Module 1 The Nervous System: 1.1.6 page 3

Lesson 1.1.6

1.1.6 page 3

Middle Ear

oval window: a membrane covered opening located between the chamber of the middle ear and wall of the inner ear; the stapes (stirrup) is attached to it and transmits sound waves by it to the inner ear

ossicles: three tiny bones located in the middle ear that are connected to each other, to the tympanum, and to the oval window; involved in amplifying sound waves

middle ear: one of the three main regions of the ear which begins just past the tympanum and consists of a chamber containing three tiny bones (ossicles) called the malleus (hammer), the incus (anvil), and the stapes (stirrup); leads into to a minute opening called the Eustachian tube

tympanum: a round elastic structure that vibrates in response to sound waves; also called the eardrum or tympanic membrane

Eustachian tube: tiny passageway extending from the middle ear to the throat (pharynx); plays an important role in equalizing air pressure on both sides of the tympanum

The middle ear extends from the tympanum, or tympanic membrane, to the oval window. The structures of the middle ear are specialized to conduct and amplify the mechanical vibrations that sound waves produced. The tympanic membrane is connected to the ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup). In the following reading, consider how the tympanic membrane vibrates, causing the ossicles to act as a lever conduction, thus amplifying vibrations through the middle ear.

malleus (hammer): the first in the sequence of the three ossicles located in the middle ear; connected to the tympanum and the second ossicle (incus); part of a lever system which amplifies sound waves

incus (anvil): the middle in the sequence of the three ossicles located in the middle ear; connected to the malleus and the third ossicle (stapes); part of a lever system which amplifies sound waves

stapes (stirrup): the third in the sequence of the three ossicles located in the middle ear; connected to the second ossicle (incus) and the oval window; part of a lever system which amplifies sound waves and causes vibrations in the fluid of the inner ear

When you have a cold and are suffering from a throat infection, your throat infection can quickly develop into a terrible ear ache. It may feel like your ear drum will break or burst from the pressure that is building in the middle ear. In some serious cases, ear infections can cause the eardrum to rupture, which can results in partial hearing loss.

Read page 420 and note Figure 12.20.

Inquiry into Biology (Whitby, ON: McGraw-Hill Ryerson, 2007), 420, fig. 12.20. Reproduced by permission.

What structures make up the middle ear? Be careful here. The diagram seems to show that the semicircular canals are part of the middle ear. This is not so! The middle ear is bordered by the tympanum and the oval window.

Did you notice the Eustachian tube? Taking into account the location of the Eustachian tube, can you hypothesize what events may have occurred as a result of your throat infection that lead to the bursting of your ear drum? Have you ever notice your ears “pop”? Your ear drum “popping” indicates the process of pressures equalizing on each side of the eardrum. This is common when you’re in an unpressurized plane, under water, or travelling in the mountains. Yawning, chewing, or swallowing can help equalize these pressures.

At this time you may wish to make summary notes, a mind map, a table of structures and functions like the one below, or you may wish to prepare a diagram of the ear with all structures and functions labeled. File your work in your course folder.

The Structures and Functions of the Middle Ear

Structure	Function(s)
Ossicles (malleus, incus, stapes)
Oval window
Eustachian tube

Self-Check

SC. To help you understand and apply the concepts on the outer and middle ear, complete the following questions. Remember that complete sentences using Biological vocabulary are necessary at the Biology 30 level. Check your answers and file the questions in your course folder. If you are unsure about an answer, consult with your instructor.

Describe what would happen if a person poked a hole in their tympanum while trying to remove ear wax.

On a plane ride, where the pilot is expecting to encounter significant turbulence, the attendants may give hard candies to the passengers. Explain the physiological basis of this gesture.

Propose a hypothesis to explain why humans do not have enormous, moveable pinnae like some animals.

Explain why middle ear infections are common.

Check your work.

Self-Check Answers

If the tympanum was punctured, then the sound waves entering the ear through the auditory canal would not effectively be transferred to the ossicles because the tympanum would not vibrate freely in the same way as it would if it were whole and in tact. Also the sound waves would not effectively be amplified by the tympanum. Note: Answering that the person would not be able to hear as well is true, but it is not an answer with sufficient detail for Biology 30.
Because the Eustachian tube connects to the throat, it is important in equalizing the pressure on the “inside” of the middle ear with the pressure on the “outside” of the outer ear by preventing the eardrum from bulging in or out. This allows the ear drum to vibrate freely. Swallowing causes the Eustachian tube to open which helps equalize pressure exerted on the tympanum.
Humans do not have big, moveable pinnae because they depend on other senses, such as sight, to a greater extent than they do on hearing.

OR

Humans have large cerebral lobes and have learned that they can channel the sound by moving their head and “cupping” the ear.
Middle ear infections are common because microorganisms, such as bacteria and viruses, can reach the middle ear by entering at the nose and at the mouth, eventually making their way to the Eustachian tube by way of the throat.