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Investigation 2: Lab

Instructional Focus

During this lab, you will:

  • Observe how reflexes demonstrate simple information processing in the nervous system.
  • Identify the major components of a reflex arc.
  • Compare touch sensitivity in different parts of the body.
  • Discover why some reflexes occur automatically while more complex thinking requires the brain.
  • Begin connecting these observations to the more advanced information processing you will study in later Investigations.

As you work:
Pay attention to how quickly your nervous system responds to different kinds of stimuli, and consider what each response reveals about how information travels through and is organized in the nervous system.

Be Prepared

For this lab, your group will need the following materials:

  • Chromebook/Laptop with Student Guide
  • Reflex hammer
  • Penlight (1 per group)
  • Caliper (1 per group)

Lab Reminder: Follow the procedures in your Student Guide carefully and record your observations as you complete each part of the Investigation.

Experiment

This experiment is made up of three parts:

  • Two-point discrimination test
  • Knee-jerk (patellar) reflex
  • Pupillary light reflex

Follow the procedures below for each part of the Investigation.

PART 1: Two-Point Discrimination

  1. Subject closes their eyes.
  2. Using a divider or compass, examiner gently touches two points on the skin at the same time.
  3. Begin with points close together and gradually increase the distance.
  4. Ask the subject whether they feel one point or two.
  5. Test different body regions (e.g., fingertip, forearm, back of hand, upper arm, calf).
  6. Record the minimum distance at which two points are felt as separate in each location.
  7. Compare results with classmates to see which body areas are most sensitive.

PART 2: Knee-Jerk Reflex

  1. Subject sits on a stool or chair with legs dangling freely.
  2. Locate the patellar tendon just below the kneecap.
  3. Tester gently taps the tendon with a reflex hammer and observes the lower leg kicking forward.
  4. Record the strength of the response using a 0–3 scale.
  5. Repeat the test on both legs.
  6. Perform the test again while the subject does the Jendrassik maneuver (clasp fingers and pull outward firmly while clenching teeth).
  7. Record and compare results with baseline trials.

PART 3: Pupillary Light Reflex

  1. Subject sits in a dimly lit room.
  2. Examiner holds a penlight in one hand and uses the other hand vertically between the subject’s eyes as a shield, so only one eye is illuminated.
  3. Shine the light briefly into the right eye and observe both pupils.
  4. Direct response: right pupil constricts.
  5. Consensual response: left pupil constricts at the same time.
  6. Record the observations.
  7. Repeat with the left eye while shielding the right.
  8. Switch roles so both partners act as subject and examiner.