Nerve Conduction Speed
Lesson 16 of 16
Objective: Students determine the speed of their nerve impulse, an electrical signal conducted from the stimulus to the brain.
It is important to branch out to other topics in science. Though high schools typically present biology, chemistry and physics as separate topics, there are many ideas and concepts that link them and many careers that require knowledge of multiple sciences. Today, students do a inter-disciplinary exercise bridging biology and physics. Students work in groups to plan and conduct an experiment that measures the speed of an electrical signal in their own sensory neurons. They use video recordings in order to time the response to a stimulus and discover that the speed of a nerve pulse through their sensory neurons is 10-40 m/s.
Students read about, summarize and teach each other some basic information about neurons in order to gain a basic understanding of what they are and how they work. This is an application of CCSS RST.11-12.2: Determine the central ideas or conclusions of a text; summarize complex concepts, processes, or information presented in a text by paraphrasing them in simpler but still accurate terms and RST.11-12.4: Determine the meaning of symbols, key terms, and other domain-specific words and phrases as they are used in a specific scientific or technical context relevant to grades 11-12 texts and topics.
Then students plan and conduct an experiment to determine the speed of an electrical impulse through the sensory neurons. This requires an application of CCSS Math Practice 2: Reason abstractly and quantitatively and Math Practice 4: Model with mathematics as well as several of the NGSS science practices: Science Practice 1: Asking questions (for science) and defining problems (for engineering), Science Practice 3: Planning and carrying out investigations, Science Practice 4: Analyzing and interpreting data, Science Practice 5: Using mathematics and computational thinking and Science Practice 8: Obtaining, evaluating, and communicating information.
Since voltage is an electric potential due to electric fields, the performance standard HS-PS3-5: Develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction, is relevant. The life sciences performance standard HS-LS1-2: Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multi-cellular organisms is also applied as students measure the time it takes to respond to a stimulus.
How do we perceive the world around us? I initiate the lesson with this question and ask students to volunteer their answers. Students supply the five senses: sight, hear, touch, smell and taste. All that we know about our world starts with these basic senses. Today, I tell the class, they learn about the physics and biology of touch!
Students work cooperatively in groups of 4. Each group member has a specific responsibility they must perform for the group to be successful. Each group gets The Nerve of Some People where students first read together from Giancoli, Physics: Principles with Applications 6th Edition, the opening paragraph of chapter 18, section 10. One group member writes a summary of what the groups learns on the handout.
Then they do a 5 minute jigsaw activity where each group member reads one portion of the text, summarizes it and explains it to the to the other group members. The four topics they review in the textbook are referenced on the hand out. The topics are: What is a neuron, types of neurons, stimulus of neurons and action potential of neurons. The students take turns, with one student taking on the role of "teacher" and another the role of recorder who puts the explanation on the handout while the other two observe. Students then switch roles with each explanation until all four are done.
Once students have a basic understanding of neurons, they perform a basic experiment to measure the speed of a stimulus neuron on one of the group members.
Nerve Stimulus Experiment
The goal of the final task is for each group to calculate the conduction speed of one group member’s sensory neuron (the type of neuron that allows us to feel or touch). The students write a plan on The Nerve of Some People handout, conduct their experiment and summarize their results. They have about 20 minutes to do this. This is an open-ended task with no single correct solution, though some solutions are more accurate than others. No matter the method, I allow groups to do their own experiments and at the end of the period, we the groups various solutions.
All groups quickly understand that they have to make two measurements in order to calculate the speed of the nerve pulse. They must measure the distance the pulse travels and the time it takes to travel that distance. The distance between the stimulus and the brain is relatively easy for groups to measure. However, because the sensory nerve conduction speed is around 20 m/s and the distance it travels is not very far (less than 2 meters), our own reaction time is a factor in data collection and stop watches cannot supply accurate times. I suggest that students use their smartphone cameras to record the experiment and use the frame-rate to get a time (e.g. if it takes 4 frames to react and the frame rate is 30 fps, then the time is 4/30 seconds).
Interestingly, all groups choose the hand or fingertip as the point to supply the stimulus. This is good because the hand is the farthest point from the brain on the upper body, and the longer the distance the longer the time which helps reduce error. Also, all students who receive the stimulus know to close their eyes so that their sight doesn't interfere with the experimental results.
Some students raise their hand as a ways to indicate they receive the stimulus, as seen in Reaction Video 1. This is not a good method, as the distance the nerve impulse travels is not certain. The signal must move from the stimulus to the brain, process in the brain and then another signal is sent across the body to the opposite hand through a motor neuron. The exact distance that signal travels is not certain as there are several muscles needed to raise ones arm, hand and finger. Also the type of nerve involved in the reaction is different. A stimulus nerve transmits information around 20 m/s, but the nerves that feed to the muscles (motor neuron) is around 80 m/s. A second method, Reaction Video 2, has a student pull their hand back in response to the stimulus. However, we run into the same problem as before where the distance the nerve impulse travels is not certain.
The groups with the best and most consistent results use the eyes as their response indicator since after reaching the brain; the response impulse does not have to travel far. For example, on Reaction Video - Best Timing 1, we see a slow motion film that shows the stimulus and then 40 frames later, the reaction. The iPhone 6 has a slow-motion frame rate of 240 fps. Both the time and the distance are relatively certain and the result is a nerve conduction speed of about 10 m/s. Another excellent test zoomed in on the stimulus and reaction in Reaction Video - Best Timing 2. Again, the distance and time here have good accuracy.
With 5 minutes left in the class, I have the groups take turns to explain their experimental plan and the results of their experiment. Groups provide nerve impulse speed numbers between 2 m/s and 300 m/s. After all groups present, I tell the class which groups had the best plan and why I feel their plans were good. I also point out that their results are very close to the expected value of 20 m/s.