Hearing and the Human Ear
Notes
Part 1:
First, we need to remember that the ear can be divided into three sections – the outer,
middle, and inner ears.
The outer ear collects sounds and funnels them to the Tympanic membrane (ear drum).
Embedded within the Tympanic membrane is the Malleus – the first of the three middle ear
ossicles. The middle ear is an air-filled cavity with the three tiniest bones in your body.
The middle ear is ventilated by the Eustachian tube and its purpose is to amplify sound
before it is sent to the fluid-filled cochlea.
The movement of the malleus starts a chain reaction of the ossicles. The last of the ossicle,
the stapes, pushes on the oval window of the cochlea. The force of the stapes movement
sends a wave of fluid up the basilar membrane of the cochlea.
The cochlea marks the start of the inner ear. It is lined with three rows of outer hair cells
and one row of inner hair cells. The outer hair cells move up and down in response to
sound stimulation. The also amplify sounds before sending them to the inner hair cells
which then tell the auditory nerve fibers to fire and send the message up to the auditory
cortex.
Part 2:
Hearing can be tested in two ways:
1) Air conduction
2) Bone conduction
With air conduction the subject wears headphones or insert earphone. Sounds are sent
through the outer ear, the middle ear, the inner ear, up the auditory nerve and to the
auditory cortex. The subject will then signal that the sound was heard. Air conduction
tests the whole auditory system.
Bone conduction bypasses the outer and middle ear and directly stimulates the fluid of the
cochlea. To test bone conduction, a person wears a bone oscillator (vibrator) somewhere
on his / her skull. The oscillator is usually placed on the mastoid bone behind the pinna of
the outer ear. The movement of the ossiclator vibrates the skull, the bones of the middle
ear, and ultimately the fluid in the cochlea. A vibration of fluid in the cochlea will cause
sounds to be sent up the auditory nerve to the auditory cortex where the subject will again
signal when the sound was heard.
