Chapter 9

Function of the Sense Organs

• 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

Introduction

         

                                                      

                                                                                                

         

                                                      

                                                                                                

• 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 ot

her 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

Sequence of Events in a ReceptorStimulus

Receptor Protein Activated

Enzyme Cascade (in some cases)

Receptor Ion Channels opened (or closed)

Receptor Current

Receptor Potential

Modulated Impulse Frequency in Second

Order Neuron

Basic Function

Reception

Amplification

Transduction

TransmissionModulated Impulse

Frequency in Receptor Cell Axon

Modulated Transmitter Release from Receptor Cell

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 adaptslowly 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:

Weak stretch causes low impulse frequency on neuron leaving receptor.

Strong stretch causes high impulse frequency on neuron leaving receptor.

Time

Membrane potential

Frequency Code

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 port

ion 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

Optic Nerve, Blind Spot, Fovea

3. The Retina and its Parts

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 working

Lens more spherical

Focus near

Muscles relaxed

Lens less spherical

Focus far

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 accommodation 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 accommodation is most significant and dramatic between the ages of 40 and 50

III Structure of Retina and its Two Photoreceptor systems

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

1. Structure of Retina

•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 CellsThe 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 cone-shaped 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)

Figure 15.10b

The Retina: Ganglion Cells and the Optic Disc

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

5 10 15 20 25 30

amount of light needed to see

Minutes in darkness

ConesRods

RodsRods• located mainly in periphery of retinalocated mainly in periphery of retina

• responsible for night visionresponsible for night vision

• detail not detecteddetail not detected

• see black, white, and gray (no color)see black, white, and gray (no color)

• several rods share 1 bipolar and 1 several rods share 1 bipolar and 1 ganglion cellganglion cell

• rod vision lacks detail, but, by rod vision lacks detail, but, by combining their efforts, groups of rods combining their efforts, groups of rods allow us to see in low lightallow us to see in low light

ConesCones• located mainly in fovealocated mainly in fovea

• work best in bright lightwork best in bright light

• enable us to see fine detailenable us to see fine detail

• responsible for color visionresponsible for color vision

• each cone has its own bipolar each cone has its own bipolar and ganglion celland ganglion cell

• this allows us to see detail but this allows us to see detail but bright light is neededbright 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

• 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 -> glutamat

e 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

Basic Mechanism

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 Pigment11-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

rhodopsin 11-cis retinal + opsin

all-trans retinal+ opsin

isomerase

11-cisretinal

All-transretinal

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 11-cis 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

wavelength

intensity

Light of a Single wavelength

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

Fig. 15.23

I Structure of the Ear

– 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

•1. Outer Ear:

–Pinna (auricle): directs sound waves into the auditory canal

• 2. Middle Ear: three tiny bones “amplify sound” and transfer sound energy to the inner ear

A: MalleusB: IncusC: 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 s

tapes 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 ConductionSound wave

Auditory Canal Auditory Canal

Tympanic membraneAir in tympanic cavity

Ossicular chain Round window

Oval window Inner ear

Bone Conduction

Sound wave

Vibration of skull

Sound wave

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 II. ROLE OF MIDDLE EAR IN SOUND TRANSMISSIONSOUND TRANSMISSION

Tympanic MembraneTympanic Membrane

Tympanic Membrane Tympanic Membrane and Ossicular Chainand Ossicular Chain

MECHANISMS INVOLVED IN MECHANISMS INVOLVED IN TRANSFORMER PROCESSTRANSFORMER PROCESS

Size difference between Tympanic Tympanic MembraneMembrane and Stapes Footplate

Lever action

First Component of Middle Ear First Component of Middle Ear Transformer ActionTransformer Action

Size Difference– Tympanic membrane

.59 cm2

– Stapes footplate .032 cm2

– Pressure formula Pressure = force/area

Impact on sound transmission

Pressure gain: 0.59/0.032 = 18.4 (times)

Transformer Action of Middle EarTransformer Action of Middle EarLever ActionLever Action

Fulcrum Effect pressure gain: 1.3 times

TRANSFORMER ACTION TRANSFORMER ACTION AMOUNT OFAMOUNT OF AMPLIFICATIONAMPLIFICATION

Pressure Gain Contribution from:

18.4 TM (Tympanic Membrane)Tympanic Membrane) to stapes footplate

1.3 Lever action

23.9 Total pressure gain

(18.6 x 1.3)

III Function of Cochlea

Cochlea - Snail-shaped organ with a series of fluid-filled tunnels; converts mechanical energy into electrical energy

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.

CochleaCochlea

CochleaCochleaFluids in the cochlea: perilymph-fills the sc

ala vestibuli and scala tympani.Endolymph fills the scala media.

CochleaCochleaAt the end of the cochlea, the helicotrema joins the scala vestibuli and the scala tympani.

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

Structures of the Inner Ear

Hearing Summary So Far!!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 CortiOrgan of Corti

a structure rests atop the basilar membrane along its length

contains approx. 16,000 cochlear hair cells

Organ of CortiOrgan 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 PotentialsElectrical 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

CochleaCochlea

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)Two DC Potentials (EP)

Endocochlear Potential (EP)– Békésy discovered EP by putting the electro

de 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

Intracellular Potential (IP) or organ of corti potential (resting potential)

–Recorded -80 mV inside cells of organ of corti

Two DC Potentials (IP)Two DC Potentials (IP)

Two AC PotentialsTwo 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

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.

There are little mechanical gates on each hair cell that open when they are bent.

Two AC PotentialsTwo 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 Summary: How Sound Travels Through The Ear...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.

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!

Summary: How Sound Travels Summary: How Sound Travels Through The Ear...Through The Ear...