20.10: Anatomy of the Ear
Auditory sensation, commonly called hearing, involves the transformation of sonic waves into neural impulses facilitated by the structures of the auditory organ. The prominent, flesh-like structure on the side of the head, called the auricle, directs sound waves towards the auditory canal. The auricle is often mislabeled as the pinna, a term more aligned with mobile structures like a feline's external ear. The auditory canal penetrates the cranium via the external auditory meatus of the temporal bone and culminates at the tympanic membrane. The tympanic membrane, more colloquially known as the eardrum, vibrates when impacted by sound waves. Collectively, the auricle, ear canal, and tympanic membrane form the external ear.
The middle ear is comprised of three small ossicles, or bones. These structures are called the malleus, incus, and stapes, derived from Latin and translated to mean hammer, anvil, and stirrup, respectively. The malleus, connected to the eardrum, meets with the incus, which connects to the stapes. The stapes link with the inner ear, where sound waves are transformed into neural signals, a pivotal point in the auditory process. The middle ear communicates with the pharynx via the Eustachian tube, which balances air pressure on either side of the tympanic membrane. This tube is generally closed, opening only when the pharyngeal muscles contract during swallowing or yawning.
The inner ear, characterized by a labyrinthine structure owing to a series of canals within the temporal bone, is subdivided into two sections. The two subsections are the cochlea and the vestibule, which facilitate hearing and balance, respectively. The neural impulses from these regions are relayed to the brainstem via separate fiber bundles from the inner ear to the brainstem as the vestibulocochlear nerve. Sound transformation into neural signals occurs within the inner ear's cochlear region, which houses the spiral ganglia's sensory neurons. The ganglia within the spiral-shaped cochlea of the inner ear is affixed to the stapes via the oval window.
The oval window forms the commencement of a fluid-filled conduit within the cochlea termed the scala vestibuli. Extending from the oval window, the scala vestibuli traverses above the cochlear duct, the median cavity of the cochlea that hosts the auditory-transducing neurons. The scala vestibuli envelop the cochlear duct near the tip of the cochlea. The fluid-filled conduit returning to the base of the cochlea is known as the scala tympani. Beneath the cochlear duct, the scala tympani ends at the round window, sealed by a membrane that encloses the fluid within the scala. The vibrations of the ossicles, transmitted through the oval window, cause the fluid within the scala vestibuli and scala tympani to undulate. The fluid waves' frequency corresponds with the sound waves' frequency. The membrane sealing the round window protrudes or invaginates in response to the fluid motion within the scala tympani.
