1. The Ear

2. Objectives
Identify the structures of the external, middle, and internal ear, and list the functions of each.
Describe how the equilibrium organs help maintains balance.
Explain the function of the organ of Corti in hearing.
Define sensorineural and conductive deafness and list possible causes of each.
Explain how we are able to localize the source of a sound.

3. The Ear: Hearing And Balance
At first glance, the machinery for hearing and balance appears very crude.
Fluids must be stirred to stimulate the receptors of the ear.
Sound vibrations move fluid to stimulate hearing receptors, where large movements of the head disturb fluids surrounding the balance organs.
Receptors that respond to such physical forces are called mechanoreceptors.

4. The Ear: Hearing And Balance
Our hearing apparatus allows us to hear an extraordinary range of sound, and our highly sensitive equilibrium receptors keep our nervous system continually up to date on the position and movements of the head.
Without this information, it would be difficult if not impossible to maintain our balance.
Although these two sense organs are housed together in the ear, their receptors respond to different stimuli and are activated independently of one another.

5. Anatomy of the Ear
The ear is divided into three major areas: the outer ear; the middle ear; and the inner ear.
The outer and middle ear structures are involved with hearing only.
The inner ear functions in both equilibrium and hearing.

7. Outer (External) Ear
The outer ear is composed of the pinna and the external auditory canal.
The pinna is what most people call the "ear" ‑ the shell‑shaped structure surrounding the auditory canal opening.
In many animals, it collects and directs sound waves into the auditory canal, but in humans this function is largely lost.
The external auditory canal is a short, narrow chamber (about 1 inch long by 1/4 inch wide) carved into the temporal bone of the skull.

8. Outer (External) Ear
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 from the middle ear.

11. Middle Ear
The middle ear is a small, air‑filled cavity within the temporal bone.
It’s flanked laterally by the eardrum and medially by a bony wall with two openings, the oval window and the membrane‑covered round window.
The Eustacian tube runs obliquely downward to link the middle ear cavity with the throat.
Normally, the Eustacian tube is flattened and closed, but swallowing or yawning can open it briefly to equalize the pressure in the middle ear cavity with the external, or atmospheric, pressure.

12. Middle Ear
The eardrum does not vibrate freely unless the pressure on both of its surfaces is the same.
When the pressures are unequal, the eardrum bulges inward or outward, causing hearing difficulty and sometimes earaches.
The ear‑popping sensation of the pressures equalizing is familiar to anyone who has flown in an airplane.
Inflammation of the middle car, otitis media, is a fairly common result of a sore throat, especially in children, whose auditory tubes run more horizontally.

13. Middle Ear
In otitis media, the eardrum bulges and often becomes inflamed.
When large amounts of fluid or pus accumulate in the cavity, an emergency myringotomy (lancing of the eardrum) may be required to relieve the pressure.
During myringotomy, a tiny tube is implanted in the eardrum that allows pus formed in the middle ear to continue to drain into the external car canal.
The tube usually falls out by itself within the year.

15. Middle Ear
The more horizontal course of the auditory tube in infants also explains why it is never a good idea to "prop" a bottle or feed them when they are lying flat.
This creates a condition that favors the entry of the food into that tube.
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 hammer (maleus), the anvil (incus), and the stirrup (stapes).

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

17. Hammer
Anvil
Stirrups
Ear Drum

18. Inner (Internal) Ear
The inner ear is a maze of bony chambers called the bony labyrinth, located deep within the temporal bone, and just behind the eye socket.
The three subdivisions of the bony labyrinth are the cochlea, the vestibule, and the semicircular canals.
The vestibule is situated between the semicircular canals and the cochlea.
The bony labyrinth is filled with a plasma-like fluid called perilymph.

19. Inner (Internal) Ear
Suspended in the perilymph is a membranous labyrinth, a system of membrane sacs that follows the shape of the bony labyrinth.
The membranous labyrinth contains a thicker fluid called endolymph.

20. Semicircular Canals
Cochlea
Vestibule

21. Mechanisms of Equilibrium
Consider why when a cat is released upside down from a height, it will land on its feet or when an infant is tilted backward, its eyes will roll downward so that its gaze remains fixed?
Both of these reactions are compensations for a disturbance in balance, reflexes that depend on the sensory receptors within the vestibule and semicircular canals.

22. Mechanisms of Equilibrium
The equilibrium receptors of the inner ear can be divided into two functional arms:
one arm responsible for monitoring static equilibrium
and the other involved with dynamic equilibrium.

23. Static Equilibrium
Within the membrane sacs of the vestibule are receptors called maculae that are essential to our sense of static equilibrium.
The maculae report on the position of the head with respect to the pull of gravity when the body is not moving.
The maculae provide information on which way is up or down, they help us keep our head erect.
They are extremely important to divers swimming in the dark depths, enabling them to tell which way is up (to the surface).

24. Static Equilibrium
Each macula is a patch of receptor cells with their "hairs" embedded in a gel or jellylike material containing otoliths, which are tiny stones made of calcium salts.
As the head moves, the otoliths roll in response to changes in the pull of gravity.
This movement creates a pull on the gel, which in turn slides like a greased plate over the hair cells, bending their hairs.
This event activates the hair cells, which send impulses along the vestibular nerve to the cerebellum of the brain, informing it of the position of the head in space.

27. Dynamic Equilibrium
The dynamic equilibrium receptors are found in the semicircular canals.
They respond to angular or rotatory movements of the head.
When you twirl on the dance floor or suffer through a rough boat ride, these receptors are working overtime.
The semicircular canals are oriented in the three planes of space.
Regardless of which plane you move in, there will be receptors to detect the movement.

28. Dynamic Equilibrium
Although the receptors of the semicircular canals and vestibule are responsible for dynamic and static equilibrium, respectively, they usually act together.
Besides these equilibrium senses, sight and the proprioceptors of the muscles and tendons are also important in providing information to the cerebellum that it uses to control balance.

31. Mechanism of Hearing
Within the membranes of the snail‑like cochlea is the organ of Corti, which contains the hearing receptors or hair cells.
Sound waves that reach the cochlea through vibrations of the eardrum, ossicles, and the oval window set the cochlear fluids into motion.
As the sound waves are transmitted by the ossicles from the eardrum to the oval window, their force (amplitude) is increased by the lever activity of the ossicles.

32. Mechanism of Hearing
Nearly the total force exerted on the much larger eardrum reaches the tiny oval window, which in turn sets the fluids of the inner ear into motion.
The receptor cells are stimulated when their "hairs" are bent or pulled by the movement of the gel‑like membrane that lies over them.
In general, high‑pitch sounds disturb receptor cells close to the oval window whereas low‑pitch sounds stimulate specific hair cells further along the cochlea.

33. Mechanism of Hearing
Once stimulated, the hair cells transmit impulses along the cochlear nerve to the auditory cortex in the temporal lobe where interpretation of the sound, or hearing, occurs.
Since sound usually reaches the two ears at different times we hear "in stereo."
Functionally, this helps us to determine where sounds are coming from in our environment.
When the same sounds, or tones, keep reaching the ears, the auditory receptors tend to adapt or stop responding to those sounds, and we are no longer aware of them.

34. Mechanism of Hearing
This is why the drone of a continually running motor does not demand our attention after the first few seconds.
Hearing is the last sense to leave our awareness when we fall asleep or receive anesthesia and is the first to return as we awaken.

36. Hearing and Equilibrium Deficits
Children with ear problems or hearing deficits often pull on their ears or fail to respond when spoken to.
Under such conditions, tuning fork tests are done to try to diagnose the problem.
Deafness is defined as hearing loss of any degree; from a slight loss to a total inability to hear sound.
There are two kinds of deafnes: conduction and sensorineural.

37. Hearing and Equilibrium Deficits
Temporary or permanent conduction deafness results when something interferes with the conduction of sound vibrations to the fluids of the inner ear.
Something as simple as a buildup of earwax may be the cause.
Other causes of conduction deafness include fusion of the ossicles, a ruptured eardrum, and otitis media.

38. Hearing and Equilibrium Deficits
Sensorineural deafness occurs when there is degeneration or damage to the receptor cells in the organ of Corti, to the cochlear nerve, or to neurons of the auditory cortex.
This often results from extended listening to excessively loud sounds.
Where conduction deafness results from mechanical factors, sensorineural deafness is a problem of nervous system structures.
A person who has a hearing loss due to conduction deafness will still be able to hear by bone conduction, even though his or her ability to hear air‑conducted sounds is decreased or lost.