Have you ever wondered how you quickly pull your hand away when you touch something hot, recognise a familiar face, or turn your head when you hear your name? These actions are possible because our body detects changes in the surroundings and responds to them through the coordinated work of our sense organs and nervous system.
SSLC Biology Chapter 3 – Behind Sensations explores how the eyes, ears, nose, tongue, and skin detect different stimuli and send information to the brain. The brain processes this information and coordinates an appropriate response. For example, when we see a friend, our eyes detect the image and send nerve impulses to the brain. The brain recognises the person and sends signals to the muscles, allowing us to smile, wave, or speak. Thus, our sense organs, brain, nerves, and muscles work together to help us respond to the world around us.
What Are Stimuli and Their Types?
A stimulus is a change that makes a living organism respond. Our body detects these changes with special cells or nerve endings called sensory receptors.
ഉദ്ദീപനങ്ങൾ
ജീവികളിൽ പ്രതികരണങ്ങൾക്കിടയാക്കുന്ന സാഹചര്യങ്ങളെയാണ് ഉദ്ദീപനങ്ങൾ എന്ന് വിളിക്കുന്നത്.
Stimuli are mainly classified into two types:
| Types of stimulus | Definition | Examples |
|---|---|---|
| External Stimuli | Changes that come from the surroundings | Touch, light, sound, heat, cold, pressure, pain, and smell |
| Internal Stimuli | Changes that occur within the body | Hunger, thirst, changes in blood glucose level, changes in body temperature, and changes in blood pressure |
How Are Receptor and Action Potentials Generated?
- Receptor potential: Sensory receptors recognise stimuli and produce electrical impulses called receptor potentials.
[ബാഹ്യ-ആന്തര ഉദ്ദീപനങ്ങൾക്ക് അനുസരിച്ച് ഗ്രാഹികളിൽ വൈദ്യുത സന്ദേശങ്ങൾ ഉണ്ടാകുന്നു. ഇത്തരം വൈദ്യുത സന്ദേശങ്ങളെ റിസപ്റ്റർ പൊട്ടൻഷ്യൽ എന്ന് വിളിക്കുന്നു. ]
2.Threshold: When the receptor potential reaches the threshold level, it can trigger an action potential in the associated sensory neuron.
3.Action potential: An action potential carries the sensory signal through the sensory neuron.
[ഗ്രാഹികളുമായി ബന്ധപ്പെട്ട ന്യൂറോണുകളിൽ ഉദ്ദീപനത്തിന്റെ ഫലമായി ഉണ്ടാകുന്ന വൈദ്യുത മാറ്റമാണ് ആക്ഷൻ പൊട്ടൻഷ്യൽ. ഇത് ന്യൂറോണുകളിലൂടെ നാഡീയ ആവേഗങ്ങളായി സഞ്ചരിക്കുന്നു.]
4.Signal transmission: The action potential travels through the sensory neuron towards the central nervous system (CNS).
5.Information carried: The frequency of action potentials can provide information about the intensity of the stimulus.
Types of Sensory Receptors and Senses
General Senses
Receptors in the skin, muscles, joints, internal organs, and blood vessels detect general sensations.
Eg: Touch, Pain, Heat, Pressure
[പൊതുസംവേദനങ്ങൾ: ശരീരത്തിന്റെ വിവിധ ഭാഗങ്ങളിലുള്ള ഗ്രാഹികൾ തിരിച്ചറിയുന്ന സംവേദനങ്ങളാണ്.]
2. Special Senses
Specialised receptors in specific sense organs detect special sensations.
Eg : Vision – Eyes, Hearing – Ears , Taste – Tongue, Smell – Nose
[പ്രത്യേക സംവേദനങ്ങൾ: പ്രത്യേക ഇന്ദ്രിയാവയവങ്ങളിൽ കാണപ്പെടുന്ന ഗ്രാഹികൾ തിരിച്ചറിയുന്ന സംവേദനങ്ങളാണ്.]
Eye Structure &Functions
The EYE is a complex sense organ made up of several parts that work together to produce vision. The cornea allows light to enter the eye, while the convex lens forms an image on the retina. The iris and pupil regulate the amount of light entering the eye. The retina contains photoreceptor cells called rods and cones. Rods help recognise objects in dim light and in shades of black and white, while cones help us see in intense light and recognise colours. Human Eye: Structure, Functions, and Common Eye Disorders explains the main parts of the eye, their functions, and common eye disorders
The key parts of the eye work together to make vision possible. This includes the protective structures, layers of the eye, lens accommodation, photoreceptors, colour perception, binocular vision, blind spot, and common eye disorders. Simple explanations, examples, tables and diagrams make these concepts easier to understand.
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Join NowKey Structures of the Eye
| Part | Main Function |
|---|---|
| Cornea | Transparent anterior part of the eye that allows light to enter |
| Iris and Pupil | Control the amount of light entering |
| Lens | Forms a small, real and inverted image on the retina |
| Retina | Inner layer of the eye that contains photoreceptor cells. Image is formed on the retina |
| Rods and Cones | Help in dim light and colour vision |
| Optic nerve | Carries visual information to the brain |
The Process of Hearing
Hearing is an important sensory process that involves the coordinated action of the ear, auditory nerves, and brain. The human ear is divided into the outer ear, middle ear, and inner ear, and each part has a specific role in receiving and processing sound. The pinna collects sound waves and directs them through the auditory canal to the tympanum or eardrum. The eardrum vibrates in response to sound, and these vibrations are passed through the three ear ossicles: malleus, incus, and stapes. The vibrations then reach the cochlea, where sensory receptors convert them into nerve signals that travel to the brain through the cochlear nerve. The ear also has protective features such as hairs and earwax, while the Eustachian tube helps regulate pressure in the middle ear.
1. Process of Hearing
2. Process of Maintaining Body Balance (Equilibrium)
Olfaction – The Sense of Smell
The nose is the sense organ responsible for detecting smells. When we breathe, tiny odor molecules enter the nasal cavity along with air. Special sensory cells in the lining of the nasal cavity detect these molecules and send information to the brain, allowing us to recognise different smells.
How Do We Smell?
Odor Molecules
- Tiny odor molecules enter the nasal cavity when we breathe.
- These molecules stimulate specialised olfactory sensory neurons in the olfactory region of the nose.
Mucous Membrane
- The inner lining of the nasal cavity produces mucus.
- The odor molecules dissolve in this mucus, allowing sensory receptors to detect them.
Olfactory Sensory Neurons
- Millions of specialised olfactory receptor neurons are present in the olfactory region of the nasal cavity.
- They detect the dissolved odor molecules and become stimulated.
Nerve Signals
- When the receptors are stimulated, they generate nerve impulses.
- These impulses carry information about the detected smell.
Olfactory Nerve
- The impulses pass through groups of olfactory nerve fibres and travel towards the brain.
Brain
- The brain receives and processes these signals.
- This allows us to recognise and identify different smells.
Flowchart – The Sense of Smell
Taste Buds and Chemoreceptors
Taste buds are sensory structures on the tongue that help us detect different tastes. They contain specialised chemoreceptors that respond to chemicals present in food.
How Taste Buds Detect Taste
Skin and Sensory Receptors
The skin is an important sensory organ that helps us understand changes in our surroundings. It allows us to sense touch, pressure, temperature, pain, and vibrations. Different types of receptors are present in the skin, and each is specialised to detect particular sensations.
Receptors in the Skin
| Receptor | Main Function |
|---|---|
| Independent nerve endings | Sense pain and temperature. |
| Merkel discs | Detect touch pressure and texture. |
| Meissner corpuscles | Detect light touch and movement across the skin. |
| Krause end bulbs | Detect cold and touch. |
| Ruffini end organs | Detect intense touch, pressure, and heat. |
| Root hair plexus | Detect movement of hairs. |
| Pacinian corpuscles | Detect vibration and high-frequency touch. |
How Does the Skin Detect Pain?
- Pain is a protective sensation that warns us about injury or harmful conditions.
- The process of detecting harmful stimuli is called nociception.
- Nociceptors are specialised sensory receptors, mainly free nerve endings, that detect harmful stimuli.
- When we touch a very hot object, nociceptors are activated and send nerve signals to the spinal cord and brain.
- The brain processes these signals, allowing us to feel pain.
- Injury or inflammation can release chemicals such as cytokines and chemokines, which can increase pain sensitivity.
- Damage to peripheral nerves can cause neuropathic pain due to abnormal nerve function.
Sensory diversity in organisms
Organisms interact with their surroundings through various senses, but their ability to detect and respond to stimuli differs greatly. While some animals mainly depend on vision and hearing, others use smell, touch, taste, or specialised senses such as detecting heat, vibrations, or electrical signals. These diverse sensory abilities help organisms locate food, recognise threats, communicate with others, and survive in their surroundings.
The Behind Sensations chapter explains how different sense organs detect stimuli and how sensory receptors, nerves, and the brain work together to produce sensations and responses. From seeing and hearing to smelling, tasting, touching, and maintaining balance, each sensory system has an important role in our daily lives. Understanding how these systems work helps us see how closely the different parts of the nervous system are connected.
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FAQ's
The eye receives light and focuses it on the retina. The photoreceptors in the retina detect the light and send visual information to the brain through the optic nerve.
The main parts include the cornea, iris, pupil, lens, retina, rods, cones, and optic nerve. The cornea allows light to enter, the iris and pupil control the amount of light, the lens focuses light, and the retina receives the image.
The ear receives sound vibrations and passes them to the cochlea. Sensory receptors in the cochlea convert the vibrations into electrical signals, which travel to the brain through the auditory nerve.
Rods and cones are photoreceptors in the retina. Rods help with vision in dim light, while cones are mainly involved in colour vision.
The ear receives sound vibrations and passes them to the cochlea. Sensory receptors in the cochlea convert the vibrations into electrical signals, which travel to the brain through the auditory nerve.
The semicircular canals and vestibular structures detect head movements and body position, helping maintain balance, posture, and coordination.
Odour molecules dissolve in nasal mucus, where olfactory receptors detect them and send signals to the brain for smell recognition.
Olfactory receptors are specialised sensory cells in the olfactory region of the nose. They detect dissolved odour molecules and generate nerve impulses that carry information to the brain.
Taste buds contain specialised chemoreceptors that respond to chemicals in food. These chemicals dissolve in saliva, stimulate the receptors, and produce nerve impulses that travel to the brain.
The skin has sensory receptors for touch and pressure. Merkel discs detect light touch, while Pacinian corpuscles detect vibration and deep pressure.
Free nerve endings detect pain and temperature. Nociceptors detect harmful stimuli and send signals to the brain.