BYPROF. SAEED ABUEL MAKAREM

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• Vision is the most highly developed and versatile of all the sensory modalities.

• The one which man The one which man are most dependent.are most dependent.

• The optic nerve and retina develop from the prosencephalon so, they are considered as an outgrowth of the brain itself.

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• The eyeball,, or globe, is almost spherical in shape.

• Near its posterior pole emerges the optic nerveoptic nerve..

• The eyeball consist of three layers of tissue, the outermost, is tough, fibrous and protective.

• Over most of the globe it forms an opaque white coat, the sclera, to which are attached the extraocular muscles.

• Over the anterior pole of the globe it forms the transparent cornea through which light enters the eye.

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• Near the anterior margin of the sclera, two rings of smooth muscletwo rings of smooth muscle extend into the lumen of the eyeball.

• The anterior one is the iris, which has a central opening, called pupil,, through which light passes to the posterior part of the eye.

• The second one is the ciliary body, ciliary process, suspensory ligament and the lens.

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• Some of the muscle fibers of the iris are arranged in a circularcircular way while others are arranged radially.radially.

• Both fibers are under the control of the autonomic nervous system. • Circular fibers are innervated by parasympathetic neuronesparasympathetic neurones,, which constrict constrict

the pupil and reduce the amount of light falling upon the retina. the pupil and reduce the amount of light falling upon the retina. • Radial fibers are innervated by sympathetic neuronessympathetic neurones to dilate the pupil.to dilate the pupil.

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• Vision starts with formation of an image on the photoreceptive retina.

• Then the image discharged to the optic nerve..

• Fibres of the optic nerve undergo hemidecussation in the optic chiasma.optic chiasma.

• Then it projects to the lateral lateral geniculategeniculate nucleus of the thalamus through the optic tract..

• Thalamo-cortical or (geniculo- calcarine) fibers in turn project to the primary visual cortex area17) through the retrolentiular part of the internal capsule to the occipital lobe where visual perception occurs.

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• Retinal photoreceptors are of two types, rods and cones..

• Rods are about 20 times more than the cones.

• These cells share many structural similarities.

• Rods are exquisitely sensitive to light.• They are particularly important for

vision in dim light. • Cones are responsible for color

vision,3 dimensions and, high visual acuity.

• Rods lie more in the periphery towards the orra serrata, while cones are predominant at the macula and fovea centralis

Prof. saeed Makarem

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In addition to photoreceptive cells, the retina contains both the first- and second-order neurones of the central visual pathway. In addition to photoreceptive cells, the retina contains both the first- and second-order neurones of the central visual pathway.

• The first-order neurone, or bipolar cell,, lies entirely within the retina.

• While the axon of the second-order neurone, or ganglion cell, forms the optic nerve.

• The first-order neurone, or bipolar cell,, lies entirely within the retina.

• While the axon of the second-order neurone, or ganglion cell, forms the optic nerve.

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• Information is transferred from photoreceptors to bipolar cells and then to ganglion cells.

• The retina also contains interneuronesinterneurones known as horizontal cells and amacrine cells.

• These modulate transmission between photoreceptors and bipolar cells and between bipolar cells & ganglion cells.

• Information is transferred from photoreceptors to bipolar cells and then to ganglion cells.

• The retina also contains interneuronesinterneurones known as horizontal cells and amacrine cells.

• These modulate transmission between photoreceptors and bipolar cells and between bipolar cells & ganglion cells.

The axons of ganglion cells assemble at the optic disc and pass into the optic nerve,, which enters the cranial cavity through the optic canal.

The axons of ganglion cells assemble at the optic disc and pass into the optic nerve,, which enters the cranial cavity through the optic canal.

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• The two optic nerves converge to form the optic chiasma on the base of the brain.

• The chiasma lies immediately rostralrostral to the tuber cinereum of the hypothalamus and betweenbetween the terminating internal carotid arteries.

• The two optic nerves converge to form the optic chiasma on the base of the brain.

• The chiasma lies immediately rostralrostral to the tuber cinereum of the hypothalamus and betweenbetween the terminating internal carotid arteries. 11Prof. Saeed Makarem

In the chiasma: • axons derived from

the nasal portions of the two retinae decussatedecussate and pass and pass into the contralateral into the contralateral optic tract,optic tract,

• while those from the temporal hemiretina remain ipsilateral. remain ipsilateral.

• Macular fibers pass to Macular fibers pass to both optic tractsboth optic tracts

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The optic tracts diverge away from the chiasma and pass round the cerebral peduncle to terminate mainly in the lateral geniculate nucleus (within the lateral geniculate body) of the thalamus.

The optic tracts diverge away from the chiasma and pass round the cerebral peduncle to terminate mainly in the lateral geniculate nucleus (within the lateral geniculate body) of the thalamus.

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A relatively small number of fibers leave the optic nerve, before reaching the before reaching the lateral geniculate nucleus,lateral geniculate nucleus, to terminate in the Pretectal areaPretectal area and the superior colliculussuperior colliculus. .

A relatively small number of fibers leave the optic nerve, before reaching the before reaching the lateral geniculate nucleus,lateral geniculate nucleus, to terminate in the Pretectal areaPretectal area and the superior colliculussuperior colliculus. .

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These fibers are involved in mediation of the pupillary light reflex.

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• From lateral geniculate nucleus, third-order neurone (thalamocortical neurones) project through the retrolenticular part of the internal capsule and form the optic radiation, which terminates in the primary visual cortex of the occipital lobe.

• The primary visual cortex is located predominantly on the medial surface of the hemisphere in the region above and below the above and below the calcarine sulcus. calcarine sulcus.

• From lateral geniculate nucleus, third-order neurone (thalamocortical neurones) project through the retrolenticular part of the internal capsule and form the optic radiation, which terminates in the primary visual cortex of the occipital lobe.

• The primary visual cortex is located predominantly on the medial surface of the hemisphere in the region above and below the above and below the calcarine sulcus. calcarine sulcus.

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• Surrounding this area, the rest of the occipital lobe constitutes the visual association cortex,( areas 18 &19).

• It is concerned with interpretation of visual images, recognition, depth perception and colour vision.

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• There is a precise point-to-point relationshipprecise point-to-point relationship between the retina and the visual cortex. • Because of the importance of the maculaimportance of the macula in vision, it is represented in both sides by

disproportionately large volumes (relative to its size) in the lateral geniculate nucleus and the visual cortex.

• Within the visual cortex the macula is represented most posteriorly,most posteriorly, in the region of the occipital pole.

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The upper half of the visual field forms images upon the lower halves of the retina, the lower visual field upon the upper hemiretina.

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• As thalamocortical fibersthalamocortical fibers leave the lateral geniculate nucleus they pass around the lateral ventricle, those representing the lower part of the visual those representing the lower part of the visual field coursing superiorly to terminate in the visual cortex above the calcarine field coursing superiorly to terminate in the visual cortex above the calcarine sulcus. sulcus.

• Those which represent the upper part of the visual fieldThose which represent the upper part of the visual field sweep into the temporal lobe (Meyer's loop)(Meyer's loop) before terminating below the calcarine sulcus.below the calcarine sulcus.

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• Visual field can be considered as comprising four quadrants (left/right, upper/lower) each projecting to its own quadrant of the primary visual cortex (left/right hemispheres, above/below the calcarine sulcus).

• There is both lateral and vertical inversion in the projection of the visual field upon the visual cortex such that, for example, the upper left quadrant of the visual field is represented in the lower right quadrant of the visual cortex.

Prof. saeed Makarem

Visual field deficits1. Disease of the eyeball

(cataract, intraocular haemorrhage, retinal detachment) & disease of the optic nerve (multiple sclerosis and optic nerve tumors) lead to loss of vision in the affected eye (monocular blindness).

2. Compression of the optic chiasm by an adjacent pituitary tumor leads to bitemporal hemianopia. .

3. Vascular and neoplastic lesions of the optic tract, optic radiation or occipital cortex produce a contralateral homonymous hemianopia.

Retinitis PigmentosaRetinitis Pigmentosa

• Retinitis pigmentosa is an inheritedinherited metabolic disorder of the photoreceptor and retinal pigment epithelial cells.

• It is due to mutation of a key protein in the retinal photoreceptors.

• Which protein?• Rhodopsin.Rhodopsin.• There is progressive night

blindness, peripheral visual field constriction and pigmentation of the retina visible on ophthalmoscopy.

• Which type of photoreceptor is affected?

• Rods.Rods.

For more information, see

http://www.emedicine.com/oph/topic704.htm

Retinitis Pigmentosa

• Can you name some of the Can you name some of the genes whose mutation leads genes whose mutation leads to retinitis Pigmentosa?to retinitis Pigmentosa?

• -- RPGR (X-linked), RP1, RPGR (X-linked), RP1, chromosome -8 chromosome -8 ((http://www.nature.com/ng/journal/v22/n3/full/ng0799_248.html))

Retinitis Pigmentosa

• Which part of the rods is affected first?Which part of the rods is affected first?• - The photoreceptor segment.- The photoreceptor segment.• What happens next?What happens next?• - The entire cells dies- The entire cells dies.• What you can do to help?What you can do to help?• - Very little.- Very little.• What drugs adversly affect RP?What drugs adversly affect RP?

– Isoretinoin (Accutane)Isoretinoin (Accutane)– Sildenafil (Viagra)Sildenafil (Viagra)- Vitamin E (high doses, >400 U/d)Vitamin E (high doses, >400 U/d)

(http://content.nejm.org/cgi/content/abstract/323/19/1302

http://www.emedicine.com/oph/topic704.htm)