A Stretch Reflex and a Crossed Extensor Reflex Model
David Journeay and Mark Little
Models are useful ways to present information to students. This activity will describe one procedure for making a stretch reflex model and a modification to expand the stetch reflex model into a crossed extensor reflex. Also, the models are made out of common materials found at most hardware and craft stores.
Polysynaptic and monosynaptic types
Reflex arc components
It took one-two hours to make.
Background Information: this action so students may have a better understanding of how reflexes work.
Clear or colored flexible tubing--1/4"; 1 foot per muscle fiber. One piece of smaller diameter tubing about one foot. For example, 5 pieces of muscle fiber is 5 feet in total length of plastic tubing.
Heat shrink tubing. Try to match heat shrink tubing color to wire color.
Duct tape and/or electrical tape
Soldering iron and solder
Needle-nosed pliers and Wire cutters
Flexible wire--2 different colors approximately 14-18 gauge--will bend and hold its shape. About 3-4 feet in total length for each color of wire.
Galvanized 18 gauge wire--approximately 5 feet in total length.
Two round balls for the cell body. (About a medium marble in size; about 3/4" in diameter) Material can be many things.
Hair dryer, boiling water or a lighter to heat the shrink tape
Teacher Preparation-Stretch reflex (Please have figure page in front of you while putting the model together for assistance)
1. Obtain the plastic tubing and cut to sections for muscle fibers about 1 foot in length. (It can be cut with a pair of scissors.) You may vary the number of fibers, depending on your budget. (About $.20/foot)
2. Cut the galvanized wire into lengths approximately 3-6 inches longer than the tubing, with wire cutters and place inside the tubes.
3. Cut a 1 inch long hole approximately in the middle of the tube with a pair of scissors.
4. Cut a piece of wire about 2 1/2-3 feet long and then cut 4 pieces of wire about 4 inches long.(The number of short pieces cut depends on the number of fibers. One short piece per fiber. All these pieces together represent the motor neuron. See figure 1
5. Using the needle nose pliers, connect short pieces of wire to the long piece of wire with about 1 inch between so it looks like a comb. Using a soldering iron, solder the ends of the shorter wire, one to the wire inside the tube and the other end to the main wire.
6. Place heat shrink tape around the connections and heat to shrink or cover with duct tape or electrical tape. Note: heat shrink tubing needs to be placed on the wire before soldering both ends. The tubing can be cut to any length but it cannot be cut lengthwise.
7. Cut another piece of clear tubing about 12 inches long. This tubing can be a different color and smaller in diameter.
8. Cut a piece of galvanized wire with wire cutters about 3 inches longer than the tube and place inside the tube.
9. Cut another piece of wire 21/2-3 feet long of the second color.
10. Wrap it a couple of times around the middle of the tube you just cut. This is a sensory neuron of the muscle spindle.
11. With the excess wire in the bottom of the muscle tubes, connect it to the bottom of the plastic tube by wrapping it all together at the base. Cover with shrink tape and/or duct or electrical tape.
12. With the excess wire in the top of the muscle tubes, connect it to the top of the plastic tube by wrapping it all together at the top. Cover with shrink tape and/or duct or electrical tape.
13. Run the two colored, long, leftover wires together and connect with heat shrink tape or duct tape to form a small triangle. When they leave the muscle the wires should be side by side. See Figure 2
14. Paint the round balls to match the color of the wires. One represents a cell body in a sensory neuron and one represents the cell body in a motor neuron.
15. Connect one ball to the sensory neuron wire and connect the other ball to the motor neuron wire.
16. Cut another small piece of the sensory neuron wire. (About 4 inches in length) and connect to the motor neuron cell body from the sensory neuron. This should complete the circle. See Figure 2
17. Cover the connections on the cell body with heat shrink tape or electrical or duct tape. If using heat shrink tape, place on the wires before connecting the ends with solder.
See Figure 3
The wire for the nerves should be long enough to reach the lumbar area on you or your subject.
A crossed extensor reflex will be very similar to the stretch reflex. There would be a few different protocols. First, the sensory neuron would run a longer distance such as from the foot up to the spinal cord. (If an individual stepped on a tack). Second, the spinal cord sections will be connected by association neurons. Third, the motor neurons will come out the opposite side of the spinal cord that the sensory neurons went into. Lastly, two sets of leg muscles need to be made. One for the leg that is extending to support the body weight and one that is being flexed by a stimulus.
Consider making a muscle component going to the muscle that is relaxing during a stretch reflex. Such as the hamstrings in a knee jerk test when the quadriceps contract.
Consider making a like model of a withdrawal reflex. See any text of anatomy and physiology for a drawing.
Have students design their own models to use and study from in class.
Make models of other tough concepts for students to understand or have the students design them meeting a specific criteria.
Hang in class from a ring stand for a more permanent display.
Drewes, Charlie, Professor of Zoology and Genetics, Iowa State University, Ames, Iowa.
Tortora, Gerald J. and Grabowski, Sandra Reynolds., Principles of Anatomy and Physiology, 8th Edition, HarperCollins College Publishers, New York, 1996.
About The Authors
David Journeay is a science teacher at Lyndon Baines Johnson High School in Austin, Texas. He can be reached at 7309 Lazy Creek Drive, Austin, Texas 78724-3299, (512)926-9900 or by e-mail at firstname.lastname@example.org
Mark Little is a science teacher at Broomfield High School in Broomfield, Colorado. He can be reached at #1 Eagle Way, Broomfield, Colorado 80020, (303)466-7344 or by email at email@example.com
See hard copy for complete diagram