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REFLEX LAB
Introduction:
A reflex arc is a way of visualizing the direction of transmission of nerve signals. The arc begins with a receptor, a
specialized cell which is stimulated by a change in the environment. For example, some receptors in the skin are sensitive
to heat, others to pressure, and so on.
If stimulation of the receptor was significant enough to initiate an action potential in the afferent (sensory) neuron,
the signal is transmitted toward the CNS.
At the peak of the arc, where the signal is ‘turned around’, the afferent neuron may synapse directly with an
efferent (motor) neuron or with an interneuron depending on whether the reflex involves 2 or 3 neurons.
The efferent (motor) neuron the proceeds to the effector which is a muscle or gland that responds in some way to
the nerve signal.
Now perform the following reflex demonstrations:
1. Patellar Reflex- “Knee- Jerk Reflex”
This reflex is mediated by a two neuron arc centered in the spinal cord. Have the subject sit on a table with the legs
dangling above the floor. Tap the knee sharply at the ligament just inferior to the patella. Record your observations
on a separate sheet of paper.
2. Achilles Reflex
Have the subject kneel on a chair, facing away from you, with the toes pointing toward the floor. Tap the middle of
the Achilles (calcaneal) tendon. Record your observations.
3. Plantar Reflex
Demonstrate this reflex by firmly sweeping your finger along the sole of the subject’s bare foot. In a normal adult,
the toes flex. In a babinski response the toes extend and move apart. The Babinski response is normal in infants
in whom the nerves have not fully myelinated but is abnormal in adults. Record your observations.
4. Pupillary reflexes
In a dimly lit area, demonstrate this reflex which is centered in the brainstem. Have the subject stare straight ahead
and turn the lights off. Observe the change in their pupil size. Record your observations. Repeat this test, turning
the lights on.
5. Corneal Reflex
While the subject stares straight ahead, quickly move a hand toward the eye but DO NOT TOUCH THE EYE.
Record the reflex you observe.
FILL IN YOUR OBSERVATIONS ON THE STUDENT ANSWER SHEET THEN ANSWER QUESTIONS.
Questions: Type the answers on a separate sheet of paper.
1. Put the following components of a reflex arc in the order in which nerve signals pass through them:
Interneuron, effector, motor neuron, receptor, sensory neuron
2. What is the advantage of pupillary and corneal reflexes?
3. In infants, the ______________ response is normal when testing the plantar reflex.
4. When testing the plantar reflex in an adult, extension of the toes may indicate damage to the ___________
somewhere along the reflex arc.
5. The center of the patellar reflex is in the _______________
6. If the Achilles (calcaneal) tendon is tapped, one would expect the ankle to (dorsiflex/ plantar flex)
7. When suddenly illuminated with a penlight, the pupil of the eye normally (dilates/constricts)
Conclusion:
Give some other examples of reflexes, write about the reflex pathway that the impulse takes and discuss the advantage of having
reflexes.