The world is giving you answers each day, learn to listen.The ear serves as both hearing and equilibrium.External Ear sound collecting organMiddle Ear sound conductor from external to the internal earInternal Ear converts sound waves into nerve impulses and registers changes in equilibrium.INTERNAL EARIts first indication can be found at 22 days.

As the oticplacodesinvaginate and form auditory vesicles (otocysts). The vesicle divides intoA) ventral component gives rise to saccule and cochlear ductB.) dorsal component forms utricle, semicircular canals and endolymphatic ductIn 6th week of devt (ventral component)a) Saccule forms a tubular outpocketing at its lower pole.b) Cochlear duct penetrates the mesenchyme in a spiral fashion.Ductusreuniensa narrow pathwayconnects the remaining portion of saccule to cochlear duct.

Cochlear duct differentiates into cartilage which undergoes vacuolization. scalavestibuliseparated with a vestibular membrane from the cochlear duct. scala tympani separated with a basilar membrane

Cochlear duct remains attached to a spiral ligament and partly supported by a future axis of the bony cochlea called modiolus.Cochlear ducts epithelial cells form two ridges:(a) Inner ridge future spiral limbus(b) Outer ridge forms hair cells covered by tectorial membrane together constitute the organ of Corti. The auditory fibers of cranial nerve VIII contributes transmission of impulsesIn 6th week of devt (dorsal component)Development of Semicircular canals (3) namely:Superior semicircular canalPosterior semicircular canalLateral semicircular canal These canals undergo apposition, fusion, and disapperance. Only five crura enter the utricle, (3) with ampulla, (2) without ampulla.Crus ampullareforms when one end of each semicircular canal dilates.Crus nonampullare (does not widen) forms when superior and posterior semicircular canal fused.

There are sensory cells for maintenance of equilibrium which includes:cristaampullarisfound in ampullae which forms a crest maculaeacusticae develops in the walls of utricle and saccule Both are carried to the brain by vestibular fibers of cranial nerve VIII.

statoacoustic ganglion supplies sensory cells to the organ of Corti, saccule, utricle, and semicircular canals.

MIDDLE EARWhich is composed of tympanic cavity, auditory/eustachian tube as well as the ossicles.

Where does the tympanic cavity came from?It is derived from the first pharyngeal pouch. When the pouch expands it comes in contact with first pharyngeal cleft and so the distal part of the pouch called tubotympanicrecesswidens and gives rise to TYMPANIC CAVITY. The proximal part narrows and forms EUSTACHIAN TUBE.

How about the OSSICLES?

It forms during 8th month of development and composed of Malleus and Incus derived from 1st pharyngeal arch, Stapes is derived from 2nd pharyngeal arch

The muscle for malleus is called tensor tympani are innervated by the mandibular branch of the trigeminal nerve.The muscle for stapes is called stapedius muscle is attached to stapes and is innervated by facial nerve.

When the tympanic cavity expands by vacuolization of surrounding tissue it forms TYMPANIC ANTRUM.

After birth, the tympanic cavity invades bone in the mastoid process, and air sacs are formed also called PNEUMATIZATION.

EXTERNAL EARexternal auditory meatuseardrumor tympanic membraneauricle.

External auditory develops from the 1st pharyngeal cleft. At the 3rd month epithelial cells proliferate forming a solid epithelial plate, the meatal plug. At 7th month the plug dissolves and the epithelial lining participates in formation of definitive eardrum.Eardrum or Tympanic membraneIt is made up of: Ectodermal epithelial lining at the bottom of the auditory meatus Endodermal epithelial lining of the tympanic cavity Intermediate layer of connective tissue

The major part of eardrum is attached to malleus and the remaining portion forms separation between external auditory meatus and tympanic cavity.AuricleIt develops from 6 mesenchymal proliferations at the dorsal ends of the 1st and 2nd pharyngeal arches. AURICULAR HILLOCKS- are swellings on each external meatus, later fuse and form the definitive auricle.CLINICAL CORRELATES: Congenital deafness - CONGENITAL DEAFNESS- (DEAF-MUTISM) Abnormal development of the membranous and bony labyrinths or by malformations of the auditory ossiclesand eardrum. Tympanic cavity and external meatus are absent External ear defects- most common, minor and severe abnormalities. May cause psychological and emotional trauma and are associated with other malformations. Maybe due to chromosomal syndromes. Preauricular appendages and pits- skintags and shallow depressions anterior to the ear, abnormal auricular hillocks.