the ear

Human Anatomy and Physiology: The Special Senses - The Ear: Hearing and Balance

1. Structure of the Ear

  • Three Major Areas:

    • External (Outer) Ear: Functions solely in hearing.

    • Middle Ear (Tympanic Cavity): Also functions solely in hearing.

    • Internal (Inner) Ear: Functions in both hearing and equilibrium.

  • Receptors:

    • Separate receptors for hearing and balance that respond to different stimuli and are activated independently.

2. External Ear

2.1 Components of the External Ear

  • Auricle (Pinna):

    • Shell-shaped structure that surrounds the ear canal.

    • Functions to funnel sound waves into the auditory canal.

    • Helix: Cartilaginous rim.

    • Lobule: Fleshy earlobe.

  • External Acoustic Meatus (Auditory Canal):

    • Short, curved tube lined with skin, hairs, sebaceous glands, and ceruminous (earwax) glands.

    • Transmits sound waves to the eardrum.

2.2 Tympanic Membrane (Eardrum)

  • Description:

    • Boundary between the external and middle ears.

    • Thin, translucent connective tissue membrane.

    • Vibrates in response to sound.

    • Transfers sound energy to the bones of the middle ear.

3. Middle Ear (Tympanic Cavity)

3.1 General Characteristics

  • Definition:

    • A small, air-filled cavity lined with mucosa, located in the temporal bone.

  • Boundaries:

    • Flanked laterally by the eardrum and medially by a bony wall that contains the oval and round membranous windows.

  • Structural Features:

    • Epitympanic Recess: Superior portion of the middle ear cavity.

    • Mastoid Antrum: Canal for communication with mastoid air cells within the mastoid process.

3.2 Pharyngotympanic (Auditory) Tube

  • Function:

    • Connects the middle ear to the nasopharynx.

    • Also known as the Eustachian tube.

    • Usually flattened but can open during yawning or swallowing.

  • Pressure Regulation:

    • Equalizes pressure in the middle ear with external air pressure; necessary for optimal tympanic membrane vibration.

    • Distorted sounds can occur if pressure is unequal.

3.3 Auditory Ossicles

  • Function:

    • Transmit vibratory motion from the tympanic membrane to the oval window.

  • Components:

    • Malleus: The “hammer,” secured to the eardrum.

    • Incus: The “anvil.”

    • Stapes: The “stirrup,” base fits into the oval window.

    • Articulated by synovial joints; suspended by ligaments.

  • Muscle Reflexes:

    • Tensor Tympani and Stapedius muscles: Contract reflexively in response to loud sounds to prevent damage to hearing receptors.

3.4 Clinical Observations

  • Otitis Media:

    • Inflammation of the middle ear, often linked to sore throats, especially in children.

    • Results from shorter and more horizontal pharyngotympanic tubes creating an environment prone to infections.

    • Most frequent cause of hearing loss in children; acute forms can cause the eardrum to bulge and become inflamed.

    • Many cases respond to antibiotics.

4. Internal Ear (Labyrinth)

4.1 Structure of the Internal Ear

  • Location:

    • Within the temporal bone, behind the eye socket.

  • Divisions:

    • Bony Labyrinth: A system of channels and cavities filled with perilymph fluid, divided into three regions: vestibule, semicircular canals, and cochlea.

    • Membranous Labyrinth: A series of membranous sacs and ducts filled with potassium-rich endolymph.

4.2 Vestibule

  • Description:

    • Central eg-shaped cavity of the bony labyrinth.

  • Components:

    • Contains two membranous sacs, Saccule (continuous with cochlear duct) and Utricle (continuous with semicircular canals).

    • House equilibrium receptor regions (maculae) that respond to gravity and head position changes.

4.3 Semicircular Canals

  • Orientation:

    • Three canals oriented at different planes: anterior, lateral, and posterior; anterior and posterior are perpendicular to each other, while the lateral canal is horizontal.

  • Function:

    • Lined with membranous semicircular ducts that communicate with the utricle.

    • Ampulla: Enlarged area of the ducts that contains equilibrium receptor regions (crista ampullaris) responsive to angular (rotational) head movements.

4.4 Cochlea

  • Description:

    • Small, spiral, conical bony chamber, analogous in size to a split pea.

    • Extends from the vestibule, coiling around a bony pillar (modiolus).

    • Contains the cochlear duct, which houses the spiral organ (organ of Corti) and ends at the cochlear apex.

  • Chamber Division:

    • Divided into three chambers:

    • Scala Vestibule: Abuts the oval window and contains perilymph.

    • Scala Media (Cochlear Duct): Contains endolymph, where the spiral organ is located.

    • Scala Tympani: Terminates at the round window, also contains perilymph.

  • Connection:

    • Scala tympani and vestibuli connect at the helicotrema at the apex of the cochlea.

  • Membrane Structures:

    • Vestibular Membrane: Roof of the cochlear duct, separates scala media from scala vestibuli.

    • Stria Vascularis: External wall of cochlear duct, secretes endolymph.

    • Floor: Composed of the bony spiral lamina and the basilar membrane, which supports the spiral organ.

  • Spiral Organ:

    • Contains cochlear hair cells arranged in one row of inner hair cells and three rows of outer hair cells.

    • Hair cells sandwiched between the tectorial and basilar membranes.

  • Nerve Pathway:

    • Cochlear branch of nerve VIII runs from the spiral organ to the brain.

4.5 Summary of Internal Ear Structures

  • Bony Labyrinth vs. Membranous Labyrinth:

    • Semicircular Canals:

    • Function: Equilibrium: rotational (angular) acceleration, Receptor Region: Crista ampullaris.

    • Vestibule:

    • Function: Equilibrium: head position relative to gravity & linear acceleration, Receptor Region: Macula.

    • Cochlea:

    • Function: Hearing, Receptor Region: Spiral organ.