Chapter 6: Sensory Organs

Learn the idea

A stimulus is a change detected by a receptor. Receptors in sense organs send signals through nerves; the nervous system processes information and can coordinate a response. Eyes detect light, ears detect sound, and receptors also support smell, taste, touch and other sensations.

Eye and ear structures

Light passes through the cornea and pupil; the lens helps focus an image on the retina. The iris controls pupil size and the optic nerve carries signals. Sound vibrations reach the eardrum, pass through the middle-ear bones and reach the cochlea; nerves carry signals to the brain. Parts of the inner ear also help balance.

Nose, tongue and skin

Smell receptors respond to substances reaching the nose; taste receptors occur in taste buds. The tongue helps move food and form speech as well as tasting. Skin contains receptors sensitive to touch, pressure, temperature and pain. Avoid the misleading idea that each taste occurs only in a separate tongue zone.

Care without risky experiments

Use comfortable light, safe listening levels and clean hands; never put objects into an ear or stare at the Sun. Do not taste unknown substances or test skin with sharp objects or extremes of heat. Persistent pain, discharge or a sudden sensory change needs a guardian and a qualified health professional.

Follow light through an eye model

The transparent cornea helps bend incoming light. The pupil is the opening; the iris controls its size. The lens helps focus an image on the retina, whose receptors produce signals carried through the optic nerve. The sclera is the tough outer white layer. Eyelids, lashes and tears help protect the surface; a model should not present the pupil as a black piece of tissue.

Trace hearing and balance separately

Sound vibrations pass from the ear canal to the eardrum and middle-ear bones. The cochlea is involved in converting vibrations into nerve signals. The auditory nerve carries signals; the brain interprets them. Semicircular canals and other vestibular structures help sense movement and balance. A tube links the middle ear with the throat and helps equalise pressure; it is not the passage that carries sound directly into the brain.

Smell, taste and skin receptors

Smell receptors respond to substances in the nose, and taste receptors in taste buds respond to substances in food. Basic tastes are detected in many tongue regions, rather than in separate exclusive sweet/sour/bitter zones. Smell contributes strongly to flavour. Skin has an epidermis and dermis, with sensory receptors and other structures; the term endodermis is used in plant anatomy, not as the correct name for a human skin layer.

Demonstrate and communicate safely

Use labelled diagrams and a care poster rather than tasting unknown substances, blindfolded walking, sharp probes or loud-sound tests. You can compare known objects by looking and gentle touch with permission, or use a supplied scenario. Write the stimulus, receptor/organ, signal route and response. Care guidance should be practical: comfortable light, clean hands, safe listening levels and telling a guardian about a concern. Persistent symptoms need professional assessment; this lesson does not diagnose them.

Check a common mistake

The retina detects light; the pupil is an opening. Also, the ear supports balance as well as hearing, so one sense organ can have more than one function.

A pupil is an opening

The iris changes pupil size when light conditions change. A diagram may colour the opening black because the interior looks dark, but black ink on the model is not the pupil’s biological structure.

A safe observation report

Look at two familiar textured materials and, with permission, touch them gently. Record how sight and touch supply different information. A supplied description can replace the observation; no unknown substance or painful stimulus is needed.

Your written practice

Show your working on paper and enter your answer here. If it does not match, compare the model and try again.