Jump to It!
For grades K-12
Here's a quick demonstration of reflexes. Talk to a group of people about
how the brain and the rest of the nervous system controls movement. Then,
suddenly slam a book on a table to create a loud noise. Ask the class
and count the number of students who:
Reflexes are used to protect the body automatically. They get us away from
objects that might hurt us,
before they hurt us. For example, if you put your hand on a hot stove,
you immediately remove your hand BEFORE the message, "Hey, my hand is on
a hot, burning stove," gets to your brain.
- Moved their heads
- Blinked their eyes
- Put their hands up
- A large book or other heavy object to make a
Knee Jerk Reflex (Patellar
Reflex) For grades 6-12
Methods The knee jerk
reflex is one that you may have had tested at a check up at the
doctor's office. In this test, the doctor hits your knee at a spot just
below your knee cap and your leg kicks out. Try it! Have a partner sit
with his or her legs crossed so that his leg can swing freely. Hit his
leg just below the knee with the side of your hand. DO NOT USE A
HAMMER!!!! The leg will kick out immediately (if you hit the right
The knee jerk
reflex (seen in the figure to the right) is called a monosynaptic reflex because there is only one synapse in the
circuit needed to complete the reflex. It only takes about 50 milliseconds
between the tap and the start of the leg kick. That is fast! The tap
below the knee causes the thigh muscle to stretch. Information is then
sent to the spinal cord. After one synapse in the ventral horn of the
spinal cord, the information is sent back out to the muscle...and there
you have the reflex.
For grades 3-9|
Our built-in reflexes really do protect us. Another demonstration of
these built-in capabilities is the blink reflex.
Have a student stand behind a see-through barrier like a window or a wire
screen. Throw a cotton ball at the person. Did he blink? Probably.
This is the blink reflex and serves to protect our eyes from damage.
- Cotton balls (or rolled-up paper towels)
- A transparent barrier (a wire screen, plastic or glass window)
|Did you know?
||People typically blink about 15 times per minute. If you are
awake for 16 hours each day, then you blink approximately 14,400
each day! (Source: Schiffman, H.R., Sensation and
Perception. An Integrated Approach, New York: John Wiley and Sons,
Inc., 2001) |
How Fast are You?
For grades K-12
Unlike the other activities on this reflex page, this project does not
test a simple reflex. Rather, this activity is designed to measure your
response time to something that you see.
Get a ruler (or a yardstick or candy bar). Hold the ruler near the end
(highest number) and let it hang down. Have another person put his or her
hand at the bottom of the ruler and have them ready to grab the ruler
(however, they should not be touching the ruler). Tell the other person
that you will drop the ruler sometime within the next 5 seconds and that
they are supposed to catch the ruler as fast as they can after it is
dropped. Record the level (inches or centimeters) at which they catch the
ruler (you can convert the distance into reaction time with the chart
below). Test the same person 3 to 5 times (vary the time of dropping the
ruler within the 5 second "drop-zone" so the other person cannot guess
when you will drop the ruler).
Here is a table to convert the distance on the ruler to reaction time.
For example, if you caught the ruler at the 8 inch mark, then your
reaction time is equal to 0.20 seconds (200 ms). Remember that there are
1,000 milliseconds (ms) in 1 second.
If you want to be more precise with your calculations, use the following
|Distance of catch||Reaction Time
|2 in (~5 cm)||0.10 sec (100 ms)|
|4 in (~10 cm)||0.14 sec (140 ms)|
|6 in (~15 cm)||0.17 sec (170 ms)|
|8 in (~20 cm)||0.20 sec (200 ms)|
|10 in (~25.5 cm)||0.23 sec (230 ms)|
|12 in (~30.5 cm)||0.25 sec (250 ms)|
|17 in (~43 cm)||0.30 sec (300 ms)|
|24 in (~61 cm)||0.35 sec (350 ms)|
|31 in (~79 cm)||0.40 sec (400 ms)|
|39 in (~99 cm)||0.45 sec (450 ms)|
|48 in (~123 cm)||0.50 sec (500 ms)|
|69 in (~175 cm)||0.60 sec (600 ms)|
|Formula 1 ||Formula 2|
|Formula 1 provides you with the distance an object will fall in
a given amount of time. By rearranging Formula 1 into Formula 2, you can
get the amount of time it takes an object to fall a
certain distance...that's what you want to find out. All you have to do is
plug in the distance (in either centimeters or inches) that the ruler
fell into Formula 2 - this will give you the reaction time.
In the formulas, t = time (in seconds); y = distance (in cm); g = 980
cm/sec2 (acceleration due to gravity). [Note: you can also
use inches in your distance measurement, but you must change g to equal
This reaction time experiment required visual information (the movement of
the ruler) to travel to your brain. Then your brain sent a motor command
("grab that falling ruler") to the muscles of
your arm and hand. If all went well, you caught the ruler!!
Questions and Comparisons
- Try the experiment in dim light. Does your reaction time increase,
decrease or stay the same? Can you explain your results?
- Test the whole class. Who is fastest?
- Compare boys vs. girls. On average, are
the boys or girls faster?
- Compare different ages. Who is
fastest?...the older students or
- Compare the scores after practice.
Does reaction time improve with
- Compare kids' scores vs. parents'
scores. Who is faster?
- Test the whole school!!
- Test the whole city!!................you
get the idea.
- Ruler or yardstick or long candy bar (give the candy bar to the person
with the fastest reaction time)
Biological Supply Company also sells a Reaction Time Ruler Set
that includes three rulers with msec gradations, one
instruction book and recording sheets. Cost = $22.50/set.