Anatomy of the Ear

• The ear is the organ for balance and hearing
• Organized structurally and functionally to transfer sound waves to auditory receptors without losing energy

Divisions of the ear

• External ear concentrates and conducts sound waves to the middle ear
• Middle ear conducts sound waves from the tympanic membrane through auditory ossicles to the inner ear
• Inner ear converts sound waves to nerve receptors in the cochlea and semicircular canals (for balance)

External Ear

• Consists of the auricle and the external acoustic meatus
• The auricle is mainly elastic cartilage, with the lobe consisting of adipose tissue, all covered by skin
• Lateral half of the auricle is cartilaginous and S-shaped and contains hairs, sebaceous glands (modified sweat glands), and ceruminous glands that secrete earwax
• Medial two thirds of the auricle consists of bone, with very thin skin
• Tympanic membrane: skin on the medial half continues on the external surface

Tympanic Membrane

• Oval-shaped, approximately 1cm in diameter
• Thin and transparent
• Consists of a superior flaccid part (pars flaccida) and a tense remainder (pars tensa)
• Concave from the outside, with the point of the funnel fixed to the malleus at Umbo
• Consists of thin skin on the outside, a mucous membrane on the inside, and connective tissue in between
• Innervated on the external surface: by the auriculotemporal nerve and the auricular branch of the vagus nerve
• Innervated on the internal surface: by the glossopharyngeal nerve

Middle Ear Walls

• Petrous part of the temporal bone forms the structure
• Lateral wall is membranous, formed by Tympanic membrane
• Medial wall is the labyrinthine wall, featuring the oval window (fenestra vestibuli) and round window (fenestra cochlea), which communicate with the inner ear
• Anterior wall is the carotid wall, separating the tympanic cavity from the carotid canal
• Contains apertures to the auditory tube and the canal for the tensor tympani muscle
• Posterior wall is the mastoid wall, featuring the aditus to the mastoid antrum to mastoid air cells
• Roof is the tegmental wall, located between the tympanic cavity and the floor of the middle cranial fossa
• Floor is the jugular wall near the tympanic cavity and the superior bulb of the internal jugular vein

Middle Ear Contents

• Auditory Ossicles form a bridge across the middle ear cavity
• Function: Receive sound waves from the tympanic membrane, transfer them from the malleus to the incus, and then to the stapes; allows stronger waves to reach the small stapes
• The stapedius originates from the posterior wall of the tympanic cavity on the pyramidal eminence and inserts on the neck of stapes
• Primary function is to protect the ear by muffling sound by tilting stapes, and is innervated by Facial n. (CNVII)
• Tensor tympani originates from the superior surface of cartilaginous auditory tube, greater wing of sphenoid, petrous part of temporal bone, and inserts on Handle of malleus
• Primary function is protective: Muffling sound by pulling handle of malleus medially, and is innervated by Mandibular n. (CNV3)
• Aperture of the Eustachian tube extends (pharyngotympanic tube / auditory tube) to nasopharynx
• Chorda tympani is a branch of the facial nerve (CNVII)
• Tympanic plexus of nerves formed by fibres of N.VII, N.IX, sympathetic nerves

Pharyngotympanic Tube

• Opens in the nasopharynx posterior to the inferior meatus
• Posterolateral ⅓ is bony, rest is cartilaginous
• Lined by a mucous membrane, continuous with the tympanic cavity and nasopharynx
• Opened by Levator veli palatini and Tensor veli palatini
• Arterial supply: Pharyngeal a., Middle Meningeal a., artery of the Pterygoid canal
• Innervated by the Tympanic plexus and Pterygopalatine ganglion (anterior)
• Receives Lymphatic drainage from deep cervical nodes

Internal Ear

• Contains end organs for hearing and balance
• Made up of petrous bone
• Bony labyrinth is a key structure
• Cochlea is a component of the bony labyrinth
• There are 2 small membranous sacs, known as the Utricle and Saccule
• Houses the Vestibule
• Features Oval and round windows
• Includes Three semicircular canals
• Internal ear is lined with endosteum and contain perilymph (liquid)
• System of continuous bony canals and cavities

Bony Labyrinth

• Cochlea: Shell-shaped part that presents as a spiral
• Cochlea: Contains the cochlear duct
• Cochlea: Contains 2.5 windings around central axis (Modiolus); has blood vessels and a cochlear nerve
• Features: Spiral plate, Scala vestibuli and Scala tympani
• Cochlear aqueduct is Linked to subarachnoid space
• Includes Round window

Cochlear Duct

• Spiral tube, blind on one side, saccule on other side
• Triangular in transverse section
• Consists of a Floor that has a Basilar membrane
• Consists of a Roof that has a Vestibular membrane
• Has a Lateral wall that consists of a Cochlear wall with epithelium
• Contains Organ of Corti

Cochlear Duct Membranes

• Contains two membranes that divide canal into three spaces: the Scala vestibule (with perilymph), Cochlear duct (with endolymph), and Scala tympani (with perilymph)
• Contains Organ of Corti, which is the receptor for hearing on the basilar membrane
• The Receptor contains hair cells which respond to vibrations in endolymph
• Hair cells are innervated by bipolar sensory neurons in spinal ganglia, the central processes of which form the cochlear nerve

Membranous Labyrinth

• Vestibule is an Oval-shaped central part of bony labyrinth
• Separated from middle ear by oval window
• Contains the Utricle and Saccule

Membranous Sacs

• Utricle and Saccule are Membranous sacs with sensory epithelium called Macula
• Macula concerns balance/ deals with linear acceleration
• Sensory epithelium innervated by bipolar sensory neurons, the central processes of which form the vestibular nerve

Semicircular Canals

• Anterior, posterior, lateral arrangement
• Positioned at right angles to each other
• Communicate with the vestibule through 5 openings

Semicircular Ducts

• Situated in semicircular bony canals
• Contains expansion (Ampulla) with sensory area called Crista ampullaris
• Crista responds to rotation (also involved in balance)
• Have 5 openings in communication with the Utricle

Conclusion on the Ear

• The ear is innervated by N. vestibulocochlearis (N. vestibularis + N. cochlearis)
• Deafness can be caused by:
• Injuries which consist of Foreign objects and cauliflower ears (blood → avascular necrosis)
• Congenital abnormalities which consist of Malformation, congenital deafness, and deaf/mute
• Infections occur in chronic middle/external ear
• Conduction deafness is caused by poor conduction in external/middle ear = otosclerosis
• Nerve deafness is caused by damage and atrophy to nerve pathway, congenital factors, and physical damage