The Special Senses

Physiology of Vision Lecture-1- The image-forming mechanism Professor A.M.A Abdel Gader MD, PhD, FRCP (Lond., Edin), FRSH (London) Professor of Physiology, College of Medicine King Khalid University Hospital Riyadh, Saudi Arabia

Sound Touch Pain Smell Receptor Action Potential CNS

The Special Senses

• Vision • Hearing • Smell • Taste

The Physiology of Vision

Objectives:

At the end of these lecture the student should be able to:

Understand the optical bases of image formation on the retina

•

Understand and explain the optical bases of common refractive errors

•

Understand the electrical bases of the photoreceptor function Understand the nature and function visual pigments Understand color vision

• •

The Physiology of Vision

Objectives:

At the end of these lecture the student should be able to:

Understand the optical bases of image formation on the retina

•

Understand and explain the optical bases of common refractive errors

•

Understand the electrical bases of the photoreceptor function Understand the nature and function visual pigments Understand color vision

• •

INL ONL Foveola Light Human foveal pit

Low Convergence Cone-Fed Circuits Retinal ganglion cell Bipolar cell High Convergence Rod-Fed Circuits Cone Retina ganglion cell Bipolar cell Rod

Convergence rod/cone cells

The Circulation of Aqueous Humor

Figure 17.8

III. L. Glaucoma

Build up of Aqueous Humor • volume Increases pressure in eye • Damages nerve Meds/surgery • •

III. L. Cataracts

Lens clouds up Must be removed Typical to replace lens with implant • • • Can get clouding repeated • Laser removal •

Glaucoma is an eye condition that develops when too much fluid pressure builds up inside of the eye. The increased internal pressure can damage the optic nerve, which transmits images to the brain. Without treatment, glaucoma can cause blindness within a few years. Glaucoma is most often inherited, meaning it is passed from parents to children. Less common causes of glaucoma include a blunt or chemical injury to the eye, severe eye infection, blockage of blood vessels in the eye and inflammatory conditions of the eye. Glaucoma usually occurs in both eyes, but it may involve each eye to a different extent.

Normal Vision Glaucoma

A cataract is a painless, cloudy area in the lens of the eye. A cataract blocks the passage of light from the lens to the nerves at the back of the eye, and it may cause vision problems. Changes in the lens of the eye are part of the aging process but normally do not develop into cataracts. However, cataracts are very common in older adults. Cataracts can also occur after an eye injury, as a result of eye disease, after the use of certain medications or as a result of medical conditions such as diabetes.

Normal Vision Cataract

The Organization of the Retina

Figure 17.6b, c

Normal Vision Diabetic Retinopathy

Diabetic retinopathy is an eye condition that affects people with diabetes who have high blood sugar over a prolonged period of time. Too much blood sugar can destroy the blood vessels in the back of the eye, causing damage to the retina. Without the retina, the eye cannot communicate with the brain, making vision impossible. In the early stages of diabetic retinopathy these blood vessels leak fluid and distort sight. In the more advanced stage of diabetic retinopathy fragile new blood vessels grow around the retina. If left untreated, these blood vessels may bleed, clouding vision or scar detaching the retina.

Normal Opthalmoscopic View of Eye

Disorders of the Eye and Vision: Retinopathy

Retinopathy in diabetes • Vessels have weak walls – causes – hemorrhaging and blindness

Optic disk Human Fundus

Papilledema

Papilledema

A peek with an ophthalmoscope

Right visual field Right Eye Optic nerve Optic chiasm Optic tract Lateral geniculate Right Visual cortex (area 17) Left visual field Left Eye Meyer ’s loop Optic radiations parietal Calcarine fissure

Summary of visual projections

Protection of the eye

• Boney wall • Lacrimal gland secretion

(Tears)

Lacrimal gland secretion (Tears)

The image-forming mechanism

Optics of the eye – Lenses

The image-forming mechanism -

Optics of the eye –

Lenses

Principles of Optics

Principles of Optics

Concave lens Convex lens

Principles of Optics-

cont.

Focal distance same as focal length Figure 17.9

when accomodated to distant vision

Eye and camera .. … similarities and differences ?

Dioptre (D)

Dioptre (s) = 1 Focal length (m) Diopteric power if the eye: Cornea …………40-45 D Lens …………… 15-20 D Accomodation …. +14 D

(according to age)

Accomodation

Focusing on a nearby object

Accomodation

Definition: Modification of the refractive power of the eye (curvature of the lens) to view a nearby object Goal: Clear vision of a nearby object

Accomodation

- cont.

– Lens changes (

accomodation)

– Changes in the pupil – Convergence of the eyes

The near response

The Near Respose

Image Focusing

Fully relaxed (unaccommodated

) • Lens accommodation Parallel light rays from distant light source Focal Distance

Fully accommodated

Focal Distance

Mechanism of accomodation

Ciliary muscle ….

Contraction: Relaxation of the suspensory ligament Lens more convex Increase diopteric power of the eye Near object focussed on the retina

Mechanism of

accomodation-

cont Ciliary muscle ….Relaxation

: Contraction of the suspensory ligament Lens less convex (Flat) Decrease diopteric power of the eye Far object focussed on the retina

Accommodation

When the cilary muscles are relaxed, the zonalus pulls tight and keeps the lens flattened for distant vision The elastic lens is attached to the circular cilary muscles by the zonalus which is made of inelastic fibres When the cilary muscles contract, it releases the tension on the zonalus and the elastic lens returns to a more rounded shape suitable for near vision

Distant Vision: Ciliary Muscle Relaxed Suspensory Ligaments Under Tension Lens is Flattened Focus on Distant Objects Accommodation: Ciliary Muscle Contracts Reduced Tension on Suspensory Ligaments Lens becomes Round Focus on Near Objects

Accomodation

cont Lens changes during accomodation:

– Affect the anterior surface of the lens

mainly

– Lens thickness increases – Lens diameter decreases

Accomodation

cont Lens changes during accomodation:

– Affect the anterior surface of the lens

mainly

– Lens thickness increases – Lens diameter decreases

Lens capsule is elastic and Lens substance is plastic

Diopter (D)

Dioptre (s) = 1 Focal length (m) Diopteric power if the eye: Cornea …………40-45 D Lens …………… 15-20 D Accomodation …. +12 D

Amplitude of Accomodation

Definition The additional diopters added by increasing the convexity of the lens Near point: The nearest point to the eye where an object can be seen clearly Presbyobia: Loss of lens elasticity in old age >>loss of accomodation

Near point and amplitude of accomodation Age (yrs) 10 20 30 40 60 70 Near point (cm) 9.0

10.0

12.5

18 83 100 Amplitude of Accomodation 11.0

10.0

8.0

5.5

1.2

1.0

The accomodation Reflex

Afferent: Retina optic nerve optic tract visual cortex optic chiasma lateral geniculate body Efferent: Occuluomotor nucleus (parasympathetic) ciliary ganglion ciliary muscle circular pupillary muscle

The Visual Pathway

The lateral geniculate body

The light reflex

Light Reflex

When an eye (Left) is subject to bright light, a direct light reflex occurs(constriction of the pupil) as well as a consensual (indirect) reflex of the other (Right) pupil

Diameter of pupil: varies from 1.5mm to 8mm (Quantity of light changes X30 fold)

The light reflex

==The constriction of the pupil in response to light Pathway: Retina optic tract colliculus pupillary muscles superior occulomotor nucleus

The lateral geniculate body

Direct and consensual light reflex

Note the pathway for pupillary contraction

Direct reflex on right Consensual reflex on left

Constriction of the pupil

The pupil constricts in response to:

• The accomodation Reflex • The light reflex

Argyll Robertson pupils (Neurosyphilis)

Pupils constrict in response: to accomodation reflex but not to the light reflex

The lateral geniculate body

Errors of Refraction

Errors of refraction

1. Hypermetropia (long sight) 2. Myopia (Short sight) 3. Astigmatism

Emmetropia

(normal vision)

Image Focusing

Fully relaxed unaccommodated lens Myopia

(Short sight)

Hyperopia

(long sight)

Hyperopia

Image Focusing

Blurred retinal image Location of focused image Myopia

Myopia -’short sight’

Concave corrective lens

Hyperopia-’

long sight’

Convex lens

Visual Abnormalities-

cont.

Figure 17.11

Emmetropia: objects focused on retina (normal)

Myopia: objects focused in front of retina

Hyperopia: objects focused behind retina

Astigmatism

III. K. Eyesight

Nearsighted Myopic – Eyeball longer than – normal Focal pt in front of – retina Retinal image out of – focus Concave lenses – •

III. K. Eyesight

Farsighted Hyperopic – Eyeball shorter – than normal Focal pt behind – retina Retinal image out – of focus Convex lenses – •

III. K. Eyesight

Astigmatic Cornea asymmetric – Results in multiple – focal points on or in front of retina Retinal images out – of focus Differential grinding – of lenses •