eardrum

The eardrum, or tympanic membrane, is a thin, cone-shaped membrane that marks the boundary between the external auditory canal and the middle ear. It functions as the first contact point for sound waves entering the ear and plays a key role in the mechanical transmission of sound to the inner ear.

Anatomy and structure: The tympanic membrane is about 8–9 mm in diameter and is concave. It consists of three layers: an outer cuticular layer of stratified squamous epithelium, a middle fibrous layer that provides tensile strength, and an inner mucosal layer continuous with the tympanic cavity. The membrane has two regions: the taut pars tensa, which makes up most of the surface, and the thinner pars flaccida at the superior part. The umbo is the point where the handle of the malleus attaches, and the cone of light is a diagnostic reflection seen on otoscopy. The membrane is anchored at the fibrocartilaginous annulus around its edge.

Function and mechanics: The tympanic membrane converts sound waves into mechanical vibrations that are conveyed to the auditory ossicles (malleus, incus, stapes). The ossicular chain then transmits vibrations to the inner ear. The middle ear muscles, the tensor tympani and stapedius, can tighten the membrane and ossicles to dampen loud sounds, forming part of the acoustic reflex.

Innervation, blood supply, and clinical relevance: The outer surface is primarily supplied by the auriculotemporal nerve (CN V3) with contributions from other nerves; the inner surface receives innervation via tympanic branches of the glossopharyngeal nerve (CN IX) and the tympanic plexus. Blood supply is from the deep auricular arteries and other tympanic branches, with venous drainage to the surrounding venous system. Clinically, perforation from trauma or infection can occur, affecting hearing; tympanoplasty and tympanometry are common diagnostic and treatment tools. Developmentally, the membrane forms from a fusion of ectoderm and endoderm layers with a fibrous middle layer.