Auditory System

Importance of Hearing

  • Hearing impairments are often considered communication disorders, but they can have wider consequences beyond difficulties in conversation.

  • Poor hearing can increase the risk of falls and injuries, directly affecting disability.

  • It may reduce activity and participation, leading to an inactive lifestyle and decreased quality of life.

  • Primary and secondary prevention of hearing loss should be a priority to promote health and well-being among older people (Viljanen 2009).

Questions to Consider

  • Do we hear with our ears or our brain?

  • What happens if one ear cannot transmit sound?

  • What happens if one Cranial Nerve 8 (CN 8) is affected (peripheral part of the pathway)?

  • What happens if one temporal lobe (one side of the cortex) is damaged?

Structures of the Ear

  • Outer ear

  • Middle ear

  • Inner ear

Vestibular and Auditory Apparatus

  • Cross-section of cochlea.

Organ of Corti

  • Zoomed-in view showing the fluid-filled spaces and the organ of Corti.

  • Basilar membrane is a key structure.

Transduction

  • Converting sounds to neural signals.

Hair Cells within Organ of Corti

  • Role of hair cell stereocilia in mechanosensory transduction.

Tonotopic Organization

  • The apex of the cochlea has the widest part of the basilar membrane and responds to BASS frequencies.

  • The base of the cochlea has the narrowest end of the basilar membrane and responds to HIGH frequencies.

  • BASILAR MEMBRANE importance.

Cranial Nerves

  • Cranial Nerve 8: Vestibulocochlear nerve.

Auditory Function in the Central Nervous System

  • Blue = sensory pathway.

  • Red = motor pathway.

  • Purple = both motor and sensory pathway.

  • Flow of signals from the hearing apparatus to the outcomes of hearing.

Auditory Pathways: Unconscious Pathways

  • To the Superior Colliculus (BRAINSTEM - midbrain): orient to sound.

  • To the Reticular Formation (BRAINSTEM - medulla, pons, midbrain): arouse to sound.

Auditory Pathways: Conscious Hearing

  • Cochlear nerve transmits to:

    • Cochlear Nuclei (BRAINSTEM – medulla/pons junction).

    • Via the Superior Olivary Nuclei (both ipsilateral and contralateral).

    • Ascends through the brainstem to the Inferior Colliculus (BRAINSTEM - Midbrain).

    • Medial Geniculate Body (THALAMUS).

    • Primary Auditory Cortex (CORTEX) - both sides.

  • Some information travels to the contralateral side = Bilateral pathway and tonotopic arrangement throughout the path.

  • Music to MGB, Light to LGB.

Information Travels Upwards

  • Information travels upwards via the Lateral Lemniscus in the brainstem to the Inferior Colliculus.

  • Lateral Lemniscus = main ascending auditory tract.

  • From MGB (in thalamus), neurons project to the primary auditory cortex in a bundle of fibers called Auditory Radiation.

Cortical Areas for Auditory Information Processing

  • Three cortical areas are dedicated to processing auditory information:

    • Primary auditory cortex: site of conscious awareness of the intensity of sounds.

    • Secondary auditory cortex: compares sounds with memories of other sounds, then categorizes the sounds as language, music, or noise.

    • Wernicke’s area: where comprehension of spoken language occurs.

Hearing Impairments

  • Conductive Hearing Loss (CHL): disorders of the external ear (e.g., canal blocked by cerumen/wax) or middle ear – sound air waves not able to transmit to the inner ear.

  • Sensorineural Hearing Loss (SNHL): disorders of the inner ear (hair cells included), cochlear nerve, or central connections.

  • Mixed hearing loss (MHL): mix of both CHL and SNHL.

Bedside Tests for Hearing Loss

  • Weber Test: vibrating tuning fork on the middle of the forehead.

    • Normal: sound is perceived equally in both ears.

  • Rinne Test: vibrating tuning fork on bone behind patient’s ear, then moved to the EAM.

    • Normal/positive test: sound will be heard again at the EAM as air conduction is better than bone conduction.

Hearing Impairments: Conditions

  • Otitis Media (with effusion): middle ear infection (with fluid/swelling).

  • Otosclerosis: fusion of the ossicles due to abnormal bony overgrowth.

  • Presbycusis: the loss of high-frequency hearing with age (likely increased by general loss of hair cells due to long-term noise exposure).

  • Acoustic Neuroma (Schwannoma): a benign tumor of Schwann cells of the CN 8; early symptom can be tinnitus, eventually leading to deafness.

  • Tinnitus: a symptom experienced with many different hearing conditions.

Meniere’s Disease

  • Affects the whole labyrinth, usually from increased endolymph in the membranous labyrinth.

  • Intermittent hearing loss/changes (including tinnitus) and vestibular symptoms (vertigo, nausea, vomiting, nystagmus).

  • Occurs in clusters of ‘attacks’ or regularly, each lasting minutes or hours.

Cortical Deafness

  • Extremely rare form of sensorineural hearing loss – BILATERAL damage to the primary and secondary auditory cortex (no apparent damage to structures of ear).

  • Often caused by stroke, head injury, or birth defect.

  • Patients ‘feel’ deaf and are aware of their inability to hear environmental sounds.

  • Reflex turning to sound can still be intact.

Auditory Agnosia

  • Destruction of secondary auditory cortex; spares the ability to perceive sound but cannot ‘recognize’ sounds.

  • Speech recognition is also affected if it is the left secondary auditory cortex that is damaged.

Auditory Verbal Agnosia

  • Pure word ‘deafness’ – inability to comprehend speech at all and cannot repeat (speech is heard as meaningless noise).

Treatment of Hearing Loss

  • Conductive Hearing Loss: clear out wax, treat infection, tube to help persistent fluid drain (grommets), and various surgical options.

  • Sensorineural Hearing Loss: cochlear implant.

  • Hearing aids: amplify the airwave signals to help them be detected by damaged ears - can be helpful for some types of mild to moderate SNHL (and/or CHL).