Ear Anatomy, Hearing, and Equilibrium

Visual Pathway (Quick Review)

• What you see is a result of electrical impulses from photoreceptors sent to the visual lobe.

• The order of the visual pathway is:

  1. Photoreceptors (in the retina)

  2. Optic nerve

  3. Optic chiasm (where the split occurs)

  4. Optic tract

  5. Lateral geniculate nucleus of the thalamus (synapse)

  6. Optic radiations

  7. Visual lobe (synapse, where perception of vision occurs)

The Ear: Structure and Function

Regions of the Ear

• The ear is divided into three main regions: External (Outer) Ear, Middle Ear, and Inner (Internal) Ear.

• Lab exams may feature matching questions where structures are identified as belonging to one of these three regions.

External (Outer) Ear

• Structures:

  • Auricle (Ear Flap): Generic term for the outer ear flap. Includes specific components like the helix, antihelix, tragus, and antitragus.

  • External Acoustic Canal / External Acoustic Meatus (Ear Hole): The canal leading inward.

  • Tympanic Membrane (Eardrum):

    • A thin, semi-transparent membrane.

    • Separates the external ear from the middle ear compartment. It is the barrier or wall between them, not strictly part of either.

• Functions:

  • Auricle Functions:

    • Protects the opening of the ear canal from foreign objects.

    • Provides directional sensitivity, helping to localize where sound is coming from.

    • Acts as a funnel, concentrating sound waves into the ear canal, thereby increasing the amount of sound heard. Without it, hearing would be diminished even if sound production remains the same.

  • Tympanic Membrane Function:

    • Vibrates when hit by sound waves, which increases the intensity of the sound waves as they are transmitted inward.

    • Aging can cause scar tissue deposition on the tympanic membrane, making it less flexible and diminishing its ability to vibrate effectively, leading to decreased hearing.

• Ceruminous Glands:

  • Located lining the external ear canal.

  • Secrete cerumen (earwax).

  • Functions of Earwax:

    • A defense mechanism: Helps keep foreign objects (dust, debris, pollutants, viruses, bacteria) from reaching the tympanic membrane, preventing pain and accelerated scar tissue formation.

    • Its sticky nature traps invaders.

    • Makes it difficult for viruses or bacteria to replicate and spread into deeper tissues where blood supply is readily available.

• Earwax and Hearing:

  • Too much earwax can block the ear canal, leading to diminished hearing (similar to wearing earplugs).

  • Proper Ear Cleaning:

    • Q-tips: Not recommended. They tend to pack earwax deeper into the ear canal, potentially pushing it against the tympanic membrane, causing trauma and accelerating scar tissue formation. Q-tips are not marketed for ear canal cleaning by the manufacturer.

    • Ear Lavage: The proper, safe, and effective method. Involves shooting water into the ear canal to loosen and flush out earwax. Drops (commercial or olive oil) can be used beforehand to soften stubborn wax.

• Swimmer's Ear: An infection of the external ear.

Middle Ear

• Location: The cavity immediately internal to the tympanic membrane.

• Structures:

  • Auditory Ossicles: Three small bones involved in sound transmission.

    • Malleus: Connects to the tympanic membrane.

    • Incus: Connects the malleus and the stapes. It is the only ossicle that touches the other two.

    • Stapes: Connects the incus to the oval window (which is part of the bony labyrinth, marking the boundary with the inner ear).

  • Opening to the Auditory Tube (Pharyngotympanic Tube / Eustachian Tube): This tube connects the middle ear to the nasopharynx (upper throat) for pressure equalization; the middle ear only contains its opening, not the entire tube.

  • Muscles: These muscles are not on lab models but are important for lecture.

    • Tensor Tympani Muscle: Attaches to the malleus and tympanic membrane.

    • Stapedius Muscle: Attaches to the stapes.

• Functions:

  • Auditory Ossicles Function:

    • Similar to the tympanic membrane, these bones vibrate and move when sound waves are transmitted to them, increasing the intensity of the sound waves.

    • Aging Impact: Arthritis can cause the articulations (joints) between these bones to fuse, preventing motion and thus diminishing hearing.

  • Middle Ear Muscles Function (Opposite of Ossicles/Tympanic Membrane):

    • They function to decrease the loudness of sound (or decrease the amount of sound waves transmitted to the inner ear).

    • They protect the inner ear from excessively loud sounds (e.g., firecrackers) by contracting in an auditory reflex. This stiffens the tympanic membrane and stops or diminishes the movement of the auditory ossicles, thus preventing the amplification of sound waves and potential permanent hearing damage (deafness).

Inner (Internal) Ear

• Location: Deepest part of the ear, containing structures for both hearing and equilibrium.

• Structures:

  • Bony Labyrinth: The hard, outer casing. Divided into three components:

    • Vestibule: Separates the semicircular canals from the cochlea.

    • Semicircular Canals.

    • Cochlea.

  • Membranous Labyrinth: A series of interconnected sacs and ducts located inside the bony labyrinth, essentially superimposed within its shape.

  • Fluids:

    • Perilymph: A water-based liquid located between the bony labyrinth and the membranous labyrinth.

    • Endolymph: A water-based liquid located inside the membranous labyrinth.

• Sensory Receptors for Hearing and Equilibrium: Hair Cells

  • Hair cells are the common sensory receptors for both hearing and equilibrium.

  • They are stimulated by physical movement: when waves generated in the perilymph and endolymph cause the hair cells to be physically bent or knocked over. This physical displacement generates an electrical impulse.

Hearing and the Cochlea

• Cochlea Functions ONLY in Hearing.

• Cochlear Duct Structure:

  • The cochlea contains a series of ducts: the vestibular duct (containing perilymph), the cochlear duct (containing endolymph), and the tympanic duct (containing perilymph).

  • Hair cells are lined up within the cochlear duct.

• Physiology of Hearing:

  1. Sound Waves: Sound waves (e.g., from a voice) are released into the air.

  2. External Ear Funneling: The auricle and external acoustic canal capture and funnel sound waves.

  3. Tympanic Membrane Vibration: Sound waves hit the tympanic membrane, causing it to vibrate.

  4. Auditory Ossicle Amplification: The vibrations are transmitted to the malleus, incus, and stapes. The movement between these tiny bones further increases the sound waves.

  5. Perilymph Waves: The stapes, attached to the oval window, transfers these amplified vibrations to the bony labyrinth, creating waves in the perilymph.

  6. Endolymph Waves: The perilymph waves, in turn, cause movement in the endolymph inside the cochlear duct.

  7. Hair Cell Stimulation: The movement (waves) of the endolymph physically knocks down the hair cells.

  8. Electrical Impulse: The stimulation of hair cells generates an electrical impulse.

  9. Brain Transmission: This electrical impulse is sent back via the cochlear branch of cranial nerve VIII to the auditory cortex in the brain, allowing us to hear.

• Characteristics of Sound:

  • Loudness / Intensity:

    • Measured in decibels ($dB$).

    • Determined by the number of hair cells stimulated. More hair cells stimulated by larger waves means a louder sound. Fewer hair cells stimulated by weaker waves mean a quieter sound.

    • Dangerous Sound Levels:

      • Regular exposure to sounds above $70$ dB can cause hearing damage over time (e.g., busy traffic, noisy restaurant).

      • $110$ dB: Chainsaw (damage possible in $2$ hours).

      • $120$ dB: Heavy metal rock concert.

      • $140$ dB: Gunshot (immediate danger of hearing damage).

      • $160$ dB: Rocket launch pad (can cause immediate and permanent deafness).

  • Pitch / Frequency:

    • Has nothing to do with loudness.

    • Determined by the location of the hair cells stimulated within the cochlear duct (different regions respond to high notes vs. low notes).

Equilibrium

• Hair cells are also the sensory receptors for equilibrium.

• Vestibule (Utricle and Saccule) Function: Static Equilibrium

  • Static equilibrium (or linear equilibrium): Sense of balance when the head is not rotating or spinning.

  • Examples of disruption: Car sickness, seasickness.

    • Occurs when there is a conflict between visual information and the body's sense of motion and gravity (e.g., on a bumpy road or a large boat with big waves).

    • The brain becomes confused, leading to nausea and discomfort without dizziness.

    • The body can often adapt over time.

• Semicircular Canals Function: Rotational Equilibrium

  • Rotational equilibrium (or dynamic equilibrium): Sense of balance when the head is spinning or rotating.

  • Example of disruption: Dizziness after spinning rapidly (e.g., on a carnival ride).

Age-Related Hearing Loss

• As a natural part of aging, most people will experience diminished hearing due to:

  • Less Flexible Tympanic Membrane: Scar tissue replaces the thin, delicate membrane, making it stiffer and reducing its vibration.

  • Fused Auditory Ossicle Joints: Arthritis can cause the articulations between the malleus, incus, and stapes to fuse, preventing their movement and sound wave amplification.

  • Hair Cell Death: The hair cells (sensory receptors for sound) gradually die off, especially with prolonged exposure to loud noises.

• Impact on Hearing Aids:

  • Hearing aids work better for mechanical issues (e.g., ossicle fusion, tympanic membrane stiffness) because they primarily amplify sound.

  • They are less effective for hearing loss caused by hair cell death, as there are fewer receptors to process the amplified sound.

  • A major limitation of hearing aids is the amplification of all background noise, which can make conversation difficult in busy environments (e.g., restaurants). Modern hearing aids connected to smartphones can help mitigate this by focusing sound input.