EAR

EXERCISE 15: THE EAR

OBJECTIVES

  • Objective 1: Identify the anatomical structures of the external (outer), middle, and inner ear on a diagram or model.

  • Objective 2: Relate the anatomical structures of the outer, middle, and inner ear to their functions of hearing and/or balance.

1. ANATOMY OF THE EAR

  • Structure Identification: Obtain a model of the ear and locate structures described below.

THE OUTER EAR

  • Auricle (Pinna):
      - The large cartilaginous structure at the outside of the ear.   - Functions to collect and direct sound waves into the external auditory canal.

  • External Auditory Canal:
      - An inch-long canal that amplifies sound waves as they travel.   - Leads to the tympanic membrane (eardrum).

  • Tympanic Membrane (Eardrum):
      - Vibrates when sound waves strike it with amplified force.

  • Cerumen (Earwax):
      - A waxy substance lining the auditory canal.   - Traps bacteria and small insects, protecting the outer and middle ear from infections.

THE MIDDLE EAR

  • Location:
      - A small chamber located deep within the temporal bone.

  • Ossicles of Hearing:
      - Three small bones (malleus, incus, stapes) span the width of the chamber:     - Malleus (Hammer): Attached to the tympanic membrane; vibrates at the same frequency as the membrane.     - Incus (Anvil): Connected to the malleus and vibrates at the same frequency.     - Stapes (Stirrup):
          - Attached to the oval window of the cochlea.       - Collectively focuses sound vibrations from the larger tympanic membrane to the smaller oval window.

  • Pharyngotympanic Tube (Eustachian Tube):
      - A flattened tube connecting the middle ear to the upper pharynx.   - Function: Equalizes air pressure within the middle ear when yawning or swallowing.

2. INNER EAR

  • Bony Labyrinth:
      - Composed of the cochlea, vestibule, and semicircular canals.   - Contains perilymph (similar to cerebrospinal fluid).

  • Membranous Labyrinth:
      - Interconnected sacs and tubes filled with endolymph (similar to intracellular fluid).
        - Elevated levels of potassium ions are present in endolymph, crucial for sensory hair cells.

INNER EAR—HEARING

  • Cochlea Structure:
      - Snail-shaped; about 1.5 inches long and coiled approximately 2.75 turns.   - Includes three compartments:     - Scala Vestibuli: Upper portion that initiates vibration from the oval window.     - Scala Tympani: Lower canal connected to the round window.     - Cochlear Duct: Middle duct filled with endolymph, contains the organ of Corti.

  • Organ of Corti (Spiral Organ):
      - Contains hair cells that respond to sound vibrations.   - Vibration of the basilar membrane causes hair cell cilia to bend against the tectorial membrane, generating electrical signals.

3. SOUND INTERPRETATION

  • Pitch Discrimination:
      - Hair cells respond to specific sound frequencies based on their location in the cochlea.   - Basilar Membrane Stiffness:     - Stiffer near the oval window for high-pitched sounds.     - More flexible further away for low-pitched sounds.

  • Signal Transmission:
      - Hair cell bending opens mechanically gated ion channels, altering membrane potential and releasing neurotransmitters to associated neurons.   - Action potentials sent via the vestibulocochlear nerve to brainstem relay centers.

  • Danger of Loud Sounds:
      - Excess energy can damage hair cells, resulting in selective deafness for certain pitches.

4. INNER EAR RECEPTOR PATHWAYS IN THE BRAIN

  • Auditory Pathway:
      - Afferent neurons from cochlear hair cells transmit signals to:     - Pons and medulla via vestibulocochlear nerve (CN VIII).     - Inferior colliculi of the midbrain for auditory reflex integration.     - Medial geniculate body of the thalamus as a relay to primary auditory cortex in the temporal lobe.

  • Cortical Analysis:
      - The brain processes auditory impulses, analyzes pitch and intensity, and recognizes sound patterns and language.

5. INNER EAR—STATIC BALANCE

  • Vestibule:
      - Egg-shaped cavity containing utricle and saccule (membranous sacs).   - Macula: Thickened area with hair cell receptors for static balance.   - Otoliths: Small calcium carbonate grains embedded in gelatinous membranes.     - Respond to head position changes under gravitational pull.

  • Signal Pathway:   - Sensory information sent to the brain to maintain posture when standing or sitting.

6. INNER EAR—DYNAMIC BALANCE

  • Role of Semicircular Canals:
      - Comprised of membranous tubes with endolymph and bony tubes filled with perilymph.   - Arranged at right angles to monitor rotational movement and acceleration.

  • Ampulla and Crista Ampullaris:
      - Hair cells within the ampulla embedded in a gelatinous mass (cupula) provide balance information.

  • Fluid Dynamics:
      - Fluid lags behind during head movement, causing hair cells to bend and initiate neuronal signaling in the vestibulocochlear nerve.

7. FUNCTIONAL CONNECTIONS

  • Integration of Balance Information:
      - Semicircular canals and vestibule provide sensory data regarding body orientation to maintain posture.   - Information transmitted to:     - Brainstem for reflexive posture adjustments.     - Cerebellum for muscle coordination.     - Cerebral cortex for conscious perception of orientation.

TERMINOLOGY

  • ampullae: Enlargements at the ends of the semicircular canals containing dynamic balance receptors.

  • auricle (pinna): Visible outer ear structure acting as a funnel.

  • bony labyrinth: A set of hollow chambers in the temporal bone incorporating the membranous labyrinth.

  • cerumen: Waxy secretion in the external auditory canal.

  • cochlear duct: Middle third of the cochlea; contains endolymph and the organ of Corti.

  • crista ampullaris: Specialized sensory organ in the ampulla for dynamic balance.

  • dynamic balance: Maintaining body position during sudden head movement.

  • endolymph: Fluid resembling tissue fluid within the membranous labyrinth.

  • external auditory canal: Tube carrying sound to the eardrum.

  • incus: Second ossicle (anvil) of the middle ear.

  • macula(e): Organs in utricle and saccule that respond to body position.

  • malleus: Hammer-shaped ossicle in the middle ear.

  • membranous labyrinth: Series of structures with endolymph inside the bony labyrinth.

  • ossicles: Tiny bones (malleus, incus, stapes) in the ear.

  • otoliths: Calcium carbonate grains in the utricle and saccule’s gelatinous layer.

  • oval window: Membrane opening at the cochlea’s entrance.

  • perilymph: Fluid similar to cerebrospinal fluid in bony labyrinths.

  • pharyngotympanic tube: Tube connecting middle ear to pharynx; regulates ear pressure.

  • pinna: The external visible part of the ear.

  • round window: Opening at the end of the tympanic canal that dampens cochlear vibrations.

  • saccule: Compartment in vestibule for static balance receptors.

  • scala tympani: Lower third of the cochlea with perilymph.

  • scala vestibuli: Upper third of the cochlea with perilymph.

  • stapes: Stirrup-shaped ossicle attached to the oval window.

  • static balance: Maintaining position in response to head orientation relative to gravity.

  • tympanic membrane: Eardrum of the ear.

  • utricle: Upper compartment of the vestibule for static balance.

  • vestibule: Inner ear part containing static balance receptors (utricle and saccule).