To better understand the total functioning of the human ear, it is best to divide the ear into three distinct regions: the outer ear, middle ear and the inner ear. The outer ear includes the pinna and the auditory canal; it ends at the tympanum or the eardrum. The middle ear includes the tympanum and the ossicles which are three small bones that are attached to the to the eardrum. The inner ear is made up of the semicircular canals and the cochlea.
The pinna is the flappy visible part of the ear. It collects sound (especially high frequencies) and directs it to the auditory canal. The pinna also allows us to determine the direction in which the sounds hear are traveling. The auditory canal acts as a funnel sending sound directly to the middle ear, via the eardrum. It is a tubular passageway lined with delicate hairs and small glands that produce a wax-like secretion called cerumen. The cerumen traps and keeps dust and dirt away from the eardrum. This keeps the eardrum from hampering its ability to vibrate.
The eardrum separates the outer ear from the middle ear. The eustachian tube is a small, narrow passageway, which connects the middle ear to the throat and the back of the nose. The primary job of the estachian tube is to keep the eardrum intact by equalizing the pressure between the middle and outer ear to prevent them from rupturing. Sometimes the estachian tube closes when it begin to feel pressure within the middle ear. This usually happens when pressure changes due to altitude occur. For example when a plane takes off or land the tube will automatically closes. Swallowing or chewing gum can open the tube.
A good way to demonstrate how the eardrum and the eustachian tube work is to have students make a replica of the eardrum and the auditory canal. This can be done using a toilet paper roll, rubber band and a piece of rubber or a balloon. Have the students stretch the rubber tightly over one end of the tube and secure it with a rubber band. Instruct the students to speak into the open end of the tube and feel what happens to the rubber. They will find that the rubber or the balloon vibrates. The eardrum reacts in the same manner. Sound vibrations enter the ear canal and hit against the eardrum.
The eardrum is attached to three bones called the ossicles. These bones are shaped like a hammer, an anvil and a stirrup. These bones are named according their shape, the hammer shaped bone is called the malleus, the anvil shaped bone is called the incus and the one shaped like a stirrup is called the stapes. If you show the students a model or drawing of the middle ear, they will clearly see how the bones in the ossicles received their names. This is also a good way to get the students to memorize the names and where the bones are located in the ear.
The hammer is located in the eardrum, and the stirrup fits into a membrane that fronts the inner ear. Vibrations from the eardrum move the hammer. Then the motion of the hammer moves the anvil, which moves the stirrup. As the sound vibrations pass from the eardrum to the ossicles it is amplified just before it passes through the oval window into the inner ear. The inner ear is protected from loud noises and pressure changes by two small muscles called the tympani and the stapedius. They protect the middle and inner ear from damage by contracting and limiting the movement of the ossicles.
The ossicles in the middle ear leads into the complex inner ear. The three main parts of the inner ear include the cochlea, the vestibule and the three semicircular canals. The cochlea is a coiled tube that looks like the shell of a snail. It is further divided into three distinct fluid-filled canals along the length of the cochlea called the vestibular canal, the cochlear canal and the tympanic canal. There is a partition between the cochlear canal and the tympanic canal called the basilar membrane. Within the basilar membrane is the is the spiral shaped organ of the Corti. There are sensory cells in the organ of Corti have several rows of hairlike projections that are attached to nerve cells. Each hairlike projection contains cilia that are bent when the basilar membrane receives sound vibrations from the middle ear. From here they are sent to the brain via the auditory nerve. The brain interprets these vibrations as specific sounds.
The best way to help students understand how the sensory cells in the organ of Corti work and to remember the terms is to have them imitate to actions of the inner ear. This can be done by having the students stand close together in an open area with their hands above their heads. Have them pretend that they are hair cells, and that their arms are hairs. Tell the students that they simulate the bending and swaying of the hairs in the organ of Corti. Instruct them that when a loud sound is heard they will bend low and if a soft sound is heard they will sway and stand tall. This is what occurs before they are sent to the brain via the auditory canal.
The second main structure of the inner ear, the vestibule, has very little to do with hearing. Its primary job is to help the body maintain balance and orientation through constantly monitoring the sensation of movement and position. The vestibule is made up of two sacs called the utriculus and the sacculus. There are special sensory areas in the wall of the utriculus, which sends impulses to the brain distinguishing the position of the head. The sensory areas have hairlike particles embedded in gelatin covered with mineral particles. The mineral particles exert pressure on the sensory cells according to the position of the head. The cells then send a specific pattern of nerve impluses to the brain. The structure of the sacculus resembles that of the utriculus. It also aids in body orientation and plays a small role in the function of hearing.
The third main structure of the inner ear is the three semicircular canals. Movement is detected from these tubes and a signal is sent to the brain. These canals direct balance as the body moves in a straight line or turns in different directions. Each of the canals contains sensory areas with hair cells that project into a cone-shaped cap of gelatin. One canal is horizontal and detects horizontal movement like turning and spinning. The other two semicircular canals are in a vertical position and detect vertical movement such as jumping or falling.
The ways in which each of the canals reacts depend on the inertia of the fluid inside. As our bodies change motion, the fluid in the canal lags behind causing the hair cells in the canal to bend. As the hair cells bend, they send nerve impulses to brain causing the canals to respond to the movement of the body. This can be demonstrated by using a half gallon plastic milk bottle filled halfway with water. By sloshing the water around in the container. The kids will get an idea of the inertia effect.
To help students better understand the importance of these balancing organs in the ear simple experiments can be performed that affects the impulses sent to the brain. Simply have the students spin around and around and tell them to walk along a straight line. If they spun fast enough, they will have trouble doing this simple task. After giving them a few seconds to regain their balance, have them spin around again and then change the direction in which they are spinning. They should also have difficulty in performing this particular task. At the end of these two exercises, have the students discuss the importance of the organs in the inner ear.
