Sense organs — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

Section 1 of 37Sense organs as receptors

1Receptors

• Groups of specialised cells that detect a change in the environment and stimulate electrical impulses.

2Sense organ

• Contains groups of receptors that respond to specific stimuli.

3Pathway

• A receptor generates an impulse, a sensory neurone carries it to the central nervous system, a motor neurone carries the response to an effector.
  • (muscle or gland)

Sense organ	Sensitive to	Sense
Skin	Pressure, heat, cold and pain	Touch and temperature
Tongue	Chemicals in food and drink	Taste
Nose	Chemicals in the air	Smell
Ear	Sound and movement	Hearing and balance
Eye	Light	Sight

Section 2 of 37Structure and function of the eye

Structure	Function
Cornea	Transparent covering at the front that refracts light
Iris	Muscle that controls how much light enters the pupil
Lens	Transparent disc that changes shape to focus light onto the retina
Retina	Layer of receptor cells that detect light intensity and colour
Optic nerve	Sensory neurone carrying impulses from the eye to the brain
Fovea (Supplement)	Region of the retina with the most cones; forms sharp coloured images

Section 3 of 37Pupil reflex

Role

• A reflex action that controls the light entering the eye by altering pupil diameter.

Dim light

• The pupil dilates to allow as much light in as possible.

Bright light

• The pupil constricts to stop too much light damaging the retina.

Section 4 of 37Antagonistic muscles of the iris (Supplement)

Stimulus	Radial muscles	Circular muscles	Pupil	Light entering
Dim light	Contract	Relax	Dilated	More
Bright light	Relax	Contract	Constricted	Less

Section 5 of 37Accommodation (Supplement)

Accommodation

• The way the eye focuses on near or distant objects by changing the shape of the lens.

Control

• It depends on the ciliary muscles and the tension in the suspensory ligaments.

Refraction

• Changing lens shape changes how much light is refracted.

Section 6 of 37Near and distant objects (Supplement)

Part of the eye	Near objects	Distant objects
Ciliary muscles	Contract	Relax
Suspensory ligaments	Loosen	Tighten
Lens	Fatter, more rounded	Thinner
Light	Refracted more	Refracted less

Section 7 of 37Rods and cones (Supplement)

Rods

• Detect light at low levels, so they are important in night vision.
• They are found all over the retina except the blind spot.

Cones

• Three kinds absorb light of different colours, giving colour vision.
• They are concentrated in the fovea.

Fovea

• The region onto which light is focused, enabling sharp coloured images.

Section 8 of 37More from the specification

1Neuronal electrical transmission versus auxin

• Neuronal electrical transmission versus auxin mammalian nerve impulses propagate rapidly along axons via action potentials, unlike polar cell-to-cell diffusion of auxin in plants.

2Accommodation in the Human Eye

• Focus on nearby objects requires contraction of ciliary muscles, slackening suspensory ligaments and allowing the elastic lens to become more convex and refractive.

3Photoreceptor Distribution in the Retina

• The fovea contains the highest density of cones for acute colour vision in bright light,
• whereas sensitive rods dominate the peripheral retina for monochrome night vision.

4Thermoregulation and Skin Adaptations

• When body temperature drops, erector muscles contract to raise hair shafts, trapping an insulating layer of warm air and minimizing conductive heat loss.

5Synapse

• A junction between two neurones where an impulse passes by neurotransmitter diffusing across a gap.

6Negative feedback

• A mechanism in which a change from the set point triggers a response that returns the factor to the set point.

7Glucose

• A simple sugar; the unit that starch, glycogen and cellulose are built from.

8Growth

• A permanent increase in size and dry mass.

9Insulin

• A pancreatic hormone that lowers blood glucose.

10Hormone

• A chemical substance produced by a gland and carried by the blood that alters the activity of specific target organs.

11Homeostasis

• The maintenance of a constant internal environment.

12Glycogen

• The polysaccharide in which glucose is stored in animals.

Section 9 of 37Sense organs and receptors

Receptors

• Receptors groups of specialised cells that detect a change in the environment and stimulate electrical impulses.

Sense organ

• Sense organ contains receptors that respond to specific stimuli.

Sense organ	Sensitive to	Sense
Skin	Pressure, heat, cold, pain	Touch and temperature
Tongue	Chemicals in food and drink	Taste
Nose	Chemicals in the air	Smell
Ear	Sound and movement	Hearing and balance
Eye	Light	Sight

Section 10 of 37Parts of the eye

Part	Function
Cornea	Transparent front covering that refracts light; does most of the focusing
Iris	Muscle that controls how much light enters the pupil
Pupil	Hole through which light enters
Lens	Transparent, flexible; changes shape to focus light onto the retina
Ciliary muscle and suspensory ligaments	Change the shape of the lens
Retina	Layer of light receptor cells (rods and cones)
Optic nerve	Sensory neurone that carries impulses from the eye to the brain
Fovea	Part of the retina with most cones; forms sharp, coloured images
Sclera	Tough white outer layer that protects the eye and keeps its shape
Choroid	Dark layer that stops internal reflection and carries blood vessels
Blind spot	Where the optic nerve leaves the eye; no receptors

Section 11 of 37Path of light and image formation

1

Light passes through the cornea, which refracts it.

2

It passes through the pupil, the amount being set by the iris.

3

The lens refracts it further and focuses it.

4

A sharp inverted image forms on the retina.

5

Receptors are stimulated and impulses pass along the optic nerve.

6

The brain interprets the image and perceives it the right way up.

Section 12 of 37Rods and cones

Rods

• Rods detect light at low levels, so are important for night vision and give black-and-white vision.
• They are found all over the retina except the blind spot.

Cones

• Cones three types detect different wavelengths, giving colour vision.
  • (red, green, blue)
• They are concentrated at the fovea, where light is focused.

Fovea

• Fovea the dense cones let the brain form sharp, coloured images.

Section 13 of 37Other protective features

Other protective features

Tears

• Tears wash the surface and contain lysozyme, an enzyme that kills bacteria.

Eyelids and eyelashes

• Eyelids and eyelashes keep out dust; blinking spreads tears over the surface.

Aqueous and vitreous humours

• Aqueous and vitreous humours support the eyeball, keep its shape and help refract light.

Section 14 of 37More from the specification

More from the specification

Negative Feedback in Homeostasis

• Negative feedback mechanisms counteract deviations from a physiological set point, ensuring internal balance in the human body.

Endocrine Control by Insulin

• Insulin is a peptide hormone produced by the pancreas that lowers elevated blood glucose concentrations by stimulating glycogen synthesis in muscle and liver cells.

Reflex Actions in the Nervous System

• Involuntary reflex responses provide immediate protective actions via fast neural pathways without requiring conscious brain processing.

Auxin and Tropisms

• Auxin functions as a plant hormone coordinating differential cellular growth in shoots and roots in response to directional environmental stimuli.

Synapse

• A junction between two neurones where an impulse passes by neurotransmitter diffusing across a gap.

Negative feedback

• A mechanism in which a change from the set point triggers a response that returns the factor to the set point.

Motor neurone

• Neurone carrying impulses from the CNS to effectors.

Radial muscle

• Iris muscle that widens the pupil when it contracts.

Circular muscle

• Iris muscle that narrows the pupil when it contracts.

Glucose

• A simple sugar; the unit that starch, glycogen and cellulose are built from.

Reflex

• Rapid, involuntary and automatic response to a stimulus.

Growth

• A permanent increase in size and dry mass.

Insulin

• A pancreatic hormone that lowers blood glucose.

Hormone

• A chemical substance produced by a gland and carried by the blood that alters the activity of specific target organs.

Homeostasis

• The maintenance of a constant internal environment.

Glycogen

• The polysaccharide in which glucose is stored in animals.

Section 15 of 37Purpose of the pupil reflex

Pupil reflex

• Pupil reflex a reflex action that controls how much light enters the eye by altering the pupil diameter.

Bright light

• Bright light the pupil constricts so too much light does not enter and damage the retina.

Dim light

• Dim light the pupil dilates to let in as much light as possible.

Section 16 of 37Antagonistic iris muscles

Circular muscles

• Circular muscles form rings around the pupil; when they contract the pupil gets smaller.

Radial muscles

• Radial muscles radiate outwards like spokes; when they contract the pupil gets larger.

Antagonistic

• Antagonistic the two sets work against each other, so when one contracts the other relaxes.

Section 17 of 37Dim and bright light compared

Stimulus	Radial muscles	Circular muscles	Pupil	Light entering
Dim light	Contract	Relax	Dilated	More
Bright light	Relax	Contract	Constricted	Less

Section 18 of 37The reflex pathway

1

Stimulus: change in light intensity.

2

Receptor: light-sensitive cells in the retina.

3

Sensory neurone: carries impulses in the optic nerve to the brain.

4

Relay neurone in the brain: the coordinator.

5

Motor neurone: carries impulses to the iris.

6

Effector: the iris muscles change the pupil size.

Section 19 of 37Features of the reflex

1Pathway

• Pathway short and without conscious thought, so the response is fast and protective.

2Negative feedback

• Negative feedback the response reverses the change and returns the light reaching the retina to a suitable level.

3Both eyes

• Both eyes light shone into one eye normally makes both pupils constrict.

4Absent reflex

• Absent reflex can indicate damage to the brain or the optic nerve, since the arc is broken.

Section 20 of 37Reflex and voluntary action compared

Feature	Reflex action	Voluntary action
Starts in	Receptor and spinal cord or brain stem, without conscious thought	Conscious part of the brain
Speed	Rapid	Slower
Control	Cannot be changed	Can be started, changed or stopped
Purpose	Protective	Deliberate

Section 21 of 37More from the specification

More from the specification

Sensory Receptors and the Nervous System

• Specialized sense organs detect changes in external stimuli and convert them into electrical impulses in the nervous system.
  • (e.g. light, temperature, touch)

Endocrine Homeostasis via Insulin

• When blood glucose levels rise after a meal, the endocrine system releases insulin into the blood to stimulate glucose uptake into cells.

Thermoregulation Set Points

• The body regulates internal temperature through sweating, shivering,
• and blood flow adjustments, maintaining homeostasis within narrow limits.

Auxin and Plant Growth Coordination

• Plant growth responses (tropisms) are mediated by the hormone auxin, which diffuses through plant tissues to alter cellular growth rates.

Homeostasis

• The maintenance of a constant internal environment.

Synapse

• A junction between two neurones where an impulse passes by neurotransmitter diffusing across a gap.

Ciliary muscle

• Muscle that changes the tension on the lens.

Suspensory ligaments

• Fibres that connect the ciliary muscle to the lens.

Glucose

• A simple sugar; the unit that starch, glycogen and cellulose are built from.

Growth

• A permanent increase in size and dry mass.

Insulin

• A pancreatic hormone that lowers blood glucose.

Hormone

• A chemical substance produced by a gland and carried by the blood that alters the activity of specific target organs.

Section 22 of 37Accommodation

Accommodation

• Accommodation the way the eye focuses on near or distant objects by changing the shape of the lens.

Mechanism

• Mechanism contraction or relaxation of the ciliary muscles changes the tension in the suspensory ligaments.

Refraction

• Refraction light from near objects diverges and needs more bending than light from distant objects, which is nearly parallel.

Section 23 of 37Near and distant objects compared

Part of the eye	Near object	Distant object
Ciliary muscles	Contract	Relax
Suspensory ligaments	Loosen	Tighten
Lens	Thicker, more rounded	Thinner, flatter
Refraction	More	Less

Section 24 of 37Focusing on a near object

1

The ciliary muscles contract.

2

The suspensory ligaments loosen and pull less on the lens.

3

The elastic lens becomes more rounded.

4

Light is refracted more and a sharp image forms on the retina.

Section 25 of 37Focusing on a distant object

1

The ciliary muscles relax.

2

The suspensory ligaments tighten and pull on the lens.

3

The lens becomes thinner.

4

Light is refracted less and a sharp image forms on the retina.

Section 26 of 37Focusing defects

Defect	Cause	Effect	Correction
Short sight (myopia)	Eyeball too long or lens too strong; image forms in front of the retina	Distant objects blurred	Concave (diverging) lens
Long sight (hypermetropia)	Eyeball too short or lens too weak; image forms behind the retina	Near objects blurred	Convex (converging) lens

• Cataract
  • the lens becomes cloudy and scatters light; treated by replacing it with an artificial lens.

Section 27 of 37Ageing and the lens

Elasticity

• Elasticity with age the lens becomes stiffer and cannot thicken as much, so the near point moves further away.

Presbyopia

• Presbyopia the loss of accommodation with age, treated with reading glasses.

Laser surgery

• Laser surgery can reshape the cornea to correct a focusing defect permanently.

Section 28 of 37More from the specification

More from the specification

Sense Organs and Receptor Cells

• Sense organs contain specialized sensory receptor cells that transduce environmental stimuli into electrical nerve impulses conveyed along sensory neurones to the brain.

Pupil Light Reflex Mechanism

• Under high illumination levels, retinal photoreceptor cells trigger a protective reflex response that activates motor neurones,
• causing circular iris muscles to contract and constricting the pupil.

Thermoreception and Temperature Regulation

• Thermoreceptive cells in the skin and hypothalamus detect deviations in blood temperature, initiating autonomic responses like vasodilation and sweating.

Neural Reflexes vs Chemical Control

• Rapid involuntary neural reflexes safeguard tissues within milliseconds,
• whereas chemical regulation like insulin modulating blood glucose or plant auxin directing growth occurs over longer timeframes.

Homeostasis

• The maintenance of a constant internal environment.

Synapse

• A junction between two neurones where an impulse passes by neurotransmitter diffusing across a gap.

Pupil reflex

• Change in pupil diameter in response to light intensity.

Negative feedback

• A mechanism in which a change from the set point triggers a response that returns the factor to the set point.

Muscle cell

• Mitochondria

Motor neurone

• Neurone carrying impulses from the CNS to effectors.

Radial muscle

• Iris muscle that widens the pupil when it contracts.

Circular muscle

• Iris muscle that narrows the pupil when it contracts.

Glucose

• A simple sugar; the unit that starch, glycogen and cellulose are built from.

Neurone

• A nerve cell that carries electrical impulses.

Reflex

• Rapid, involuntary and automatic response to a stimulus.

Growth

• A permanent increase in size and dry mass.

Insulin

• A pancreatic hormone that lowers blood glucose.

Section 29 of 37Structure of the eye

Part	Function
Cornea	Refracts light entering the eye
Iris	Controls the amount of light entering
Lens	Focuses light onto the retina
Retina	Contains light receptors
Optic nerve	Carries impulses to the brain
Ciliary muscle and suspensory ligaments	Alter lens shape

Section 30 of 37Rods and cones

Rods

• Rods contain a pigment that responds to dim light and give black-and-white vision.
• Many rods share one neurone, so images are less sharp.

Cones

• Cones need bright light; three types give colour vision.
  • (red, green, blue)
• Each cone has its own neurone, so detail is sharper.

Fovea

• Fovea the part of the retina with most cones, giving the sharpest vision.

Choroid

• Choroid absorbs stray light, helping give a clear image.

Section 31 of 37Accommodation

Near object

• Near object ciliary muscle contracts, ligaments loosen, lens becomes thicker and refracts more.

Distant object

• Distant object ciliary muscle relaxes, ligaments tighten, lens becomes thinner and refracts less.

Section 32 of 37The pupil reflex

The pupil reflex

Bright light

• Bright light circular muscles contract and radial muscles relax, so the pupil gets smaller and protects the retina.

Dim light

• Dim light radial muscles contract and circular muscles relax, so the pupil gets larger and lets more light in.

Type of response

• Type of response involuntary reflex.

Section 33 of 37Other sense organs

Sense organ	Receptors detect	Detail
Skin	Touch, pressure, temperature, pain	More touch receptors in the fingertips
Ear	Sound and balance	Cochlea detects sound; semicircular canals detect head movement
Tongue	Chemicals in food	Taste buds detect sweet, sour, salty, bitter and umami
Nose	Airborne chemicals	Olfactory cells detect smell

• Receptor cells
  • each responds to one stimulus and sends impulses along sensory neurones to the brain.

Section 34 of 37Sound and balance in the ear

Pinna

• Pinna collects sound.

Eardrum and ossicles

• Eardrum and ossicles transmit vibrations.

Cochlea

• Cochlea receptor cells convert the vibrations to nerve impulses.

Semicircular canals

• Semicircular canals detect head movements for balance.

Section 35 of 37More from the specification

More from the specification

Sensory reception and the nervous system

• Sense organs contain specialized receptor cells that convert environmental stimuli into electrical impulses transmitted through the peripheral nervous system to the brain.

Insulin and endocrine regulation

• The hormone insulin is produced by beta cells of the islets of Langerhans in the pancreas and secreted into the blood to regulate blood glucose levels.

Auxin and directional plant growth

• Auxin is a plant growth hormone synthesized at shoot tips that regulates cell elongation and directional growth responses to light and gravity.

Coordination systems comparison

• Animals coordinate physiological functions through fast-acting electrical impulses in the nervous system and slower, prolonged chemical signalling via hormones circulating in blood.

Homeostasis

• The maintenance of a constant internal environment.

Synapse

• A junction between two neurones where an impulse passes by neurotransmitter diffusing across a gap.

Negative feedback

• A mechanism in which a change from the set point triggers a response that returns the factor to the set point.

Motor neurone

• Neurone carrying impulses from the CNS to effectors.

Glucose

• A simple sugar; the unit that starch, glycogen and cellulose are built from.

Growth

• A permanent increase in size and dry mass.

Insulin

• A pancreatic hormone that lowers blood glucose.

Hormone

• A chemical substance produced by a gland and carried by the blood that alters the activity of specific target organs.

Section 36 of 37Key terms

Term	Meaning
Receptor	A cell that detects a stimulus and generates an impulse.
Pupil reflex	Change in pupil diameter in response to light intensity.
Accommodation	Change in lens shape to focus on near or distant objects.
Retina	Layer of the eye containing light receptors.
Fovea	Region of the retina with the most cones.
Rod	Receptor cell sensitive to dim light.
Cone	Receptor cell for colour vision.
Ciliary muscle	Muscle that changes the tension on the lens.

Term	Meaning
Cornea	The transparent front of the eye that refracts light.
Retina	The light-sensitive layer containing rods and cones.
Iris	The muscle that controls pupil size.
Optic nerve	The nerve that carries impulses from the retina to the brain.
Fovea	The part of the retina with the highest density of cones.
Blind spot	The area of the retina with no receptors where the optic nerve leaves.

Term	Meaning
Reflex action	A rapid, automatic response to a stimulus.
Antagonistic muscles	A pair of muscles with opposite effects.
Circular muscle	Iris muscle that narrows the pupil when it contracts.
Radial muscle	Iris muscle that widens the pupil when it contracts.
Constrict	Become narrower.
Dilate	Become wider.

Term	Meaning
Accommodation	The change in lens shape that focuses light from objects at different distances on the retina.
Ciliary muscle	Muscle that alters the tension on the suspensory ligaments.
Suspensory ligaments	Fibres that connect the ciliary muscle to the lens.
Short sight	A defect in which the image of a distant object forms in front of the retina.
Long sight	A defect in which the image of a near object forms behind the retina.
Cataract	A clouding of the lens.

Term	Meaning
Receptor	A cell or group of cells that detects a stimulus.
Rod cell	A retina receptor sensitive to dim light.
Cone cell	A retina receptor that detects colour in bright light.
Cochlea	The part of the ear whose receptors detect sound.
Semicircular canals	Ear structures that detect head movement for balance.
Pinna	The outer ear that collects sound.

Section 37 of 37Exam technique

1

Saying the lens does most of the refraction — the cornea refracts light and the lens fine-focuses.

2

Reversing the iris muscles — in bright light the circular muscles contract and the radial relax.

3

Saying near objects need a thinner lens — the lens becomes fatter for near objects.

4

Saying rods give colour vision — cones give colour vision.

1

Saying the lens does most of the focusing — the cornea does most of the refraction.

2

Saying the image on the retina is upright — it is inverted.

3

Confusing the iris with the pupil — the iris is the muscle and the pupil is the hole.

4

Saying the blind spot has many receptors — it has none.

1

Saying the pupil is a muscle — the iris is the muscle and the pupil is the hole.

2

Saying the radial muscles contract in bright light — they relax and the circular muscles contract.

3

Saying the lens controls how much light enters — the iris does.

4

Saying the brain consciously decides to constrict the pupil — it is an involuntary reflex.

1

Saying the ciliary muscle relaxes for near vision — it contracts.

2

Mixing up the corrections — short sight needs a concave lens and long sight a convex lens.

3

Saying the cornea changes shape during accommodation — the lens does.

4

Saying the lens is rigid — it is elastic and changes shape.

1

Saying the lens is the main refracting structure — the cornea is.

2

Mixing up radial and circular muscles — radial contract in dim light, circular in bright light.

3

Saying cones work in dim light — rods do.

4

Saying the optic nerve detects light — the retina's receptors do.