Hearing and Chemical Senses Lecture Review

Hearing: The Nature of Sound

• Stimulus for Sound Perception: Waves of depression (compression) and rarefaction (pulling apart) in a medium, typically air, but also possible in water.

• Demonstration: Ringing a bell causes air molecules to be pushed closer (compression) and pulled apart (rarefaction), creating sound waves.

• Dimensions of Sound Waves:

  • Amplitude: Represents the energy of the sound wave. Perceptually equates to loudness.

    • Measured in decibels (dB).

  • Frequency: Refers to the number of cycles per unit of time (how many times air molecules are pushed together/pulled apart).

    • Perceptually equates to pitch.

    • Higher frequency = higher pitch; lower frequency = lower pitch.

    • Measured in hertz (Hz), which signifies cycles per second.

Anatomy of the Ear: From Sound Waves to Neural Signals

• Outer Ear: Comprises the initial structures that capture and direct sound.

  • Pinna (k p I n n a): The oddly shaped visible part of the ear. Its channels and whorls capture and direct sound energy inward.

    • Crucial for sound localization (determining where a sound is coming from), although humans use subtle head movements rather than independent ear rotation like cats.

  • External Auditory Canal (External Auditory Meatus): The tube extending from the pinna, leading towards the skull. The ear's delicate structures are protected by the surrounding skull bones.

  • Tympanic Membrane (Eardrum): A membrane stretched across the end of the auditory canal.

    • Sound waves channeled in strike the tympanic membrane, causing it to vibrate at the same frequency as the incoming sound wave (e.g., a 500 Hz tone causes the tympanic membrane to vibrate 500 times per second).

• Middle Ear: An air-filled space on the other side of the tympanic membrane.

  • Pressure Equalization: Essential for changes in elevation (e.g., airplane travel). The Eustachian tube connects the middle ear to the sinuses, allowing pressure to equalize. Blockage can cause pain and pressure; yawning helps release air pressure.

  • Ossicles: Three tiny bones, the smallest in the human body, located within the middle ear. These bones transmit mechanical vibrations.

    • Malleus (hammer): Attached to the inside of the tympanic membrane; vibrates in response to its movement.

    • Incus (anvil): Hinged with the malleus; vibrates as the malleus moves.

    • Stapes (stirrup): Hinged with the incus; vibrates as the incus moves.

    • Together, they act as a lever system to amplify and transmit vibrations.

• Inner Ear: Begins with a membrane connected to the stapes.

  • Oval Window: A membrane at the interface of the middle and inner ear. The vibrating stapes pushes on the oval window.

  • Cochlea: A snail-shell shaped bony structure containing three fluid-filled tunnels or tubes.

    • The stapes pushing on the oval window causes the fluid within the cochlea to vibrate, creating waves.

    • Scala Tympani: The bottom tube of the cochlea, through which fluid vibrations travel.

    • Round Window: A pressure release valve at the far end of the scala tympani, dissipating the fluid waves.

  • Cochlear Canal: The middle section of the cochlea, which houses the Organ of Corti.

  • Organ of Corti: A complex grouping of structures responsible for transducing mechanical energy into neural signals.

    • Tectorial Membrane: A stiff, rigid membrane that remains stationary.

    • Basilar Membrane: A membrane separating the cochlear canal from the scala tympani. Unlike the tectorial membrane, it moves by riding the fluid waves, flexing up and down.

    • Hair Cells: Specialized sensory receptors (neurons) lodged within the basilar membrane. They are named for their hair-like projections.

      • Cilia: Little fiber-like projections from the hair cells, clumped together in stacks, with connections between their tips.

      • Transduction Process: As the basilar membrane flexes, the hair cells (and their cilia) are brushed against the rigid tectorial membrane.

        • When cilia are pushed towards their tall end, ion channels at their tips are pulled open.

        • Ions enter the cell, leading to the release of neurotransmitters, generating a neural signal.

        • When cilia are pushed towards their short end, ion channels close, stopping ion entry and neurotransmitter release.

      • Inner Hair Cells: These are the primary cells responsible for coding sound information.

Pitch Perception

• Frequency Theory (Helmholtz, late 1800s):

  • Core Idea: The basilar membrane and hair cells vibrate at the same frequency as the sound energy, directly coding pitch.

  • Example: A 200 Hz tone causes hair cells to bend 200 times per second, resulting in 200 'send signal/stop signal' cycles per second.

  • Limitation/Problem: Neurons have a maximal firing rate (approximately 1000 action potentials per second). Humans can hear pitches up to 20,000 Hz (babies up to 40,000 Hz), which a single neuron cannot achieve.

• Place Theory (Von Békésy, ~50 years later):

  • Core Idea: Different locations on the basilar membrane resonate and vibrate maximally to specific sound frequencies based on its varying physical properties.

  • Basilar Membrane Properties: It is thick and narrow near the oval window, and wide and thin at its opposite end.

  • Principle of Resonance: Similar to an opera singer shattering a wine glass or a bridge resonating to wind speed, different parts of the basilar membrane vibrate strongly at their specific resonant frequency.

  • Mechanism: High pitches cause a significant bend at a specific, localized place on the basilar membrane (near the oval window), where hair cells are then activated. Other areas show a diffuse response.

  • Tuning of Cells: Individual hair cells show