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Structure and function of eye and ear

Eye

ComponentsThe eye is made up of three coats, enclosing

three transparent structures.: The outermost layer, known as the fibrous

tunic, is composed of the cornea and sclera. The middle layer, known as the vascular tunic

or uvea, consists of the choroid, ciliary body, and iris.

The innermost is the retina, which gets its circulation from the vessels of the choroid as well as the retinal vessels, which can be seen in an ophthalmoscope.

Blood vessels can be seen within the sclera, as well as a strong limbal ring around the iris.

Within these coats are the aqueous humour, the vitreous body, and the flexible lens.

The aqueous humour is a clear fluid that is contained in two areas: the anterior chamber between the cornea and the iris, and the posterior chamber between the iris and the lens. The lens is suspended to the ciliary body by the suspensory ligament (Zonule of Zinn), made up of fine transparent fibers.

The vitreous body is a clear jelly that is much larger than the aqueous humour present behind the lens, and the rest is bordered by the sclera, zonule, and lens. They are connected via the pupil.

Working of the eyesVision begins when light rays are reflected off an

object and enter the eyes through the cornea, the transparent outer covering of the eye.

The cornea bends or refracts the rays that pass through a round hole called the pupil.

The iris, or colored portion of the eye that surrounds the pupil, opens and closes (making the pupil bigger or smaller) to regulate the amount of light passing through.

The light rays then pass through the lens, which actually changes shape so it can further bend the rays and focus them on the retina at the back of the eye.

The retina is a thin layer of tissue at the back of the eye that contains millions of tiny light-sensing nerve cells called rods and cones, which are named for their distinct shapes.

Cones are concentrated in the center of the retina, in an area called the macula. In bright light conditions, cones provide clear, sharp central vision and detect colors and fine details.

Rods are located outside the macula and extend all the way to the outer edge of the retina. They provide peripheral or side vision. Rods also allow the eyes to detect motion and help us see in dim light and at night.

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Some important pointsIt is estimated that the human eye contains about

120 million rods and about 6 million cones.The blind spot is the region of the retina where the

optic nerve fibers leave and where the blood vessels enter and leave the retina.

Cones are the most numerous in a specialized region of the retina known as fovea.

Rods occur more abundantly 20 degrees around the back of the eyeball.

Visual acuity is greatest at the fovea, non-existent at the blind spot .

It is said that we have two visual systems- a cone system and a rode system. It is known as duplicity theory of vision.

Cones are the retinal elements active in bright light.

Rods are the retinal elements active in very dim light.

Cones are necessary for color vision.Color-blind persons have deficiencies in their cone

functioning.

Transduction in visionThe rods and cones contain photosensitive pigments. When the electromagnetic energy in the visible

spectrum strikes these pigments, some of the light energy is absorbed by the pigments and causes them to change their configuration and thus creates electrical energy.

This results in receptor voltage which can be recorded.

Through a series of further electrical steps involving the horizontal, bipolar and amacrine cells of the retina, electrical activity is passed from the rods and cons to the ganglion cells of the retina.

The electrical events that have travelled across the retina trigger nerve impulses in the ganglion cells..

The visual pathway in the brainGanglion cells have long axons that leave the retina through the optic disc to make up the optic nerve.

The patterns of nerve impulses in these fibers carry information about the light that struck the rods and cones.

The axons in the optic nerve reach the lateral geniculate body of the thalamus.

There they make synapses with the lateral geniculate body.

Then fibers from the lateral geniculate cells carry nerve impulses to the primary visual sensory area at the back of the brain.

Ear

Broken into three sections: outer Middle inner.

Outer Ear

The auricle (pinna) is the visible portion of the outer ear. It collects sound waves and channels them into the external auditory meatus (ear canal) where the sound is amplified.

The sound waves then travel toward a flexible, oval membrane at the end of the external auditory meatus called the tympanic membrane (eardrum). The tympanic membrane begins to vibrate.

Middle Ear

The vibrations from the eardrum set the ossicles into motion. The ossicles are three tiny bones (smallest in the human body): malleus (hammer), incus (anvil) and stapes (stirrup) which further amplify the sound.

The stapes attaches to the oval window that connects the middle ear to the inner ear. The Eustachian tube, which opens into the middle ear, is responsible for equalizing the pressure between the air outside the ear to that within the middle ear.

Inner EarThe sound waves enter the inner ear and then into

the cochlea, a snail shaped organ.The cochlea is filled with a fluid that moves in

response to the vibrations from the oval window.As the fluid moves, 25,000 nerve endings are set

into motion.These nerve endings transform the vibrations into

electrical impulses that then travel along the VIII cranial nerve (auditory nerve) to the brain.

The brain then interprets these signals and this is how we hear. The inner ear also contains the vestibular organ that is responsible for balance.

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