Human Ear

The mammalian ear is built in three connected parts arranged one behind the other, and the simplest way to remember them is that each division does a separate stage of the work — the outer (external) ear collects sound, the middle ear carries and amplifies the vibrations, and the inner ear converts them into nerve signals and also senses balance. The ear is meant for both balancing and hearing, and the sense of hearing is finally interpreted in the auditory area of the temporal lobe of the cerebral cortex, so the ear gathers and converts the signal but the brain is what actually "hears" it.

• The mammalian ear is made of three parts — external (outer) ear, middle ear and inner ear.
• A useful rule is that the eardrum marks the boundary between the outer and middle ear, while the oval window marks the boundary between the middle and inner ear.

Diagram questions are common, so it helps to know every label that appears on the standard cross-section of the ear, grouped by which division it belongs to. The three ossicles are stapes, incus and malleus, all sitting in the middle ear.

• The vestibulocochlear nerve splits into a vestibular branch (balance) and a cochlear branch (hearing).

The outer ear is the visible part of the ear together with the short tube that leads inward, and its task is simply to collect sound waves from the air and direct them onto the eardrum. The flap of skin and cartilage we see on the side of the head is the pinna (also called the auricle), which acts like a funnel; the sound it catches then passes down the auditory canal, a short tube that ends at the eardrum or tympanic membrane.

• The external ear collects sounds.
• Sound travels through the auditory canal, a short tube that ends at the ear drum.

The middle ear is a small air-filled cavity (the tympanic cavity) that lies just beyond the eardrum, and its key feature is a chain of three tiny bones called the ear ossicles. These three bones pass the vibration of the eardrum across the cavity to the inner ear, and as they do so they amplify the force of the vibration before handing it on at the oval window. The middle ear is connected to the throat by the Eustachian (auditory) tube, which keeps the air pressure on both sides of the eardrum equal.

• The 3 ear ossicles are Malleus (Hammer), Incus (Anvil) and Stapes (Stirrup), in that order.
• The stapes is the smallest bone in the body.

The inner ear is a maze of fluid-filled chambers carved into the temporal bone, and it carries out two completely separate senses with two different structures. The coiled, snail-shaped cochlea is the organ of hearing, while the three loop-shaped semicircular canals, together with the vestibule, form the organ of balance.

• The Organ of Corti is a structure that contains thousands of specialised sensory hair cells with hair-like projections called cilia.
• The inner ear is served by the vestibulocochlear (auditory) nerve — cochlear branch for hearing, vestibular branch for balance.

Hearing is a smoothly co-ordinated chain in which sound energy in the air is changed first into mechanical vibration, then into a fluid wave, and finally into an electrical nerve impulse the brain can read. Tracing this path in order is one of the most reliable ways to answer a "working of the ear" question.

• When sound waves fall on the ear drum, the eardrum starts vibrating back and forth rapidly.
• The stapes strikes the membrane of the oval window and passes its vibration to the liquid in the cochlea, which begins to vibrate.
• The round window (fenestra cochleae) is a membrane connecting the cochlea to the middle ear; it helps dampen the vibrations in the cochlea.

Besides hearing, the ear is the body's main organ of equilibrium, and this job belongs to the vestibular apparatus of the inner ear rather than to the cochlea. The three semicircular canals sense whichever way the head turns, while the vestibule below them senses gravity and straight-line motion. The Eustachian tube and the vestibular complex are the parts responsible for balance.

• The vestibular complex contains receptors that maintain body balance.
• Hearing helps us to listen, learn and communicate; balance helps us to stand, walk and move properly — both are essential for daily life.

To finish, the table below pulls together the single-fact answers that examiners ask most often. The ear converts sound energy into electrical energy for the brain.