1. Module 1

1.37. Page 4

Lesson 6

Module 1—The Nervous System

The Inner Ear

 

cochlea: contains the organ of Corti, which functions to convert the mechanical energy of sound waves into a nerve impulse

 

round window: a membrane-covered opening between the chamber of the middle ear and the inner ear located below the oval window

 

The round window functions in dissipating sound waves in the inner ear.

 

auditory nerve: a nerve composed of sensory fibres from the organ of Corti, the vestibular apparatus that conducts impulses to the temporal lobe of the cerebrum

Sound has been collected, transmitted, and amplified through the outer and middle ear, but you haven’t heard anything yet. The inner ear is responsible for converting mechanical stimulation to a nerve impulse to be communicated to the brain. The inner ear also converts mechanical stimulation into information about balance to be interpreted by the brain.

 

Can you remember which structures make up the inner ear? The inner ear is composed of two organs: one for hearing and one for balance. Note the structures for hearing, which include the oval window, cochlea, round window, and auditory nerve. Note also the utricle, saccule, and semi-circular canals, which all have functions in maintaining balance.

 

You may wish to record the information in summary notes, a concept map, or as additions to your diagram or table. Store your work in your course folder.


 

THE STRUCTURES AND FUNCTIONS OF THE INNER EAR

Structure

Function(s)

oval window

 

cochlea

 

round window

 

auditory nerve

 

 

Organ of Corti

 

Special parts of the inner ear must be functional in order to facilitate hearing. So far, you have not encountered the mechanoreceptors of these parts and how they function in hearing. Mechanoreceptors are specialized receptor cells that detect a stimulus, such as pressure or vibration.

 

As you study the parts of the inner ear, you will discover why people can lose their range of hearing as they age.

 

Read

 

organ of Corti: a structure of the inner ear that contains hair cells that detect vibrations in the fluid of the inner ear and intiates a nerve impulse that is transmitted to the auditory nerve

Read the section about the inner ear and the organ of Corti on pages 420 and 421 of your textbook. Study “Figure 12.21” on page 420, which shows a cross-section of the cochlea and the location of the organ of Corti (blown up on the far left of the diagram). The receptors for sound waves are located inside the organ of Corti. Make summary notes or summarize the functions of the structures of the organ of Corti in a table similar to the following. Save your work in your course folder.

 

THE STRUCTURES AND FUNCTIONS OF THE ORGAN OF CORTI

Structure

Function(s)

basilar membrane

 

hair cells

 

tectorial membrane

 

auditory nerve

 

sensory neurons

 



basilar membrane: one of two parallel membranes that comprise the organ of Corti in the inner ear

 

Hair cells are attached to the basilar membrane.

 

hair cells: sensory mechanoreceptors attached to the basilar membrane in the organ of Corti

 

tectorial membrane: one of two parallel membranes found in the organ of Corti

 

During the transmission of sound waves, the basilar membrane vibrates and causes the sensory hairs to flex against the tectorial membrane.

 

sensory neurons: nerve cells that are stimulated by hair cells in the organ of Corti to conduct messages toward the temporal lobe of the brain

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, which physically moves to equalize pressure. It is important to 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 water plants moving with the current in a stream. Movement of the fluid in the inner ear causes vibration of the basilar membrane, which flexes the hair cells in the organ of Corti. The hair cells bend, like the plants in the stream, against the tectorial membrane. This mechanical action stimulates the dendrites of the sensory neurons and an electrochemical nerve impulse is initiated. The auditory nerve then transmits the impulse to the temporal lobe of the cerebrum where it will be interpreted as sound.

 

Watch and Listen

 

To see this in action, visit the McGraw-Hill Ryerson online learning centre at www.albertabiology.ca. Navigate to the Student Resources, and select “Chapter 12” from the left column. Then choose “Animations.” Select “Effect of Sound Waves on Cochlear Structures.”