Reflex Arc Lab
Name: Ozanalp
Eryılmaz
Purpose
To study the simplest functional unit of the nervous system: the reflex arc.
Materials
Ruler, glass rod, piece of paper, nail, flashlight, piece of paper with writing on it, beaker.
Method
For the duration of this lab students should work in pairs.
Patellar (Knee-jerk) Reflex
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Have the subject sit on the edge of a table with one leg dangling freely. Strike the tendon just below the
knee-cap with the edge of a ruler.
Record the results in your lab book.
The subject sitting moved his left leg suddenly when the edge of a ruler was struck the left tendon by me. He
raised his leg quickly (in one second) as well.
Uvular Reflex
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Have the subject open his/her mouth widely and touch the uvula with a clean Q-tip.
Record the results in your lab book.
Of what value is this response to humans?
Pupillar Reflex
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Have the subject close his/her eyes for 2 minutes.
Observe the pupils of both eyes as they open their eyes and record the response.
The pupils of both eyes were relatively large before the subject closed his eyes for 2 minutes. When the
subject opened his eyes after 2 minutes, the pupils of both eyes became even smaller compared to the
beginning as observed.
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Explain the results in your lab book.
The eyes of the subject were completely adapted to darkness when his eyes were close. The pupils of both
eyes became larger in order to absorb much more intensity of light. When the subject opened his eyes, more
light penetrated into his eyes so that the eyes tried to adapt to more brightness. Therefore, the pupils turned
out to be smaller in order to let less light pass through the eyes and prevent the unlikely effects probable to
the eyes.
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Place a piece of paper along the bridge of your subject’s nose. Shine light only into the left eye using a
small flashlight.
Observe the pupils of both eyes as he/she opens his/her eyes and record the response.
There was no change on the pupil of his right eye in terms of size while the left eye’s pupil became smaller.
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Explain the results in your lab book
The right eye of him was adapted to the normal surroundings while the left eye was exposed to light. For the
left eye was exposed to light, the pupil of the left eye got smaller to limit the intensity of light passing through.
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What could happen if this reflex did NOT exist?
The eyes could surely have been blind due to being exposed to excessive intensity of light. The pupils
recognise the optimum intensity of light coming through thanks to the reflex arc.
a) Accommodation Reflex
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Focus both eyes on the tip of a pencil held at eye level and at arm's length. While focusing on the pencil tip,
indicate whether or not the objects in the background are in focus.
They are not focused on and seem to be in blurry.
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Keeping the same line of sight, focus on the objects in the background. What happens to the image of the
pencil tip?
The image of the pencil tip gets into blurry as well.
b) Near Point Vision (The distance from the eye to the nearest object that can be focused on clearly)
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Cover one eye, and focus on a piece of paper with writing on it. Gradually bring the paper closer and closer
to your eye until the letters just go out of focus.
Have your partner measure the distance (in cm) between the paper and your eye in your lab book
7 cm
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Repeat these last two steps with the other eye.
Record the near point for both eyes.
8 cm
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Give a physiological explanation for what has occurred in both parts of this experiment.
Why can a person not see objects clearly when they are positioned closer to the eye than the near point?
Light reflects on an image and travels into our eyes; this is the way an object is seen. The reflected light
cannot pass through the eye properly when the object gets closer to the eye. This is the physiological reason
why a person cannot actually see objects positioned closer to the eye than the near point.
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What happens to the near point as a person ages? Why does this occur?
The distance to the near point is greater since the eye gradually loses its ability to refract the incident light.