Visual system

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Visual system by komal soomro: 

Visual system by komal soomro




FUNCTIONAL ANATOMY OF EYE The human eye is globular in shape made up of 2 segments. Anterior part and posterior part. Orbital cavity: It is a bony cavity in which eye ball is present It is separated from it by alveolar tissues to protect it from the external shock.

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eye lid : it is a muscular covering for protection It is covered by cilia. conjunctiva: thin layer of mucous membrane which cover exposed part of the eye. Walls of the eye ball: there are three main layers. External layer: tunica fibrosa or externa. Middle layer: includes choroid, iris and ciliary body. Inner layer: retina. Tunica fibrosa: it consists of: Sclera: it is a fibrous tissue. Cornea: it consists of 5 layers.

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Layers of cornea: epithelium Bowman’s membrane Substantia proper Descemet’s membrane endothelium

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Middle layer: Choroid: outer pigmented and inner highly vascularized layer. Ciliary body: lies behind iris. Composed of: Ciliary ring Ciliary processes Muscles Iris inner layer: it consist of retina: Retina has 10 layers Internal ocular fluid: Vitreous humour Aqueous humour


PHOTORECEPTOR It is a specialized type of neuron found in the retina that is capable of photo transduction. The photo transduction is the process by which light is converted into electrical signals in the rod cells and the cons cells. There are two classical photoreceptor cells, rods and cons. They are capable of converting light into signals that can stimulate the biological processes. Both contributing information used by the visual system to form the representation of the visual world, the sight.


RODS AND CONES They are found on the outer most layer of the retina. The human retina has approximately 5 million cons and 120 million rods. Rods are used for scotopic vision means vision under low light condition or darkness. Cons are used for photopic vision means vision under high light condition or brightness. Rods are not present in fovea and the cons are concentrated in fovea. The fovea is the region which is responsible of producing highest visual acuity.


STRUCTURE OF RODS They are cylindrical structure. Each rod is composed of four structures: 1: outer segment 2: inner segment 3: cell body 4: synaptic terminal


OUTER SEGMENT Outer segment is long and slender. It is formed by the modified cells and it contains the membranous disk . There are about 1000 disks In each rod. The disks in rod cells contain the photosensitive pigment the rodopsin. INNER SEGMENT The inner segment is connected to the outer segment by the modified cilium. It contains many types of organelles with large number of mitochondria.


CELL BODY A cylinder fiber called rod fiber arises from the inner segment of the rod cell and passes to the outer nuclear layer . In the outer nuclear layer, the enlarged portion of this fiber forms the cell body or red granule that contains the nucleus. SYNAPTIC TERMINAL A thick fiber arising from the cell body passes to the outer layer and ends in a small enlarged synaptic or terminal body. The synaptic vesicle present in the synaptic terminal possesses the neurotransmitter, the glutamate.

FUNCTIONS OF RODS They are extremely sensitive to the light and have a low threshold so, the rods are responsible for dim light vision or night vision but rods do not take part in resolving the details and boundaries of objects(visual acuity) or the color of the objects.


RHODOPSIN Rhodopsin is the photosensitive pigment of rod cells. It is present in membranous disks located in outer segment of rod cells. It is a conjugated protein with a molecular weight of 40, is made up of the protein called opsin and a chromophore. the chromophore present in the rod cells is called retinal. Retinal is present in the form of 11-cis retinal called as retinine 1 in the human eyes. its significance is this that it can combine only in this form with the opsin and synthesize the rhodopsin.


RHODOPSIN -RETINAL VISUAL CYCLE According to this cycle when rhopopsin absorbs the light that falls on the retina, it is split into retinine and the protein called SCOTOPSIN through various photochemical reactions and in the end the activated rhodopsin called the metarhodopsin 2 is synthesized, which is responsible for the receptor potential in the rod cells.


STRUCTURE OF CONS It is the visual receptor and flask shaped in structure The shape and length of the cons vary in different parts of the retina The cons in the fovea are long and narrow The cons in the periphery are short and broad It consists of the four parts: 1: outer segment 2: inner segment 3: cell body 4: synaptic terminal


OUTER SEGMENT It is small and conical and does not contain membranous disk as in rods In cons, the infoldings of the cell membrane form the saccules . The photo pigment of the cone which is called as the color pigment OR The photopsin is synthesized in the inner segment and incorporated in the saccules . INNER SEGMENT The inner segment is connected to the outer segment by a modified cilium. The various types of the organelles are present and the mitochondria are more.


CELL BODY In cons fiber arising from the inner segment Is thick and it enters the inner nuclear layer through external limiting membrane. In the nuclear layer the cone forms the cell body that possesses the nucleus. SYNAPTIC TERMINAL The fiber from the cell body leaves the inner nuclear layer and enters the outer plexiform layer, it ends in the form of an enlarged synaptic terminal or body. The synaptic vesicle present in the synaptic terminal possesses the neurotransmitter, the glutamate.



FUNCTION OF CONS Cons have high threshold for light stimulus so cons are sensitive to bright light therefore con cells are called receptors of bright light vision or day light vision. They are also responsible for visual acuity and the color vision.

Neurophysiology of vision: 

Neurophysiology of vision Layers of the retina:





Visual fibers also passes to the:: 

Visual fibers also passes to the: suprachiasmatic nucleus of the hypothalamus to control the circadian rhythms. Pretectal nucleus to control visual reflexes. Superior colliculus to control directional movements of the two eyes. Ventral lateral geniculate nucleus of the thalamus and surrounding basal regions for controlling behavioral functions. Visual pathway division: old system: signals passes to the midbrain and base of the forebrain. new system: signals passes directly to the primary visual cortex.

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Location of primary visual cortex Location of secondary visual cortex Layers of primary visual cortex Layer 1,2,3,4,5,6




LATERAL GENICULATE BODY It has 6 layers. 2,3,5 receives signals from lateral ½ of the ipsilateral retina. 1,4,6 receives signals from the medial ½ of the contra-lateral retina. In another way it has 4 layers: layer 1 and 2 are called magnocellular layers. Layer 3 to 6 are called parvocellular layer.

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ACCOMODATION Accommoda tion is the adjustment of the eye to see either near or distant objects clearly. Mechanism of accommodation : The mechanism of accommodation in human eye was first represented by the young and then by the Helmholtz. Young-Helmholtz theory : This theory describes how the curvature of lens increases and thereby, the refractive power of the lens is enhanced. During distant vision During near vision

ACCOMODATION REFLEX Accommodation is a reflex action when a person looks at a near object after seeing a far object. Pathway for accommodation reflex:


CLINICALS ARGYLL ROBERTSON PUPIL: In this condition light reflex is lost but the accommodation is present.

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HORNER’S SYNDROME: It is characterized by ptosis, constriction of pupil (miosis), exopthalamus and decreased sweating of the face on the same side (hemi facial anhidrosis).

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PRESBYOPIA: It is the problem of old age in which amplitude of the accommodation decreases and the near point is away from the eye.

Clinical of the visual system Myopia: (nearsightedness) it is refractive error when distance object come into focus in front of the retina of the eye as a result of longer than normal eyeball so the distance object blur to the myopic eyes.

Hypermetropia (long-sightedness) it is a refractive error where distance object come into focus behind the retina when the eye is relaxed.

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CATARACT It is clouding of the lens of the eye. It affects your vision and very common in older people specially in the united states. common symptoms: Blurred vision Colors that seemed faded Not being able to see well at night Double vision Frequent perception changes

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Factors that causes cataract: Age Diabetes Long-term exposure to bright sunlight Previous eye injury Treatment: Surgery is an option. It involve removing the cloudy lens and replace it with an artificial lens.