Module 3: Proprioception - the Sixth Sense

Specifications and Materials
This unit can be adapted to accommodate students in the sixth grade and up in most public and private schools. It is best to break up a class into groups of 10 or less, with two instructors per group (one instructor per group will work also). This demonstration is relatively short compared to the first two - it will take about 20 minutes to complete. Only one piece of equipment is required: standard physiotherapy vibrators (100cps, 2.5-3mm displacement). Two vibrators are required for each group of students.

Description
This lab introduces students to the reflex arc, motor neuron connections from spinal cord to muscle fibers and feedback via sensory neurons returning to the spinal cord. It illustrates how muscle spindles, the sensory organs within muscles, convey input from the muscle to give us a sense of where our limbs are in space and in relation to each other, allowing us to respond by adjusting our movements. This sense of limb positioning is known as proprioception or kinesthesia (our “sixth sense”).

Prior to starting the experiments, introduce the neuron (if this has not already been done in the anatomy lab). Explain that there are neurons in your spinal cord as well as in your brain, and that motor neurons from the spinal cord project to muscle fibers. Introduce the muscle spindle as a sensor within your muscles that can detect the length of the muscles. (Some students will look at their muscles to try to find the spindles -- explain that they cannot be seen from the outside). Sensory neurons from the muscle spindles project back to the spinal cord, bringing information about the length of the muscle. This allows the motor neurons to change the muscle length accordingly. This muscle-spinal cord-muscle neuronal circuit is the basis of the reflex motion. This same circuit allows us the sense of kinesthesia. Have all of the students try to touch their noses with their eyes closed. They should all have no problem doing this task. This illustrates how they can use their muscles to move their arms even though they can’t see their arms (with no other sensory inputs).

The demonstrations use physiotherapy vibrators (refer to them as “stimulators” in the classroom!) to artificially stimulate muscle fibers, thereby altering the perception of the position of those muscles. The lab involves three experiments to illustrate the role of the muscle spindle in sensing limb position: 1) the ability to match the position between both arms: 2) vibration induced movement and perception of movement of the arm in relation to the head; and 3) loss of balance due to stimulation of the muscle spindles used for standing.

**Note: You will have an easier time with these demonstrations if you choose volunteers who are neither very heavy nor very thin. If the student is wearing heavy clothing, have her pull up the sleeves or remove a heavy sweater.

Experiment 1
This experiment illustrates how muscle spindles allow us to determine positioning of the arms. Choose a student volunteer. Have the student sit in front of a table with her elbows resting at the edge of the table, relaxing the arm muscles, palms towards the face. While the student closes her eyes, hold the right wrist and move the arm up and down slowly. Ask the student to match the position of the right arm with the left arm. Then place the vibrator on the biceps muscle of the right arm and move the arm up and down. Ask the student to match the movement again. Everyone should observe that the student now has a difficult time performing this task. Ask the students to explain why this happened.

Experiment 2
This experiment demonstrates how information from muscle spindles allows 1) perception of the limbs in relation to the head and 2) holding the position of a limb in space. Choose a new volunteer. With eyes closed, have the student extend his arm out in front of him, then reach up and touch his nose. Then ask him to move his finger about an inch from his nose. Place the vibrator on the triceps muscle. You should see the arm move away from the face in response to this stimulation. Artificial stimulation of the muscle disrupted the ability to hold the limb in one place.
Now with his hand about a foot from his face, stimulate the triceps and ask the student to try to touch his nose. The student will go past his nose until his hand touches his face. Often subjects describe this sensation as “pointing right through the face.” Try the same experiment, but stimulating the biceps muscle. The subject’s hand will stop about an inch from his nose. It feels as if the hand should already be at his nose at this position!

Experiment 3
This experiment shows how kinesthesia and muscle spindles are essential for standing. For this experiment you will need two vibrators. Choose a volunteer without high-top shoes, or have her remove her shoes. This demonstration uses the Achilles tendon, which connects the calf muscle to the heel. The subject should stand with feet together, arms crossed at the chest, eyes closed, and head back. This removes any visual, tactile, or vestibular (balance) sensory inputs. Have two volunteers stand in front and behind the subject as spotters. With the assistance of another instructor stimulate both Achilles tendons (found just above the heels). The subject will feel as if he is falling forward. In compensation for this feeling, the calf muscles will contract and send the subject leaning (or falling) backwards. This illustrates how important the information from the muscle spindles is even for simple tasks such as standing.