Biology 30 · The senses
Seeing, hearing and balance
Your senses do not send pictures or sounds to the brain — they send nerve impulses. Each sense organ is really a device for turning one kind of energy into those impulses.
Sensory receptors
The idea: a receptor converts a stimulus into nerve impulses. The brain decides what the impulse means based on where it came from, not what caused it.
| Receptor | Responds to | Examples |
|---|---|---|
| Photoreceptors | light | rods and cones in the retina |
| Mechanoreceptors | pressure, vibration, movement | touch receptors, hair cells in the ear |
| Chemoreceptors | chemicals | taste buds, smell receptors |
| Thermoreceptors | temperature | heat and cold receptors in skin |
That is why pressing gently on closed eyes can make you "see" flashes: the pressure starts impulses in the optic nerve, and the brain reads anything from that nerve as light.
Sensory adaptation is receptors responding less to a steady stimulus — why you stop feeling your clothes, or stop noticing a smell after a few minutes.
The eye
The idea: the cornea and lens focus light onto the retina, where rods and cones turn it into impulses that travel down the optic nerve.
| Rods | Cones |
|---|---|
| Work in dim light | Need bright light |
| Black, white and grey only | Detect colour (three types) |
| Mostly around the edges of the retina | Packed into the fovea |
- Iris: the coloured muscle that changes pupil size to control how much light enters.
- Accommodation: for close objects, the ciliary muscles contract and the lens becomes rounder to bend light more.
- Blind spot: where the optic nerve leaves, there are no rods or cones.
Looking slightly to the side of a faint star helps because its image lands on the rods around the fovea, which are far more sensitive to dim light.
Vision problems
The idea: most vision problems are focusing problems — the image lands in front of or behind the retina — and a lens moves it back into place.
| Condition | What goes wrong | Correction |
|---|---|---|
| Myopia (nearsighted) | Image focuses in front of the retina; distant objects blur | Concave lens |
| Hyperopia (farsighted) | Image focuses behind the retina; near objects blur | Convex lens |
| Astigmatism | Irregularly curved cornea or lens; blurry at all distances | Specially shaped lens |
| Presbyopia | The lens stiffens with age and cannot round up for close work | Convex reading glasses |
| Cataracts | The lens becomes cloudy | Surgery to replace the lens |
Watch out: nearsighted means you see near things clearly — it is far things that blur. The name tells you what still works.
The ear, hearing and balance
The idea: sound is passed along as vibration through the outer and middle ear, then turned into impulses by hair cells in the cochlea. The inner ear also handles balance.
The path of sound
- Outer ear: the pinna and auditory canal funnel sound to the eardrum (tympanic membrane).
- Middle ear: three bones — malleus, incus and stapes — amplify the vibration and pass it to the oval window.
- Inner ear: fluid in the cochlea vibrates, bending hair cells in the organ of Corti, which start impulses in the auditory nerve.
The Eustachian tube links the middle ear to the throat and equalizes pressure — why your ears pop on a plane.
Balance
- Semicircular canals detect rotation of the head.
- The vestibule (utricle and saccule) detects the head's position relative to gravity.
Because hearing and balance use different structures, damage to the semicircular canals can cause dizziness while hearing stays normal.
Watch out: loud noise does not just "wear out" hearing temporarily. It destroys cochlear hair cells, which humans cannot regrow, so the loss is permanent.