Auditory System and Aphasia Notes

The Auditory System

Overview of the Auditory System

• The auditory system is an exterioceptive system involved in the perception of sound.
• It consists of three main anatomical regions:
  • External or outer ear
  • Middle ear
  • Inner ear

External Ear

• Extends from the auricle to the tympanic membrane (eardrum).
• Auricle:
  • Trumpet-shaped flag of elastic cartilage.
  • Attached by ligaments and muscles.
  • Collects sound waves.
• External Auditory Canal (Meatus):
  • Approximately 2.5 cm tube in the temporal bone.
  • Located between the auricle and the tympanic membrane.
  • Contains hairs and ceruminous glands (specialized subcutaneous oil glands).
  • Ceruminous glands secrete cerumen (earwax).
  • Hairs and cerumen prevent dust and foreign objects from entering the ear.
• Tympanic Membrane (Eardrum):
  • Thin, semi-transparent partition made of skin-like tissue.
  • Located between the external auditory meatus (EAM) and the middle ear.
  • Receives sound vibrations and transmits them to the auditory ossicles.

Middle Ear

• Small, air-filled cavity lined by epithelium.
• Lies between the tympanic membrane and the inner ear.
• Separated from the inner ear by a thin bony partition with two small membrane-covered openings:
  • Round window (below)
  • Oval window (above), covered by the stapes.
• Auditory Tube (Eustachian Tube):
  • Connects the middle ear with the nasopharynx.
  • Equalizes pressure in the middle ear with atmospheric pressure.
  • The nasopharynx entrance is usually closed but opens during swallowing and yawning.
  • Equalization prevents reduction in sound transmission and rupture of the eardrum due to sudden pressure changes (e.g., diving).
• Auditory Ossicles:
  • Three tiny bones that bridge the middle ear from the tympanic membrane to the oval window.
  • Malleus (hammer)
  • Incus (anvil)
  • Stapes (stirrup)
  • Couple sound energy from the eardrum to the oval window in the cochlea.
• Functions of the Middle Ear:
  • Equalize pressure on both sides of the tympanic membrane.
  • Ensure free vibration of the tympanic membrane when struck by sound waves.

Inner Ear

• Functions in both hearing and balance.
• Complicated series of canals called labyrinths.
• Two main subdivisions:
  • Bony Labyrinth:
    • Bony cavity filled with watery fluid called perilymph.
  • Membranous Labyrinth:
    • Membrane-lined sac containing endolymph inside.
    • Located within the bony labyrinth.
    • Analogous to a fluid-filled balloon inside a bottle filled with a different fluid.
• Divisions of the Bony Labyrinth:
  • Semicircular canals
  • Vestibule
  • Cochlea

Vestibule

• Oval central portion of the bony labyrinth.
• Contains two membranous sacs:
  • Utricle
  • Saccule
  • Known as otolith organs; respond to gravitational forces.
  • Also called gravity receptors.
• Macula:
  • Patch of sensory cells on the inner surface of each sac.
  • Approximately 2 mm in diameter.
  • Monitors the head's position relative to the vertical position.

Semicircular Canals

• Three canals in the bony labyrinth:
  • Superior (anterior)
  • Horizontal (lateral)
  • Posterior
  • The superior and posterior canals are in diagonal vertical planes that intersect at right angles.
• Ampulla:
  • Expanded end of each canal that opens into the vestibule.
  • The vestibule completes the circle for each of the semicircular canals.
• The canals are mainly concerned with vestibular function and balance.

Cochlea

• Part of the inner ear concerned with hearing.
• Contains the sensory organ of hearing.
• Resembles a snail shell (named from the Greek word for snail).
• Divided into three channels, each separated by a membrane:
  • Scala vestibuli: Superior channel, ends at the oval window
  • Scala tympani: Inferior channel, ends at the round window
  • Scala media (cochlear duct): Contains the organ of Corti
• Organ of Corti:
  • Contains hair cells that generate nerve signals in response to sound vibrations.
  • Known as the organ of hearing.
• Membranes:
  • Vestibular membrane separates scala media from scala vestibuli.
  • Basilar membrane separates scala media from scala tympani.
  • The organ of Corti rests on the basilar membrane.

Organ of Corti Functions

• Transduction: Converts mechanical energy into electrical energy.
• Signal Transmission: Dispatches a coded version of the original sound to the brain, including fundamental frequency, intensity, and timbre.
• Cell Types:
  • Hair cells
  • Support cells
• Structure:
  • Shaped like a "jam roll," spiraling around with the cochlea.
  • The basilar membrane supports the organ at its base.
  • Contains approximately 23,500 hair cells arranged in orderly rows.
  • Hairs protrude through the top of the organ and are embedded in the tectorial membrane.
  • Hairs are the receptors that transduce sound.
• Mechanism:
  • As the basilar membrane moves with sound waves, it pushes and pulls on the tissues above it.
  • The motion of the basilar membrane bends, twists, pulls, and pushes the hairs.
  • Under these physical stresses, the hairs generate electrical signals, stimulating the auditory nerve (acoustic nerve, eighth cranial nerve).
• Auditory Nerve:
  • Bundle of approximately 30,000 individual fibers.
  • Transmits the electrical signal to the hearing centers of the brain.
  • Ultimately stimulates the auditory cortex.

Recap of Auditory Pathway

1. Outer Ear: Sound waves are collected by the auricle and directed along the auditory canal to the tympanic membrane.
2. Middle Ear: The tympanic membrane vibrates, sending vibrations along the malleus, incus, and stapes. The stapes vibrate the oval window.
3. Inner Ear: Vibration produces movement of the basilar membrane.
4. Organ of Corti: As the basilar membrane moves, hair cells of the organ of Corti bend against the tectorial membrane, initiating a nerve impulse.
5. Auditory Cortex: Receives the impulse, which travels along the cochlear branch of the auditory nerve to the medulla. The impulse then travels to the thalamus and the primary auditory area, located in the temporal lobe.

Auditory Cortex

• The ultimate target of afferent auditory information.
• Subdivisions:
  • Primary auditory cortex (A1)
  • Peripheral or belt areas
• Primary Auditory Cortex (A1):
  • Located in the superior temporal gyrus of the temporal lobe.
  • Receives point-to-point input from the ventral divisions of the medial geniculate complex.
  • Contains a precise tonotopic map.
• Belt Areas:
  • Receive more diffuse input from the belt areas of the medial geniculate complex.
  • Less precise tonotopic organization.
• The primary auditory cortex has a tonotopic map of the cochlea, similar to topographical maps in the visual and somatosensory cortices.

Common Auditory Conditions: Aphasia

• Aphasia is a language disorder caused by damage to the brain's language centers, often in the left hemisphere.
• Results from stroke, brain injuries, tumors, infections, or neurodegenerative diseases.
• Primary Characteristic: Anomia (difficulty retrieving words).
• May also experience challenges in comprehension, reading, writing, and mathematical abilities based on the type and severity.

Boston Classification System for Aphasia Subtypes

• Commonly used, though some cases don't fit neatly.
• Aphasia can evolve over time due to recovery or intervention.
• A thorough speech and language evaluation is essential to understand an individual's strengths and difficulties in speaking, listening, reading, writing, and functional communication.

Types of Aphasia

1. Broca's Aphasia (Expressive Aphasia):

   • Non-fluent aphasia where speech output is limited, short, and effortful.
   • Primarily consists of nouns and verbs with little use of functional words.
   • Struggle to form complete sentences, leading to hesitant and laborious speech.
   • Comprehension is generally strong but understanding complex sentences might be challenging.

2. Wernicke's Aphasia (Receptive Aphasia):

   • Fluent aphasia where speech remains fluid and retains normal intonation, but meaning is distorted due to frequent word substitution errors and jargon.
   • Comprehension is typically impaired.
   • Individuals might be unaware of their communication difficulties, leading to frequent misunderstandings.
   • Reading and writing skills are also typically affected.

3. Global Aphasia:

   • Most severe form of non-fluent aphasia.
   • Both speech production and comprehension are profoundly impaired.
   • Individuals often have extreme difficulty finding words and may rely on repetitive phrases or stereotypical utterances.

4. Primary Progressive Aphasia:

   • Neurodegenerative condition that gradually worsens over time due to progressive brain cell degeneration.
   • Primarily linked to frontotemporal degeneration or Alzheimer's disease.
   • Affects language skills without significant recovery.

Occupational Therapy and Auditory Changes

• Occupational therapists accommodate auditory loss during assessments.

Principles for Assessment with Auditory Changes

Intervention and Assistive Technology

• Computers, tablets, and cell phones can be operated through touch technology and voice text.
• Occupational therapists play a role in providing appropriate assistive technology.
• Collaboration with speech therapists to train communication partners.
• Communication is reciprocal; partner training addresses the ability to understand what is being said.

Strategies for Care Partners

• Providing written choices
• Asking closed-ended questions
• Making available writing papers, pens, papers, maps, photos, objects
• Listening actively without interrupting
• Allowing extended response time
• Encouraging them

Supporting Client Advocacy

• Use of aphasia support or information cards to explain communication difficulties to others.