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Chapter 9
Function of the Sense Organs
Introduction
• Human life would be very different without the
ability to sense and perceive external stimuli
• Imagine your world without the ability to see, hear,
smell, touch, and feel
• Environmental sensation is limited to those forms
of energy that sensory receptors are designed to
detect.
• Sensory receptors may convey information to the
cortex with awareness or perception and may lead
to cerebrally controlled responses.
• Sensory receptors also serve as afferent pathways
for reflex action with or without conscious
sensation.
Section 1 Physiology of the Receptor and
Sense Organs
I Concept and Classification of the
Receptor and Sense Organs
Sensory Receptors
• Receptors are specialized nerve cells that transduce
energy into neural signals
–
Receptors lack axons, form synapses with dendrites of
other sensory neurons
• Receptors are “mode” specific
– “Law of Specific Nerve Energies”: sensory messages are
carried on separate channels to different areas of the
brain
•
Receptors detect a small range of energy levels
–
–
–
Eye: 400-700 nM
Ear: 20-20,000 Hz
Taste buds: specific chemicals
Spectrum of the Electromagnetic Wave
Which receptor?
General sensory receptor
structure
• Free nerve endings:
dendrites interspersed
among other
cells/tissues (pain,
temperature, touch)
General sensory
receptor structure
• Encapsulated
nerve endings:
dendrites with
special supporting
structures
(mechanoreceptors
and proprioceptors)
Classification of Receptors:
1. Location
1) Externoceptors
Located on the body surface or specialized to detect external
stimuli
Pressure, pain, temp, touch, etc.
2) Visceroceptors
Located within internal organs, detect internal stimuli
Blood pressure, pain, fullness.
3) Proprioceptors
Found in the joints and muscles
Also in the vestibular structures and the semicircular canals of the
inner ear. Limb and body position and movment.
2 Modalities
1) Mechanoceptive
Detects stimuli which mechanically deform the receptor;
Pressure, vibration, touch, sound.
2) Thermoceptive
Detects changes in temperature; hot/cold
3) Nociceptive (pain)
Detects damage to the structures
4) Photoreceptors
Detect light; vision, retinal of the eye
5) Chemoceptive
Detect chemical stimuli; CO2 and O2 in the blood, glucose, small,
taste.
3. Complexity
1) Simple receptors
Usually a single modified dendrite
General sense
Touch, pressure, pain, vibration, temperature
2) Complexity
High modified dendrites, organized into complex structures;
ear, eye.
Special senses:
Vision, hearing, smell, taste
Sensation: Receiving messages
• Stimuli: What messages can be received?
– Anything capable of exciting a sensory receptor
cell can be defined as a “stimulus”
– Examples include: sound, light, heat, cold, odor,
color, touch, and pressure
Sensation: Receiving messages
about the world
• Sense organs operate through sensory
receptor cells that receive external forms of
energy and translate these external forms
into neural impulses that can be transmitted
to the brain
• There are two types of sense organs which
we will examine in this chapter
Basic Function
Sequence of Events in a Receptor
Stimulus
Reception
Amplification
Receptor Protein Activated
Enzyme Cascade (in some cases)
Receptor Ion Channels opened (or closed)
Receptor Current
Transduction
Receptor Potential
Modulated Transmitter
Release from Receptor Cell
Transmission
Modulated Impulse
Frequency in Receptor
Cell Axon
Modulated Impulse
Frequency in Second
Order Neuron
II Properties of the Receptors
1. Adequate Stimulus of Sensory Receptors
Each type of receptor is highly sensitive to one type of
stimulus for which it is designed and yet is almost
nonresponsive to normal intensities of other type of
stimuli.
The stimulus to which a given receptor has the lowest
threshold is termed the adequate stimulus of the
sensory receptor.
For instance, the roes and cones are highly responsive
to light but almost completely nonresponsive to
heat and cold.
2. Transduction of Sensory Receptors
Transduction: The process by which an environmental
stimulus becomes encoded as a sequence of nerve impulses in
an afferent nerve fiber is called sensory transduction
–Sense orgrans transduce sensory energy into neural
(bioelectrical) energy
–Converting one type of energy into another type is the
process of transduction
–Your brain only deals with bioelectrical impulses so
transduction must occur; what cannot be transduced cannot
be a stimulus
Principles of Transduction
• Different kinds of receptor are activated in different ways
but the first stage in sensory transduction is the generation
of a graded receptor potential.
• The magnitude of the stimulus is related to that of the
receptor potential which in turn is related to either
a) the sequence or frequency of all-or-none action
potentials generated in the afferent nerve fiber;
b) modulated release of transmitter from the receptor
cell generating a sequence of action potentials in a second
order neurone.
Receptor/Generator Potential
Receptor potentials: Changes in the transmembrane
potential of a receptor caused by the stimulus.
Generator Potential: A receptor potential that is strong
enough (reaches threshold) to generate an action potential.
Remember that APs are all-or-none. The stronger the
sitmulus (above threshold) the more APs are fired over a given
time period; this is translated by the CNS as a strong sensation.
3. Adaptation of Sensory Receptor
Sensory Adaptation is one form
of Integration
Phasic receptors quickly
adapt. The frequency of
action potentials
diminishes or stops if the
stimulus is unchanging.
Tonic receptors adapt
slowly or not at all.
Most exteroreceptors
(receptors that monitor
the external environment)
are phasic receptors.
Phasic receptors alert us to changes in sensory stimuli
and are in part responsible for the fact that we can
cease paying attention to constant stimuli.
The slowly adapting receptors (tonic receptors), such
as the pain receptors and the baroreceptors of the
arterial tree, are useful in situations requiring
maintained information about a stimulus.
4. Encoding of Sensory Receptor
The quality of the stimulus is encoded in the frequency of the
action potentials transmitted down the afferent fibre and the
number of sensory receptors activated.
Stretch
Receptors:
Frequency Code
Weak stretch
causes low
impulse
frequency on
neuron leaving
receptor.
Strong stretch
causes high
impulse
frequency on
neuron leaving
receptor.Membrane
potential
Time
Summary
• The external & internal environments are monitored
by sensory receptors.
• Each type of receptor is excited most effectively by
only one modality of stimulus known as the adequate
stimulus.
• The stimulus is converted into an electrical potential.
• Stimuli are detected as either static or dynamic
events.
• The intensity & duration of the stimulus is frequency
coded as bursts of action potentials in the primary
afferent nerve.
Section 2 Visual Sense Organ
Functions of the Complete Eye
• Eye functions like a camera
• Iris allows light into eye
• Cornea, Lens & humors focus light onto
retina
• Light striking retina is converted into action
potentials relayed to brain
I. Structure of
the Eyeball
• A slightly irregular hollow sphere with anterior
and posterior poles
• The wall is composed of three tunics – fibrous,
vascular, and sensory
• The internal cavity is filled with fluids called
humors
• The lens separates the internal cavity into anterior
and posterior segments
Anatomy of the Eye
• Three coats or tunics
– Fibrous: Consists of sclera and cornea
– Vascular: Consists of choroid, ciliary body, iris
– Nervous: Consists of retina
1. Fibrous
Tunic
• Forms the outermost coat of the eye and is
composed of:
– Opaque sclera
– Clear cornea
• The sclera protects the eye and anchors extrinsic
muscles
• The cornea lets light enter the eye
2. Vascular Tunic (uvea):
• Has three regions: choroid, ciliary body, and iris
• Choroid region
– A dark brown membrane that forms the posterior
portion of the uvea
– Supplies blood to all eye tunics
Vascular Tunic: Ciliary Body
• A thickened ring of
tissue surrounding
the lens
• Composed of smooth
muscle bundles
(ciliary muscles)
• Anchors the
suspensory ligament
that holds the lens in
place
Vascular Tunic: Iris
• The colored part of the eye
• Pupil – central opening of the iris
– Regulates the amount of light entering the eye
during:
• Close vision and bright light – pupils constrict
• Distant vision and dim light – pupils dilate
Pupil Dilation and Constriction
Figure 15.9
3. The Retina and its Parts
Optic Nerve, Blind Spot, Fovea
The Retina and its Parts
• Retina: inner layer on back of eye that contains
“light-sensitive” rods and cones
• Optic Nerve: bundle of axons running from retina to
visual (occipital) cortex
• Blind Spot: spot on the retina where optic nerve exits
eye, there are no receptors (rods or cones) there
• Fovea: center of the retina where “acuity” (ability to
see fine detail) is greatest
II The Image-Forming Mechanism
The images of objects in the environment
are focused on the retina.
1. Principle of Optics
Light rays are bent (refracted) when they pass from
one medium into a medium of a different density.
Parallel light rays striking a biconvex lens are
refracted to a point (principal focus) behind the
lens.
The principle focus is on a line passing through the
centers of a curvature of the lens, at the principal
focal distance.
For practical purpose, light rays from an object that
strike a lens more than 20 ft (6 m) away are
considered to be parallel.
The rays from an object closer than 20 ft are
diverging and are therefore brought to a focus
farther back on the principal axis than the principal
focus.
Biconcave lenses cause light rays to diverge.
2. Optic Characteristics of Refractive System
in Human Eye
The refractive system of the human eye is composed of
the cornea, aqueous humor, crystalline lens, and vitreous
humor.
When light coming from an object is brought to a focus,
an image is formed.
In the normal human eye, parallel rays of light entering
the eye are focused to an image just on the retina.
This ideal condition is called emmertropia.
So the image of distant object (6 m away) will be
focused on the retina in the emmertropic eye.
3. Accommodation
When the ciliary muscle is relaxed, parallel light rays
striking the optically normal (emmetropic) eyes are
brought to a focus on the retina.
As long as this relaxation is maintained, rays from
objects closer than 6 m from the observer are brought
to a focus behind the retina, and consequently the
objects appear blurred.
This problem can be
solved by increasing the
curvature or refractive
power of the lens.
The process whereby near objects are brought to a
sharp focus on the retina is called accommodation of
eye or visual accommodation.
Accommodation involves following reflexes.
(1) Accommodation of lens
– Increase bulging (refraction) of lens
– Via contraction of ciliary muscle, relaxes the
suspensory ligaments (parasympathetic fibers)
Accommodation of Lens. The solid lines represent
the shape of the lens, iris, and ciliary body at rest,
and the dotted lines represent the shape during
accommodation.
Focusing
Muscles
relaxed
Lens less
spherical
Focus far
Muscles
working
Lens more
spherical
Focus near
The visual accommodation is mainly through
increasing curvature of crystalline lens.
The power of accommodation is limited.
If a object is close enough to the eye, the increased
refracting power of the crystalline lens is insufficient
to overcome the light divergence, and the object will
be blurred.
The nearest distance of the eye at which an object can
seen distinctly is called the near point.
At this point, the visual accommodation is at a
maximum.
Decline in
the
amplitude
of
accommod
ation in
human
with
advancing
age.
(2) Pupillary reflex
–Decrease size of pupils (parasympathetic) prevents
divergent light rays from entering
(3) Convergence of eyeballs
Viewing near object causes reflexly both eyes to move
inward to focus on a near object, this process is called
convergence reflex.
4. Error of Refraction
Caused by shape of eye and/or power of
lens
• Farsightedness: is less common
– eye too short and/or lens too weak
– light focuses behind retinal
– correct with “convex” lens to add power
FARSIGHTEDNESS
(HYPEROPIA)
UNCORRECTED
CORRECTED
•Nearsightedness: is more common
–eye is too long and/or lens is too powerful
–light focuses in front of retina
–correct with “concave” lens to reduce power
NEARSIGHTEDNESS (MYOPIA)
UNCORRECTED
CORRECTED
•Astigmatism:
–abnormal curvature of the cornea
–Light from vertical and horizontal direction do not
focuses in the same point
–correct with “cylindrical” lens to compensate
•Presbyopia: Oldsightedness
–The crystalline lens tends to harden and the
capsule itself becomes less elastic with age
–The near point of distinct vision moves further and
further away from the eye with age.
–The far point is normal
–May be compensated by placing a converging lens
in front of the eye.
The loss in
power of
accommod
ation is
most
significant
and
dramatic
between
the ages of
40 and 50
III Structure of Retina and its Two
Photoreceptor systems
1. Structure of Retina
The retina is the light
sensitive portion of
the eye.
From outside to inside:
(1) Pigment layer
(2) Photoreceptor cell
layer
(3) Bipolar cell layer
(4) Ganglion cell layer
•Receptor cells: rods and cones, sensitive to light
•bipolar cells: carry signals from receptors to ganglion cells
•Ganglion cells: axons of ganglion cells form the optic nerve
Sensory Receptor Cells
The photoreceptor cells are two types, rod cells
(rods) and cone cells (cones)
Rods and Cones
The outer segment of a
rod cell has a rod-like
appearance, whereas that
of a cone cell has a coneshaped appearance.
The outer segments of the photoreceptor cell contain
stacks of membranous discs.
The visual pigments appear to be built into the disc
membranes.
At the inner ends of
the photoreceptor
cells, the part that
synapses with bipolar
and horizontal cells,
contains numerous
vesicle that is filled
with chemical
transmitters.
The inner pole of bipolar cells synapses with the ganglion cells.
When the photoreceptor cells are excited, signals are
transmitted through successive neurons in the retina itself and,
finally, into the nerve fibers and cerebral cortex.
Wiring: each
cone has its
own bipolar and
ganglion cell
while several
rods share one
bipolar and
ganglion cell
The Retina: Ganglion Cells and the
Optic Disc
• Ganglion cell axons:
– Run along the inner surface of the retina
– Leave the eye as the optic nerve
• The optic disc:
– Is the site where the optic nerve leaves the eye
– Lacks photoreceptors (the blind spot)
The Retina:
Ganglion Cells
and the Optic
Disc
Figure 15.10b
2. Two Photoreceptor System of Retina
Rod system and cone system
The rod system consists of rods and subsequent
bipolar cells and ganglion cells, which correlate with
the rods.
The cone system is composed of cones and
subsequent bipolar cells and ganglion cells.
Distribution of the
cones and rods on the
retina.
Cones see detail but require bright light
Rods see in low light but lack detail
Rods vs. Cones
Cones
Rods
amount
of light
needed
to see
5
10
15
20
Minutes in darkness
25
30
Rods
•
•
•
•
•
located mainly in periphery of retina
responsible for night vision
detail not detected
see black, white, and gray (no color)
several rods share 1 bipolar and 1
ganglion cell
• rod vision lacks detail, but, by
combining their efforts, groups of rods
allow us to see in low light
Cones
•
•
•
•
•
located mainly in fovea
work best in bright light
enable us to see fine detail
responsible for color vision
each cone has its own bipolar
and ganglion cell
• this allows us to see detail but
bright light is needed
Other evidence that two photoreceptor system of
retina exit.
1) The nocturnal animals have a preponderance
of rods, whereas the diurnal animals have a
preponderance of cones in their retina.
2) The visual pigment in the rods is only
rhodopsin. There are three classes of cones in
the retina, each containing different pigment
sensitive to particular region of visible
spectrum.
IV Transduction of Light Energy by Rod
Cell
Basic Mechanism
• Photopigments are located in the membrane of the
outer segment of rods and cones
• Each pigment consists of an opsin (a protein) and
retinal (a lipid)
– In the dark, membrane Na+ channels are open ->
glutamate is released which depolarizes the membrane
– Light splits the opsin and retinal apart->
•
•
•
Activates transduction (G protein)->
Activates phosphodiesterase->
Reduces cGMP -> closes Na+ channels
• The net effect of light is to hyperpolarize the retinal
receptor and reduce the release of glutamate
1. Photochemical Reaction and Metabolism of
Rhodopsin
The visual pigment (light-sensitive pigment) in the outer
segment of rods is the rhodopsin.
The rhodopsin is a combination of a protein part,
scotopsin (opsin), and a carotenoid pigment, 11-cis retinal.
This cis form of the retinal is important because only this
form can bind with scotopsin to synthesize rhodopsin.
11-cis retinal
Retinal – Light Sensitive Pigment
11-cis-Retinal - All-trans-Retinal
Light
Dark
When light energy is absorbed by rhodopsin, the
rhodopsin begin within trillionths of a second to
decompose.
This decomposition converts 11-cis retinal (bent
shape form) into all-trans retinal (straight chain
form)
In order to maintain the ability to detect light, the rods
must reconvert the all-trans retinal into 11-cis retinal.
This process requires metabolic energy and is catalyzed
by the retinal isomerase.
This process only occurs under the dark environment.
Under the dim light, the 11-cis and 11-trans keep
dynamic balance
Light
Dark
11-cis retinal
+ opsin
rhodopsin
all-trans retinal
+ opsin
isomerase
All-trans
retinal
11-cis
retinal
opsin
opsin
Retinal – derived from
Retinoic Acid or Retinol
or -Carotene
All-trans retinol is one form of vitamin A
The all-trans retinol is converted into 11-cis retinol
under the influence of isomerase, and finally into 11cis retinal that combines with opsin to form rhodopsin.
Vitamin A is present both in the cytoplasm of the rods
and in the pigment layer of the retina.
Therefore, vitamin A is always available to form new
retinal when needed.
Night blindness occurs in any person with severe
vitamin A deficiency.
The reason for this is that not enough vitamin A is then
available to form adequate quantities of retinal.
Rhodopsin Cycle
2. Receptor Potential of Rods
Rod Cell Hyperpolarization
V Color Vision
1. Photochemistry of Color Vision by Cones
The light sensitive substances in the cones have almost exactly
the same chemical composition as that of rhodopsin in the
rods.
The only difference is that the protein portion, the opsin,
called photopsin (as scotopsin in rods) in the cones, are
different from the scotopsin of the rods.
Therefore, the color sensitive pigments in the cones are
combinations of 11-cis retinal and photopsin.
When the cones are exposed to light, the resulting receptor
potential is also hyperpolarization potential.
2. Trichromatic Theory of Color Vision
Light of a Single wavelength
wavelength
intensity
Visible spectrum: 380-760 nm (nm is a billionth of a meter)
Theories of Color Vision
• Trichromatic theory
– Occurs at the receptor level
– Each cone is coated by one
of 3 photopigments
• Short-wave (blue)
• Medium-wave (green)
• Long-wave (red)
– Ratio of activated cones =
color differentiation
Primary Colors
• Primary Colors: sets of 3 colors that can be
mixed to produce any other color
• For Visual System: set of interest is “Red
Green and Blue”
Color Sensitivity of Different Cones
All of the colors of the visible spectrum can be
produced by mixing these three primary colors or by
stimulating different combination of the three of cones
in the proper proportions
Color Blindness
• Sex-linked conditions: Genes on X
chromosome, so more common in men.
– Protanopia, missing red photopigment
– Deuteranopia, missing green photopigment
• Non-sex-linked condition
– Tritanopia, missing blue photopigment or blue
cones
– monochromats: people who are totally
colorblind, more severe
Color Vision Systems
Tritanopia
deuteranopia
protanopia
VI Dark Adaptation and Light Adaptation
Range of
luminance to which
the human eye
respond
Dark Adaptation
Concept
From brightly lighted surrounding to a dimly
lighted environment, the retinas slowly become
more sensitive to light.
This decline in visual visual threshold is known
as Dark Adaptation.
Components:
Cones
adaptation:
early, rapid
but small in
amplitude
Rods
adaptation:
late, slowly
and great in
amplitude
Mechanism:
In bright light for a long time:
All the rhodopsin in his rods undergoes decomposition.
The receptor cells (cones) undergo a process of
adaptation due to persistent exposure to a strong
stimulus.
First phase is due to neural adaptation.
Receptor cells (cones), which had adapted to persistent
strong stimulation, recover their sensitivity when the
strong stimulus is withdrawn.
The time required for
dark adaptation is
determined in part by the
time required to build up
the rhodopsin stores.
The second phase is due
to chemical adaptation.
During this phase, rods,
which had got depleted
of rhodopsin in bright
light, replenish their
rhodopsin.
Light Adaptation:
When one passes suddenly from a dim to a brightly
lighted environment, the light seems intensely and
even uncomfortably bright until the eyes adapt to the
increased illumination and the visual threshold rises.
Mechanism?…
Dark Adaptation
• dark adaptation: increased sensitivity of
rods and cones in darkness
—e.g., entering a darkened room
• Cones: adapt for 10 minutes bet never
become very sensitive
• Rods: continue adapting for 30 minutes and
become much more sensitive
Light & Dark Adaptation
• Exposure to very intense light  initial
glare  large scale bleaching of pigment
•  Decrease of sensitivity of retina
•  Retinal neurons switch from rod to cone
system for reception
• < 5-10 min.  high visual acuity
• Exposure to dark restores pigments in rods
(from bleached pool) takes more than 20-30
min.
Hearing
I Structure of the Ear
Fig. 15.23
•1. Outer Ear:
–Pinna (auricle):
directs sound
waves into the
auditory canal
– Auditory Canal: conducts sound to the eardrum
– Tympanic membrane (Eardrum): thin membrane
that vibrates in response to sound, and transfers
sound energy to bones of the middle ear
• 2. Middle Ear: three tiny bones “amplify sound” and
transfer sound energy to the inner ear
A:
Malleus
B: Incus
C: Stapes
–Ossicles are smallest
bones in the body
–Act as a lever system
–Footplate of stapes
enters oval window of
the cochlea
• 3. Inner Ear: where sound energy is transduced
– Cochlea: snail shaped fluid-filled structure
– Oval window: thin membrane, transfers vibrations from
stapes to fluid of cochlea
–Basilar membrane: runs the length of the cochlea
–Organ of Corti: rests on basilar membrane, contains
“receptor” cells
–Round window: absorbs energy and equalizes pressure in
the cochlea
4. Pathway Transmitting Sound Wave from
External Environment to Inner Ear
Air Conduction
Sound wave
Auditory Canal
Sound wave
Auditory Canal
Air in tympanic cavity
Tympanic membrane
Ossicular chain
Round window
Bone Conduction
Sound wave
Oval window
Inner ear
Vibration of skull
5. Properties of Sound
Sound travels in waves as does light
• 1. Pitch: determined by “frequency,” the number
of cycles per second of a sound wave, measured in
hertz (Hz)
• 2. Loudness: determined by “amplitude” (height)
of the sound wave, measured in decibels (dB)
• 3. Timbre: determined by “complexity and shape”
of the sound wave, gives each sound its unique
quality
Loudness of Sound
• 0 dB = hearing threshold
• 50 dB = normal conversation
• 90 dB = danger zone
• 120 dB = Rock concert
• 130 dB = Pain threshold
II. ROLE OF MIDDLE EAR IN
SOUND TRANSMISSION
Tympanic Membrane
Tympanic Membrane
and Ossicular Chain
MECHANISMS INVOLVED IN
TRANSFORMER PROCESS
 Size
difference between Tympanic
Membrane and Stapes Footplate
 Lever
action
First Component of Middle Ear
Transformer Action

Size Difference
– Tympanic membrane
 .59 cm2
– Stapes footplate
2
 .032 cm
– Pressure formula
 Pressure = force/area

Impact on sound
transmission
Pressure gain: 0.59/0.032 = 18.4 (times)
Transformer Action of Middle Ear
Lever Action
Fulcrum Effect
pressure gain: 1.3 times
TRANSFORMER ACTION
AMOUNT OF AMPLIFICATION
Pressure Gain
18.4
1.3
23.9
Contribution from:
TM (Tympanic Membrane) to stapes footplate
Lever action
Total pressure gain
(18.6 x 1.3)
III Function of Cochlea
Cochlea - Snail-shaped organ with a series of fluidfilled tunnels;
converts mechanical energy into electrical energy
Cochlea
The
scala vestibuli is separated from the scala media
by vestibular membrane.
The
scala media is in turn separated from the scala
tympani by the basilar membrane.
Cochlea
 Fluids
in the cochlea: perilymph-fills the
scala vestibuli and scala tympani.
 Endolymph fills the scala media.
Cochlea
At
the end of the cochlea, the helicotrema
joins the scala vestibuli and the scala tympani.
Structures of the Inner Ear


Oval Window – located at the footplate of the
stapes; when the footplate vibrates, the cochlear
fluid is set into motion
Round Window – functions as the pressure relief
port for the fluid set into motion initially by the
movement of the stapes in the oval window
Hearing Summary So Far!!
Sound Waves  movement of tympanic
membrane  movement of Malleus 
movement of Incus  movement of Stapes 
movement oval window  movement of fluid
inside the cochlea  Movement of round
window
Organ of Corti
a
structure rests atop the basilar membrane
along its length
 contains approx. 16,000 cochlear hair cells
Organ of Corti
a
structure rests atop the basilar membrane
along its length
contains
approx. 16,000 cochlear hair cells
How to discriminate the frequency of the
sound? --Traveling Wave Theory
Vibration of Basilar Membrane and the
Traveling Wave Theory
• Sound wave entering at the oval window is
to cause the basilar membrane at the base
of the cochlea to vibrate
• different frequencies cause vibrations at
different locations (places) along basilar
membrane
• higher frequencies at base, lower
frequencies at top
Electrical Potentials
 DC vs. AC
– Direct Current (DC) = stimulus doesn’t
change with time, constant; i.e. battery
– Alternating Current (AC) = always changing
over time, looks like a sine wave
Cochlea
 Perilymph-similar
in composition to
extracellular fluid. High in Na+ and low in
K +.
 Endolymph-found
in the scala media.
Similar to intracellular fluid. High in K+
and low in Na+
Two DC Potentials (EP)

Endocochlear Potential (EP)
– Békésy discovered EP by putting the
electrode in the scala media and discovered
a +80 mV potential with respect to a neutral
point on the body
– Tasaki discovered EP was due to the Stria
Vascularis
The endocochlear potential (EP) is the positive
voltage of 80 - 100 mV seen in the endolymphatic
space in the scala media of the cochlea
+80 mV
Reticular Lamina
-80 mV
Two DC Potentials (IP)
Intracellular Potential (IP)
or organ of corti
potential (resting potential)
–Recorded -80 mV inside cells of organ of corti
Two AC Potentials

Cochlear Microphonic Potential
– Reproduces frequency and waveform of a
sinusoid perfectly
– Generated from HC

Action Potential (AP)
– Electrical activity from the VIII Nerve
– Can be measured from anywhere in the cochlea
or in the auditory nerve
Hair Cell in the Organ of Corti
When the basilar
membrane moves,
a shearing action
between the
tectorial
membrane and the
organ of Corti
causes hair cells to
bend
There are little mechanical gates on each
hair cell that open when they are bent.
K+ comes into the hair cell and
depolarizes the hair cell.
The concentration of K+ in the endolymph
is very high so when it comes into the hair
the +(positive) ions come to the cell
causing a depolarization.
Two AC Potentials

Cochlear Microphonic Potential
– Reproduces frequency and waveform of a
sinusoid perfectly
– Generated from HC

Action Potential (AP)
– Electrical activity from the VIII Nerve
– Can be measured from anywhere in the cochlea
or in the auditory nerve
Homeostatic imbalances of hearing.
• Deafness.
– Conduction deafness • possible causes include: perforated eardrum,
inflammation, otosclerosis
– Sensineural deafness - nerve damage
• Tinnitus - ringing in the ear
• Meniere's syndrome - attacks of dizziness,
nausea, caused by excess endolymph in the
media canal
Summary: How Sound
Travels Through The Ear...
Acoustic energy, in the form of sound waves, is
channeled into the ear canal by the pinna.
Sound waves strike the tympanic membrane, causing it to
vibrate like a drum, and changing it into mechanical energy.
The malleus, which is attached to the tympanic membrane,
starts the ossicles into motion.
The middle ear components mechanically amplify sound.
The stapes moves in and out of the oval window of the
cochlea creating a fluid motion.
Summary: How Sound Travels
Through The Ear...
The fluid movement within the cochlea causes
membranes in the Organ of Corti to shear
against the hair cells.
This creates an electrical signal which is sent
via the Auditory Nerve to the brain, where
sound is interpreted!