Sense of Vision

lecture 10

Abbas A. A. Shawka

Medical student

2nd grade

Subjects

• Sense of vision

- Anatomy of the eyeball

- Notes related to the function of eye

structures that will be the BASICS for you

in your medical learning !!

Eyeball

• The globe-shaped eyeball occupies the anterior part of the orbit.

• Its rounded shape is disrupted anteriorly, where it bulges outward. This outward projection represents about one-sixth of the total area of the eyeball and is the transparent cornea.

• Posterior to the cornea and in order from front to back are the anterior chamber, the iris and pupil, the posterior chamber, the lens, the postremal (vitreous) chamber, and the retina.

Anterior and posterior chamber • The anterior chamber (1) is the area

directly posterior to the cornea and anterior to the colored part of the eye (iris).

• The central opening in the iris is the pupil. Posterior to the iris and anterior to the lens is the smaller posterior chamber (2).

• The anterior and posterior chambers are continuous with each other through the pupillary opening. They are filled with a fluid (aqueous humor), which is secreted into the posterior chamber, flows into the anterior chamber through the pupil, and is absorbed into the scleral venous sinus (3) (the canal of Schlemm), which is a circular venous channel at the junction between the cornea and the iris.

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Anterior and posterior chamber

• The aqueous humor supplies nutrients to the avascular cornea and lens and maintains the intra-ocular pressure. If the normal cycle of its production and absorption is disturbed so that the amount of fluid increases, intra-ocular pressure will increase. This condition (glaucoma) can lead to a variety of visual problems.

Lens and vitreous humour• The lens separates the anterior one-fifth

of the eyeball from the posterior four-fifths.

• It is a transparent, biconvex elastic disc attached circumferentially to muscles associated with the outer wall of the eyeball.

• This lateral attachment provides the lens with the aability to change its refractive ability to maintain visual acuity.

• The clinical term for opacity of the lens is a cataract.

• The posterior four-fifths of the eyeball, from the lens to the retina, is occupied by the postremal (vitreous) chamber (arrow)

• This segment is filled with a transparent, gelatinous substance-the vitreous body (vitreous humor). This substance, unlike aqueous humor, cannot be replaced.

Walls of the eyeball • Surrounding the internal components

of the eyeball are the walls of the eyeball. They consist of three layers: an outer fibrous layer, a middle vascular layer, and an inner retinal layer.

1. The outer fibrous layer consists of the sclera posteriorly and the cornea anteriorly.

2. The middle vascular layer consists of the choroid posteriorly and is continuous with the ciliary body and iris anteriorly.

3. The inner layer consists of the optic part of the retina posteriorly and the nonvisual retina that covers the internal surface of the ciliary body and iris anteriorly.

Arterial supply • The arterial supply to the eyeball is from

several sources:

1. The short posterior ciliary arteries are branches from the ophthalmic artery that pierce the sclera around the optic nerve and enter the choroid layer.

2. The long posterior ciliary arteries, usually two, enter the sclera on the medial and lateral sides of the optic nerve and proceed anteriorly in the choroid layer to anastomose with the anterior ciliary arteries.

3. The anterior ciliary arteries ( derived from the muscular branches of the ophthalmic artery ) are branches of the arteries supplying the muscles -as the muscles attach to the sclera, these arteries pierce the sclera to anastomose with the long posterior ciliary arteries in the choroid layer.

4. The central retinal artery that has traversed the optic nerve and enters the area of the retina at the optic disc.

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Venous drainage

• Venous drainage of the eyeball is primarily related to drainage of the choroid layer.

• Four large veins (the vorticose veins) are involved in this process.

• They exit through the sclera from each of the posterior quadrants of the eyeball and enter the superior and inferior ophthalmic veins.

• There is also a central retinal vein accompanying the central retinal artery.

Fibrous layer of the eyeball

• fibrous layer of the eyeball consists of two components

• The sclera covers the posterior and lateral parts of the eyeball, about five-sixth of the surface.

• The cornea covers the anterior part

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Sclera • The sclera is an opaque layer of

dense connective tissue that can be seen anteriorly through its conjunctival covering as the "white of the eye."

• It is pierced by numerous vessels and nerves, including the optic nerve posteriorly and provides attachment for the various muscles involved in eyeball movements.

• The fascial sheath of the eyeball covers the surface of the scleraexternally from the entrance of the optic nerve to the corneoscleraljunction while internally the surface of the sclera is loosely attached to the choroid of the vascular layer.

Cornea

• Continuous with the sclera anteriorly is the transparent cornea.

• It covers the anterior one-sixth of the surface of the eyeball and, being transparent, allows light to enter the eyeball.

Vascular layer of the eyeball

• The vascular layer of the eyeball consists of three continuous parts

1. the choroid,

2. the ciliary body,

3. the iris

from posterior to anterior

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The choroid

• The choroid is posterior and represents approximately two thirds of the vascular layer.

• It is a thin, highly vascular, pigmented layer consisting of smaller vessels adjacent to the retina and larger vessels more peripherally.

• It is firmly attached to the retina internally and loosely attached to the sclera externally.

Ciliary body • Extending from the anterior border

of the choroid is the ciliary body .

• This triangular-shaped structure, between the choroid and the iris, forms a complete ring around the eyeball.

• Its components include the ciliary muscle and the ciliary processes

• The ciliary muscle consists of smooth muscle fibers arranged longitudinally, circularly, and radially.

• Controlled by parasympatheticstraveling to the orbit in the oculomotor nerve [III], these muscle fibers, on contraction, decrease the size of the ring formed by the ciliary body.

Ciliary body • The ciliary processes ( arrow ) are

longitudinal ridges projecting from the inner surface of the ciliary body.

• Extending from them are zonular fibers attached to the lens of the eyeball, which suspend the lens in its proper position and collectively form the suspensory ligament of the lens.

• Contraction of the ciliary muscle decreases the size of the ring formed by the ciliary body. This reduces tension on the suspensory ligament of the lens. The lens therefore becomes more rounded (relaxed) resulting in accommodation of the lens for near vision.

• Ciliary processes also contribute to the formation of aqueous humor.

Iris • Completing the vascular layer of the

eyeball anteriorly is the iris ( arrow ) .

• This circular structure, projecting outward from the ciliary body, is the colored part of the eye with a central opening (the pupil).

• Controlling the size of the pupil are smooth muscle fibers within the iris

Fibers arranged in a circular pattern (1) make up the sphincter pupillae muscle, which is innervated by parasympathetics, contraction of its fibers decreases or constricts the pupillary opening.

Fibers arranged in a radial pattern (2) make up the dilator pupillae muscle, which is innervated by sympatheticscontraction of its fibers increases or dilates the pupillary opening.

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Inner layer of the eyeball

• The inner layer of the eyeball is the retina.

• It consists of two parts.

1. Posteriorly and laterally is the optic part of the retina, which is sensitive to light,

2. Anteriorly is the nonvisual part, which covers the internal surface of the ciliary body and the iris.

• The junction between these parts is an irregular line the ora serrata ( arrows )

Optic part of the retina • The optic part of the retina consists

of two layers, an outer pigmented layer and an inner neural layer:

1. The pigmented layer is firmly attached to the choroid and continues anteriorly over the internal surface of the ciliary body and iris.

2. The neural layer, which can be further subdivided into its various neural components, is only attached to the pigmented layer around the optic nerve and at the ora serrata.

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• It is the neural layer that separates in the case of a detached retina.

obvious features are visible on the posterior surface of the optic part of the retina1. Optic disk

• The optic disc is where the optic nerve leaves the retina

• It is lighter than the surrounding retina and branches of the central retinal artery spread from this point outward to supply the retina.

• As there are no lightsensitivereceptor cells in the optic disc, it is referred to as a blind spot in the retina.

2. Macula lutea

• Lateral to the optic disc a small area with a hint of yellowish coloration is the macula lutea

3. Fovea centralis

• Central depression in macula lutea

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Macula lutea

• This is the thinnest area of the retina and visual sensitivity here I higher than elsewhere in the retina because it has fewer rods (light-sensitive receptor cells that function in di light and are insensitive to color) and more cones (lightsensitiv receptor cells that respond to bright light and ar sensitive to color).

Summary of intrinsic muscles of eye

Muscle Location Innervation function

Ciliary Muscle fibers in the ciliary body

Parasympathetics from the oculomotor nerve [Ill]

Constricts ciliary body, relaxes tension on lens, lens becomes more rounded

Sphincter pupillae

Circularly arranged fibers in the iris

Parasympathetics from the oculomotor nerve [Ill]

Constricts pupil

Dilator pupillae

Radially arranged fibers in the iris

Sympathetics from the superior cervical ganglion (Tl )

dilate pupil

Notes about function of eye structures

• Visual pathway

• Refractive power of the eye

• Eye disorders

– Presbyopia

– Myopia

– Hypermetropia

– Glucoma

– Cataract

• Eye reflexes

• Eye muscles

Visual pathway • Divided into four quadrants by

horizontal and vertical meridians (superior/inferior and nasal/ temporal)

• Vision occurs by photoreceptors ( rodes and cones ) which contain light sensitive pigments

• Rodes = absent in macula and optic disk ( blind spot )

• Cones = color vision ( Red, Green, Blue, RGB ) tightly packed mostly in macula !!

• Flow of information is from photoreceptors bipolar cells ganglionic cells

• Axon of ganglionic cells forms the optic nerve !!

• Cells in retina !!

• R= rodes

• C = cones

• B = bipolar cells

• G = ganglionic cells

• Aid in VISION

• A= amacrine cell ( neuron )

• H = horizontal cell ( neuron )

• P = pigment cell

• Aid in SUROOUNDING INHIBITION

• It is important to know that ….

• Nasal part of retina see the temporal part of visual field !!

• Temporal part of retina see the nasal part of visual field !!

• OF HIGHLY IMPORTANT !!

Optic nerve and optic chiasma • Macular fibers on center and non

macular fibers on periphery !!

• 4 segments for optic n. ( intraocular –intraorbital, intracalinicular, intracranial ).

• Last segment will lie superior to ICA , sphenoidal sinus ( medial ) and other cortical structures superiorly …

• Two optical nerves converge at the optic chiasma when the axons from the nasal part of each retina decussate and the axons from the temporal part of each retina remain ipsilateral …

• Optic chiasma located below the suprachiasmatic recess , lamina terminalis and anterior commissure. The pituitary stalk is immediately posterior to it.

Retinal targets Retina

Superchiasmatic n.

Superior colliculi Pretectal area

To LGB

Primary visual pathway Circadian rhythm and neuroendocrine function

Pupillary light reflex Conjugate eye movement with head movement

Optic tract & lateral geniculate nuclei

• Extend from the chiasm poteriolaterally around hypothalamus around cerebral peduncles to lateral geniculate body LGB …

• Lateral geniculate body ( or nucleus ) lies in the inferolateral thalamus.

• LGB contain teritery neurons that will form the optic radiation to the primary visual cortex surrounding the calcarine fissure …

Optic radiation

• Axons of LGN neurons that project to calacarine cortex of occipital lobe …

• There is

1. Parietal lobe optic radiation :- Subserve inferior visual field

2. Temporal lobe optic radiation : - Subservesuperior visual field

• Ventral optic radiation is closely related to posterior limb of the internal capsule

Visual cortex • Primary visual cortex ( area 17 ) is

in the wall and floor of calcarinesulcus in the occipital lobe

• Macula roject to posterior 1/3 of the calcarine cortex

• Macula = posteriorly

• Peripheral retina = anteriorly

• Fovea = at occipital pole

• Upper field = inferior

• Right field = left

• Secondary visual cortex ( area 18 & 19 ) located in adjacent occipital lobe cortex …

Lesions • The defects are conventionally

described with reference to the visual fields and not to the retina.

1. A complete lesion of the left optic nerve gives rise to complete blindness in the left eye.

2. Compression of the optic chiasma, as by a pituitary tumour, causes bitemporal hemianopia (blind ness in the temporal half of both visual fields) because the nasal fibres from each retinas are interrupted. This effectively narrows the outer part of each visual field, so that the patient complains of bumping into the sides of a doorway or into people on each side.

What hemianopia look like ?!

Lesions 3. A lesion of the left optic tract gives a

right homonymous hemianopia, due to interruption of fibres from the same (left) sides of both retinas (hence homonymous, meaning same-sided). The field defects are therefore right-sided.

4. A lesion of the lower fibres in the left optic radiation (as from an abscess in the temporal lobe spreading upwards from the middle ear) causes a right upper quadrantichomonymous hemianopia, because the lower fibres in the optic radiation (from the lower part of the retina) are represented in the upper part of the visual field.

Lesions 5. lesion of the upper fibres in the left

optic radiation (as from a parietal lobe lesion, and in practice very rare) gives a right lower quadrantichomonymous hemianopia. ( similar to 4 )

6. A lesion of the anterior part of the left visual cortex (as from occlusion of the posterior cerebral artery) gives a right homonymous hemianopia similar to the optic tract lesion in (3), but there is sparing of the macular (central) vision because the most posterior part of the visual cortex at the very tip of the occipital lobe where macular vision is represented is supplied by the middle cerebral artery.

Lesions 7. Traumatic damage to the tip of

the left occipital lobe, i.e. to the macular area, gives a right homonymous macular defect.

Test • The Optic nerve (CN II) is tested in

five ways:

1. Acuity

2. Colour

3. Fields

4. Reflexes

5. Fundoscopy

• The acuity is easily tested with Snellen charts. If the patient normally wears glasses or contact lenses, then this test should be assessed both with and without their vision aids.

Test

• Colour vision is tested using Ishihara plates which identify patients who are colour blind.

Test

• Visual fields are tested by asking the patient to look directly at you whilst you wiggle one of your fingers in each of the four quadrants. Ask the patient to identify which finger is moving.

• Visual inattention can be tested by moving both fingers at the same time and checking the patient identifies this.

One quadrants Alternative quadrants

Test • Visual reflexes comprise direct and

concentric reflexes. Place one hand vertically along the patients nose to block any light from entering the eye which is not being tested. Shine a pen torch into one eye and check that the pupils on both sides constrict. This should be tested on both sides.

• Reflexes WILL BE STUDIED later !!!

• Fundoscopy : should be performed on both eyes

Refractive power of the eye • is the degree to which an optical

system converges or diverges light.

• Refractive media in eye include :-

1. Cornea

2. Aqueous humor

3. Lens

4. Vitreous body

• Cornea can have 75% of the refractive power of the eye !!!

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Refractive errors in eye • An eye that has too much or too

little refractive power to focus light onto the retina has a refractive error.

• A myopic eye has too much power so light is focused in front of the retina.

• So we used a divergence lenses in treatment !!!

• Near sishtedness WHY ?!!

Refractive errors in eye

• Conversely, a hyperopic eye has too little power so when the eye is relaxed, light is focused behind the retina.

• So we used a convergence lensesin treatment !!!

• Far sightedness WHY ?!!

Refractive errors in eye

• An eye with a refractive power in one meridian that is different from the refractive power of the other meridians has astigmatism.

• Corrected with eyeglasses that have 2 different refractive index as need …

Why near-sightedness and far-sightedness ?!

• Focusing of the retinal image is mainly regulated through the process of accommodation.

• Accommodation is mediated mainly by changes in the shape of the lens.

1. A more globular lens results in a larger refraction of the light to focus on near objects.

2. A flat lens refracts light less and is suited for focusing in the distance.

Near sightedness • Looking at a near object

• Lens needs to be short and fat

• as light rays are diverging as they hit the eye

• and so require a lot of bending.

• The lens will adopt a short, fat shape with

• no tension in the suspensory ligaments (the ligaments are slack) and this is achieved by contracting the ciliary muscle.

• In Myopic eye, picture locate in front of retina and hence farther object send parallel rays lens need to be flat and hence lens can not be relaxed so far object will be blurry ..

Far sightedness • Looking at a far object

• Lens needs to be long and thin

• as light rays are almost parallelas they hit the eye

• and so require little bending.

• To pull the lens long and thin requires suspensory ligaments to be taut and this is achieved by the ciliary muscle relaxing.

• In hypermetropic eye, picture locate behind retina and hence near object send divergancerays lens need to be more cuvature and hence lens can not be bend so near object will be blurry ..

Presbyopia • Age related : >40

• a common age-related problem

• caused when the natural lens in the eye loses its elasticity, making it difficult to focus on near objects (hyperopia).

• So corrected by a convergence lenses ( picture located behind the retina ) …

Glucoma

• Aqueous humour is produced by the ciliary processes by diffusion from the capillaries and transported by the ciliary epithelium into the posterior chamber; it passes through the pupil into the anterior chamber.

• At the margin of the anterior chamber is the iridocorneal angle and here aqueous humour filters through trabecular tissue into the canal of Schlemm

Glucoma

• Obliteration of the angle therefore prevents absorption of aqueous humour, with consequent rise of intraocular tension, leading to the condition of glaucoma.

• Aqueous humour is an avenue for nutrients and metabolic exchange for the avascular cornea and lens.

pupil

Open Vs. Closed Angel Glaucoma

Cataract • A cataract is a clouding of the lens in

the eye which leads to a decrease in vision.

• Causes

1. Age

2. Trauma

3. Radiation

4. Genetics

5. Skin diseases

6. Smoking and alcohol

7. Inadequate vitamin C

8. Medications

9. Post-operative

10. Other disease

• symptoms may include faded colors, blurry vision, halos around light, trouble with bright lights, and trouble seeing at night.

Summary of intrinsic muscles of eye

Muscle Location Innervation function

Ciliary Muscle fibers in the ciliary body

Parasympathetics from the oculomotor nerve [Ill]

Constricts ciliary body, relaxes tension on lens, lens becomes more rounded

Sphincter pupillae

Circularly arranged fibers in the iris

Parasympathetics from the oculomotor nerve [Ill]

Constricts pupil

Dilator pupillae

Radially arranged fibers in the iris

Sympathetics from the superior cervical ganglion (Tl )

dilate pupil

YOU SHOULD UNDERSTAND THIS IN ORDER TO UNDERSTAND EYE REFLEXES !!

Direct and Consensual light reflexes

• When we have a light directed to one of the eyes , the pupil of this eye will constrict ( direct ) and also the pupil of the other eye will constrict ( consensual ) ….

• Pathway from optic n. to LGB then to pretectal n. the to EW nuclei ( parasympathetic nucleus of III cranial nerves ) which give bilaterally innervation to III CN then to ciliary ganglion then to sphincter pupillae

• This reflex examine the optic n. of one eye and the parasympath. Component of the two eyes !!

Accommodation reflex • Three things that responsible for accommodation ….

• 1- Contraction of the medial recti brings about convergence of the ocular axes.

• 2- Lens thickens to increase its refractive power by contraction of the ciliary muscle.

• 3- Pupils constrict to restrict the light waves to the thickest central part of the lens.

• Afferent :- Optic n.

• Efferent :- oculomotor nerve

• Observe the motor and sensory function of III CN ( EW n. and motor n. )

Corneal reflex

• When touching the cornea, the reflex in blinking

• Afferent :- V CN

• Efferent :- VII CN

• This test examine both the V CN and VII CN

Thank you