14.2 Sense organs
- Syllabus
- 0610–2026–2027
- Topic
- 14.2
- Level
- —
Learning objectives
14.2.1Sense organs as groups of receptor• Describe sense organs as groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals14.2.2Structures of the eye• Identify in diagrams and images the structures of the eye, limited to: cornea, iris, pupil, lens, retina, optic nerve and blind spot14.2.3Function of each part of the eye• Describe the function of each part of the eye, limited to: (a) cornea – refracts light (b) iris – controls how much light enters the pupil (c) lens – focuses light on to the retina (d) retina – contains light receptors, some sensitive to light of different colours (e) optic nerve – carries impulses to the brain14.2.4Pupil reflex• Explain the pupil reflex, limited to changes in light intensity and pupil diameter14.2.5Pupil reflex in terms• Explain the pupil reflex in terms of the antagonistic action of circular and radial muscles in the iris14.2.6Accommodation to view near and distant• Explain accommodation to view near and distant objects in terms of the contraction and relaxation of the ciliary muscles, tension in the suspensory ligaments, shape of the lens and refraction of light14.2.7Distribution of rods and cones• Describe the distribution of rods and cones in the retina of a human14.2.8Function of rods and cones• Outline the function of rods and cones, limited to: (a) greater sensitivity of rods for night vision (b) three different kinds of cones, absorbing light of different colours, for colour vision14.2.9Position of the fovea and state its• Identify in diagrams and images the position of the fovea and state its function
Describe sense organs
A sense organ is a group of receptor cells that responds to specific stimuli.
| Stimulus | Example sense organ |
|---|---|
| light | eye |
| sound | ear |
| touch or temperature | skin |
| chemicals | nose or tongue |
A sense organ contains receptor cells; it is not itself a stimulus, and different receptors respond to particular kinds of change.
Identify the required structures of the eye
Use position and connection to identify the seven required structures in an eye section.
| Structure | Identification cue |
|---|---|
| cornea | transparent curved front surface |
| iris | ring of tissue in front of the lens |
| pupil | opening in the centre of the iris |
| lens | transparent biconvex structure behind the pupil |
| retina | light-sensitive layer lining the back of the eye |
| optic nerve | bundle leaving the back of the eye |
| blind spot | point where the optic nerve leaves the retina |
The pupil is an opening, not a tissue. The blind spot is on the retina at the optic-nerve exit.
Describe the functions of the eye structures
Each required eye structure has a distinct job in controlling, focusing, detecting or transmitting light information.
| Structure | Required function |
|---|---|
| cornea | refracts light |
| iris | controls how much light enters the pupil |
| pupil | opening through which light enters |
| lens | focuses light onto the retina |
| retina | contains light receptors, including receptors sensitive to different colours |
| optic nerve | carries impulses to the brain |
The cornea refracts incoming light and the lens focuses it; the retina detects light, while the optic nerve carries impulses rather than light.
Explain the pupil reflex using light intensity
The pupil reflex changes pupil diameter as light intensity changes.
| Light intensity | Pupil diameter | Effect |
|---|---|---|
| bright light | decreases; pupil constricts | less light enters |
| dim light | increases; pupil dilates | more light enters |
The pupil is the opening that changes diameter; the iris is the tissue that causes this change.
Explain antagonistic action in the pupil reflex
Circular and radial muscles in the iris act antagonistically: when one set contracts, the other relaxes.
| Condition | Circular muscles | Radial muscles | Pupil |
|---|---|---|---|
| bright light | contract | relax | constricts |
| dim light | relax | contract | dilates |
Both muscle sets do not contract together. Circular contraction narrows the pupil; radial contraction widens it.
Explain accommodation for near and distant objects
Accommodation changes lens shape so light from objects at different distances is focused on the retina.
| Viewing | Ciliary muscles | Suspensory ligaments | Lens | Refraction |
|---|---|---|---|---|
| near object | contract | slacken; less tension | thicker, more convex | more |
| distant object | relax | tighten; more tension | thinner, less convex | less |
Near: ciliary muscles contract → ligaments slacken → lens becomes thicker → light is refracted more. Distant: every change reverses.
Ciliary-muscle contraction reduces tension in the suspensory ligaments; it does not pull the lens flatter.
Describe the distribution of rods and cones
Rods and cones are unevenly distributed across the retina.
| Retinal region | Rods | Cones |
|---|---|---|
| fovea | absent | highest concentration |
| peripheral retina | numerous | present in lower numbers |
| blind spot | absent | absent |
Overall, rods are more numerous across much of the retina, while cones form a sharp concentration at the fovea.
The blind spot has neither rods nor cones; the fovea has the highest cone concentration and no rods.
Outline the functions of rods and cones
Rods and cones are light receptors with different sensitivities and visual roles.
| Receptor | Sensitivity and role |
|---|---|
| rods | more sensitive in low light; provide night vision |
| cones | three different kinds absorb light of different colours; provide colour vision |
Rods are more sensitive for dim-light vision but do not provide colour vision. Colour vision depends on three kinds of cone.
Identify the fovea and state its function
The fovea is a small region of the retina with the highest concentration of cone cells.
In an eye section, locate it on the retina opposite the lens and separate from the blind spot where the optic nerve exits.
The fovea provides the sharpest, most detailed colour vision because images of objects viewed directly are focused onto its densely packed cones.
The fovea is not the blind spot: the fovea is cone-rich, while the blind spot has no light receptors.