Understanding Hearing Loss

The external ear is made up of two parts:

1. Pinna: external, visible portion of the ear.
2. External Auditory Meatus: outer ear canal, extending from pinna to the tympanic membrane ™ or eardrum.

The pinna consists of folds of skin and cartilage with some muscular attachments to the back. Sound waves are directed by the pinna into the ear canal. The pinna’s twists and folds enhance high-pitched (or high-frequency) sounds, and these changes help the brain determine the direction of the sound source. Sounds coming from the front and sides are slightly enhanced as they are funneled into the ear canal, while sounds from behind are slightly reduced. This improves our ability to hear what we are looking at while reducing the level of distracting background noise coming from behind. Cupping your hand behind the pinna slightly increases amplification of sounds coming from the front because it effectively enlarges the sound collection surface area.

External auditory meatus is a small, twisting, tunnel-like tube connecting the pinna of the outer ear to the tympanic membrane in the middle ear. The human ear canal is approximately one inch in length (2.5 cm) and 7 mm in diameter (about the size of a pencil eraser). The size and shape of the canal vary among individuals which is an important factor to consider when fitting hearing aids and protective hearing devices. A cranial nerve branch passes just below the back canal wall surface. The walls of the ear canal are sensitive to touch and if the ear canal is touched near this nerve, many people react by coughing (arnold’s reflex). The outer half to two-thirds of the canal is surrounded by cartilage and contains glands that produce cerumen (ear wax), while the inner one-third to one-half is surrounded by bone. The ear canal has a natural resonance that typically boosts sound pitches around 3000 hertz (also referred to as cycles per second) by approximately 10-15 decibels (db).

The middle ear consists of the cavity between your eardrum and the inner ear. It contains the three smallest bones in your body (the malleus, incus and stapes), also called the ossicles. These bones are linked together in a bony chain supported by tiny ligaments and muscles that adjust the tension. The middle ear connects the tympanic membrane to an opening in the inner ear.

Tympanic membrane ™, also called the eardrum, is a concave-shaped layer of membrane at the end of the ear canal. Sounds waves travels down the ear canal until they hit the eardrum, causing it to vibrate. These vibrations are then transferred through the ossicles to the cochlea, a fluid-filled part of the inner ear. The first bone is the malleus (hammer), which is attached to the inside surface of the eardrum. The second bone is the incus (anvil) and the innermost bone is the stapes (stirrup). Sounds set this structure into vibration, and the footplate of the stapes vibrates the oval window opening of the cochlea, transferring sound energy to the fluids and tissues of the cochlea.
Eustachian tube is a small tube connecting the middle ear space to the back of the throat. This tube is normally closed, but opens momentarily with yawning or swallowing. This periodic opening maintains equalization of the air pressure between the middle ear and the outside air. For effective transfer of sound through the middle ear the pressure must be equalized, and if not, the sudden opening of the eustachian tube produces a pop, after which hearing improves because the pressure balance is restored.

The inner ear has two functionally separate sections, one for balance and one for hearing:

1. Vestibular (balance related)
2. Cochlea (hearing related)

these two parts are interconnected, yet each serves its own vital function.
The vestibular rotational and linear motion, and to sense head position relative to gravity. Equally important, it is part of a reflex arc that makes it possible for us to maintain visual focus in the presence of many small and rapid motions of the head that occur as we engage in walking, riding, chewing, etc.

The cochlea is a coiled canal (resembling a snail’s shell) located in the dense bone tissue of the skull. It houses three fluid-filled, membranous canals, which extend along its full length. The central canal houses the organ of corti, which is comprised of specialized cells and their supporting tissues. Vibratory energy propagated through the canals’ fluid produces deformation of the organ of corti, which results in shearing forces on tiny tufts of hair (cilia) extending from the upper surfaces of the specialized cells. This shearing action triggers an electro-chemical signal that is sent to the auditory nerve.

The auditory nerve carries neural impulses from the cochlea through the internal auditory canal to the brainstem. Eventually, the signals reach the brain’s auditory processing center in the cortex.

Neural auditory processing is a complex series of functions that take place in the auditory center of the brain, and which are responsible for interpreting sound. The primary auditory cortex, located in the temporal lobe of the brain, is where aural perceptual functions are performed. These include sound localization, speech interpretation, focusing on individual sounds within noisy sonic environments (“cocktail party effect”) and other complex tasks such as music perception.