L42. GA - Orbit & Eye

Gross Anatomy: Ear - Session Objectives

• Students will accurately discuss the functional anatomy of the external ear, applying anatomical knowledge to clinical problems.
• Students will accurately discuss the functional anatomy of the middle ear, applying anatomical knowledge to clinical problems.
• Students will accurately discuss the anatomy of the internal ear, applying anatomical knowledge to clinical problems.

Supplemental Reading

• Gray's Anatomy for Students, 4th Edition (2020) by Drake, Vogl, and Mitchell (Elsevier), chapter 8, section on the ear.

Session Outline - Anatomy of the External Ear

• Auricle: A single elastic cartilage structure, covered on both sides by hairy skin.
• External Acoustic Meatus (EAM): A cartilaginous (outer 1/3) and bony (inner 2/3) tunnel leading to the tympanic membrane. Contains modified sweat glands (cerumen production).
• Sensory Innervation: Innervation from multiple nerves, including the great auricular, lesser occipital, auriculotemporal, auricular branch of facial, and auricular branch of vagus nerves.
• Tympanic Membrane (TM) / Ear Drum: A thin, semitransparent membrane separating the external and middle ear. Attached to the temporal bone by fibrocartilage.

Session Outline - Anatomy of the Middle Ear

• Bony Tympanic Cavity: A narrow cavity within the petrous part of the temporal bone (middle ear cavity).
• Boundaries/Walls: Includes roof (tegmen tympani), floor (internal jugular vein), lateral wall (tympanic membrane), medial wall (internal ear compartment), anterior wall (auditory tube and carotid canal), and posterior wall (mastoid).
• Structural Relationships: Connects to the nasopharynx via the pharyngotympanic (Eustachian) tube, enabling air pressure equalization.
• Functions: Amplification of sound waves and regulation of sound intensity via the ossicle chain (malleus, incus, stapes).
• Clinical Applications (Middle Ear): Include otosclerosis, otitis media, CN VII paralysis, tonic tensor tympani syndrome.

Session Outline - Anatomy of the Internal Ear

• Bony and Membranous Labyrinths: The cochlea, vestibule, and semicircular canals within the temporal bone.
• Cochlea: Fluid-filled structures responsible for sound transduction into electrical signals. Includes scala vestibuli, scala tympani, and scala media (cochlear duct).
• Function in Hearing: The transmission of sound waves through fluid-filled chambers, converting them to electrical signals sent to the CNS.
• Clinical Applications (Internal Ear): Sensorineural deafness, and imaging approaches.

Ear Compartments and Sound Transmission

• External ear: Collects and channels sound waves to the tympanic membrane.
• Middle ear: Amplifies sound waves and transmits them to the inner ear by way of ossicle chain.
• Inner ear: Transforms sound waves into electrical signals, relaying this information to the brain via the auditory nerve.

Clinical Applications of External Ear Anatomy

• External ear disorders: Cartilaginous auricle damage (cauliflower ear), bacterial infections (swimmer's ear).
• Otoscopic TM assessment: Examination of the tympanic membrane for perforations or other abnormalities

Clinical Applications of Middle Ear Anatomy

• Otosclerosis (otospongiosis): Middle ear disorder causing hearing loss due to abnormal bone growth.
• Otitis media: Middle ear infection often associated with the auditory tube and spread into the middle ear.
• Stapedius muscle paralysis (e.g., Bell's palsy): Facial nerve dysfunction causing loud sound perception.
• Tensor tympani syndrome: Muscle spasms affecting the tensor tympani, leading to various symptoms.