SCN3230-ABED_1: Module 1 Lesson 3 - 3

Lesson 3 — Mechanoreception: The Ear

The Inner Ear

Read the remainder of pages 420 - 421

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

Sound has been collected, transmitted, and amplified, but you have not heard anything yet. The inner ear is responsible for converting mechanical stimulation to a nerve impulse to be communicated to the brain. In addition, the inner ear converts mechanical stimulation into information on balance to be interpreted by the brain.

Study Figure 12.20 on page 420. Can you determine which structures make up the inner ear?

The inner ear is composed of two organs: one for hearing and one for balance. The structures for hearing are the oval window, cochlea, round window, and auditory nerve. The structures involved in balance are the utricle, saccule, and semi-circular canals.

Cochlea - Organ of Corti

Critical parts of the inner ear must be functional to facilitate hearing. Mechanoreceptors are specialized receptor cells that detect a stimulus such as pressure or vibration. As you study these parts of the inner ear, you will discover why people can lose their range of hearing as they age.

Study the diagram. It shows a cross-section of the cochlea and the location of the organ of Corti. The receptors for sound waves are located inside the organ of Corti.

You should now realize that the vibrating ossicles cause the oval window to vibrate. This causes pressure waves in the fluid-filled canals of the cochlea. The waves travel to the round window that moves to equalize pressure. Note how the structures in the cochlea function. The Organ of Corti, resting on the basilar membrane, consists of hair cells, a tectorial membrane, and sensory nerve fibres.

Imagine underwater plants moving with the current in a stream. The movement of fluid in the inner ear causes vibration of the basilar membrane, which flexes the hair cells of the Organ of Corti. Similar to plants in a stream, the hair cells bend against the tectorial membrane. This mechanical action stimulates the dendrites of the sensory neurons, and an electrochemical nerve impulse is initiated. Then, the auditory nerve transmits the impulse to the temporal lobe of the cerebrum where it is interpreted as sound.

Pathway of Sound

pinna → auditory canal → tympanum (eardrum) → ossicles → oval window → cochlea (organ of Corti) → auditory nerve → brain (temporal lobe)

Try This

To ensure that you understand the mechanisms of the inner ear that are involved in hearing, complete the drag-and-drop activity below.

Did You Know?

The most sensitive mechanoreceptors are the hair cells in the cochlea (ears).

Watch and Listen

To consolidate your understanding of the concepts to this point, watch this animated clip on how sound waves are captured, transformed into mechanical vibrations, amplified, and then converted into nerve impulses that are transmitted to the brain by the various structures of the ear.

Watch the following segment of BiologiX-07 video on the Anatomy and Physiology of the Ear.

Self-Check

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

The following self check will help you review the concepts concerning the outer, the middle, and the inner ear.

Imagine that you are a sound wave. In order, describe the structures of the ear that you encounter, starting with the outer ear and finishing with the organ of Corti.
What happens to the energy of sound waves when they reach the tympanum?
How is the surrounding medium of the inner ear different from that of the middle ear? What has to happen to the vibrations in the middle ear to accommodate this difference?

What is the role of the hair cells located in the organ of Corti?
To which part of the brain are nerve impulses sent after being initiated in the organ of Corti?
Which three parts of the ear are involved in amplifying sound waves?
In which structure does the initiation of nerve impulses occur?
Which structure first directs sound waves into the ear?
Which structure aids in the equalization of pressure between the outer ear and the inner ear?
Which structure is most affected if pressure is not equalized?
Where is the organ of Corti?
Which structure is affected by middle ear infections?
Which structure causes the ossicles to vibrate?

Check your work.

Self-Check Answers

In correct sequence, sound waves encounter the following structures: pinna, auditory canal, tympanum, ossicles (malleus, incus, and stapes), oval window, cochlea, and organ of Corti.
When sound waves contact the tympanum, they cause it to vibrate. Its vibrations are passed on and amplified by the neighbouring ossicles. Each bone vibrates more than the next one so that the vibrations are amplified as they pass from one to the other. The last bone concentrates the vibrations onto the oval window. The vibrations cause the fluid in the cochlea to vibrate, which causes pressure waves.
The middle ear is filled with air whereas the inner ear is filled with fluid. To convert the sound vibrations of the middle ear into the pressure waves of the inner ear, the sound vibrations must be amplified a great amount. (Fluid is much more difficult to move than air is.)
The role or function of the hair cells is to initiate a nerve impulse when they are stimulated by the pressure waves in the fluid of the inner ear.
The auditory nerve transmits nerve impulses initiated by the hair cells to the temporal lobe of the cerebrum.

The three parts of the ear involved in amplifying sound waves are auditory canal, tympanum, and ossicles.
The initiation of nerve impulses occurs in the cochlea, more specifically, the organ of Corti.
Sound waves are directed first into the ear by the pinna.
The Eustachian tube aids in the equalization of pressure between the outer ear and the inner ear.
The structure most affected if pressure is not equalized is the tympanum (eardrum).
The organ of Corti is in the cochlea.
The structure affected by middle ear infection is ossicles.
The tympanum causes the ossicles to vibrate.