Ear – 3 Anatomic Parts

The Ear External, Middle, Inner Ear – 3 anatomic parts

Function: • Hearing and balance

Consists of 3 parts: • External ear • Middle ear Contained within • Internal ear the petrous portion of temporal bone

External ear Air-filled spaces; obstruction leads to Middle ear conductive hearing loss Inner ear – Fluid-filled space (cochlear duct); dysfunction leads to sensorimotor hearing loss Development of the Ear

• The ear is a composite structure having multiple embryonic origins. o External and middle ears arise from the 1st and 2nd pharyngeal arches and the intervening pharyngeal cleft, membrane, and pouch. o The inner ear develops from an ectodermal otic placode that appears on either side of the developing brain/neural tube Development of the Inner Ear

• The inner ear develops from an ectodermal otic placode that appears on either side of the developing brain rd o 3 week, the otic placode invaginates and pinches off to form an otic vesicle within the head mesenchyme. Differentiates into: o Endolymphatic duct o Pars superior 3 semicircular canals and utricle o Pars inferior cochlear duct and saccule

Membranous labyrinth • The otic placode also gives rise to the sensory ganglia of the vestibulocochlear nerve (CN VIII) Development of the Inner Ear

Membranous labyrinth • The otic placode also gives rise to the sensory ganglia of the vestibulocochlear nerve (CN VIII) • Week 9-23, mesenchyme surrounding the membranous labyrinth (=otic capsule) chondrifies, then ossifies to form the bony labyrinth within the petrous portion of temporal bone Development of the Inner Ear

The otic vesicle stimulates the mesenchyme around the vesicle to condense and differentiate into a cartilaginous otic capsule. Later, vacuoles appear in the otic capsule and coalesce to form the perilymphatic space. The membranous labyrinth becomes suspended in perilymph fluid and the perilymphatic space subsequently develops two divisions: • Scala tympani • Scala vestibuli.

Subsequent ossification of the cartilaginous capsule produces the bony labyrinth.

• Cochlea – perceives sound waves • Vestibular apparatus – perceives orientation, movement, and gravity necessary for balance Development of the Middle Ear

Middle Ear • The 1st pharyngeal pouch elongates to form the pharyngotympanic tube and tympanic cavity. • The 3 auditory ossicles: malleus, incus, stapes develop in mesenchyme adjacent to the tympanic cavity st o Malleus and incus – from 1 arch mesenchyme nd o Stapes – from 2 arch mesenchyme • Last month of gestation – mesenchyme surrounding ossicles regresses and the tympanic cavity expands to enclose the ossicles. Development of the External Ear

External Ear • Auricle (pinna) of the external ear forms from 6 auricular hillocks lined along the lateral edges of the 1st and 2nd pharyngeal arches • The 1st pharyngeal cleft lengthens to form the external auditory canal • The tympanic membrane is a derivative of the 1st pharyngeal membrane that separates the 1st pharyngeal pouch and cleft

1st arch

2nd arch Auricular Abnormalities

Auricular appendage • May result from accessory auricular hillocks

Microtia • Small auricle • Result from suppressed mesenchymal proliferation in * auricular hillocks

Absence of external auditory meatus • May result from atresia of EAM Features of the External Ear

A. Auricle (Pinna) • Consists of elastic cartilage (continuous with cartilage of EAM) covered by perichondrium and thin skin; cartilage gives shape to the ear • Lobule – fat-fill appendage that hangs down from auricle • Attached to side of head by ligaments and muscle • Collects sound waves and directs these into the EAM • Sensory innervation provided by: . Great auricular nerve . Lesser occipital nerve . Auriculotemporal nerve . Vagus nerve . Facial nerve Features of the External Ear

A. Auricle (Pinna) • Blood supply – principally the superficial temporal and posterior auricular arteries

Normal ear Auricular hematoma “Cauliflower” ear Features of the External Ear B. External auditory meatus (~2.5 cm long) • Extends from concha of auricle to tympanic membrane • Propagates sound waves to tympanic membrane • “S”-shaped canal – superior wall is shorter than inferior wall (eardrum slants) o outer 1 cm is cartilaginous and lined by skin which contains hairs and modified sweat glands (ceruminous, ear wax-producing glands) o inner 2.5 cm is bony and lined by thin skin in direct contact with periosteum of bone – explains pain due to inflammation or instrumentation of canal • Sensory innervation (principal nerves) provided by: Isthmus – junction between the cartilaginous and . Auriculotemporal nerve – to anterior and superior walls bony portions of canal; narrowest region of canal . Vagus nerve – to posterior and inferior walls

Tympanic membrane To exam the tympanic membrane via otoscope, the auricle needs to be pulled upward and backward to straighten out the canal.

Otitis externa – “Swimmer’s ear” • A painful condition resulting from an infection in the skin lining the EAM; usually occurs in swimmers (repeated exposure to water results in removal of cerumen and drying of the EAM leading to bacterial or fungal proliferation) Tympanic Membrane = Eardrum Right eardrum

• Dimensions: 9-10 mm long, 8-9 mm wide, 0.1 mm thick Superior • Thin, translucent, shiny, pearly gray in health; reflects a cone of light on otoscopic exam (5 o’clock position in right ear, 7 o’clock position in left ear) • Fiery red, dull, when inflamed • Conical in shape, not flat; apex (=umbo) is directed inward toward the middle ear space; handle of malleus attaches to internal surface of membrane and its tip ends at the umbo Posterior Anterior Umbo

Consists of 3 layers (derivative of 1st pharyngeal membrane): Inferior • Outer layer – continuous with skin on ear canal (ectoderm) • Middle layer – consists of a fibrous sheet that gives the membrane its tension and stiffness (mesoderm) • Inner layer – continuous with mucous membrane lining the middle ear cavity (endoderm)

Two parts: • Pars tensa – represents the bulk of the membrane • Pars flaccida – a small area superiorly between the anterior and posterior malleolar folds that is lacking the middle layer

Sensory innervation: • Anterior half of lateral surface – auriculotemporal nerve • Posterior half of lateral surface – vagus nerve • Internal surface – tympanic branch of glossopharyngeal nerve Inflammation or Perforation of Tympanic Membrane Inflamed left eardrum • Middle ear infections (otitis media) can inflame the tympanic membrane and produce fluid that cause the membrane to bulge toward the examiner and an air-fluid level may be discernible

• Tympanic membrane perforations can be surgically created (for ventilating tubes) or result from infections, trauma (e.g., blows to ear, explosions), or improper removal of ear wax.

Pressure-equalizing (PE) tubes Middle Ear Cavity or Space

• An air-filled, mucous membrane lined space between the tympanic membrane and inner ear structures; mucosa is continuous with the lining of the pharyngotympanic tube, the mastoid Middle Ear (Lateral wall removed) antrum, and mastoid air cells; sensory innervation provided by tympanic plexus (tympanic br of CN IX) • Derivative of the 1st pharyngeal pouch • Space is normally sterile and aerated (devoid of fluid)

Contents: • Ossicles (malleus, incus, stapes) • Two muscles – tensor tympani, stapedius • Chorda tympani nerve • Tympanic plexus (on promontory) – via tympanic branch of glossopharyngeal nerve; gives rise to lesser petrosal nerve

Walls: • Roof – tegmen tympani. Borders the floor of the MCF • Floor – formed by a layer of bone that borders the IJV • Lateral wall – tympanic membrane • Medial wall – features the promontory of the cochlea, oval and round windows, and prominences of the facial and lateral semicircular canals • Posterior wall – aditus to the mastoid antrum, descending portion of facial canal, pyramidal eminence containing the stapedius, and opening for chorda tympani • Anterior wall – features the pharyngotympanic tube and canal for the tensor tympani Middle Ear (Lateral wall removed) Middle Ear Cavity (Anterior wall removed)

Ossicles and Muscles – malleus, incus, stapes; ligaments to hold bones within the cavity; 2 synovial joints (between malleus and incus, and incus and stapes)

• Malleus – attaches via its manubrium to internal surface of tympanic membrane; chorda tympani nerve passes forward between the malleus and incus • Tensor tympani attaches to its handle/manubrium – pulls handle medially tensing the membrane and reducing the amplitude of its vibration; helps prevent injury to internal ear with loud sounds

• Stapes – its footplate attaches to the oval window • Stapedius – attaches to the neck of the stapes; pulls stapes posteriorly to prevent excessive movement to prevent damage to internal ear with loud sounds

Otosclerosis – caused by abnormal bone growth around the stapes and its fixation to the oval window. Leads to slow progressive hearing loss. Middle ear and mastoid air cells

The tympanic cavity is continuous posteriorly with the mastoid air cells via the mastoid antrum. Middle ear infections that enter here can infect the air cells = mastoiditis.

Axial CT Pharyngotympanic Tube/Auditory Tube/Eustachian Tube

Opening of bony portion of auditory tube

Tympanic canaliculus – for tympanic branch of CN IX Cartilaginous portion of auditory tube Pharyngotympanic Tube/Auditory Tube/Eustachian Tube

• Connects the middle ear cavity and nasopharynx • Derivative of the 1st pharyngeal pouch • Consists of a: • Bony part – 1/3 its length (nearest the middle ear cavity) • Cartilaginous/membranous part – 2/3s its length (opens at lateral wall of nasopharynx) • Normally closed, except when we swallow or yawn - serves as a pressure-equalizing valve • Tensor veli palatini muscle is principal muscle for opening tube • Ciliary action moves fluid in the direction of the nasopharynx

Sensory innervation: • Tympanic plexus (via tympanic branch of Blockage of the Eustachian tube isolates the middle glossopharyngeal nerve) ear cavity from the outside environment. The lining of the cavity absorbs trapped air and creates a negative pressure that pulls the eardrum inward. This leads to pain, pressure, and hearing loss. It also leads to accumulation of fluid that may become infected to produce acute otitis media. Auditory Tube: Adult vs Child

Anatomic basis for increased susceptibility of of young children (1-6 yrs of age) to middle ear infections (otitis media)

Netter. 6th edition

~3.5 cm long ~1.3 cm long ~ 45° to horizontal plane ~ 10° to horizontal plane Inner Ear Right side • Located between the middle ear cavity Posterior and the internal auditory meatus • Functions in hearing and balance; mediated by the vestibulocochlear nerve (CN VIII) Bony labyrinth • Membranous labyrinth derived from the otic placodes

Lateral Left side Anterior

Membranous labyrinth Inner Ear – Vestibulocochlear Nerve (CN VIII) Acoustic Neuroma (Vestibular Schwannoma)

• A benign, usually slow growing tumor that develops on the vestibular nerve (from Schwann cells) • Common signs and symptoms: • Gradual hearing loss • Ringing (tinnitus) in the ear • Unsteadiness/loss of balance • Facial muscle weakness Autonomic Pathways in the Head