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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
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