The Cochlear Implant (Bionic Ear)
How the ear functions in health and disease
The
cochlea (inner ear)
With normal hearing, sound vibrations are transmitted along the ear
canal to the ear drum and then via the small middle ear bones to the
sense organ of hearing or organ of Corti, housed in the cochlea or inner
ear.
The
organ of Corti in the cochlea
The organ of Corti is a small structure that rests on the basilar
membrane lying across the spiral cavity of the cochlea. Variations in
stiffness of this membrane allows it to vibrate selectively to
different sound frequencies, and so it acts as a sound filter. High
frequencies produce maximal vibrations at the basal (base) end, and low
frequencies at the apical end.
The
organ of Corti
The vibrations are converted into electrical signals by inner hair cells
in the organ of Corti. The cells have hairs on their surface, and these
hairs bend to and fro in the overlying gelatinous (tectorial) membrane and act like
a switch allowing electrical current to flow through the cells to the
nerve fibres. The electrical responses in the nerve fibres are conducted up the
central auditory pathways where they are interpreted as speech and other
sounds.
Temporal
code
The coding of sound frequencies, normally occurs through a temporal and
place code. The temporal code is illustrated in this slide. The sound
waves at a frequency of 500Hz are on the top, and the auditory nerve
action potentials, which are in time with the sound waves, are on the
bottom. Note too that the intervals between the nerve action potentials
are important for coding the frequency.
Place
code
With place coding the inner ear filters the frequencies, and high
frequencies excite the basal end of the cochlea and low frequencies the
apical end. The different frequency regions are connected spatially to
all the centres in the brain so that a frequency scale is preserved. In
other words, we recognize the pitch of a sound according to the site of
stimulation in the brain.
The
organ of Corti
With severe-to-profound deafness there is a marked loss of hair cells,
and so amplifying sound with a hearing aid will not lead to speech
comprehension.
Diagram
of electrical stimulation
So the challenge was could electrical stimulation of the auditory nerve
bypass the cochlea and give speech understanding?