SPECIAL SENSES

Physiology 2

Presented by: Dr. Shaimaa Nasr Amin

Lecturer of Medical Physiology

General Education Program

Special Senses

Vision Hearing Smell Taste

Optic axis

Visual Axis

Light Waves

Part of electromagnetic spectrum

Visible part of the spectrum 400 – 700 nm

Formed of Photons, but has wave properties:

Wave length Frequency

Velocity of light 300’000 km/s in vacuum and gases,

slower in transparent liquids and solids

OPTICS OF THE EYE

Light strikes a surface

Absorbed

Reflected

With Refraction

Transparent medium Opaque medium

Without Refraction

Refraction of Light

Angle of incidence Refractive index

Dense medium

Higher refractive index

Less dense medium

Lower refractive index

Velocity of light in medium

Velocity of light in air

Refraction of Light

Angle of incidence Refractive index

Dense medium

Higher refractive index

Less dense medium

Lower refractive index

Refraction of Light

Angle of incidence Refractive index

Dense medium

Higher refractive index

Less dense medium

Lower refractive index

Convex Spherical Lens

Nodal point

Power of the lens:

Diopter =

Focal

point

Focal length

1

Focal length (in meters)

1 1 1

Object distance Image distance Focal length + =

15 mm

∞ 0.015

zero + 67 = 67 D

Power

Object size Object distance

Image size Image distance =

Protective mechanisms of the eye:

1- Bony orbit :

2- Eye lids:

3- Tears:

Eyelids:

Eyelids:

Types of closure of eyelids

Blinking

Voluntary winking

Blepharo-spasm

Eyelids:

Types of closure of eyelids

I- Blinking:

1- Spontaneous blinking:

- Rate: 10-12/min.

- Duration: 0.3 sec.

- Occurs in blind people.

- More in women than men.

- More with air currents, irritation of

conjunctiva and emotions.

-Function: spreads tears and help their

drainage

Rapid closure of both eyes

Eyelids:

Types of closure of eyelids

I- Blinking:

2- Reflex blinking:

Occurs as a result of:

-Touching the cornea, eye lashes, or conjunctiva.

- Dazzle reflex.

- Menace reflex.

- Auriculo-palpebral reflex.

3- Voluntary blinking.

Eyelids:

Types of closure of eyelids

II- Voluntary winking: Rapid closure of one eye

III- Blepharo-spasm: Due to eye injury.

The Lacrimal Apparatus

Functions of Tears:

1- Maintain corneal surface optically uniform.

2- Corneal nutrition.

3- Bactericidal: lysozymes, lactoferin and IgA, G, M.

4- Flushing foreign bodies.

5- Lubrication.

Precorneal film of tears: Formed of 3 layers:

1- External lipid (1%).

2- Water (> 90%).

3- Mucin.

Intraocular Fluids

Aqueous Humour

Secretion: Ciliary processes 2-3 ml /min. 3.5 ml/24h

Mechanism:

Active Na+ K+ ATPase

Cl- & HCO3-

Carbonic anhydrase

Drainage

Functions of Aqueous Humour:

1- Nutrition of lens and cornea.

2- Drainage of waste products.

3- Optical medium (refractive index is 1.33).

4- Intraocular pressure.

Intraocular Pressure

12 – 20 mmHg.

Higher in the morning Maintained by balance between

rate of secretion = rate of drainage

of aqueous humour

Regulated by changes in

rate of outflow

Functions:

1- maintains shape.

2- Normal focusing.

Glaucoma:

IOP > 20 mmHg.

Usually due to blocking of drainage

of aqueous humour

It leads to: Disturbance of focusing for near vision

Pressure on retinal arteries and nerves

causing their atrophy Blindness.

Pain.

Treatment: 1- Decrease aqueous secretion (acetazolamide)

2- Open spaces of Fontana (pilocarpine).

3- Surgical opening of spaces of Fontana.

Refractive media of the eye

Refractive

index Air = 1.0

Cornea

= 1.38

Aqueous

= 1.33

Lens

=1.40 Vitreous

= 1.34

Power of the

cornea = 45 D

Power of the

lens = 20 D

The Cornea

The cornea is transparent. It absorbs ultraviolet rays.

Causes of corneal transparency:

Avascular

Unmyelinated nerve endings

Collagen fibers uniform, parallel, equally spaced

Relative dehydration:

Metabolic pump Na+ K+ pump

Osmosis.

The Cornea

Strong refractive power = 45 D, but FIXED

- High regular curvature.

High R.I. 1.38 compared to air 1.0

The Corneal

Reflex

Ophthalmic

division of V

Facial nuclei

Facial nerve

Orbicularis muscles

The Lens

Capsule

Nucleus

Cortex

60-70% water

35% proteins

The Lens

Transparent due to:

- Avascular

- lens fibers are uniform and densely packed.

- different components have the same R.I.

Metabolism: - Low metabolic rate.

- mainly anaerobic oxidation of glucose.

- Some aerobic oxidation (glutathione).

Diopteric power:

- 20 D without accommodation.

- increases to 34 D in full accommodation.

Cataract: Loss of transparency of the lens

Causes: Decreased glutathione.

Degeneration & coagulation of lens proteins

and ppt of Ca++

Senile cataract, D.M., UV rays

Treatment: Surgical removal + Convex lens

Accommodation:

Emmetropic eye

Accommodation:

Accommodation:

Accommodation:

Power of accommodation:

power of lens

without accommodation - power of lens

With full accommodation

20 D 34 D

14 D

Near point:

Far point: ≈ 6 meters

10 cm

Range of accommodation

Near Response (Near Reflex)

miosi

s

Edinger

Westphal

nucleus

Occulomotor

nerve

Ciliary ganglion

convergence

Pathway of Near

Reflex Accommodation

miosis

Presbyopia:

Presbyopia:

Errors of Refraction

Astigmatism:

The Uveal Tract

Iris Ciliary Body Choroid

The Iris

The Iris

Radial fibers

Dilator pupillae

Circular fibers

Constrictor pupillae

pupil

Sympathetic nerves

Parasympathetic

nerves

Functions of the iris:

1- Control the amount of light entering the eye.

2- Increase the depth of focus:

Functions of the iris:

1- Control the amount of light entering the eye.

2- Increase the depth of focus:

3- Prevent spherical aberration:

Functions of the iris:

1- Control the amount of light entering the eye.

2- Increase the depth of focus:

3- Prevent spherical aberration:

4- Prevent chromatic aberration:

Pretectal

nucleus

Edinger Westphal

nucleus

Ciliary ganglion

Pupilloconstriction

of both eyes

Argyl Robertson

pupil

Changes in pupil during different

stages of anesthesia:

First stage: pupil normal in size & reactive to light.

Second stage: pupil dilated & reactive to light.

Third stage (surgical stage): pupil constricted.

Fourth stage : pupil dilated & not reactive.

Conditions which produce pupilloconstriction

(Miosis):

1. pupillary light reflex.

2. Near response.

3. Horner’s syndrome.

4. Sleep.

5. Drugs (parasympathomimetics, morphine,

histamine.

6. Third stage of anesthesia.

Conditions which produce pupillodilatation

(Mydriasis):

1. Dark adaptation.

2. Distant vision.

3. Oculomotor nerve lesion.

4. Sympathetic stimulation (fear, anger, emotions).

5. Drugs (parasympatholytics, adrenaline, cocaine,

alcohol).

6. Second stage of anesthesia.

7. Increased intra-ocular pressure.

Ciliary Body:

Ciliary muscle: Accommodation

Ciliary processes: Aqueous humor

Choroid:

1. Blood supply to the eye.

2. Melanin: prevents reflection of light.

3. Attachment of the ciliary muscle.

The Retina

The Retina

The Retina

The Retina

Pigment layer of the retina

Melanin

Functions:

1- Absorbs light and prevents reflection.

2- Stores vitamin A.

3- Phagocytic.

Macula Lutea

Right Eye Temporal

Fovea Centralis

Structure of Photoreceptors:

Rods (120 millions)

Cones (6 millions)

Distribution

in Retina

More in periphery

Absent at fovea

Mainly at the fovea

Much less at periphery

Photopigment Scotopsin. Large amount. Photopsin. (3 types)

Sensitivity High. (Night vision). Low. (Day vision).

Receptor

potential

Longer , easily summate. Shorter.

Convergence High degree of convergence No convergence at fovea.

Visual Acuity Low. High.

Colour vision Not seen Seen

Duplicity Theory of Vision

2 separate mechanisms for vision

Scotopic

Vision

Photopic

Vision Vision in dim light

(Dark vision)

Vision in bright light

(Day vision)

Rods Cones

Cannot detect

boundaries, colors

or details

boundaries, colors

or details are seen

Spectral Sensitivity of Photopigments

% l

um

ino

sit

y

505 nm

Blue-green

550 nm

green-Yellow

Purkinje Shift

phenomenon

Photoreceptor Potential

Photochemical changes (Bleaching).

Genesis of electrical response.

The excitation cascade.

Signal transmission in the retina.

Photopigments

4 types:

One types in rods Rhodopsin

Three types in cones

Scotopsin

11-cis-retinal

Opsin + Chromophore

Photopsin (3 types)

Dark Current

Photochemical Changes

Rhodopsin Bathorhodopsin

Lumirhodopsin

Metarhodopsin I

Metarhodopsin II

n sec

m sec

m sec

sec

p sec

scotopsin

all-trans-Retinal

all-trans-Retinol

(vitamin A)

11-cis-Retinal

11-cis-Retinol

minutes

isomerase

isomerase

Rhodopsin Kinase

Na+

Na+

Ca++

Na+

Signal transmission in the retina

Electrotonic conduction Action potential

Organization of receptive fields

of Ganglion Cells

Bipolar

Cells

Ganglion

Cell

Types of Ganglion Cells

Parvo-cells

(P-cells) Magno-cells

(M-cells)

- Large size.

- wide dendritic field.

- Large receptive field.

- Transient discharge.

- Rapid conduction.

- Detect movement and

changes in illumination.

- Small size.

- Narrow dedritic field.

- Small receptive field.

- Sustained discharge.

- Slow conduction.

- Detect details, texture

and colors.

Automatic Regulation of

Retinal Sensitivity to Light:

Retinal

Adaptation

Light Adaptation:

Change in retinal sensitivity ( 100’000 – 500’000 times)

Decreased retinal sensitivity

Decreased photopigments in both rods & cones.

Increased visual threshold

Decreased signal intensity in retinal neurons

Miosis

Dark Adaptation

Decline in visual Threshold Increased retinal sensitivity

Regeneration of photopigments 30 minutes

Rapid Phase 5-10 minutes

Regeneration of cone

pigments

Slow phase 30 minutes

Regeneration of rod

pigments

Colour Vision

The ability of the retina to distinguish different wavelengths

Function of cones

Characteristics of colours

Hue is the actual colour

Saturation is the purity of colour

brightness number of photons / unit area / second

Primary Colours Red Blue Green

Complementary Colours When mixed give white

Yellow + Blue Red+ Green

Mixing light of different wavelengths

Mixing paints is different !!

Yellow + Blue = Green

Mechanism of Colour Vision

Trichromatic Theory of Colour Vision

Young – Helmholtz Theory

Thomas Young, 1802 Hermann von Helmholtz, 1851

Mechanism of Colour Vision

Trichromatic Theory of Colour Vision

Young – Helmholtz Theory

Suggest the existence of 3 types of cones

1- Short wave (blue) sensitive cones: 445 nm

2- Medium wave (green) sensitive cones: 535 nm

3- Long wave (red) sensitive cones: 565 nm

445 nm 535 nm 565 nm

580 nm Red : 99%

Green: 42%

Blue: 0%

100%

Orange

Neural Mechanism of Colour Vision

Optic nerve Lateral

Geniculate

body

Primary Visual cortex

Parvo cellular

neurons

Parvo

ganglion

cells

Blobs

Cones

Colour Blindness

Inability to perceive portion of the spectrum

Inability to distinguish between colours

Recessive sex-linked trait.

Males much more affected than females (8% : 0.4%)

Colour Blindness

Colour Anomaly

Colour Anopia

Anomalous Trichromats

Protanomaly: weakness of red colour

Deuteranomaly: weakness of green colour

Tritanomaly: weakness of blue colour

Colour weakness

Loss of perception of one colour or more

Dichromats

Monochromats

Protanopia: red blindness

Deuteranopia: green blindness

Tritanopia: blue blindness

After-Image Phenomenon

Visual sensation perceived after removal of the visual stimulus

Negative After-Image

Negative After-image

Stare, unfocused, at the red cross for 10 seconds then look at white wall

Negative After-image

Cyan

Negative After-image

Stare, unfocused, at the flag for 10 seconds then look at white wall

Negative After-image

Cyan Magenta Yellow

Positive after-Image

Flicker

Intermittent light sensation by successive visual stimuli

Critical fusion frequency (CFF) Log light intensity

CFF for rods = 20 Hz

CFF for cones = 50 Hz

Ferry-Porter Law

Electroretinogram (ERG)

a

b

c

d

off on +

-

Rods &

Cones

response

Ganglion

cell activity

Recovery of

receptors

ERG is useful in diagnosis

Retinitis pigmentosa Rods affected

ERG of dark adapted eye shows absent “a” wave.

Neural Pathway of Vision Visual Pathway

Neural Pathway of Vision Visual Pathway

Neural Pathway of Vision Visual Pathway

Neural Pathway of Vision Visual Pathway

Visual Pathway

Retina

Photoreceptors → Bipolar cells →

Ganglion cells

Optic Nerve

Optic Chiasma Decussation of nasal fibers

Optic Tract Temporal fibers of the same-side retina

Nasal fibers of the opposite-side retina

Ends in Lateral Geniculate Body

Suprachiasmatic nucleus of hypothalamus (circadian rhythm)

Pretectal nucleus (pupillary light reflex)

Superior colliculus of midbrain (reflex eye directional movement)

Visual Pathway

Lateral Geniculate Body

Formed of 6 layers

Layers 2,3,5 connected

to same-side eye

Layers 1,4,6 connected

to opposite-side eye

Magnocellular Layers (1,2)

Receive from large ganglion cells Detect movement and

stereoscopic perception Discharge to occipital cortex

Parvocellular Layers (3-6)

Receive from small ganglion cells Detect fine details and

colour Discharge to blobs

Visual Pathway

The Visual Cortex in occipital lobe

Visual Pathway

The Visual Cortex

Visual Pathway

The Visual Cortex

Macula

Intermediate

retina

Peripheral

retina

Visutopic organization

Primary visual cortex

Visual Pathway

The Visual Cortex Primary visual cortex

Formed of 6 layers

Geniculocalcarine fibers end in layer 4

Feature detectors

2 types of neurons

Simple cells

Complex cells

Visual Pathway

The Visual Cortex Primary visual cortex

Simple Cells Receptive field

(retina)

Simple cell

response

Rectangular receptive

field with specific

orientation

Visual Pathway

The Visual Cortex Primary visual cortex

Complex Cells

Rectangular

receptive field

Respond to

moving bars of

light

Visual Pathway

The Visual Cortex Primary visual cortex

Organized into Orientation Columns

Blobs

Visual Pathway

The Visual Cortex Primary visual cortex

Decodes information about: form - contour - contrast -

colour – depth - movement.

The Visual Cortex Secondary visual area

Areas 18 & 19

= Visual Association Areas

= Peristriate cortex

Analyze visual information to detect:

Nature of objects

Visual orientation and depth perception.

Send information to: Posterior parietal area (area 7)

Frontal eye field (area 8)

Inferior temporal area (areas 21,22)

Blindness of right eye.

Bitemporal hemianopia

Contralateral homonymous

hemianopia.

Contralateral homonymous

quadrant anopia.

Contralateral homonymous

quadrant anopia. Macular sparing

Contralateral homonymous

hemianopia. Macular sparing Contralateral hemianopic

scotoma

Lesions in Visual Pathway

Visual Field

Visual Field

Visual Field

Importance of visual field determination:

1- Determination of site of lesion in visual pathway.

2- Localization of the site of scotoma.

3- Diagnosis of some diseases e.g. retinitis pigmentosa.

Visual Acuity

Degree of perception of details & contour of objects

Measured by the minimum separable distance.

Visual angle ONE MINUTE

Determination of visual acuity

C C C

Determination of visual acuity

Factors affecting visual acuity

1- Refractive power of the eye.

2- Degree of illumination and contrast.

3- Pupilloconstriction increases visual acuity.

4- Eye diseases e.g. cataract, glaucoma and retinal

detachment.

5- Maximal at the fovea.

Binocular Vision

Ability to see one object with the two eyes without diplopia

Right visual cortex

Binocular Vision

Requirements of binocular vision

1- Considerable overlap of both visual field.

2- Nearly equal diopteric power of both eyes.

3- Normal extraocular muscles.

4- Normal visual cortex.

Advantages of binocular vision:

1- Wider visual field.

2- Minimizes the effect of retinal defects in one eye.

3- Masks abnormal refraction of one eye.

4- Better depth perception and stereoscopic vision.

Depth perception

The ability to determine distance by vision.

Depends on:

1- Relative size of objects.

2- moving parallax

Depth perception

The ability to determine distance by vision.

Depends on:

1- Relative size of objects.

2- Moving parallax

3- Fade of colours and details.

Depth perception

The ability to determine distance by vision.

Depends on:

1- Relative size of objects.

2- Moving parallax

3- Fade of colours and details.

4- Parallel lines appear to converge (perspective).

Depth perception

The ability to determine distance by vision.

Depends on:

1- Relative size of objects.

2- Moving parallax

3- Fade of colours and details.

4- Parallel lines appear to converge (perspective).

5- Occlusion of part of far object by nearer one.

Depth perception

The ability to determine distance by vision.

Depends on:

1- Relative size of objects.

2- Moving parallax

3- Fade of colours and details.

4- Parallel lines appear to converge (perspective).

5- Occlusion of part of far object by nearer one.

6- Distribution of light and shadows on the surface.

Depth perception

The ability to determine distance by vision.

Depends on:

1- Relative size of objects.

2- Moving parallax

3- Fade of colours and details.

4- Parallel lines appear to converge (perspective).

5- Occlusion of part of far object by nearer one.

6- Distribution of light and shadows on the surface.

7- Shadows on surroundings.

Stereoscopic vision

The perception of the 3 dimensions of objects:

height - width - depth.

Depends on:

1- All factors that determine depth perception.

2- Formation of slightly dissimilar images on both images.

Eye Movements

Squint

Incoordination of external

ocular muscles

Image falls on non-corresponding retinal points Diplopia

If neglected in children → neglected eye

Amblyopia exanopsia

Treatment:

1- Exercise of extraocular muscles.

2- Special glasses.

2- Surgical operations.

Types of Eye Movements

Fixation movements

1- Voluntary fixation: Frontal eye field (area 8)

Searching movement

2- Involuntary fixation:

Following movement

Secondary visual area

(area 19)

Frontal eye field (area 8) unlocks fixation from one

object to another.

Types of Eye Movements

Fixation movements

Saccadic movements

Sudden jerky movements when the gaze changes

from one object to another e.g. reading

Types of Eye Movements

Fixation movements

Saccadic movements

Smooth pursuit movements

Smooth movement while fixing on moving object.

Types of Eye Movements

Fixation movements

Saccadic movements

Smooth pursuit movements

Convergence movement

Fixing on near object to keep images on foveae

Types of Eye Movements

Fixation movements

Saccadic movements

Smooth pursuit movements

Convergence movement

Nystagmus

Nystagmus

Physiological nystagmus

Fine oscillatory movements during prolonged fixation.

Prevents adaptation of photoreceptors.

Optokinetic nystagmus

Smooth pursuit movements + Rapid saccade

Vestibular nystagmus

Maintains visual fixation as the head rotates

Ophthalmoscopic Examination

Ophthalmoscope

Ophthalmoscopic Examination

Ophthalmoscope

1- Retinal blood vessels:

Hypertension

diabetes

Renal disease

Ophthalmoscopic Examination

Ophthalmoscope

1- Retinal blood vessels: Hypertension - diabetes - renal

disease

2- Fovea & optic disc : cupping in glaucoma.

1- Retinal blood vessels: Hypertension - diabetes - renal

disease

2- Fovea & optic disc : cupping in glaucoma.

3- Retinal diseases e.g. detachment.

Ophthalmoscopic Examination

Ophthalmoscope

1- Retinal blood vessels: Hypertension - diabetes - renal

disease

2- Fovea & optic disc : cupping in glaucoma.

3- Retinal diseases e.g. detachment.

4- Determination of errors of refraction.

Ophthalmoscopic Examination

Ophthalmoscope

Physiology of Hearing

Ear

It consists of:

• External ear

• Middle ear

• Inner ear

External Ear

• Ear Pinna

• Collect &Direct sound to External auditory meatus.

• Sound localization.

• ITS loss decrease hearing power by 30%

• External AUDITORY MEATUS

• Conduct & Concentrate sound tympanic membrane

• Resonance amplify sound.

• Protection of tympanic membrane by: Mucus, Hair and Antibacterial effect.

• its anatomy long & tortuous protect it from trauma.

Middle Ear

• Air filled cavity bounded laterally by tympanic membrane.

• Connected to nasopharyx.

• Content of middle ear: • 3 bony ossciles (Malleus, incus & stapes)

• Tympanic membrane.

• 2 muscles:

• Tensor tympani supplied by 5th cranial N

• Stapedius muscle supplied by 7th crainal nerve

Inner ear

Vestibular apparatus

Posture & equilibrium

Vestibular nerve

Cochlea

Hearing

cochlear nerve

cochlea