Anatomy & Physiology of Hearing Study Notes

Anatomy & Physiology of Hearing

Pinna

  • Pinna (auricle) landmarks include:
      - Helix and antihelix
      - Concha (entrance to External Auditory Meatus, EAM)
      - Scaphoid fossa
      - Tragus
      - Antitragus
      - Lobule

  • Function: Channels sound waves into the ear canal.

  • Tissue: Covered with epithelial tissue invested with fine hair.

External Auditory Meatus (EAM)

  • Length: Approximately 2.5 cm long from the concha.

  • Lining: Lined with epithelial tissue (same tissue as pinna).

  • Composition:
      - Outer 1/3 is the Cartilaginous Meatus (cartilage, courses upwards).
      - Inner 2/3 is the Osseous Meatus (temporal bone, courses downwards) which protects the deeper parts of the ear.

  • Function: Channels sound waves towards the tympanic membrane (ear drum).

  • Additional Function: Hair and cerumen (earwax) trap particles to protect the ear.

Tympanic Membrane (TM)

  • Role: Acts as a boundary between the outer and middle ear.

  • Size: Approximately 55 mm² in area.

  • Layers of the TM:
      - Outer Layer: Continuation of the EAM and pinna lining (epithelial tissue).
      - Intermediate Layer: Composed of fibrous (connective) tissue that enhances strength and flexibility.
      - Inner Layer: Continuous with middle ear mucosa (epithelial tissue).

  • Key Landmarks:
      - Umbo: Attachment point of the TM to the malleus.
      - Manubrium: Visible behind TM, aids in orientation.

  • Function: The TM vibrates with sound waves and transfers these vibrations to the ossicles in the middle ear.

Middle Ear Ossicles

  • Ossicular Chain: Composed of three small bones:
      - Malleus (Hammer)
        - Largest ossicle.
        - Attaches to the tympanic membrane.
        - Transmits sound vibrations from the TM to the incus.
      - Incus (Anvil)
        - The middle bone acts as a bridge between malleus and stapes.
        - Transmits sound vibrations along the chain.
      - Stapes (Stirrup)
        - Smallest bone in the body.
        - Connects to the oval window of the inner ear through its footplate.
        - Final point of sound transmission in the middle ear.

Middle Ear Muscles

  • Tensor Tympani
      - Function: Dampens loud sounds by pulling the malleus inward, thus tightening the tympanic membrane.
      - More active in response to self-generated sounds (e.g., chewing, speaking).
      - Innervation: Mandibular branch of the Trigeminal Nerve (CN V).

  • Stapedius
      - Function: Pulls the stapes away from the oval window, stiffening the ossicular chain and reducing movement of the stapes to protect the inner ear from loud sounds.
      - This protective mechanism is termed the "acoustic reflex", minimizing potential damage from loud noises.
      - Innervation: Facial Nerve (CN VII).

  • Both the tensor tympani and stapedius work together to manage sound vibrations entering the inner ear.

Inner Ear

  • Contains structures for hearing and balance.

  • Main parts include:
      - Cochlea (hearing)
      - Semicircular Canals (balance)

  • Function of Cochlea: The hearing organ, where sound vibrations are converted into electrical signals for the brain.

  • Sound vibrations enter through the oval window connected to the stapes.

  • Vibrations create waves in the cochlear fluid; different frequencies activate specific regions in the cochlea.

  • Presbycusis: Age-related hearing loss, commonly characterized by a gradual decrease in the ability to hear high-frequency sounds, affecting many individuals as they grow older.

    • trouble with fricatives

Cochlear Anatomy

  • Inside the cochlea lies the Organ of Corti which contains sensory hair cells (stereocilia).

  • Movement of cochlear fluid bends these hair cells, which leads to the release of neurotransmitters.

  • These neurotransmitters activate the auditory nerve (part of CN VIII).

Auditory Pathway

  • The auditory nerve carries signals through the brainstem, which helps process basic sound information.

  • Signals reach the primary auditory cortex for interpretation of sound as speech, music, or environmental noise.

Auditory Transduction

  1. Outer Ear: Collects and directs sound waves to the eardrum.

  2. Middle Ear: The tympanic membrane vibrates with sound waves, and tiny bones (ossicles) enhance strength through impedance matching, transmitting the vibrations to the oval window.

  3. Inner Ear: Sound vibrations reach the cochlea, creating waves in cochlear fluid that cause hair cells to bend, triggering neurotransmitter release and activating the auditory nerve fibers.

  4. Initial Signal Processing: Signals travel along the auditory nerve to the brainstem for initial sound analysis, aiding in sound localization and pitch recognition.

  5. Advanced Processing: Signals are processed in the primary auditory cortex, where they are interpreted as recognizable sounds such as speech or music.