Anatomy of the Ear
The ear has two functions:
- The sense of hearing.
- The sense of balance.
The ear is divided into three parts:
- The outer ear.
- The middle ear.
- The inner ear.
The outer and middle ear react to vibration and are solely involved in hearing.
The middle ear is an air filled cavity and connected to the back of the nasal passage by the Eustachian Tube. This tube provides the means of equalising pressure between the outer ear and the middle ear.
The inner ear is divided into two parts:
- Cochlea: Responsible for the transduction of the atmospheric vibrations into electrical energy transmission to the brain.
- Vestibular Apparatus: Responsible for balance.
Sound waves travel through the outer ear and cause the ear drum (tympanic membrane) to vibrate. The vibrations are amplified and conducted across the middle ear (the conductive system) by the Ossicles (Malleus, Incus and Stapes) to the inner ear.
The cochlea converts the vibrations to nerve impulses which are then relayed to the brain and interpreted as sound.
All living organisms monitor their environment and one important aspect of that environment is gravity and the orientation of the body with respect to gravity. In addition, animals that locomote must be able to adjust their orientation with respect to self generated movements, as well as forces that are exerted upon them from the outside world. The vestibular system performs these essential tasks.
It engages a number of reflex pathways that are responsible for making compensatory movements and adjustments in body position. It also engages pathways that project to the cortex to provide perceptions of gravity and movement. The first section of the Chapter begins with a description of the components of the peripheral sensory apparatus and describes the ways in which specialized receptors transduce mechanical signals into electrical events.
The second section describes the projections of the vestibular afferents to the vestibular nuclei, and projection pathways from the vestibular nuclei to other brain structures such as the cerebellum.
The membranous labyrinth of the inner ear consists of three semicircular ducts (horizontal, anterior and posterior), two otolith organs (saccule and utricle), and the cochlea.
There are three fluid filled semi-circular canals in each ear. The canals are set in three planes at right angles to each other and are named the Lateral Canal, the Anterior Canal and the Posterior Canal.
These movements are turned into electrical signals - since there are 3 canals at right angles the brain can use these signals to give 3-dimensional information to help control balance and tell us which way up we are.
Since the semi-circular canals are at right angles to one another the forces of acceleration in yaw, pitch and roll can be detected.
In the absence of visual cues, the brain will interpret these stimuli as:
- Acceleration as movement.
- Simple acceleration.
- Changes of acceleration.
- Constant velocity.
In general terms, the semi-circular canals sense any angular movement by the body.
At the base of each canal is a sensory organ, the cupola. The cupola is a saucer shaped valve anchored at one end to the semi-circular canal, detecting movements of the fluid it contains.
With no acceleration the cupula remains vertical.
During any lateral motion the fluid in the canal begins to move. The cupula is then deflected in the direction of fluid movement.
As a state of equilibrium is reached the cupula overcomes the movement of the fluid and returns to a state of rest.
When rotation stops the fluid within the canals, because of the inertia, will cause a deflection of the cupula in the opposite direction.
The two Otoliths, positioned below the semi-circular canals in the inner ear are made of calcium carbonate.
They are sensitive to linear movement and the force of gravity. Movement in a linear sense can give a false impression of climbing or descending.
Sound vibrations or pressure waves (noise) have two variable factors which directly affect any damage to the ear:
- Intensity of Sound: This depends on the amplitude of the sound waves and is registered as loudness.
- Frequency: The number of cycles per second, this is pitch.
The frequency range of human hearing extends from 20 - 20,000 Hertz.
- 50 -100 Hz: The hum from a mains voltage system.
- 256 Hz: Middle C on the piano.
- 300 - 500 Hz: Speech range.
- 8000 Hz: The upper level of the speech range
Sound intensity is usually registered in decibels (dB). A list of sounds and their noise rating are given below:
- 0 dB: Threshold of hearing.
- 15 dB: Whisper.
- 30 dB: Conversation.
- 45 dB: Conversation in a busy office.
- 60 dB: An orchestra playing loud music.
- 90 dB: Pneumatic drill.
- 120 dB: Piston aircraft engine a few feet away.
- 125 dB: Disco.
- 130 dB: Jet aircraft noise a few feet away.
- 150 dB: Jet aircraft with afterburner selected.
Effects of Noise
Damage to the ears depends on:
- The intensity of the noise.
- The duration of the noise.
If the hearing system is subjected to noises in excess of 85 dB temporary hearing loss can occur. Where there is exposure above 85 dB for more than 8 hours a day over a long period permanent hearing loss may occur. Excessive exposure to noises above 120 dB for several hours a day for 3-6 months will cause Noise Induced Hearing Loss (NIHL) or deafness. With noises above 120 dB:
- 120 dB: Discomfort to the ears.
- 140 dB: Pain to the ears.
Other problems associated with noise include.
Where 100 dB is heard:
- A frequency below 100 Hz causes the body to sweat.
- High intensity noise can affect mental and physical co-ordination, and lead to disorientation.
- High intensity noise below this danger level must be regarded as a stress factor and can lead to a decrease in efficiency.
Deafness because of damage to the middle ear can be caused by:
- The perforation of the eardrum, or
- Infection to the middle ear, or
- Displacement of the Ossicles.
This damage can often be repaired and does not necessarily result in hearing loss; if permanent hearing loss occurs it is known as conductive deafness.
Damage to the cochlea is more serious and can be irreversible. Cochlea! implants are possible and these return some of the hearing loss. The cochlea is full of sensitive membranes connected to nerve ends which respond to vibrations. This vibration generates movement in the nerve impulses which the brain translates as sound.
Noise Induced Hearing Loss (NIHL)
If the membranes in the cochlea are over vibrated" then they can be permanently damaged. These hairy membranes are bent over permanently and are unable to recover. High frequency acuity is usually the first area of the auditory range to be lost; known as high tone deafness.
Protection Against Noise
Hearing protection is simple and should be used by pilots. The effects of jet engines, engineering sheds, car noise, discotheques or even personal stereos can damage the hearing. Ear defenders (ear muffs) or ear plugs are very effective in attenuating (weakening) noise:
- Ear Plugs: Protection of up to 25 dB.
- Headset or Ear Defenders: Protection up to 45 dB.
Presbycusis is the loss of hearing that gradually occurs in most individuals as they grow older. Hearing loss is a common disorder associated with aging. About 30 - 35% of adults between the ages of 65 and 75 years have some hearing loss.
Age reduces the effectiveness of the auditory system; high frequency acuity is usually affected first.
Vibration affects both the visual and psychomotor performance.
Where frequencies between 1 - 20 Hz are experienced the following physical symptoms may occur:
- 1 - 4 Hz: Breathing problems.
- 4 - 10 Hz: Possible chest and stomach pains.
- 8 - 12 Hz: Lumbar region pains.
- 10 - 20 Hz: Headaches and possible eyestrain.
Vestiublar System: Consists of semicircular canals, the saccules and utricles and vestibular nerve.
Saccule and Utricle: Structures which detect linear acceleration - for example moving forward, backward, side to side, up and down. These structures depend on gravity.
Otoliths: "Ear Stones" microscopic crystals of calcium carbonate inside saccules and utiricles, which shift position in response to linear acceleration.
Hair Cells: Sensing cells found in the saccules and utricles and semicircular canals which when stimulated due to motion send a message to the brain via the vestibular nerve.
Vestibular Nerve: The nerve, which sends vestibular information to the brain.
Semicircular Canals: Three fluid-filled tubes within the vestibular system positioned at right angles to each other, which detect rotational or angular acceleration.
Ampulla: The widened area of each semicircular canal containing hair cells which detect movement of the fluid in the ampulla and thereby sending a message to the brain.
Pitch: Nodding in an up and down manner
Yaw: Shaking your head in a 'no' side-to-side motion (pitch, roll and yaw are all detected by the semicircular canals)
Roll: Head tilt to the sides - or shoulders
Space Motion Sickness: The disorientation, confusion and ill feelings resulting from conflicting signals from our senses.
Sensory Conflict: When the brain receives very different signals from the senses of sight and touch than it receives from the otolith organs - often resulting in space motion sickness.