section Ⅸ -...

Section Ⅸ

Physiology of the Sense OrgansYing Huang

yinghuang@shmu.edu.cn

Schedule for section Ⅸ

•Today------General principles, Vision

•Next Wednesday------Hearing & Equilibrium, Other senses

Clinical Investigation

After a visit to a brothel in Arles, France, on December 23, 1888, Vincent Van Gogh, the nineteenth-century French painter, returned to his room, picked up a knife, and cut off his own ear. A local physician, Dr. Felix Ray, examined Van Gogh that night and wrote that the painter had been “assailed by auditory hallucinations” and in an effort to relieve them, “mutilated himself by cutting off his ear.” A few months later, Van Gogh committed himself to a lunatic asylum. By 1890, Van Gogh was dead by his own hand. Historians have assumed that Van Gogh suffered from epilepsy. But the painter’s strange attacks of dizziness, nausea, and overwhelming tinnitus, which he recorded in desperate letters to his relatives, are more consistent with Ménière's disease, a condition that affects the inner ear.

GENERAL PRINCIPLES OF SENSORY PHYSIOLOGY

What is the Sensory System?•Part of the nervous system consisting of sensory receptors that receive stimuli from internal and external environment and conduct this information to brain that processes this information.

What is it made of?•Nose•Eyes•Ears•Tongue•Skin•Brain•Neurons•Receptors

Traditional Classification of Sensory System

General sensesTouchTemperaturePain

Special sensesVisionHearingTasteSmellEquilibrium

What Triggers These Senses

•Receptors–Specialized endings–Collect information about external and internal environment through a stimulus.

•Each receptor is specific to a certaintype of stimulus

–An exception: a receptor can be activated by a nonspecific stimulus if its intensity is sufficiently high

Events occurring within a sensation

1.stimulation of the receptor2.transduction (conversion) of stimulus into a graded potential–vary in amplitude and are not propagated3.generation of impulses when graded potential reaches threshold4. Information is sent to CNS5.integration of sensory input by the CNS

Receptors and Sense Organs

•Receptor is referred to the structure located on the body surface or within tissues to detect the changes in internal or external environments and to convert stimulus into electrical signals. •The receptor is often associated with nonneural cells that surround it, forming a sense organ.

Receptors and Sense Organs

Hair cells Ear

Receptors and Sense Organs

•Receptors detect a small range of energy levels –Ear, 20～20,000 Hz vibration–Taste buds, specific chemicals–Eye, 380～750 nM electromagnetic wave

Complexity Range of Receptors

Types of Receptors-Location

1.Exteroceptors (外感受器)–Teleceptors (距离感受器)

•Sight, hearing–Contact receptor (接触感受器)

•Pressure, temp., touch, taste2.Interoceptors (内感受器)

–Visceroceptors (内脏感受器)•Blood pressure, pain

–Proprioceptors (本体感受器)•Muscle spindle

Types of Receptors-Modalities1.Mechanoreceptors

•respond to mechanical stimulus2.Thermoreceptors

•changes of temperature can stimulate these receptors3.Nociceptors

•brings information concerning pain4.Electromagnetic receptors

•rod cells and cone cells of the eye which are stimulated by changes of intensity and wavelengths of the light

5.Chemoreceptors•respond to chemical stimulus, such as taste, smell andblood oxygen

Properties of the Receptors

1.Adequate stimulus2.Transducer function3.Coding4.Adaptation

1、Adequate Stimulus of Receptors

• Each type of receptor is highly sensitive to one type of stimulus

• The particular form of energy to which a receptor ismost sensitive is called its adequate stimulus.

• E.g. The adequate stimulus for the rods and conesis light. The threshold for their nonspecific responsesis much higher. Pressure on the eyeball will stimulate the rods and cones, but the threshold of these receptors to pressure is much higher than the threshold in the skin.

2、Transducer function

• The receptors translate the energy of the stimulusinto an electrical change in specific sensory cells orafferent nerve endings.

• In the former case the electrical change is called receptor potential, and in the later case it is called generator potential.

• Converting one type of energy into another type(bioelectrical energy) is the process of transduction

• Our brain only deals with bioelectrical impulses sotransduction must occur!

Generator (Receptor) Potential

Character: local excitation–Not all-or-none–electrotonic propagation–temporal & spatial summation

It can be summated to reach the threshold to produce AP in afferent fibers.

3、Coding of Sensory Information

•The quality of sensation is dependent on sensory pathway including receptor, afferent fiber and cortex, which the stimulus eventually activates.

•The quantity (intensity) of sensation is associated with the frequency of APs in afferent fibers and the number of receptors activated by stimulus. –The stronger the stimulus, the more APs are fired over a given time period

E.g. Stretch Receptors:Weak stretch causes low impulse frequency on neuron leaving receptor.Strong stretch causes high impulse frequency on neuron leaving receptor.

4、Adaptation of Receptors• Definition: When a maintained stimulus of constant

strength is applied to a receptor, the frequency ofthe action potentials in its sensory nerve declinesover time

1.Rapidly adapting receptor (Phasic Receptors):The frequency of APs diminishes or stops if the stimulusis unchanging.

2.Slowly adapting receptor (Tonic Receptors):adapt slowly or not at all.

The height of the curve indicates the frequency of the discharge in afferent nerve fibers at various times after beginning sustained stimulation.

Adaptation of Receptors

Significance of Adaptation

•The rapidly adapting receptors cease firing if strength of a continuous stimulus remains constant. Allow body to ignore constant unimportant information, e.g. Smell

•The slowly adapting receptors continue signal transmission for duration of stimulus. Monitoring of parameters that must be continually evaluated, e.g. baroreceptors.

Visual Sense Organ

• Structure of the Eye Refractive systemA lens system for focusing light on the receptors.

RetinaA light-sensitive tissue lining the inner surface of the eye.Blind spot

Function ofRefractiveSystem

Reduced EyeRefraction of Light in the Eye

Vitreous humor

Accommodation of Refractive Function

Gaze at a near object (less than 6m)：

1. Accommodation of lens

2. Accommodation of Pupils

3. Convergence of Eye Balls

Clear image on retina

Accommodation of Refractive FunctionAccommodation of Lens

Lens ligaments

A more convex shape

Accommodation of Refractive Function

Accommodation of Pupils

Sphincter pupillae contracts, the size of thepupil is decreasedPupillary light reflex

Significance: regulate the amount of lightentering the eye

Pupillary light reflexdirect light reflexconsensual light reflex

Bright light is shone on the eye light sensitive cells in the retina, including rod and cone photoreceptors---the optic nerve fiber that carry the impulses initiating pupillaryresponses end in the pretectal region and the superior colliculi--- send signals to the oculomotor nerve, which terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil.

Pupillary light reflex•Direct light reflex

•Consensual light reflex

Accommodation of Pupils Near reflex of the pupil: The pupil constricts when an individual looks at a near object

Near point of vision:The nearest point at which an object can be

seen distinctly by the eye

Far point of vision:The farthest point at which an object can be

seen distinctly by the eye

Abnormalities of Refractive Function andAccommodation in the Eyes

EmmetropiaAmetropia

Myopia

The anteroposterior diameter of the eyeball is too long. This defect can be corrected by the use of glasses with biconcave lenses so that parallel light rays are made to diverge slightly before they strike the eye.

• Hyperopia

The eyeball is shorter than normal and parallel rays of light are brought to a focus behind the retina. The use of glasses with convex lenses to aid the refractive power of the eye in shortening the focal distance corrects the defect.

Astigmatism

The curvature of the cornea is not uniform. When the curvature in one meridian is different from that in others, light rays in that meridian are refracted to a different focus, so that part of the retinal image is blurred.Corrected with cylindric lenses placed in such a way that they equalize the refraction in all meridians

Presbyopia

Glasses with convex lenses

The photoreceptor mechanism

Retina: A light-sensitive tissue lining the inner surface of the eye.

• The optics of the eye: create an image of the visual world on the retina, which serves the same function as the film in a camera.

• Rod cells and cone cells• Light activating rod cells and cone cells on the retina

initiates chemical and electrical events that ultimately trigger nerve impulses. These are sent to visual centers of the brain through the fibers of the optic nerve.

Structure of retina

The photoreceptor mechanism

Photoreceptors in Retina

1) Scotopic vision systemRod cell:The photosensitive pigment in the rods is called Rhodopsin

• Rod cellNot color sensitiveMainly more sensitive to light (scotopic vision)

Peripheral area of retinaNight blindness

Photoreceptors in Retina

2) Photopic vision system

Cone cell:

• Cone cellColor sensitiveLess sensitive to lightMainly fovea in retina

Visual Transduction in Rods System

Photochemistry of rhodopsin-retinal visual cycle in the rod

Rhodopsin: opsinretinene

Mechanism for Rod Receptor Potential

Generation of a hyperpolarization receptor potential

In dark In the presence of light

Ionic currents in the photoreceptor cell( The terminal releases less transmitters when

hyperpolarized )

Hyperpolarization of photoreceptor in response to light• Intensity-dependent • Saturation

Color Vision (Cone, Photopic vision)

Trichromatic theory

Color Blindness

Information Process in Retina

Other Phenomina Related with Vision

Visual acuity:

The degree to which the details and contours of objects are perceived

Snellen visual acuity chart

Other Phenomina Related with Vision Dark adaptation: If a person spends a considerable period of time

in brightly lighted surroundings and than moves to a dimly lighted enviroment, the retinas slowly become more sensitive to light. The decline in visual threshold during this process is known as darkadaptation.

Light adaptationMechanisms of adaptations:Changes in concentrations of rhodopsin or color photochemicals

to resolve fine detail

Visual field:Not circularWhite > yellow > red > greenBlind spotular

White＞yellow＞red＞green

Binocular vision and stereopsis

The central parts of the visual fields of the 2 eyes coincide; therefore, anything in this portion of the field is viewed with binocular vision. Binocular vision is often assigned an importantrole in the perception of depth to produce stereopsis.

HEARING

Outline

• Belongs to special senses• Receptors: Hair cells in the inner ear• Sense organ: Ear• Adequate stimulus: Sound wave• Coding: Pitch and loudness

Structure and function of earExternal ear-funnels sound waves from environment

Middle ear-cavity between tympanic membraneand cochlea

Inner ear-where sound is actually transmittedto nerves (It alsoreceives the changes of the position of the head to balance centers in the brain.)

1. External Ear:

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

–External auditory Canal:conducts sound to the eardrum

2. Middle Ear−Tympanic membrane (Eardrum): thin membrane that

vibrates in response to sound, and transfers sound energy to bones of the middle ear

−Ossicles:three tiny bones “amplify sound” and transfer sound energy to the inner ear

−Tympanic cavity: small cavity surrounding ossicles−Auditory tube: connecting the middle ear with the throat,

equalizing pressure during yawning or swallowing

Role of middle ear in sound transmission

• Sound Force Amplification by the Ossicles– Greater pressure at oval window than tympanic

membrane, moves fluid of cochlea• The Attenuation Reflex

– Response where onset of loud sound causes tensor tympani and stapedius muscle contraction

– Function: Adapt ear to loud sounds

Inward movement of the tympanicmembrane by a sound pressure wave causes the chain of ossiclesto push the footplate of the stapes into the oval window

Mechanisms involved in transformer process

1.Size difference between Tympanic Membrane and Stapes Footplate

2.Lever action

Impact of size difference on Middle Ear Transformer Action

Tympanic membrane59.4 mm2

Stapes footplate3.2 mm2

Pressure = force/area

Pressure gain: 59.4/3.2 = 18.6 (times)

Impact of Lever Action on Middle Ear Transformer Action

pressure gain: 1.3 times

Total Amount of Amplification

Pressure Gain Contribution from

18.6 Size difference1.3 Lever action24.2 Total pressure gain

(18.6 x 1.3)

The malleus takes the pressure from the inner surface of the tympanic membrane and passes it by means of the incus to the stapes in such a way that the pressure is amplified about 24 times as it moves.

3. Inner EarIncludes sense organs for hearing and balanceFilled with perilymph

Inner Ear

A maze of bony chambers•Cochlea: snail shaped fluid-filled structure•Vestibule

Oval window: thin membrane, transfers vibrations from stapes to fluid of cochleaRound window: absorbs energy and equalizes pressure in the cochlea

Cochlea•Snail-shaped organ with a series of fluid-filled tunnels. Converts mechanical energy into electrical energy•Three chambers: scala vestibuli, scala tympani, scalamedia (cochlear duct)•Basiliar and vestibular membranes

CochleaAt the end of the cochlea, the helicotrema joins the scalavestibuli and the scala tympani.

Fluids in the cochlea

Perilymph: fills the scala vestibuli and scala tympani. similar in composition to extracellular fluid (High in Na+and low in K+).

Endolymph: fills the scala media. Similar to intracellular fluid (High in K+and low in Na+).

Cochlea - Organ of Cortia structure contains approx. 16,000 cochlear hair cellslocated on basilar membrane Stereocillia, kinocilium : at apex of hair cells, embedded in tectorial membraneGel-like tectorial membrane is capable of bending stereocillia, konocilium and activating receptorsCochlear nerve attached to hair cells transmits nerveimpulses to auditory cortex

Function of Corti

Relative shearing motion of basilar membrane and tectorial membrane makes the hair cells bend and depolarize, changes transmitters release

Properties of SoundSound travels in waves as light does

•Pitch: determined by “frequency,” the number of cycles per second of a sound wave, measured in hertz (Hz)

•Loudness:determined by “amplitude” (height) of the sound wave, measured in decibels(dB)

•Timbre: determined by “complexity and shape” of the sound wave, gives each sound its unique quality

Pitch of sound

Loudness of Sound•0 dB = hearing threshold•50 dB = normal conversation•90 dB = danger zone•120 dB = Rock concert•130 dB = Pain threshold

Sound Transmission and Transduction

Sound waves

Tympanic membrane vibrations

Ossicles transmit & amplify vibration

Via oval window to perilymph then endolymph

basal membrane resonance oscillation

hair cells translate the vibration into generator potentials

auditory nerves transmits nerve impulses to auditory cortex

Conduction of Sound1. The air conduction

Air Conduction of Sound

Conduction of Sound

2. The bone conduction

Sound Transmission and Transduction

Sound waves

Tympanic membrane vibrations

Ossicles transmit & amplify vibration

Via oval window to perilymph then endolymph

basilar membrane resonance oscillation

hair cells translate the vibration into generator potentials

auditory nerves transmits nerve impulses to auditory cortex

Electrical Potentials in Cochlea1, Endocochlear Potential (EP)

–Putting the electrode in the scala media and a +80 mV potential with respect to a neutral point of perilymph in scala tympani can be discovered

Intracellular Potential (IP) or hair cell resting potential: -80 mV

Difference between extracellular and intracellular potential:

Top: 150-160mV endolymph in scala media

Base: 80mV perilymph in scala tympani

Electrical Potentials in Cochlea

Mechanoreceptor in hair cell

When the stereocilia of a hair cell move toward

the tallest cilium, the hair cell is depolarized;

when the stereocilia bend in the opposite direction,

the hair cell is hyperpolarized.

Electrical Potentials in Cochlea

Sound Transduction　

• Vibrations from sound waves move tectorialmembrane

•　 Hair cells (stereocillia) are bent by the membrane•　 Generator potential is induced in hair cell•　 An action potential starts in the cochlear nerve

2,Cochlear Microphonic Potential (CM)–When cochlear is activated by sound, the membrane potential recorded in cochlear or near cochlear which is generated from hair cells. It reproduces frequency of a sound wave perfectly.

3, Action Potential (AP)–Electrical activity from the auditory nerve–Can be measured from anywhere in the cochlea or in the auditory nerve

Electrical Potentials in Cochlea

Coding of sound

•Coding Information About Sound Intensity–Firing rates of neurons–Number of active neurons

•Coding Information About Sound Frequency–Location of Basilar membrane activated–Frequency: Highest at base, lowest at cochlea apex

How to discriminate the frequency of the sound?

Traveling Wave Theory

The Traveling Wave Theory

• Sound wave entering at the oval window is to cause the basilar membrane to vibrate

• different frequencies cause vibrations at different locations (places) along basilar membrane

• higher frequencies at base, lower frequencies at top

The Traveling Wave Theory

Hearing impairments

• Deafness • Conduction deafness • Sensorineural deafness

• Tinnitus • Ménière's syndrome

• attacks of dizziness, nausea, caused by excess endolymph in the media canal

Deafness Conduction deafness • Transmission of sound waves through middle ear is impaired. • Impairs all sound frequencies. • May be caused by damage to ossicles by an infection of the middle ear or immobilization of the stapes in otosclerosis. • Treatment: Hearing aids- amplify sounds

Sensorineural/perceptive deafness• Transmission of nerve impulses anywhere from the cochlea to the auditory cortex is impaired. • Often impairs ability to hear some pitches more than others. • May be caused by many pathological processes or exposure to extremely loud sounds. • Treatment: Cochlear implants- electrically stimulate nerve fibers in response to sound

Outline

• Belongs to special senses • Receptors: Hair cells in the inner ear • Sense organ: Vestibular organ in the

inner ear • Adequate stimulus: Rotational and

linear acceleration

Structure of vestibular organ

Ability to detect head position and movement (or acceleration) to maintain a steady balance when it is still or moving

•　 Semicircular canals •　 Utricle •　 Saccule

Semicircular canals•　 Three semicircular canals on one side on three

directions, helps sense all possible head-rotation angles•　 Receptors in the semicircular canals

– Crista ampullaris, located in the expanded end (ampulla) of the canal

– Consists of hair cells – Cupula (gelatinous cap) covers the hair cells

Utricle and saccule•Receptors in the Utricle and saccule: OtolithicOrgan (Maculae)•Hair cells are embedded in the otolithic membrane•Otoliths (tiny stones) float in a gel around the hair cells

Function of Semicircular canals•Detects rotational acceleration•To maintain balance while turning

Function of Utricle•Detects changes relative to gravity (linear acceleration) from vertical movement

Function of Saccule•Detects backward-frontward (horizontal)movement (linear acceleration) from acceleration

Vestibular reaction

•Vestibular autonomic reaction–Motion sickness: “mismatch” betweenvisually perceived movement and the vestibular system's sense of movement

– Dizziness, fatigue and nausea

–Treatment: Antimotion drugs (e.g. Dramamine), depression of vestibular inputs

Vestibular reaction

• Nystagmus (The Vestibulo-Ocular Reflex, VOR)–When spinning is suddenly stopped, eyes continue to move inthe direction opposite of the spin, then jerk rapidly back to themidline.When a person begins spinning, the cupula bends in the opposite direction,and eyes slowly drift in opposite direction, then jerk rapidly back to the midline.

–If the spining suddenly stops, inertia of endolymph causes it tocontinue moving in the direction of spin,and eyes movement isopposite to those at the begining .

–This is a normal phenomenon that helps maintain balance during spinning.

Otoscopy is used to have a visual examination of the earAudiometer measures various frequencies to test hearingA tuning fork compares the conduction of sound in one ear or between the two ears.

Summary

• Each type of receptor is excited most effectively byonly one modality of stimulus known as the adequate stimulus.

• The stimulus is converted into an electrical potential.• The intensity of the stimulus is associated with frequency of APs and the number of receptors activated by stimulus.

• Ear is the sense organ for both hearing and equilibrium, both use hair cells in the inner ear as receptors.

Ear care Day

March 3rd

Olfaction-the sense of smell•The sense of smell is activated by neurons called olfactory receptors which are covered with cilia.•Olfactory receptors are yellowish-brown masses along the top of the nasal cavity.•Responds to molecules dissolved in mucus or lipids

Gustation-the sense of taste

•Humans perceive five types of taste: bitter, sour, salty, sweet, and umami(meaty flavor) .•Papillae are structures on the surface of the tongue that contain the taste buds.•Taste Buds are organs that sense the taste of food, contain receptor cells called taste cells responsive to each of the taste categories.

Somatic senses1.Pain

•Fast pain: The nervous system quickly responds to a pain initiating event to the central processing unit •Slow pain: persistent, indistinct source

•Referred pain is a phenomenon of pain perceived at a site adjacent to or at a distance from the site ofan injury's origin.

(during ischemia brought by a myocardial infarction where pain is often felt in the neck, shoulders, and back rather thanin the chest.)

2. Warmth and cold• Changes in temperature in dermis, skeletal muscles, liver and hypothalamus

• Free nerve endings• Cold receptors are much more than warm receptors

3.Touch-pressure

• Unencapsulated receptors: free nerve endingsMerkelsdics-fine touch

• Encapsulated receptors: Meissners corpuscles -fine touchPacinian corpuscles -deep pressure

4. Proprioception

monitors of muscle stretch

•Muscle spinder

•Tendon organ