Ear MI

Outer Ear:

  • Main purpose is to capture sound

  • Pinna = The curved flap of cartilage. The funnel shape catches sound waves and funnels them into the ear canal. It also helps you figure out which direction a sound is coming from.

  • External Auditory Meatus = The tube from the pinna to the eardrum. Moves sound inward and amplifies speech range frequencies. Also produces cerumen (aka earwax) to trap dust and bacteria.

Middle Ear:

  • Tympanic Membrane = (Aka eardrum) The membrane that vibrates when sound hits it (also amplifies the sound) Helps move the sound through the oval window. Vibrations cause the ossicles to amplify the sounds.

    • A perforated/ruptured eardrum (caused by infection, pressure change, or trauma) causes muffled hearing and an increased risk of infection.

  • Ossicles:

    • Malleus = (AKA hammer) Attached to the tympanic membrane—when it vibrates the malleus strikes the incus.

    • Incus = (AKA anvil) Sends the movement to the stapes.

    • Stapes = (AKA Stirrup) Increases the pressure of vibrations and pushes it through the oval window of the cochlea.

      • Otosclerosis is an abnormal bone growth, fusing the stapes in place so it can’t vibrate, leading to conductive hearing loss.

Inner Ear:

  • Cochlea = A snail shaped, fluid filled tube that is lined with hair cells sitting on the basilar membrane. Stapes pushes fluid into cochlea, causing the membrane to move and bending the hair cell’s bristles. This triggers an electrical nerve signal.

    • Base = high frequency

    • Apex = low frequency

    • Cochlear hair cells don’t regenerate, thus noise reduced damage is permanent. (e.g. loud music hearing loss is irreversible)

  • Semicircular Canals = 3 fluid filled loops, one for nodding, shaking, or tilting your head. When you rotate your head, fluid bends the hair cell bundles, signaling the brain about movement.

    • BPPV = tiny calcium crystals fall into the canals and cause dizziness when head moves.

  • Auditory Nerve = Vestibulocochlear nerve with two branches

    1. Cochlear Nerve = Carries hearing signals.

    2. Vestibular Nerve: carries brain signals.

    • Carries everything to brain stem and to the auditory cortex where sound is perceived.

Hearing Loss:

  • Conductive Hearing Loss = Blockage in outer/middle ear (NOT inner ear). Can be caused by ear wax, fluid, ruptured drums, and otosclerosis. Usually fixable.

  • Sensorineural Loss = Cochlear hair cells Cochlear hair cells/auditory nerve damage. Caused by noise, aging, genetics, and infection. May need a cochlear implant. Usually permanent.

  • Mixed loss = Both types at once.

  • Tinnitus = A ringing/buzzing sound only the patient can hear. Caused by cochlear or neural damage, often from nerve exposure. No cure.