The Reflex Arc

Physiology-The Nerve Impulse Lab

Purpose: To determine the speed of a nerve impulse. Introduction: In 1850, Hermann von Helmholtz succeeded in measuring the speed of the nerve impulse and found it to be much slower than previously believed, at between 50 and 100 meters/second in humans. You will conduct an investigation in which you attempt to estimate the speed of the nerve impulse in human beings. You will measure the time between ankle stimulation and response and between the upper arm stimulation and response using multiple subjects. The ankle condition should result in a longer reaction time than the upper arm condition. The difference between these reaction times corresponds to the time it takes for the nerve impulse to travel a distance equal to the difference between the ankle and the finger, and upper arm and the finger. Notice that this difference excludes any time due to such things as decision-making processes. So the nerve conduction time can be estimated by subtracting the distance of the upper arm to the brain from the distance of the ankle to the brain (the distance from the brain to finger is a constant), and dividing by the reaction time difference. The reaction time for either stimulation of the ankle or of the upper arm is quite small, and hence a clock that measures time in hundredths of a second would be necessary. This problem can be solved by adding together the reaction times of several people; after obtaining the total time, simply divide it by the number of people to get the average individual time. This general mass reaction time technique will be used to measure the speed of the nerve impulse in this experiment.

Materials: Each team of 6 students needs a stopwatch and a ruler or meter stick.

Procedure:

1. 6 students should form a team. 1 student in the group will time all others. The remaining 5 people should form a circle with each person sitting in a chair and very loosely clasping the ankle of his or her neighbor to the right.

2. All subjects should close their eyes during the study. The measurer should tell each person to squeeze the ankle he or she is holding, when he or she feels his or her ankle squeezed.

3. The measurer starts the study by squeezing one person's ankle and simultaneously starting a stopwatch. Watching the ankle that was initially squeezed, the measurer notes the time it takes for the fifth squeeze to occur. Repeat this procedure for a total of five times, each time recording the time in the space provided in part 1 of the data sheet you were given in class. Your group should show more efficiency over time.

4. Next have each person in the circle release the ankle they are holding and grasp the upper arm, just below the shoulder of the person to their right. Run another five trials exactly as you did for the ankle trials, each time recording the time in part 2 of the data sheet. You will probably note that the total reaction time dropped within each set of five trials. Why? Hopefully, the last two to three trials yielded about the same values.

5. These ten trials serve as practice. The group of five subjects and the measurer must "learn" in some general way to do this task efficiently. Having completed practice, you are now ready to begin the measurement of the speed of the nerve impulse. Run four more trials as before, the first and fourth with ankle stimulation, and the second and third with the upper arm. This will generate two ankle and two upper arm mass reaction times (record these in part 3 of the data sheet).

6. Each reaction time represents the sum of twenty-five reaction times (five subjects, five times each). Obtain the average individual reaction time by dividing the total reaction times by twenty-five. Now average the two ankle reaction times and, separately, the two upper arm reaction times. Subtract the averaged arm time from the averaged ankle time. This is the amount of time it takes the nerve impulse to go the extra distance from the ankle to the level of the shoulder. To calculate the speed of the nerve impulse, you must estimate the magnitude of this distance. 7. Measure the distance for the third tallest person in your group of five subjects. Measure the distance from ankle to the base of the neck and from the upper arm to the base of the neck. Take the difference between the numbers, divide by the time difference, and you will have an estimate of the speed of the nerve impulse.

8. How does it compare with Helmholtz' estimate? Helmholtz estimated a speed of from 50 to 100 meters per second. Modern measurements range from 6 to 122 meters per second, depending on the type of nerve fiber.

Data: Part I -Practice (5 subjects, 5 times around) Part I -Practice (5 subjects, 5 times around) TRIAL TIME (s) TRIAL TIME (s) 1-Ankle 1-Upper Arm 2-Ankle 2-Upper Arm 3-Ankle 3-Upper Arm 4-Ankle 4-Upper Arm 5-Ankle 5-Upper Arm

Actual Test-Part II (5 subjects, 5 times around) TRIAL TIME (s) (Time in seconds/25) 1-Ankle

2-Upper Arm

3-Upper Arm

4-Ankle

Calculations:(from ACTUAL data above)

Trial 1 + Trial 4 =______(average ankle time) 2

Trial 2 + Trial 3=______(average upper arm time) 2 Average ankle time-average upper arm time=______(Difference 1)

Distance of ankle to brain (for third tallest person) = ______(Distance 1)

Distance of upper arm to brain (for third tallest person)=______(Distance 2)

Distance 1- Distance 2 =______(Difference 2)

Difference 2 = ______(speed of nerve impulse) Difference 1

Analysis Questions: 1) How does your predicted rate for a nerve impulse compare to the actual value we determined? 2) What factors influenced the outcome of our data? 3) Summarize the importance of sensory neurons, motor neurons and inter-neurons in this simulation. 4) Why is it important that messages travel to the brain at a speedy rate? 5) How does this lesson relate to the concept of homeostasis and balance? Write a Conclusion