Our ear is divided into three main parts, each containing different structures responsible for different roles. This lesson focuses on our cochlea, which is found in our inner ear and is structured so that it is able to carry out its part in allowing us to hear.
Have you ever held a seashell up to your ear and listened for the ocean wondering how the noise got trapped inside the shell? Actually, you’re hearing the disturbances from air inside the shell and surrounding noise. The sounds from your environment enter the shell and bounce around the shell’s interior walls mixing their frequencies and changing what you hear, thus creating an illusion of hearing ocean waves. Since that mystery has been unveiled, let’s tackle another: how do we hear, anyway?
How We Hear
The human ear is divided into three parts: the outer ear, middle ear, and inner ear. The outer ear is made of cartilage and aids in collecting information through sounds around us (that’s the part of our ear we can see). The eardrum, also known as the tympanic membrane, is located in the middle ear, just after the ear canal, and vibrates when sound waves reach it.
When there is a disturbance in the air, sound waves travel away from the disturbance like ripples in a pond. The eardrum vibrates with the same frequency as the sound waves and transfers those vibrations through three middle ear bones that amplify the vibrations to the inner ear. The inner ear is made up of the cochlea (which contains sound detectors) and semicircular canals (which control balance).
The Cochlea’s Role
The cochlea is named after the Latin word for snail shell because of its coiled snail-like shape. The walls are made of bone with a thin lining of tissue encompassing three chambers.
The two large chambers include the upper vestibular canal and lower tympanic canal, which both contain fluid called perilymph. The two canals are separated by a smaller chamber called the cochlear duct, which is lined with the basilar membrane and filled with fluid called endolymph. In biology, the prefix peri- means around, and the prefix endo- means within. At the floor of the cochlear duct is the organ of Corti, which is lined with hair cells that act as receptors.
Just above the organ of Corti is the tectorial membrane.When the pressure of vibrations reaches the cochlea from the middle ear, the movement of the fluid inside the cochlea stimulates the hair receptors, which brush against the tectorial membrane. The ear then converts the energy of pressure waves into nerve impulses. Sensory neurons send nerve impulses to the cerebrum, a part of your brain, through the auditory nerves. Your brain turns those signals into sound.
All this happens within fractions of a second!
Volume and Pitch
Two properties of sound are volume and pitch. Consider two dogs barking. One is a large dog with a loud, low-pitched bark while the other is a small dog with a quieter, high-pitched bark. The louder the noise, the more the hair receptors in the organ of Corti will bend, allowing our brain to interpret louder noise. When the small dog barks, hair receptors will bend because the volume is fairly low; but when the large dog barks, hair receptors will bend even more because its volume is greater than the small dog.Pitch is determined quite differently. Different frequencies of pressure waves, caused by a sound’s pitch, are determined by specific places on the basilar membrane that lines the cochlear duct.
The small dog will have a higher pitched bark than the large dog. Higher pitches stimulate the membrane closer to the outer ear while lower pitches stimulate the membrane deeper in the cochlea. When each dog barks, a different spot on the basilar membrane will be stimulated.The basilar membrane responds to different pitches based on what specific area of the membrane is affected by a sound wave.
Auditory areas in the brain are stimulated according to which region of the basilar membrane was stimulated. Also, the actual perception of pitch depends on the brain but relies on the basilar membrane’s function.
The mystery of hearing has now been unveiled.
We have determined that our ability to hear depends on more than one structure inside our ears. Of those structures, the cochlea, a structure resembling a snail shell in our inner ear, is responsible for the transfer of pressure waves into nerve impulses. A sound wave travels through the ear canal to the tympanic membrane or eardrum, where vibrations are amplified. The fluids inside the canals of the cochlea (upper vestibular canal, lower tympanic canal, and cochlear duct) then stimulate hair receptors in the organ of Corti. This allows our brain to interpret the disturbances of things within our environment, whether it is the illusion of crashing waves in a sea shell or the sound of barking dogs.