Nervous System and the Senses, Grade 12: Reaction Times and Eye Dissection
Two of the four hands-on practicals assessed in Grade 12 Life Sciences sit in this one topic, and together they are worth 80 marks.
One of them needs a 30 cm ruler and nothing else. The other needs a sheep's eye and a sharp blade. Both are marked, both are prescribed, and both are regularly skipped because nobody has the method written out in one place.
This page gives you the reflex arc the marks actually turn on, the reaction times investigation with its mark split, the pupil reflex demonstration, and the full eye dissection. It also corrects six things the printed material gets wrong, including one that contradicts a diagram on the same page.
Where this fits in the curriculum
| Subject | Life Sciences |
|---|---|
| Grade | 12 |
| Term | 2 |
| Topic | Topic 6, Responding to the environment: humans |
| Status | Carries two formally assessed practical tasks |
| Marks | 40 for the reaction times investigation, 40 for the eye dissection, plus a 20-mark pupil reflex activity that is not formally assessed |
The reflex arc, which is where the marks are
A reflex action is a fast involuntary response to a stimulus. The pathway it travels is the reflex arc, and learners are asked to reproduce it constantly.
stimulus, receptor, sensory neuron, coordinator, motor neuron, effector, response
Learn it as seven words rather than five. Most printed versions give the five boxes and leave the two neurons as labels on the arrows between them. Examiners ask for the neurons by name, so put them in the sequence from the start.
| Step | What happens when you prick a finger |
|---|---|
| Stimulus | The pin |
| Receptor | Pain receptors in the skin of the finger detect it |
| Sensory neuron | Carries the impulse to the spinal cord through the dorsal root |
| Coordinator | An interneuron inside the spinal cord. This is the central nervous system, and it is the whole reason a reflex is quick |
| Motor neuron | Leaves the spinal cord through the ventral root |
| Effector | The muscle in the finger |
| Response | The finger is withdrawn |
Dorsal in, ventral out. That is the pair most often swapped in an exam, and it costs two marks every time.
A correction worth making out loud
You will read that reflex actions are an important part of the peripheral nervous system. The diagram printed beside that sentence labels the coordinator as the central nervous system.
Both halves cannot be right. The sensory and motor neurons are peripheral. The arc is completed in the spinal cord, which is central, and that is exactly why the response happens before the brain has been consulted. A learner who answers "peripheral nervous system" to a question about where a reflex is coordinated will lose the mark.
Practical 1: the reaction times investigation, 40 marks
This is the cheapest assessed practical in Grade 12 Life Sciences. It needs a 30 cm ruler, a calculator and a sheet of paper.
Method
- Work in pairs. One learner holds the ruler vertically by the top, with the 0 cm end down
- The second learner puts a thumb and forefinger on either side of the 0 cm mark, not touching the ruler
- The holder drops the ruler without warning. No countdown, because a countdown measures anticipation rather than reaction
- Read the centimetre mark where the ruler was caught
- Repeat and take a mean. The task asks learners to state how many repeats they chose and why, so decide that before you start rather than afterwards
- Convert the distance to a time using the conversion table, then draw the bar graph
The conversion is not linear and learners should be told so. A ruler caught at 10 cm is 0,143 s. At 20 cm it is 0,202 s, which is not double. The distance depends on the square of the time, because the ruler is in free fall.
Where the 40 marks sit
| Part | What is asked | Marks |
|---|---|---|
| A | Hypothesis | 2 |
| A | Dependent, independent and fixed variables | 4 |
| A | Number of repeats, with a reason | 1 |
| A | Method in point form | 3 |
| A | Results table and the mean | 5 |
| A | Converting centimetres to seconds | 2 |
| A | Bar graph | 5 |
| A | Does the result support the hypothesis | 2 |
| A | Two improvements to the test | 4 |
| A | Two other factors affecting reaction time | 2 |
| B | Mean reaction time, and the stopping distance at 100 km/h | 5 |
| B | Following distance, and the reasons for the rule | 5 |
| Total | 40 |
Nine of the 40 marks are for the variables and the graph. Neither has anything to do with reaction time, and both are where the marks are quietly lost.
The road safety figures in this task are from 2011
The printed version asks learners to reason from 1 230 road deaths recorded between 1 December 2011 and 10 January 2012. A learner answering it in 2026 is working from numbers that are fourteen years old, in a question about a risk that is current.
The arithmetic is unaffected and the safe following distance rule has not changed, so the fix is simple: use the most recent festive season figures published by the Road Traffic Management Corporation beside the printed ones, and let the class compare. Our worksheet is written so that the calculation works with whichever year you give it.
The sum that makes the point: at 100 km/h a car covers 27,8 metres every second. A reaction time of 0,2 s means the car travels about 5,6 m before the brakes are even touched, and that is before any braking distance at all.
Practical 2: the pupil reflex, 20 marks
Not formally assessed, worth doing anyway, and it needs a torch.
- Let the class sit in normal room light for a minute, then look at a partner's pupils and note the size
- Shine a torch towards, not into, one eye and watch that pupil
- Cover both eyes with cupped hands for thirty seconds, then uncover and watch
| Condition | Radial muscles | Circular muscles | Pupil |
|---|---|---|---|
| Dim light | Contract | Relax | Dilates |
| Bright light | Relax | Contract | Constricts |
The two sets of muscles are antagonistic, and the autonomic nervous system runs both: sympathetic dilates, parasympathetic constricts.
The glossary definition of the pupil reflex is half a definition. You will see it given as "the contraction of the pupil in response to light". The text beside it describes dilation in dim light and constriction in bright light, and the autonomic nervous system table earlier in the same topic lists both. A reflex that only works one way would leave you blind every time you walked into a dark room.
Practical 3: dissect a mammalian eye, 40 marks
The biggest assessed practical in the grade, and the one we can supply completely.
What you need
| Item | Why |
|---|---|
| Dissecting kit of instruments | The whole job in one box. If you buy one thing for this practical, buy this |
| Scalpel handle and blades | The incision where the cornea meets the sclera needs a sharp blade, not a blunt one |
| Dissecting scissors | Cutting away the fat and muscle |
| Forceps | Lifting the retina off the choroid without tearing it |
| Dissecting tray with wax | Pins hold the specimen. A plate does not |
| Nitrile gloves or latex gloves | One pair per learner, not one pair per group |
The eye itself comes from a butcher, not from us. Ask for sheep or pig eyes with the muscle and fat left on, and order them a day ahead. Fresh eyes dissect far better than frozen ones, which go cloudy and tear.
Method
- Sketch and label the outside of the eyeball before you cut anything. This is 10 of the 40 marks and it cannot be recovered later
- Trim the fat and muscle away with the scissors, holding the eyeball on the tray
- Cut where the cornea meets the sclera. Aqueous humour is released. Name it and say what it does
- Cut right around the cornea and lift it off
- Squeeze the eyeball gently. The lens pops up, hard and disc shaped
- Make two cuts into the sclera and turn the eyeball inside out to expose the choroid and retina
- Put the eyeball into a container of clean water. The retina floats free of the choroid and stays attached at one point only, which is the blind spot
- Pin paper flags to label the parts for your teacher to assess
Step 7 is the one worth slowing down for. The retina lifting away in water and staying attached at a single point is the clearest demonstration of the blind spot anyone will ever see, and most classes rush past it.
What you should see
- A ruler caught between about 10 cm and 25 cm for most Grade 12 learners, which is roughly 0,14 s to 0,23 s
- A pupil that visibly changes size within a second of the light changing
- Aqueous humour, which is watery, from the front of the eye, and vitreous humour, which is jelly, from behind the lens
- A choroid that is very dark, and in sheep eyes often iridescent blue or green
- A retina that is cream coloured and detaches in water, holding at the blind spot
If it does not work
| What happens | What caused it |
|---|---|
| Reaction times are impossibly fast, under 0,10 s | The catcher is anticipating. Vary the delay before dropping, and do not count down |
| The ruler is missed completely | Fingers started too far from the 0 cm mark. They should be a centimetre apart, level with 0 |
| The pupil reflex is not visible | The room is already bright. The eye has nowhere to go. Dim the room first |
| The eyeball is too tough to cut | It is frozen, or the blade is blunt. Use a fresh blade |
| The retina will not come away | The eye is not in enough water. Submerge it completely and let it sit |
| The lens is cloudy rather than clear | The eye is old or was frozen. A fresh lens is clear enough to read print through |
Safety
- Gloves for every learner handling the eye, and hands washed with soap afterwards
- Scalpel blades are fitted and removed by the teacher, never by a learner
- Cut away from the body, on the tray, never in the hand
- Never shine a torch directly into the eye during the pupil reflex activity. Shine it towards the face from the side
- All eye material goes into a sealed bag and into the bin, and the benches are washed down
Four more things the printed material gets wrong
We check what we teach, and in two of these four the printed text contradicts the diagram printed beside it.
| What is printed | What is correct |
|---|---|
| The choroid's dark colour "prevents refraction of light inside the eyeball" | It prevents reflection. Absorbing light stops it scattering around inside the eye. Refraction is the bending of light as it crosses a boundary, and it is precisely what the cornea and lens are for. In a topic where accommodation is examined separately, the wrong word does real damage |
| White matter is "the inner section of the brain and spinal cord" | True of the brain, false of the spinal cord. In the cord the grey matter is central, in a butterfly shape, and the white matter is on the outside. Reflex arc diagrams draw it correctly, so the written description and the picture disagree |
| "The brain is the enlarged upper part of the spinal cord" | The brain and the spinal cord both develop from the same neural tube. The brain is not an enlargement of the cord, and describing it that way leaves learners unable to explain why damage high in the cord is so much worse than damage low down |
| Laser surgery for long-sightedness "thickens the cornea" | Laser refractive surgery only removes tissue. It cannot add any. For long-sightedness the laser steepens the centre of the cornea by removing tissue from around it. The cornea ends up thinner, not thicker |
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 100 marks, covering the reflex arc, the full reaction times investigation with its variables and graph, the pupil reflex and the eye dissection
- Marking memorandum, with the mark split for both assessed tasks and the six answers that look right and score nothing
Related practicals
- Mitosis and meiosis, Grade 12. The other Grade 12 Life Sciences topic we have built, with the slide practical and the contradiction that would make alleles impossible
- Circulatory and respiratory systems, Grade 9. The other organ dissection on our site, and the place to learn the knife work before you meet a 40-mark eye
- Plant and animal cells under a microscope, Grade 9. Where learners first focus a microscope, which is the skill the prepared slide work in this grade assumes
- Visible light, the spectrum, colour and the eye, Grade 8. The pinhole camera and the lens, four years before accommodation is examined
- Human reproduction and puberty, Grade 7. Where the hormonal half of "nerves and hormones" starts
Buy this experiment
The eye dissection needs one thing: the dissecting kit of instruments at R271,29. Everything in it is used, and the kit costs less than buying the scalpel, scissors and forceps separately.
Add gloves and a tray and you are done. Nitrile gloves at R99 a box, and the wax dissecting tray at R900 if you do not already have one. The tray lasts decades and runs every dissection from Grade 9 upwards.
The reaction times investigation needs nothing from us, and we would rather say so. A 30 cm ruler and a calculator. It is 40 marks and it costs a photocopy.
Everything for dissection work is in safety gear and the instruments are listed with the rest of our microscopy and biology range.