Human Sensory Organs | 5 Sensory Organs in English

There are a total of five sense organs:

1. 👁️ Eye – Vision
2. 👂 Ear – Hearing and Body Balance
3. 👃 Nose – Smell (Olfaction)
4. 👅 Tongue – Taste
5. ✋ Skin – Touch/Tactile Sense

Based on the stimuli they receive from the environment, their receptors are called as follows:

• 👁️ Eye: Receives light and provides vision – Photoreceptors
• 👂 Ear: Receives sound waves – Phonoreceptors
• 👃 Nose: Detects smell – Olfactory receptors
• 👅 Tongue: Identifies tastes – Gustatory receptors
• ✋ Skin: Perceives touch and heat/cold – Tangoreceptors (Touch) & Thermoreceptors (Temperature).

Importance of Sense Organs:

• Protection: To identify danger (e.g., immediately withdrawing your hand when you touch a hot object).
• Pleasure: To experience joy through beautiful sights, pleasant music, and delicious food.
• Communication: To talk to others and understand their feelings or expressions.
• Learning: They are the very first step to understanding the world and learning new things.

In short, sense organs act as a vital bridge between the external world and our internal world (the brain). If they do not function properly, understanding and interacting with the world becomes exceedingly difficult.

Human Eye – Structure (Sensory Organs)

The scientific study of the eye is called Ophthalmology. The eye helps us perceive colors, shapes, sizes, and distances. This vision is possible because light enters our eyes. The eye is protected by eyelids, eyelashes, eyebrows, and lacrimal glands (tear glands). The front part of the eye is covered by a thin transparent membrane called the Conjunctiva. The eye is a spherical organ, hence called the Eyeball. The eyeball is safely situated in the eye socket of the skull. Generally, when we look at someone, only 1/6th of their eyeball is visible outside. The rest is safely embedded inside the eye socket. The human eye works like a camera. The eye is mainly protected by three layers. They are:

1. Outer Layer (Sclera)
2. Middle Layer (Choroid)
3. Inner Layer (Retina)

1. Outer Layer (Sclera)

This is the outermost tough layer that protects the eye. It is white in color and very firm. The front part of the sclera bulges out to form a transparent layer called the Cornea. During an eye donation, doctors actually extract this Cornea. It does not have a direct blood supply. At the back end of the sclera, the Optic nerve emerges and connects to the brain. The fluid secreted by the lacrimal glands keeps the conjunctiva moist and protects the eye.

2. Middle Layer (Choroid)

The choroid is generally black in color. This layer prevents internal reflection of light inside the eye. The choroid contains numerous blood vessels that supply essential nutrients and oxygen to the eye. It covers almost the entire eye except for the pupil area. The part of the choroid that surrounds the pupil is called the “Iris”. A pigment called Melanin present in the iris determines the color of our eyes (black, brown, blue, etc.). The iris consists of radial muscles and circular muscles. Behind the pupil, there is a Biconvex Lens. This lens is held in place by Ciliary muscles and Suspensory ligaments. The ciliary muscles control the biconvex lens to adjust its focal length. The small hole in the center of the iris is called the ‘Pupil’. It controls the amount of light entering the eye. The function of the iris and pupil is often compared to the diaphragm of a camera.

3. Inner Layer (Retina)

The retina is the third and innermost layer of the eye. It is the most sensitive layer. There are two types of light-sensitive photoreceptor cells in the retina: 1. Rods, 2. Cones.

Rods: These help us see in dim light (in the dark). They contain a pigment called ‘Rhodopsin’. Rhodopsin is synthesized using Vitamin A.
Cones: These help us see in bright daylight and identify colors. They contain a pigment called ‘Iodopsin’.

In the human eye, the normal ratio of rods to cones is 15:1. Nocturnal animals like owls have a higher number of rods. The part of the retina where there is absolutely no vision is called the ‘Blind spot’, and the part where the sharpest and clearest vision occurs is called the ‘Macula Lutea’ or ‘Yellow Spot’. The image formed on the retina is always real and inverted. There are two main fluid-filled chambers inside the eye: 1. Aqueous chamber 2. Vitreous chamber.

1. Aqueous chamber:

The space between the Cornea and the Lens is called the Aqueous Chamber. It is located at the front part of the eye and is filled with a thin, watery fluid called aqueous humor.

2. Vitreous chamber:

The large space behind the Lens, surrounded by the retina, is called the Vitreous Chamber. It is located in the back part of the eye and is filled with a thick, jelly-like fluid called vitreous humor.

1. Night Blindness (Nyctalopia)
2. Dry Eyes (Xerophthalmia)
3. Short Sight (Myopia)
4. Long Sight (Hypermetropia)
5. Glaucoma
6. Cataract
7. Color Blindness

1. Night Blindness (Nyctalopia)

The inability to see objects clearly in low light or at night is called “Night Blindness”. In medical terms, it is known as Nyctalopia.

Cause: It is primarily caused by a deficiency of Vitamin-A (Retinol). Our retina contains photoreceptor cells called ‘Rods’ which help us see in the dark. Due to a lack of Vitamin-A, these rod cells fail to function properly.

Symptoms: Affected individuals can see normally in daylight, but as dusk approaches and light dims, their vision becomes blurred. They struggle in dimly lit rooms.

Prevention: Consuming a diet rich in Vitamin-A, such as carrots, spinach, papaya, eggs, and milk.

2. Dry Eyes (Xerophthalmia)

The condition where the eyes dry out due to insufficient production of tears is called “Dry Eyes” or Xerophthalmia. It can also occur due to prolonged screen time (mobile, computer, TV).

Symptoms: Burning sensation, redness, eye fatigue, and a gritty feeling in the eyes.

Prevention: Giving adequate rest to the eyes, using prescribed eye drops, and staying well-hydrated.

3. Short Sight (Myopia)

Nearby objects can be seen clearly, but distant objects appear blurry. In this defect, the image of an object is formed in front of the retina rather than on it. Myopia is corrected by using spectacles with a Concave lens.

Cause: The eyeball being too long or the lens having excessive curvature.

4. Long Sight (Hypermetropia)

Distant objects can be seen clearly, but nearby objects appear blurry. In this defect, the image is formed behind the retina. Hypermetropia is corrected by using spectacles with a Convex lens.

Cause: The eyeball being too short or the lens having insufficient curvature.

5. Glaucoma

This disease occurs when the fluid pressure inside the eye becomes abnormally high, damaging the optic nerve. An increase in Intraocular Pressure damages the Optic Nerve. The fluid in the anterior chamber (Aqueous Humor) is continuously produced and drained out through a specific pathway to maintain stable pressure. When these drainage angles are blocked or malfunction, the fluid builds up, increasing the internal eye pressure. This high pressure damages the optic nerve. If left untreated, it can lead to permanent blindness.

Symptoms:

• Eye pain
• Headaches
• Loss of vision
• Seeing halos around lights

There are primarily two types of Glaucoma:

1. Open-Angle Glaucoma
2. Angle-Closure Glaucoma

6. Trachoma

Trachoma is a contagious bacterial eye infection caused by the bacterium ‘Chlamydia trachomatis’. It initially affects the inner surface of the eyelids (conjunctiva). Early symptoms include mild itching, irritation, and watery eyes. If untreated, repeated infections can cause severe complications. Trachoma spreads very quickly.

Symptoms:

• Itching and burning sensation in the eyes.
• Redness of the eyes.
• Watery eyes.
• Discharge of pus or mucus from the eyes.
• Sensitivity to light (Photophobia).

7. Cataract

This condition occurs in old age when a cloudy, white layer forms over the eye’s natural lens, leading to blurred vision. It is treated surgically by implanting an artificial lens. This disease is most commonly seen in the elderly.

Symptoms:

• Cloudy or blurred vision.
• Difficulty seeing in bright light or glare.

Treatment: Surgically removing the clouded lens and replacing it with an Artificial lens.

8. Color Blindness

This is generally a hereditary condition. The inability to distinguish between certain colors is the main symptom of this disease. It occurs due to a defect or absence of Cone cells. Affected individuals typically cannot differentiate between red and green colors. The ‘Ishihara Test’ is used to diagnose this condition. Because the famous scientist John Dalton suffered from this, it is also known as ‘Daltonism’.

Cause: Malfunctioning or absence of Cones in the retina.

9. Conjunctivitis (Pink Eye)

In medical terminology, a pink eye or ‘Eye Flu’ is called Conjunctivitis. It occurs when the transparent membrane covering the eye (the conjunctiva) becomes inflamed or infected. Redness, watery discharge, itching, and eyelids sticking together are the main symptoms of conjunctivitis. Viral and bacterial eye flus are highly contagious and spread rapidly from person to person.

The ear has three main parts:

1. Outer Ear
2. Middle Ear
3. Inner Ear

1. Outer Ear

● Pinna: The outer visible part of the ear on either side of the head is called the Pinna or Auricle. The pinna is made of Cartilage. It opens into the Auditory Meatus (Ear canal).

● Two types of glands in the auditory meatus:

1. Ceruminous glands → Secrete Ear wax
2. Sebaceous glands → Secrete sebum (oil)

● These glands keep the ear canal lubricated and trap dust and debris from entering deep inside.

● Tympanic membrane / Eardrum: At the end of the auditory canal, there is a thin membrane called the Eardrum. It separates the outer ear from the middle ear. It is cone-shaped. The inner end of the eardrum is connected to the Malleus, the first bone of the middle ear.

2. Middle Ear

● The middle ear plays a crucial role in amplifying the sound vibrations that strike the eardrum. It contains a chain of three tiny bones (ossicles):

1. Malleus (Hammer)
2. Incus (Anvil)
3. Stapes (Stirrup)

● These three bones amplify the sound intensity and transmit it to the inner ear. Eardrum + Malleus + Incus → form a chain and finally connect to the Stapes.

● The end of the middle ear is covered by a membrane called the Oval window. The middle ear connects to the inner ear through the Round window.

3. Inner Ear

● The inner ear is the most important part for hearing. It contains the Bony labyrinth, which consists of three parts: 1. Vestibule 2. Semicircular canals 3. Cochlea.

● 1. Vestibule: It has two parts:
1. Anterior part → Saccule
2. Posterior part → Utricle.
The nerve fibers from these join to form the Vestibular nerve.

● 2. Semicircular Canals: These are connected to the vestibule. They are filled with a fluid called Endolymph. The Vestibule + Semicircular canals together control body balance.

● 3. Cochlea: The cochlea has a spiral-shaped structure. It contains three canals:

• Scala vestibuli → Filled with Perilymph.
• Scala media → Filled with Endolymph.
• Scala tympani → Filled with Perilymph.

● Nerve fibers from the cochlea form the Cochlear nerve. (Vestibular nerve + Cochlear nerve = Auditory nerve).

● The nose has two Nostrils. These open into the Nasal cavity.

● The Nasal septum divides the nasal cavity into two halves (right and left).

● The walls of the nasal cavity are covered with a Mucous membrane and tiny hairs.

● These hairs and mucus prevent dust, microbes, and unwanted substances from entering the body.

● The mucous membrane contains Olfactory receptors. These receptors detect odor molecules, convert them into nerve impulses, and send them to the olfactory centers in the brain.

● The sense of smell in humans is comparatively lower than in many other animals.

● The receptors that perceive smell and taste are collectively known as Chemoreceptors.

● Taste Buds: The taste buds, located in the elevated structures on the tongue called Papillae, convey the taste of the food we eat to the brain. The human tongue has about 10,000 taste buds (also known as taste receptors or gustatory calyculi). These taste buds are located in the walls of the papillae on the tongue.

● Receptors: The nerve cells that perceive taste are called ‘Gustatory receptors’.

Four Primary Tastes

1. Sweet → Front part of the tongue
2. Salty → Front edges of the tongue
3. Sour → Sides of the tongue
4. Bitter → Back part of the tongue

● Along with these, the fifth primary taste discovered by Japanese scientist Kikunae Ikeda is ‘Umami’. It is a unique savory (protein) taste found in meat, tomatoes, cheese, and soy sauce.

Types of Papillae on the tongue:

1. Filiform papillae ━━━━ Scale-like structures. They do not contain taste buds.
2. Fungiform papillae ━━━━ Mushroom-shaped (round) structures.
3. Circumvallate papillae ━━━━ Large papillae located at the back of the tongue.
4. Foliate papillae ━━━━ Ridges or bumps located on the sides of the tongue.

● Note: Except for Filiform papillae, all other papillae contain taste buds.

Structure of the Skin (Layers):

The skin consists of three main layers. They are:

1) Epidermis:

This is the outermost layer. It does not contain blood vessels. The pigment called ‘Melanin’, which gives color to the skin, is produced here. It protects us from the sun’s harmful UV rays. Layers within the epidermis include:

i. Stratum corneum:

• It is the outermost layer of the epidermis.
• It consists of dead cells.
• It contains a protein called Keratin.
• Keratin is also found in nails and hair.
• The cells in the stratum corneum constantly flake off as scales.

ii. Malpighian layer:

• It is the inner layer of the epidermis.
• It contains living cells.

2) Dermis:

• The dermis is the layer situated beneath the epidermis.
• Fat deposits are found below this layer.
• It contains ridges and grooves. These structures form Fingerprints.
• Fingerprints are unique to every individual, even identical twins, making them useful for identifying people.

Structures in the Dermis:

i. Hair follicles: Hair grows from the hair follicles located in the dermis.

ii. Sebaceous glands: These secrete an oily substance called Sebum. This oil prevents the skin from drying out.

3) Hypodermis:

This is the fat layer located beneath the skin. It acts as a shock absorber and a thermal insulator for the body.

Sweat glands:

• They secrete sweat. Sweat is released out through sweat pores.
• Sweat evaporates and keeps the body cool.
• Sweat glands excrete excess water, sodium chloride, and urea from the blood.

Melanin:

• The pigment responsible for the color of the skin and hair is Melanin.
• The color of the skin and hair varies depending on the amount of melanin. If melanin is abundant, the skin appears dark.
• When the skin is exposed to excess sunlight, melanin production increases, turning the skin darker. This is called Tanning.
• Melanin protects the skin from harmful Ultraviolet (UV) rays.

Skin Receptors:

• The receptors in the skin that perceive stimuli like touch, heat, and pressure are called Skin receptors.
• Receptors that detect touch are called Tactile receptors (found in higher numbers especially on fingertips and lips).
• Receptors that detect pressure are called Pacinian corpuscles.
• Receptors that detect pain in the skin are called Nociceptors.