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How We Hear

The human ear is functionally broken down into four primary portions: the external, middle, and inner ear, as well as the hearing control center of the brain. These four components work collectively to conduct noises from outside the person to the brain and then process them. This document will clarify the purpose of each portion of the hearing structure. Hearing systems of the brain are not discussed here because of their complexity.

How We Hear: The External Ear

How We Hear - The Anatomy of the EarDivided into two parts, the external ear is comprised of the pinna which is the exterior ear that is seen on the side of the head, and the external auditory meatus also known as the ear canal. The ear canal stretches from the pinna to the tympanic membrane (TM), commonly known as the eardrum.

The pinna is mainly skin and cartilage with muscular attachments that allow people to wiggle their ears. The pinna gathers and disseminates sounds through the ear canal. The folds and twists of the pinna boost high frequency tones and also assist in determining the source’s direction. Sounds arriving from the front and sides are marginally enhanced as they’re directed towards the canal while noises coming from behind are somewhat diminished. This assists us to notice what we’re focusing on while decreasing distracting background sounds coming from behind. Using the hand to cup behind the pinna, provides listeners with a slightly increased amplification since it essentially enlarges the audio collection area of the outer ear.

The ear canal is a little, tunnel-like, twisting tube that links the pinna to the ear drum. It’s about an inch long (2.5 cm) and possesses a diameter around the size of a pencil’s eraser. The surfaces of the ear canal are delicate to touch and contain a cranial nerve branch which passes just beneath the back surface. In the event that the ear canal is actually contacted around this nerve, people will involuntarily cough which is called Arnold’s reflex. The exterior two thirds surface of the ear canal is encircled by cartilage and glands that will create cerumen or ear wax; while the inner third is bound by bone tissue. The ear canal provides a organic resonance that generally increases sound pitches by around 3000 Hz and approximately 10-15 decibels. Ear wax serves several beneficial purposes in the ear canal.

How We Hear: The Middle Ear

The middle ear is made up of an air-filled gap in between the tympanic membrane and the inner ear. It includes three small bones (ossicles)joined together that join the ear canal to an opening at the inner ear. It contains several tiny muscles and ligaments that adjust and support tension in chain.

The tympanic membrane consists of a concave shaped membrane covering at the located at the final section of the ear canal. Sounds journey down the canal and contact the ear drum, triggering a vibration. These vibrations then are transmitted through the ossicles to the inner ear or cochlea. The first ossicle bone is the malleus or hammer bone and is joined to the inner surface of the ear drum. The second ossicle bone is the incus or anvil, and the innermost bone is called the stapes or stirrup. Sound begins the vibration of the entire system. The stapes’ footplate vibrates in the oval shaped opening of the cochlea, which then transfers audio energy down to the tissues and fluids of the cochlea.

There is a little tube that links the middle ear with the rear of the throat, which is called the eustachian tube. It is usually closed but will open briefly with swallowing and yawning. This intermittent opening preserves equalization of the air pressure among the middle ear and exterior air pressure. The pressure needs to be equalized in order for effective exchange of sound through the middle ear. When pressure becomes unequal, like with rapid altitude change, the abrupt opening of the Eustachian tube generates a “pop” that improves hearing since the balance of pressure is restored.

How We Hear: The Inner Ear

The inner ear is consists of two separate portions: The vestibular, which provides balance, and the cochlea, that functions in hearing. These interconnected parts each serve their own important function.

The vestibule’s purpose is to aid in sensing acceleration and deceleration of motion and relation head position to gravitational forces. It’s a part of the reflex arc that allows us to preserve sharp visual focus without disruption from the many small and rapid motions that occur while walking, chewing, riding, and so on.

The cochlea is made up of a coiled canal inside the thick tissue of the skull. It’s shape to some degree resembles a snail. It contains three fluid-filled membranous pathways extending down the full span. The central canal stores the organ of Corti that consists of specialized cells and supporting tissues. Vibratory electrical signals are propagated through the fluid to produce deformation in the organ of Corti. This in turn results in shearing pressure on tiny hairs or cilia extending from the upper surfaces. The shearing activates an electro-chemical signal which travels down the nervous pathway and passes through the internal canal to the brain. Then the signal moves upward to the processing centers of the brain’s temporal lobes. This simplified description of complex actions that ultimately result in how we hear.

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