How We Hear: A Complex Process

The ear is a complex organ of hearing and balance. It is capable of perceiving an incredible range from the tiniest audible sound, which would be 1 trillion times weaker than that of a sound tha twould cause pain. This range is about 130 decibels. Besides loudness, the human ear can detect pitch. This relates to the sound vibrations per second or the frequency, which is measure in Hertz. The higher the frequency, the higher the pitch. This range is incredible, from about 20 to 20,000 Hertz. Finally, the human ear can detect tone. This allows us to tell the difference between flute and a clarinet when both instruments play the same note.

How We Hear: Parts of the Ear

In order for the ear to perform these extraordinary functions, it must detect, decipher and analyze the sounds. The ear is divided into four parts:

1. The outer ear
2. The middle ear
3. The inner ear
4. Auditory nerve leading to the brain

How We Hear: Pathway of Sound

The outer ear is a funnel that directs waves from th eair to the ear drum, sending vibrations that cause the three bones in the middle ear to move mechanically. These mechanical vibrations move to the inner ear, where they are turned into electrical energy by the tiny hair cell of the cochlea. This impulse travels the auditory nerve to the brain, where they are translated into useful sound information that we understand.

How We Hear: The Outer Ear

The outer ear is composed of 2 parts: the pinna, which is the visible part of the ear and the ear canal, or the external auditory meatus.

The pinna is composed mainly of skin and cartilage. It collects and funnels sound into the external auditory canal to the eardrum.

The ear canal is a twisted passageway that connects the pinna to the ear drum. It is about 2 o 3 cm in length. Since it is not straight, it makes it difficult for foreigh objects or insects to enter. Also, there are cerumen glands (earwax) in the ear canal that help keep the dust, dirt and bugs from entering.

How We Hear: The Middle Ear

The middle ear space is filled with air and is located between the outer ear canal and the inner ear. It is composed of the ear drum, the ossicles (bones), Eustachian tube, muscles and ligaments.

The ear drum (tympanic membrane) is an opaque conical shaped tissue located at the end of the ear cnal and is only about 0.1mm thick. Sound waves travel down the ear canal and strike the ear drum that causes it to vibrate.

The sound is transferred to the middle ear bones. These bones are the smallest bones in out body. They are held in place by muscles an dligaments that also allow them to rock back and forth with the sound.

How We Hear: The Middle Ear

The first bone in the chain is the malleus (hammer). It is attached to the ear drum.

The second bone is the incus (anvil).

The third bone in the chain is the smallest bone in our body and is called the stapes (stirrup). This bone is attached to the oval window, which is a membrane that covers the opening to the inner ear.

How We Hear: The Middle Ear

The Eustachian tube connects the middle ear to the nasopharynx. The purpose is to equalize pressure between the outer ear and the middle ear. Swallowing or yawning allows the tube to open and air enters into the middle ear. Swallowing or yawning allows the tube to open and air enters in to the middle ear. If it becomes blocked, for example by infection, fluid may accumulate in the middle ear space. This is calle dOtitis Media.

Sound vibrations cause the ear drum to move, which moves the bones in the middle ear. They work as a lever system that causes the sound to amplify before it enters the inner ear. A problem with the outer or middle ear that results in a reduction of sound being transferred into the inner canal will result in a conductive hearing loss.

How We Hear: The Inner Ear

The inner ear is a structure that contains the cochlea (hearing apparatus) and the labyrinth (organ of balance). It is a capsule filled with fluid.

The cochela is a bony tube that resembles a snail, having 2.5 turns. It is a complex organ that is partitioned into 3 parts.

The fluid moves as a wave sets along the entire length of the cochlea. As the fluid moves, approximately 12,000 hair cells will bend. The frequency and intestity of the sound is determined by which hair cells bend. This action causes nerve impulses to travel up the auditory nerve to the brain for processing. Any damage to the hair cells will affect our hearing.

The labyrinth contains the semicircular canals. It is their function to help us maintain our equilibrium.

How We Hear: The Auditory Nerve

Once the hair cells move in the inner ear, electrical signals are sent to the auditory nerve, which is connected to the auditory center of the brain (temporal lobe). Input frome ach ear will end up in the brain on each side of the brain. The brain will translate the electrical enery into sounds that we can recognize.