EAR

I. Overview - specialized for sound detection

A. Outer ear - funnel shaped structure of cartilage and skin that leads to tympanic membrane; directs sound toward tympanic membrane and helps detect source of sound (binaural hearing)

B. Middle ear - air filled chamber that contains bones that link tympanic membrane to cochlea, muscles that dampen sounds; middle ear is linked to nasopharynx by auditory tube; allows for equilibration of air pressure on inner side of tympanic membrane.

C. Inner ear – fluid filled chamber in petrous part of temporal bone; inner ear contains cochlea (hearing) and vestibular apparatus for gravity detection (both innervated by CN VIII) (Note: Inner Ear is covered in Neuro Course)

II. Outer Ear - composed of two parts:

A. Auricle (pinna) - elastic cartilage covered with skin; functions to reflect sound waves. Parts: helix, antihelix, tragus and lobule (Atlas Figs. 7.77, 7.78; Snell 11-68).

B. External auditory meatus -- tube from auricle to the tympanic membrane; posterior to parotid gland and TMJ; anterior to mastoid process. (Atlas Fig. 7.79); Outer third consists of elastic cartilage; contains hairs, sebaceous glands and ceruminous glands (produce cerumen = ear wax); serves to protect tympanic membrane; Inner two thirds is composed of bone lined with skin.

Clinical note: External auditory meatus is curved anteriorly in adults, is straight in children; in adults auricle is pulled up and back to insert otoscope.

III. Middle Ear (Tympanic cavity) - cavity in the petrous portion of the temporal bone that is hard to visualize. (Snell Figs. 11-68 to 11-71); lies below middle cranial fossa

A. Boundaries (Atlas Figs. 7.81, 7.82; Snell Figs. 11-70, 11-71)

1. Roof - tegmen tympani (thin plate of petrous part of temporal bone) separates tympanic cavity from middle cranial fossa.

2. Floor - Jugular foramen lies below cavity; rupture of the internal jugular vein can result in hemorrhaging into the tympanic cavity.

3. Anterior wall – has opening of auditory tube (posterior 1/3 of tube is canal in bony, anterior 2/3 is cartilage); anterior wall also has bony canal containing tensor tympani muscle.

4. Posterior wall – leads to mastoid air cells in mastoid process (opening is call aditus); canal for Facial nerve (CN VII) courses in posterior wall (after passing from medial wall).

5. Medial wall – is lateral wall of inner ear; landmarks – Oval window (fenestra vestibuli) is attachment for stapes; Round window (fenestra cochlea) is other end of coiled cochlea; landmarks – promontory is bulge in wall from first turn of cochlea; prominence of facial nerve canal – horizontal ridge from underlying facial nerve.

6. Lateral wall – tympanic membrane.

Note: Otitis media (middle ear infection) is common in children. Middle ear is functionally a dead end cavity that opens to nasopharynx. Infection can spread from upper respiratory system. Damage to auditory ossicles can cause hearing loss. Prolonged infection in tympanic cavity can spread through tegmen tympani to brain.

B. Auditory ossicles - from lateral to medial: malleus (hammer), incus (anvil) and stapes (stirrup); ossicles amplify effect of vibration (tympanic membrane has 15-20 times greater area than footplate of stapes); this increases force per unit area and helps transmit sound vibrations from air to fluid in inner ear. (Atlas Fig. 7.81; Snell Fig. 11-69)

C. Muscles (Snell Fig 11-70, 11-71)

1.Tensor tympani muscle – origin – canal in anterior wall; insertion - handle of malleus; innervation – V3

2. Stapedius – origin – posterior wall (landmark is pyramid), insertion – neck of stapes; innervation - VII

Actions – both muscles act to dampen movements of ossicles (decrease intensity of sound); tensor also makes tympanic membrane tighter; prevents damage to inner ear; paralysis of muscles produces hyperacousia (sounds seem too loud).

D. Innervation – Tympanic nerve (GVA branch of IX that enters cavity) – nerve forms tympanic plexus that also innervates mastoid air sinus and auditory tube; can give rise to Lesser Petrosal nerve. (Atlas Fig. page 817)

Note: Chorda tympani – Chorda tympani has no function in middle ear; it provides taste to anterior 2/3 of tongue, parasympathetics to submandibular ganglion; however, it leaves facial canal and passes through tympanic cavity and crosses over handle of malleus before exiting via petrotympanic fissure; if pierce tympanic membrane, can lose taste to tongue on that side; this fact may have baffled early physicians and patients. (Atlas Figs. 7.82, 7.83, page 813)