Introduction
A photoreceptor cell, or photoreceptors, is a type of neuron that can transmit light and is found in the retina of the eye.
The photoreceptor delivers information to other nerve cells via a change in membrane potential when photons are absorbed.
Photoreceptors are specialized cells found in the retina of the eye.
Photoreceptors have such a particular structure and function that they can function in a variety of situations. The eye's receptors are highly specialized neuroepithelial cells.
This is due to the epithelial and neuronal activity of the cells, which enable them to convey visual information. These photoreceptors are vital to life because they have the ability to transform light from the environment.
Types of photoreceptors:
There are three types of photoreceptor cells in mammalian cells.
The three categories are rods, cones, and intrinsically photosensitive retinal ganglion/bipolar cells.
The most well-known and traditional photoreceptors are rods and cones. The information that the visual system utilizes to generate the representation of the visual world that we perceive comes from both of the eye's receptors.
In most cases, the photoreceptors are organized in an uneven pattern.
There are 120 million rod cells and 6 million cone cells in the human retina.
Pigments are known to be present in all types of photoreceptors, making the cells suited for picture visualization.
Photoreceptor cells can attain high photopigment density because they are tightly packed, allowing a large number of photoreceptors to absorb enormous amounts of light photons. This allows the brain to analyze images more efficiently.
Structure and Function of Photoreceptors:
The mechanism by which photoreceptors detect light through the eyes is referred to as photoreception.
Light is absorbed by specialized cells known as photoreceptors, which convert the light information into nerve impulses using this approach.
Rod cells and cone cells are the two primary types of photoreceptors, as previously explained.
Night vision is controlled by the rods, whereas daylight vision is controlled by the cones.
Eye Anatomy: Parts of the Eye Outside the Eyeball
The eye sits in a protective bony socket called the orbit. Six extraocular muscles in the orbit are attached to the eye. These muscles move the eye up and down, side to side, and rotate the eye.
The extraocular muscles are attached to the white part of the eye called the sclera. This is a strong layer of tissue that covers nearly the entire surface of the eyeball.
The Surface of the Eye
The surface of the eye and the inner surface of the eyelids are covered with a clear membrane called the conjunctiva.
Tears lubricate the eye and are made up of three layers. These three layers together are called the tear film. The mucous layer is made by the conjunctiva. The watery part of the tears is made by the lacrimal gland. The eye’s lacrimal gland sits under the outside edge of the eyebrow (away from the nose) in the orbit. The meibomian gland makes the oil that becomes another part of the tear film. Tears drain from the eye through the tear duct.
The Front of the Eye
Light is focused into the eye through the clear, dome-shaped front portion of the eye called the cornea.
Behind the cornea is a fluid-filled space called the anterior chamber. The fluid is called aqueous humor. The eye is always producing aqueous humor. To maintain a constant eye pressure, aqueous humor also drains from the eye in an area called the drainage angle.
Behind the anterior chamber is the eye’s iris (the colored part of the eye) and the dark hole in the middle called the pupil. Muscles in the iris dilate (widen) or constrict (narrow) the pupil to control the amount of light reaching the back of the eye.
Directly behind the pupil sits the lens. The lens focuses light toward the back of the eye. The lens changes shape to help the eye focus on objects up close. Small fibers called zonules are attached to the capsule holding the lens, suspending it from the eye wall. The lens is surrounded by the lens capsule, which is left in place when the lens is removed during cataract surgery. Some types of replacement intraocular lenses go inside the capsule, where the natural lens was.
By helping to focus light as it enters the eye, the cornea and the lens both play important roles in giving us clear vision. In fact, 70% of the eye's focusing power comes from the cornea and 30% from the lens.
The Back of the Eye
The vitreous cavity lies between the lens and the back of the eye. A jellylike substance called vitreous humor fills the cavity.
Light that is focused into the eye by the cornea and lens passes through the vitreous onto the retina — the light-sensitive tissue lining the back of the eye.
A tiny but very specialized area of the retina called the macula is responsible for giving us our detailed, central vision. The other part of the retina, the peripheral retina, provides us with our peripheral (side) vision.
The retina has special cells called photoreceptors. These cells change light into energy that is transmitted to the brain. There are two types of photoreceptors: rods and cones. Rods perceive black and white, and enable night vision. Cones perceive color, and provide central (detail) vision.
The retina sends light as electrical impulses through the optic nerve to the brain. The optic nerve is made up of millions of nerve fibers that transmit these impulses to the visual cortex — the part of the brain responsible for our sight.
Eye Defects and Correction
The eye lens is composed of fibrous, jelly-type material. The curvature of the eye lens can be adjusted to a certain level with the aid of ciliary muscles.
A change in the curvature of the eye lens can change its focal length. The eye lens becomes thin, and the focal length increases when the muscles of the eyes are relaxed. The objects at a distance can be viewed clearly when the focal length increases.
To see the objects that are close by, the ciliary muscles contract and increase the curvature of the lens and hence decrease the focal length. The ability of the eye lens to adjust its focal length is called accommodation.
Defects of Vision And Their Correction
Defects in the eye happen due to many reasons. Due to growing age, the vision also decreases, and when the focal length alters, the vision also alters. We know that cataract is a common defect seen in the eye. Cataracts cause partial or sometimes complete vision loss when not treated properly. When the crystalline lens at old age becomes milky and cloudy, it is known as a cataract. When a person undergoes cataract surgery, the vision can be restored.
When the eye loses its ability to adjust its focal length, problems appear like a person cannot see the image correctly (blurring of vision), unable to view nearby objects or far away objects. When the defect in the refractive index occurs, the person cannot see the objects comfortably and distinctly. If not taken timely care of, the eyes might completely lose the power of accommodation. In this article, let us learn about various vision defects and their correction.
Refractive Defects of Vision
Some of the common defects of vision are:
(i) Myopia or near-sightedness
(ii) Hypermetropia or far-sightedness
(iii) Presbyopia
Myopia or Near-Sightedness
Myopia is commonly known as near-sightedness. In this condition, the person can see the objects nearby but cannot see distant objects clearly.
Faraway objects appear blurry, and a person will not be comfortable seeing them. Myopia condition takes place when the shape of the eyes leads the light rays to bend in a wrong way, focusing images in front of the retina rather than focusing on the retina.
Symptoms:
Blurry vision.
Difficulty in seeing while driving, particularly during night times.
Headaches due to eyestrain.
Correction : When a concave lens of suitable power is used, it assists in focusing the image onto the retina.
Hypermetropia or Far-Sightedness
Hypermetropia is commonly known as far-sightedness. In this condition, the person can see objects at a distance but cannot see nearby objects clearly. Usually, the person with this disorder squints to see nearby objects. Hypermetropia is caused when the light rays from a closeby object are focussed at a point behind the retina.
Symptoms:
Blurry vision.
Headaches due to eyestrain.
Squinting.
Correction : Using spectacles with a converging lens imparts additional focusing power and thus helps form the image on the retina.
Presbyopia
We know that along with age, the power of the accommodation factor to adjust the focal length also decreases. People have difficulties viewing nearby objects clearly without the assistance of corrective eyeglasses. This condition is referred to as presbyopia. Presbyopia happens when the ciliary muscles weaken and diminish the elasticity of the eye lens. Presbyopia can be seen in people above the age of 40 years.
Symptoms:
Blurred vision due to ageing.
Headaches due to eyestrain.
Correction : This condition can be corrected by using proper eyeglasses or contact lenses. Minor surgery also helps in restoring the vision with better clarity. Advancement in technology has made it easy to correct the refractive defects with contact lenses or through surgical interventions.
Person With Myopia and Hypermetropia Disorders
We can see some people who suffer from myopia and hypermetropia disorders. In such conditions, it is advised to use bifocal lenses. Usually, bi-focal lenses consist of concave as well as convex lenses. The bi-focal lens has a concave lens in the upper portion and a convex lens in the lower portion to facilitate distant vision and near vision.
