Science 7–10 · Year 9

Involuntary responses: the knee-jerk reflex and the pupil reflex

Science understanding: Biological sciences

PracticalLow risk

The idea

Some responses bypass conscious control: the knee-jerk kick follows the tap too fast to feel as a delay because the pathway runs through the spinal cord, and the iris adjusts the pupil to light without any decision being made.

Safety card

Low riskAn adult supervises

Hazards

Striking the kneecap rather than the tendon
Bright light in the eyes

Controls

Tap gently below the kneecap with the side of the hand only, never a hammer
Room lighting only, no torches or lasers; anyone with a photosensitive condition observes rather than participates

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

What you need

A chair high enough for the subject's legs to hang or cross; the tap is given with the side of the hand (the University of Washington activity warns against using a hammer)
A room whose lights can be dimmed, and a mirror or a partner to observe the pupils
A results sheet

How to do it

Knee-jerk: the subject sits with legs crossed so the upper foot hangs free, as the University of Washington activity describes. The tester taps just below the kneecap of the upper leg with the side of the hand and both watch the lower leg.
Repeat three times; ask the subject to try to stop the kick on the third tap.
Pupil: with the room lights on, the partner notes the pupil size. Dim the lights for a few minutes, observe the pupils, then bring the lights back up and watch the pupils straight away.
Record what changed, how fast, and whether the subject could control it.
Do not shine a torch into anyone's eyes; use room lighting only.

What you should see

A well-placed tap below the kneecap produces a kick at once, too fast to feel as a delay, because the pathway runs through the spinal cord without waiting for a decision. No time is given as an expected result: this practical times nothing, and the published figures disagree, the University of Washington activity saying about 50 milliseconds between the tap and the start of the kick while the latency between the stretch of the patellar tendon and the start of quadriceps contraction is reported as about 18 milliseconds. Record whether trying to stop the kick changed it. The pupils are larger after a few minutes in the dim room and smaller once the lights return. The learner knows it worked when the kick appears on well-placed taps and both pupils change together.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

All responses are decided by the brain.
The pupil is a black object that grows; it is an opening in the iris.
A reflex is a decision made by the brain; the knee-jerk pathway runs through the spinal cord without waiting for a decision.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC5-DIS-01 SC5-WS-01
Australian Curriculum v9AC9S9U01AC9S9I03

Sources

The pages the author read to write this activity.

All Concept Studio activities