What is a sense organ?
A stimulus is a change around us or inside us, like light, sound or heat. A sense organ is a body part that picks up one kind of stimulus. Inside it are receptors: special cells that turn the stimulus into electrical nerve signals.
The organ, its nerve and the part of the brain that reads the signal work as one team. Some books call this team an analyser. Your brain, not your eye, finally "sees".
The eye and seeing
Light enters through the clear cornea and the pupil (the black hole). The coloured iris changes the pupil's size: small in bright light, wide in dim light. The lens bends light and makes a small, upside-down image on the retina at the back.
The retina has two kinds of receptors. Rods work in dim light (black and white). Cones see colour in good light. The optic nerve carries signals to the brain, which reads the image the right way up.
Common eye problems
Short sight (far things blurred) and long sight (near things blurred) are fixed with lenses. Too much screen time and poor light strain the eyes.
The ear: hearing and balance
The outer ear collects sound and sends it down the ear canal. Sound makes the eardrum vibrate. In the middle ear, three tiny bones (hammer, anvil and stirrup, the smallest bones in the body) pass the vibration on and make it stronger. In the inner ear, the snail-shaped cochlea is full of liquid and hair cells. The hair cells bend and send signals along the auditory nerve.
Next to the cochlea are three loops, the semicircular canals. The liquid in them moves when you turn your head, so they sense balance.
Very loud sound (above about 85 decibels for a long time) can damage hair cells for ever. Keep earphone volume low.
Nose, tongue and skin
Smell and taste are chemical senses. Tiny molecules from food or flowers float into the nose and fit receptors high inside it. The tongue has taste buds that detect five basic tastes: sweet, salty, sour, bitter and umami (a savoury taste, as in tomatoes and cheese). What we call flavour is taste plus smell.
The skin is the biggest sense organ. It has separate receptors for light touch, pressure, heat, cold and pain. Fingertips and lips have the most receptors, so they are the most sensitive.
From stimulus to response, and keeping senses healthy
Every sense follows the same path: stimulus → receptor → sensory nerve → brain or spinal cord → motor nerve → muscle or gland (response). For fast danger, the spinal cord answers by itself: this is a reflex, like pulling your hand from a hot pan.
Perception is the brain making sense of signals, using memory and attention. That is why an optical illusion can fool you: the eye is fine, the brain guesses wrongly.
Look after your senses and brain: good light for reading, the 20-20-20 rule for screens (every 20 minutes look 20 feet away for 20 seconds), low volume, no sharp objects in ears, enough sleep (8–10 hours for teenagers) and a helmet on a bike.
Key formulas and definitions
- Stimulus: a change the body can detect (light, sound, chemicals, touch, heat)
- Receptor: a cell that turns a stimulus into a nerve signal
- Rods: dim light · Cones: colour
- Cochlea: hearing · Semicircular canals: balance
- Five tastes: sweet, salty, sour, bitter, umami
- Path: stimulus → receptor → sensory nerve → brain / spinal cord → motor nerve → effector
Worked examples
1. You walk from bright sunlight into a dark cinema. At first you see nothing; after a minute you can see seats. Explain.
Step 1: in bright light the iris kept the pupil small. Step 2: in the dark the pupil slowly widens to let in more light. Step 3: rods, which work in dim light, slowly become more sensitive. Together these let you see after a short time.
2. Why does food taste flat when you have a blocked nose?
Step 1: flavour = taste + smell. Step 2: the tongue only detects five basic tastes. Step 3: most of the rich flavour comes from smell molecules reaching the nose. Blocked nose → no smell → food seems tasteless.
3. Trace the path when a mosquito bites your arm and you slap it.
Stimulus: the bite. Receptor: touch and pain receptors in the skin. Sensory nerve carries the signal to the brain. Brain decides to slap. Motor nerve carries the order to arm muscles. Response: the slap.
Common mistakes
- Thinking the eye "sees". The eye only collects light and makes signals; the brain does the seeing.
- Saying the tongue has separate zones for each taste (the old "tongue map"). Taste buds for all five tastes are spread over the tongue.
- Forgetting that the ear also controls balance through the semicircular canals.
- Mixing up rods and cones: rods are for dim light, cones are for colour.