The human ear can be divided into three sections: (1) the outer ear, (2) the middle ear, and (3) the inner ear. Each section has a different role in transmitting sound waves to the brain. View the diagrams below to learn more about the different sections of the ear and how we hear.
Parts of the Outer Ear
The outer ear consists of the visible portion on the side of the head, known as the pinna (1), and the external auditory canal or ear canal (2). The purpose of the pinna is to catch sound waves, amplify them slightly, and funnel them down the ear canal to the tympanic membrane or eardrum (3).
Parts of the Middle Ear
The middle ear is an air-filled cavity that sits between the tympanic membrane (3) and the inner ear. The middle ear consists of the tympanic membrane (3), three tiny bones called ossicles (4), the round window (5), the oval window (6), and the Eustachian tube (7). The tympanic membrane is a very thin structure that separates the external ear canal from the middle ear space. The first ossicle, called the malleus (commonly known as the hammer) is attached to the tympanic membrane. The other end of the malleus is attached to the second ossicle, called the incus (anvil). The incus, in turn, is attached to the third ossicle, called the stapes (stirrup). The base of the stapes (footplate) is located in a depression called the oval window (6). The oval window membrane is one of two membranes that separate the middle ear space from the inner ear. The other is the round window membrane.
The Eustachian tube (7) connects the middle ear space to the upper part of the throat. In its normal state, the Eustachian tube stays closed, but it will open when you yawn, swallow, chew, or hold your nose and blow. The purpose of the Eustachian tube is to provide fresh air to the middle ear space and to equalize pressure between the outer ear and the middle ear. Ever wonder why your ears “pop” when you go up or down in an airplane or an elevator in a tall building? That sound is your Eustachian tube(s) opening and closing to equalize the air pressure in your ears.
Parts of the Inner Ear
The inner ear is encased in the temporal bone (8). The inner ear consists of three parts: the vestibule (9), the cochlea (10), and the semicircular canals (11). The vestibule is the central inner ear cavity, the cochlea is the organ of hearing, and the semicircular canals are part of the balance system.
The cochlea is made up of three compartments (scala tympani, scala media, scala vestibuli) that are separated from each other by two membranes (basilar membrane and Reissner’s membrane). A tiny organ (organ of Corti) sits on top of the basilar membrane. This organ contains hair cells, which convert the mechanical energy from the vibrations of the basilar membrane into electrical impulses. Those electrical impulses are sent to the auditory nerve (12), which transmits the information up the brainstem to the auditory cortex.
How We Hear
The pinna catches sound waves and channels them down the external auditory canal, where they hit the tympanic membrane and make it vibrate. Those vibrations cause the three ossicles to move. The stapes footplate pushes on the oval window membrane, which sets the cochlear fluid in motion. This wave-like motion causes the basilar membrane to vibrate. As the basilar membrane moves up and down, the tiny “hairs” (stereocilia) on top of the hair cells open and close to change the electrical charge of the cell. This results in a release of chemicals (neurotransmitter), which signals auditory nerve fibers to fire. The auditory nerve sends these impulses up to the brain, where the signal is interpreted as sound.
- Normal Hearing - Video clip courtesy of Cochlear Corporation.