Vision
The eye is organ of sight. It is situated in orbital cavity and supplied by optic nerve (ll cranial nerve).
Size - Almost spherical in shape and about 2.5cm in diameter.
The space between eye and orbital cavity is occupied by adipose tissue ( Help to eye protect from injury).
Anatomy of Eye
There are three layers of tissue in wall of eye-
1. Outer supporting layer (protective cover)
2. Middle vascular coat (Nutritive coat); Highly blood supply
Choroid
Ciliary body
Iris
3. Retinal layer (Sensory layer;inner layer of eye ball)
Neural part (Highly sensitive part)
Containing rod and cons
Pigment layer
1. Outer Supporting layer
Outer layer of eye ball consist mainly of thick membrane of tough, fibrous connective tissue. Composed dense avascular connective tissue.
Outermost layer of eyeball consist of two parts-
1. Sclera
2. Cornea
1. Sclera
Sclera or white of eye, form outermost layer of posterior and lateral aspect of eyeball which cover 5/6 the tough outer layer. and it continuous anteriorly with transparent cornea at sclerocorneal junction.
Sclera aanterior part is covered by conjunctiva tissue.
Sclera is most almost avascular.
Functions of sclera
1. Protective outer layer of eye.
2. Mataintain shape of eye ball.
3. Gives attachment to extrinsic muscle of eye.
4. Forms white of eye
2. Cornea
Anterior outer layer is transparent cornea, which cover 1/6th of outer layer.
Cornea is more convex than sclera and cornea is avascular.
Cornea is covered by epithelial sheet.
Cornea consist of -
Collagen fibers
Elastic fibers
Proteoglycans
Light rays pass through cornea to reach retina.
Cornea is convex anteriorly and is involved in refracting light rays to focus them on retina.
2. Middle vascular layer
Vascular layer or middle coat of eye ball and it provide nutrient and oxygen.
It consist of -
Choroid
Ciliary body
Iris
Lens
1. Choroid
Choroid lines posterior 2/3rd of vascular layer, thin membrane called choroid and thin pigment layer.
It is very rich in blood vessels and is deep chocolate brown in color.
Light enters the eye through pupil, stimulate Sensory receptors in retina and is then absorbed by choroid.
2. Ciliary body
Ciliary body is anterior continuation of choroid consisting of ciliary muscle and secretary epithelial cells.
It is continuous anteriorly with iris and posteriorly with choroid.
Ciliary body consist ciliary muscle, which are important in controlling lense shape.
The lense is attached to cilary body by radiating sensory ligament.
Contraction and relaxation of ciliary muscle fibers, which are attach to suspensory ligaments, control shape of lens.
Epithelial cells secrete aqueous fluid into anterior segment of eye i.e spa e between lens and cornea.
3. Iris
Iris is visible colored part of eye and extends anteriorly from ciliary body, lying behind cornea and in front of lens.
It divide anterior segment of eye into anterior and posterior chamber, which contain aqueous fluid secreted by ciliary body.
It is circular body composed of pigment cells and two layer of smooth muscle fibers are circular layer and radiating layer in center an aperture called pupil
Iris supplied is supplied by parasympathetic and sympathetic nerves.
Parasympathetic stimulation constricts pupil and sympathetic stimulation dilates it.
Colour of iris is determined by genetically and depend on number of pigment cell present in connective tissue.
In close vision and bright light spinchter pupillae (circular muscle) contract and pupil constrict.
In distant vision and dim light dilator pupillae (radial muscles) contract and pupil dilates, allowing more light to enter eye.
4. Lens
Lens is flexible, transparent, colourless, avascular, highly elastic circular biconvex body of epithelial cell immediately behind pupil and iris.
It is enclosed in thin, transparent, highly elastic fibrous capsule and is held in place by suspensory ligament from ciliary body process.
It's thickness is controlled by ciliary muscle through suspensory ligament.
When ciliary muscle contracts, it move forward, releasing it's pull in lense, increasing it's thickness.
Nearer object being viewed, thicker lens become to allow focusing.
Lens bends light rays reflected by object in front of eye.
It is the only structure in eye that can vary it's refractory power, which is achieved by changing it's thickness.
3. Inner layer; Retinal layer
Retina is innermost layer of wall of eye. It is an extremely delicate structure and is well adopted for stimulation by light rays.
It is composed of sever layers of nerve cell bodies and their axons, lying on pigmented layer of epithelial cells which attach it to choroid.
Retina has thin pigmented and thick neural layer or light-sensitive layer consist of sensory receptor cell rod cells and cons cell, which contain photosensitive pigment that convert light energy to nerve impulse.
1. Thin layer pigmented retina
This layer is heavily pigmented layer of retina, this layer adjoins vascular layer before sensory retina is thin layer of pigmented epithelium.
These cells absorb excess light and prevent it from scattering in eye and prevent reflecting back to sensory retina.
2. Sensory retina; thick layer
Thick layer is nervous tissue called Sensory retina that connects with optic nerve.
Neuroretina consist highly specialized photoreceptor nerve cells rods and cones.
Rods and cones are light receptors of eye.
1. Cones
Respond only to bright light and are sensitive to color, most of cones are contracted. In center of retina directly behind lens in are called macula lutea or yellow spot in center of yellow spot little depression called fovea centralis
2. Rods
Rods are dim-light
Sensors for perception of black to white shades.
Neural layer composed of 3 main layer-
Photoreceptor--- Bipolar cells -------Ganglion cells
Optic Nerve Disc
Optic disc, where optic nerve exits eye, is a spot in fundus (posterior wall) of eye.
About 0.5cm to nasal side of macula lutea all nerve fibers of retina converge to form optic nerve.
The portion of retina where optic nerve exit from eyeball contains neither rods or cons. It is called optic disc or blind spot, because it lacks photoreceptors and not sensitive to light.
Blood Supply to Eye
Eye is supplied with arterial blood by ciliary arteries and central retinal artery.
These are branches of ophthalmic artery, one of branches of internal carotid artery.
Venous drainage is by number of vein including central retinal vein, which eventually empty into deep venous sinus.
Central retinal artery and vein are encased in optic nerve, which enters the eye at optic disc.
Internal Structure of Eye
Cavity of Eye ball
Eyeball divided into two segments-
1. Anterior segment
2. Posterior Segment
1. Anterior segment
Space between cornea and lens is anterior segment. It divided by iris into-
1. Anterior chamber
Region between cornea and iris
2. Posterior chamber
Region between iris and lens
Production and Absorption of Aqueous humor
Both chamber is filled with aqueous humor. Aqueous humor is produced by capillaries in ciliary body. It circulates Infront of lens, through pupil into anterior chamber and returns to venous circulation through sclera venous sinus (canal of Schlemm) into angle between iris and cornea. There is continuous production and drainage of aqueous humor.
Functions
Aqueous fluid supplies nutrients and remove waste from transparent structures in front of eye that have no blood supply i.e cornea, lens and lens capsule.
Aqueous humor responsible for maintaining constant pressure within eyeball which maintains constantcy of opical dimension of eye.
It provide essential nourishment as oxygen, glucose for avascular tissue and remove waste from lens and cornea, which don't have blood vessel to nourishment.
2. Posterior Segment or Vitreous
Behind lens and filling posterior Segment (cavity) of eyeball is Vitreous body.
Vitreous humor is soft, colorless, transparent, jelly like substance composed of 99% water, some salts and mucoprotein.
Function
It keeps eyeball from collapsing as result of external pressure.
It supports posterior surface of lens and hold neural retina against pigmented layer.
It maintain sufficient intraocular pressure to support retina against choroid and prevent wall of eyeball from collapsing.
Formation-
Vitreous humor is formed by ciliary body.
Eye keeps it's shape because of intraocular pressure exerted by aqueous fluid and Vitreous body.
Physiology of Vision
Visual pathway to the Brain
Retina rods and cones are photoreceptors. They responds to light waves and generate neural signals that are process by retinal cell and stimulate ganglion cells to produce action potential.
Then nerve fibers/ axon of ganglion cells carry impulses along optic nerves
Optic Nerves (IInd cranial nerves)
The fibres of optic nerve originate in retina of Eye and they converge to form optic nerve about 2.5 cm to nasal side of macula lutea.
The nerve go into choroid and sclera to pass backward and medially through orbital cavity.
It then passes through optic foramen of sphenoid bone backwards and medially to meet nerve from other eye at optic chaisma.
Optic chiasma
Optic chiasma is situated immediately in front of and above pituitary gland.
In optic chiasma nerve fibers of optic nerve from nasal side of each retina crossover to opposite side.
Fibers from temporal side don't cross but continue backwards on same side.
This crossing over provide both cerebral hemisphere with sensory input from each eye
Visual formation from right half of each field is conveyed to left side of brain and visual formation from left half of each field is conveyed to right side of brain.
Optic Tracts
After passing through optic chiasma, nerve fibers are called optic tracts.
Each tract consist of nasal fibers from retina of one eye and temporal fibers from retina of the other
The optic tracts pass backwards to synapse with nerve cell of lateral geniculate bodies of thalamus.
From there nerve fibers proceed backwards and medially optic radiation to terminate visual area of cerebral cortex in occipital lobes of cerebrum where conscious perception of visual image occurs.
Other neurons origination in lateral geniculate body transmission impulses from eyes to cerebellum where, together with impulse from semicircular canals of ears and from skeletal muscles and joints, they contribute to maintenance of posture and balance.
How we see an object?
Light ray enter through cornea, light ray move through pupil which surround by iris to keep out extra light, then light rays move through lens then light ray move through Vitreous humor, light rays fall on retina and convert light to neuron signal using rod and cones cells.
Neuron signals transmitted through optic nerve.
Neuron signal move through visual pathway optic nerve, optic chaisma, optic tract, optic radiatios, visual neuron signal reach occipital (visual) cortex for brain processing,
Visual cortex interpret signals as images; and along with other parts of brain interpret image to exact form meaning, memory and context of image.
Physiology of Vision
Light waves travel at speed of 300000 kilometers per second. Light is reflected into eyes by objects within field of vision.
White light is combination of all colours of visual spectrum; red, orange, yellow, green, blue, indigo, violet
Red light has longest wavelength and lowest energy and violet is shortest wavelength and most energetic.
White things reflects all wavelength of light, wheareas black objects absorb them all.
Reflection and Refraction
Reflection
The ray of light fall into surface and then it will bounce back the same medium known as reflection.
Refraction
Refraction is bending of light rays when they pass from one medium to another medium with different intensity. It's speed changes.
Example, in eye, biconvex lens bends and focuses light rays.
This principles is used to focus light on retina.
Formation of Image on Retina
Four processes focus light rays so that they form clear image on retina.
1. Refraction of light rays entering eye
2. Accommodation of lense
3. Constriction of pupil
4. Movement of eyeball
1. Refraction of light rays entering eye
Light rays inflected from object are bent (refracted) by lense when they enter eyes.
Abnormal refraction within eye corrected using biconvex or biconcave lenses.
2. Accommodation of lense
Lense is biconvex elastic transparent body suspended behind iris from ciliary body by suspensory ligament.
It is the only structure in eye that changes it's refractive power.
Light rays entering eye need to be refracted to focus them on retina.
Light from distant objects need least refraction and as object comes closer, amount of refraction needed increased.
To increase refractive power, ciliary muscle contracts.
This moves ciliary body inwards towards lens (contracts and sphincter)
Relaxing pull on suspensory ligaments and allow lens to increasing it's convexity.
The focus light rays from near object on retina.
When ciliary muscle relaxes, it slip backward,
Increasing it's pull on suspensory ligament, making less thinner.
Focuses light rays from distant object on retina.
3. Size of Pupils
Pupil size influences accommodation by controlling amount of light entering eye.
In bright light pupils are constricted. In dim light they are dilated.
If pupil were dilated in bright light, too much light would enter eye and damage sensitive retina.
In dim light, if pupils were constricted, insufficient light would enter eye to activate light-sensitive pigments in rods and cones which stimulate nerve ending in retina.
Iris consist of - one layer of circulatory and one layer of radiating smooth muscle fibers.
Contraction of circular fibers constrict pupil and contraction of radiating fibers dilates it.
Size of pupil is controlled by Autonomic nervous system-
Sympathetic stimulation dilates pupils
Parasympathetic stimulation causes constriction.
4. Convergence (movement of eyeballs)
Light rays from near by object enter two eye at different angles and for clear vision they must stimulate crossponding area of two ratinae.
Extrinsic muscle move eyes and to obtain a clear image, they rotate eye so that converge on object view.
This coordinated muscle activity is under autonomic control.
When there is voluntary movement of eye both eye move and convergence is maintained.
Nearer object as to eye greater eye rotation needed to achieve convergence.
If convergence is not complete eye are focused on different objects or on different point of same object.
There are then two messages sent brain and this lead to double vision, diplopia.
Object more than 6 meter away from eye are focused on retina without adjustment of lens or convergence of eyes.
Light and Dark Adaptation
Light and dark adaptation is adjustment of eyes to change in light.
Light adaptation occurs when move from darkness into bright light.
When exposed to bright light, rhodopsin with in sensitive rode I'd degraded, when there is enough light to activate cones.
Light adaption, may take only 5 min to complete.
Dark adaptation is when go from light are to dark one, rhodopsin accumulates and retinal sensitivity increases.
Dark adaptation is slower than light adaptation.
Binocular vision
Human beings have binocular vision meaning that we have 2 eyes, we perceive only one image, the image from two eyes are fused in cerebrum so that only one image is perceived.
Extraocular muscles of eye
These include muscles of eyelids and those that move eye balls. The eyeball moved by six extrinsic muscles, attached at one end of eyeball and at other to wall of orbital cavity.
The oculomotor (device that track eye movement) nerve supply intrinsic eye muscle of iris and ciliary body.
Refractory Error
Myopia
Hyperopia
0 Comments