149. Functional Anatomy and Functions of the Ear – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• The ear is concerned with hearing and maintenance of balance. It is divided into external, middle and inner ear. The external ear collects sound, the middle ear transmits and amplifies it, and the inner ear contains receptors for hearing and equilibrium.
• The external ear consists of the pinna and external auditory canal. The pinna collects sound and helps determine its direction. The external auditory canal is about 2.5 centimetres long and conducts sound to the tympanic membrane. Cerumen and hairs protect the deeper structures, while the pinna and canal amplify incoming sound by about 10–15 decibels.
• The tympanic membrane converts sound waves into mechanical vibrations and transmits them to the malleus. It also acts as a protective barrier. Its central depression is the umbo.
• The three auditory ossicles are malleus, incus and stapes. The stapes footplate fits into the oval window and transmits vibrations to the inner ear. The round window permits pressure release within the cochlea. Fixation of the stapes in otosclerosis causes conductive hearing loss.
• The middle ear contains tensor tympani and stapedius muscles. Contraction of the stapedius and tensor tympani reduces excessive sound transmission and forms the acoustic or tympanic reflex. It protects cochlear hair cells from prolonged loud sounds. Stapedius paralysis causes hyperacusis. The reflex is too slow to protect against sudden explosive sounds.
• The Eustachian tube connects the middle ear with the nasopharynx and equalizes pressure on both sides of the tympanic membrane. It opens during swallowing, yawning or sneezing. Dysfunction may cause ear pain, fullness and conductive hearing loss, particularly during air travel or upper respiratory infection.
• An important function of the middle ear is impedance matching, which allows efficient transfer of sound from air to cochlear fluid. The tympanic membrane-to-oval-window area ratio and lever action of the ossicles together increase sound pressure by approximately 22-fold.
• The inner ear contains a bony labyrinth and a membranous labyrinth. Perilymph lies between them and resembles cerebrospinal fluid, whereas endolymph fills the membranous labyrinth and is rich in potassium. The cochlea is responsible for hearing, while the vestibular apparatus detects head position and movement.
• The cochlea has three compartments: scala vestibuli, scala media and scala tympani. The scala vestibuli and scala tympani contain perilymph, while the scala media contains potassium-rich endolymph. The scala vestibuli begins at the oval window and communicates with the scala tympani at the helicotrema.
• The stria vascularis maintains the high potassium concentration of endolymph. The organ of Corti rests on the basilar membrane and contains the sensory hair cells responsible for auditory transduction.
• The organ of Corti contains inner and outer hair cells. About 3,500 inner hair cells form a single row and receive approximately 90–95% of the afferent nerve supply; they are the principal receptors for hearing. Outer hair cells occur in three rows and mainly amplify and fine-tune sound, improving sensitivity and frequency discrimination.
• Cochlear hair cells have stereocilia containing actin filaments and tip links. Deflection of stereocilia toward the tallest stereocilium opens mechanically gated cation channels and produces depolarization; movement away produces repolarization or hyperpolarization.
• Afferent fibres form the cochlear division of the vestibulocochlear nerve and carry auditory information to the brainstem. Efferent olivocochlear fibres mainly influence outer hair cells and help regulate cochlear sensitivity and protect against excessive acoustic stimulation.