Detecting and responding to stimuli

Receptors

Every response starts with a receptor, a cell or group of cells that detects a particular kind of stimulus and turns it into nerve impulses. Sense organs such as the eye and ear are packed with receptors, with other tissues that help them work.

Receptors only detect. You don't see with your eyes or hear with your ears; you see and hear with your brain, which makes sense of the impulses. That is why optical illusions work, and why a blow to the back of the head can make you "see stars".

More than five senses

The old list of five senses (sight, hearing, taste, smell, touch) leaves a lot out. As well as these, you can sense:

• balance and acceleration, from the inner ear
• proprioception, knowing where your body parts are without looking, from stretch receptors in your muscles and joints. Close your eyes and touch your nose: that's proprioception.
• temperature and pain, which use different receptors from touch
• internal states such as hunger, thirst, a full bladder and your heartbeat.

Scientists still argue about the exact number, but it is more than five.

The eye

• The cornea is the clear window at the front. Because it is curved, it does most of the bending of light.
• The iris is the coloured ring of muscle. It changes the size of the pupil, the hole that lets light in: small in bright light, wide in the dark. This is the pupil reflex (4B).
• The lens fine-tunes the focus. Muscles change its shape: fatter for close objects, thinner for distant ones.
• The retina is the light-sensitive layer at the back. It has about 120 million rods, which work in dim light but only see in shades of grey, and about 6 million cones, which need brighter light and detect colour. There are three types of cone, most sensitive to red, green and blue light.
• The optic nerve carries impulses to the occipital lobe. Where it leaves the eye there are no receptors at all, so you have a blind spot in each eye. Your brain fills in the gap so you never notice.

The image on the retina is upside down. The brain learns to interpret it the right way up.

Ears, tongue and nose

Hearing. Sound waves make the eardrum vibrate. Three tiny bones pass the vibrations to the cochlea, a coiled tube full of liquid, where about 15 000 hair cells turn them into impulses. Loud noise can bend and kill these hair cells, and they do not grow back, so noise damage is permanent. Ear infections are also a big problem: many First Nations children, especially in remote communities, have repeated middle ear infections (otitis media) that cause hearing loss during the years they are learning to talk and read. Hearing checks and early treatment in schools and clinics are an important part of closing that gap.

Balance. Next to the cochlea are three fluid-filled semicircular canals at right angles to each other. When your head turns, the fluid lags behind and bends hair cells, telling your brain which way you are moving. Spin around and stop suddenly, and the fluid keeps swirling, which is why you feel dizzy.

Taste and smell. Taste buds on the tongue have chemoreceptors for five basic tastes: sweet, sour, salty, bitter and umami (savoury). Every part of the tongue can detect all five; the "tongue map" in old textbooks is a myth. Most of what we call flavour is actually smell. Your nose has around 400 types of smell receptor, which together can tell apart a huge number of odours. That's why food tastes bland when you have a blocked nose.

Touch

Your skin contains several kinds of touch and pressure receptors, but they are not spread evenly. Fingertips and lips are packed with them; your back has very few. You can measure this with a two-point test: touch the skin with two points close together and find the smallest gap that still feels like two separate points. The more receptors there are, the smaller that gap. Areas with many receptors also have a bigger share of the parietal lobe devoted to them.

Senses in other living things

Other animals detect stimuli we can't. Bees and many birds see ultraviolet light, which shows patterns on flowers invisible to us. Sharks and rays, like the platypus, have electroreceptors. Bats and dolphins use echolocation: they make high-pitched clicks and listen for the echoes. Many migrating birds can sense Earth's magnetic field.

Plants have no nervous system, but they still respond to stimuli, mostly by growing in a particular direction. You met these tropisms in Year 8: shoots grow towards light and roots grow downwards with gravity, controlled by plant hormones such as auxin. Some plants respond faster. The leaves of the sensitive plant (Mimosa pudica) fold up within seconds of being touched, as water rushes out of cells at the base of each leaflet. A Venus flytrap snaps shut when an insect touches two of its trigger hairs, using electrical signals that travel across the leaf.