination

  • Takes place along the basilar membrane

    • A long pressure wave wanders along the membrane and causes it to vibrate.

- The membrane is narrow and stiff at the base of the cochlea, and broad and flexible at the apex.

  • Low sound frequencies cause higher amplitudes of vibration at the apex end

  • High frequencies cause larger vibrations at the basal end

Other vertebrates

  • Many mammals have large pinnae that are movable.

    • Can listen to sounds coming from a certain direction

  • Large animals are generally more sensitive to low frequencies, while small animals are usually more sensitive to high frequencies.

  • Bats and toothed whales use ultrasound for echolocation, since small prey animals only reflect sounds of short wavelength

  • Birds, reptiles, and amphibians do not have outer ears and pinnae

    • Instead, the tympanum lies exposed

      • Only one or two middle ear bones (the columella, homologous to the stapes + extra columella (missing in some groups)

        • Attached to the oval window just like in mammals

    • Birds and reptiles have the lagena (homologous to mammalian cochlea) and builds the inner ear

    • Barn owls have facial feathers (facial ruff), which collects and directs sound to the ears

      • Have spatial differences in the vertical position of the ears, thus allowing them to locate sounds in more planes than the horizontal plane (due to sound waves hitting the ears at milliseconds difference)

    • Amphibians have a third bone in the middle ear (operculum)

      • Inner ear (sacculus) contains hearing organs

      • Has both basilar papilla and amphibian papilla

        • Much simpler than in other tetrapods

        • Many frogs produce calls with two frequency components, heard by the two different hearing organs in the sacculus

  • Fish hear through their whole bodies (hearing in water is much easier)

    • Some species, bones or swim bladder amplify sound waves

Auditory processing

  • Auditory nerve is part of the cranial nerve VIII

    • Projects to the cochlear nucleus in the medulla

    • Signals from both ears are integrated in the olive nuclei and then sent to the thalamus and finally the primary auditory cortex.