Special Senses – The EAR

Overview

Hearing is one of the major senses and like vision is important for distant warning and communication. It can be used to alert, to communicate pleasure and fear. It is a conscious appreciation of vibration perceived as sound. In order to do this, the appropriate signal must reach the higher parts of the brain.

Functions of the ear

The ears are paired sensory organs comprising the auditory system, involved in the detection of sound, and the vestibular system, involved with maintaining body balance or the equilibrium. The ear divides anatomically and functionally into three regions: the external ear, the middle ear, and the inner ear. All three regions are involved in hearing. Only the inner ear functions in the vestibular system.

1. SOUND DETECTION. The function of the ear is to convert physical vibration into an encoded nervous impulse. It can be thought of as a biological microphone. Like a microphone the ear is stimulated by vibration: in the microphone the vibrationis transduced into an electrical signal, in the ear into a nervous impulse which in turn is then processed by the central auditory pathways of the brain.
2. MAINTAINING BODY BALANCE or EQUILIBRIUM. The prime function of the vestibular system is to detect and compensate for movement. This includes the ability to maintain optic fixation despite movement and to initiate muscle reflexes to maintain balance.

Anatomy of the Ear

The ear is divided into three major areas: the outer or external ear, the middle ear and the inner or internal ear. The outer and middle ear are involved with hearing only whilst the inner ear functions both equilibrium and hearing.

OUTER (EXTERNAL) EAR

The outer ear collects sound waves in the air and channels them to the inner parts of the ear. The outer ear along with its canal has been shown to enhance sounds within a certain frequency range. That range just happens to be the same range that most of the characteristics of human speech sounds fall into. This allows the sounds to be boosted to twice their original intensity. Parts of the outer ear are the following:

1. Pinna – Also called the auricle. It is the part which protrudes from the side of the skull made of cartilage covered by skin. The pinna collects sound and channels it into the ear canal. The pinna’s shape enables it to funnel sound waves into the external auditory meatus. The various folds in the pinna’s structure amplify some high frequency components of the sound. They also help in the localization of sound in the vertical plane. As sounds hit the pinna from above and below, their paths to the external auditory meatus vary in length. This means that they take different times to reach the meatus.
2. External acoustic meatus or external auditory canal – this is a short chamber about 1 inch long by ¼ inch wide. It is carved into the temporal bone of the skull. The canal has bends in both the vertical and horizontal planes. This means that it is difficult for anything poked into the meatus to hit the drum. Any trauma is likely to be to the walls of the canal. In its skin-lined walls are the ceruminous glands, which secrete a waxy yellow substance, called earwax or cerumen.

Sound waves entering the external auditory canal eventually hit the tympanic membrane or eardrum and cause it to vibrate. The canal ends at the eardrum, which separates the outer ear from the middle ear.

MIDDLE EAR

The middle ear or tympanic cavity is an air filled space within the temporal bone. It transforms the acoustical vibration of the sound wave into mechanical vibration and passes it onto the inner ear. The three tiny bones of the middle ear act as a lever to bridge the eardrum with the oval window. Incoming forces are magnified by about 30 %. This increased force allows the fluid in the cochlea of the inner ear to be activated.

The tympanic cavity is spanned by the three smallest bones in the body, the ossicles which transmit the vibratory motion of the eardrum to the fluids of the inner ear. These bones, named for their shape, are the following:

• Hammer or malleus
• Anvil or incus
• Stirrup or stapes

When the eardrum moves the hammer moves with it and transfers the vibration to the anvil. In response to this, the anvil passes it on to the stirrup which in turn presses on the oval window of the inner ear. The movement of the oval window sets the fluids of the inner ear into motion, eventually exciting the hearing receptors.

The Eustachian tube is a canal that links the middle ear with the throat area. This tube helps to equalize the pressure between the outer ear and the middle ear. Having the same pressure allows for the proper transfer of sound waves. The eustachian tube is lined with mucous, just like the inside of the nose and throat.

Inner Ear

The inner ear consists of a maze of fluid-filled tubules running through the temporal bone of the skull. The bony tubes, the bony labyrinth, are filled with a fluid called perilymph.  Within this bony labyrinth is a second series of delicate cellular tubes, called the membranous labyrinth, filled with the fluid called endolymph. This membranous labyrinth contains the actual hearing cells, the hair cells of the organ of Corti.

There are three major sections of the bony labyrinth:

1. The front portion is the snail-shaped cochlea, which functions in hearing
2. The rear part, the semicircular canals, helps maintain balance
3. Interconnecting the cochlea and the semicircular canals is thevestibule, containing the sense organs responsible for balance, the utricle and saccule.

The bony cochlea is so called because it is shaped like a snail shell It has two and a half turns and houses the organ of hearing known as the membranous labyrinth surrounded by fluid called the perilymph. The cochlea has a volume of about 0.2 of a millilitre. In this space lie up to 30,000 hair cells which transduce vibration into nervous impulses and about 19,000 nerve fibers which transmit the signals to and from the brain.

The inner ear has two membrane-covered outlets into the air-filled middle ear – the oval window and the round window. The oval window sits immediately behind the stapes, the third middle ear bone, and begins vibrating when “struck” by the stapes. This sets the fluid of the inner ear sloshing back and forth. The round window serves as a pressure valve, bulging outward as fluid pressure rises in the inner ear. Nerve impulses generated in the inner ear travel along the vestibulocochlear area (cranial nerve VIII), which leads to the brain. This is actually two nerves, somewhat joined together, the cochlear nerve for hearing and the vestibular nerve for equilibrium.

How Do We Hear?

The range of audible sound is approximately 10 octaves from somewhere between 16 and 32 Hz (cycles per second) to somewhere between 16,000 and 20,000 Hz. The sensitivity is low at the extremes but becomes much more sensitive above 128 Hz up to about 4,000 Hz when it again becomes rapidly less sensitive. The range of maximum sensitivity and audibility diminishes with age.

What Do We Hear?

All sounds (music, voice, a mouse-click, etc.) send out vibrations, or sound waves. Sound waves do not travel in a vacuum, but rather require a medium for sound transmission, e.g. air or fluid. What actually travels are alternating successions of increased pressure in the medium, followed by decreased pressure.

Hearing starts with the outer ear. When a sound is made outside the outer ear, the sound waves, or vibrations, travel down the external auditory canal and strike the eardrum (tympanic membrane). The eardrum vibrates. The vibrations are then passed to three tiny bones in the middle ear called the ossicles. The ossicles amplify the sound and send the sound waves to the inner ear and into the fluid-filled hearing organ (cochlea).

Once the sound waves reach the inner ear, they are converted into electrical impulses which the auditory nerve sends to the brain. The brain then translates these electrical impulses as sound.

The mechanism of hearing is summarized below:

1. Sound
2. Pinna collects the sound heard
3. External auditory canal
4. Vibration in the ear drum is produced
5. Amplified by the ossicular chain. The sound is then transmitted to the inner ear.
6. The cochlea converts the sound vibration to electrical impulses. Within the cochlear duct, the endolymph-containing membranous labyrinth of the snail-like cochlea is the organ of Corti, which contains the hearing receptors or hair cells. The chambers above and below the cochlear duct contain perilymph. Sound waves that reach the cochlea through vibrations of the eardrum, ossiscles and oval window set cochlear fluids into motion.
7. The hair cells transmit impulses along the cochlear nerve or auditory nerve (a division of cranial nerve VIII – the vestibulocochlear nerve) to the auditory cortex in the temporal lobe.
8. Auditory cortex interprets the sound, or hearing occurs.

Since sound usually reaches the two ears at different times, a person can hear “in stereo,” which functionally helps humans differentiate where sounds are coming from the environment. In cases where the sounds or tones keep reaching the ears, the auditory receptors tend to adapt or stop responding to these sounds, thus the person becomes no longer aware of them. Important information about hearing is the fact that it is the last sense to leave the awareness when a person falls asleep or receives anesthesia or dies. As a person awakens from sleep, it is the first sense to return.

image courtesy of umm.edu