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SBI 4U
Ms. Girvan
Name: ______________________
Date: _______________________
Investigation: The Reflex Arc
Purpose: to study a number of different reflex arcs found in the body.
Background: All reflex arcs have a minimum of five basic components:
1. A receptor, which detects stimuli and converts them into impulses.
2. A sensory (afferent) neuron which conducts the impulses to the central nervous
system.
3. A synapse in the central nervous system between a sensory and motor (efferent)
neuron.
4. A motor neuron which carries the impulses to an effector.
5. An effector (muscle or gland) which initiates a response.
Materials:
Glass rod
Flash light
Ruler
Q-tip
Method:
1. Work in pairs for the following activities.
A. Patellar (knee-jerk) reflex
2. Have the subject sit on the tabletop. Be sure your partner’s legs are relaxed. Find the
patellar tendon, in the gap just below the kneecap and above the tibia. Gently strike the leg
just below the knee with the side of your hand. The leg will kick out immediately (if you hit
the right place).
B. Achilles Reflex
3. Have the subject remove a shoe. Ask your subject to kneel on a chair so that his feet hang
over the edge of the chair. Tell him to relax the leg muscles and then LIGHTLY tap the
Achilles tendon extending up from the heel with your hand.
C. Babinski Reflex
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4. Now ask the subject to remove a sock. Have the subject sit on a chair, then place the heel
of the bare foot on another chair for support. Quickly slide the glass rod along the sole of the
subject’s foot, beginning at the heel and moving toward the toes.
D. Pupillary Reflex
5. Have the subject close one eye for approximately 1 min. Ask him to open the closed eye.
Compare the size of the pupils.
6. Ask the subject to close both eyes for one minute, and then open both eyes. Shine a
penlight in 1 of the eyes.
7. Select a student with light-coloured eyes to be the subject. With at least two observers
carefully watching the subject’s eyes, gently stroke the fine hairs on the nape of the subject’s
neck.
E. Uvular Reflex
8. Have the subject open his mouth widely and touch the uvula with a clean Q-tip.
F: Measure Your Response Time
9. Get a ruler.
10. Hold the ruler near the end (highest number) and let it hang down.
11. Have another person put his hand at the bottom of the ruler and have them ready to grab
the ruler (however, they should not be touching the ruler).
12. Tell the other person 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.
15. Record the level (inches or centimeters) at which they catch the ruler (you can convert the
distance into reaction time with the chart below).
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16. 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).
Table 2: Convert the distance on the ruler to the reaction time.
Distance
2 in (~5 cm)
4 in (~10 cm)
6 in (~15 cm)
8 in (~20 cm)
10 in (~25.5 cm)
12 in (~30.5 cm)
17 in (~ 43 cm)
24 in (~61 cm)
31 in (~79 cm)
Time
0.10 sec (100 ms)
0.14 sec (140 ms)
0.17 sec (170 ms)
0.20 sec (200 ms)
0.23 sec (230 ms)
0.25 sec (250 ms)
0.30 sec (300 ms)
0.35 sec (350 ms)
0.40 sec (400 ms)
Record the reaction times: ______ ______ ______ ______ ______
Average reaction time: ____________________