Measuring the Speed of the Nerve Impulse

For this assignment, you are asked to conduct an investigation which you attempt to estimate the speed of the nerve impulse in human beings. To conduct your experiment, follow the instructions below. Record your data on the data sheet you were given in class. Follow the instructions below and on the data sheet to calculate your results. Then, take a few minutes to brainstorm with your team about the answers to the last few questions on your data sheet. Turn in one data sheet per team of students before you leave class. Be sure all of your teammates' names are on the sheet, so everyone gets credit!

One of the great stumbling blocks to advances in biological psychology was the belief that thought, and hence nervous impulses, occurred instantaneously, or nearly so. In fact, the German physiologist Johannes Mueller once estimated that the speed of the nerve impulse was eleven million miles per second! Naturally, this claim that nerve impulses travel at immeasurable speed discouraged scientific research on the physiology of the nervous system, and encouraged mystical or dualistic interpretations of the mind. It also discouraged study of the speed of various mental activities, research that is today an important cornerstone of the fields of cognitive and perception psychology.

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 fifty and one hundred meters per second in humans. This finding was followed by intensive investigation of the nervous system within the framework of the physical and biological sciences. It also opened the door to the use of reaction time as a tool in the study of thought and perception processes (as we saw on the first day of class!).

This experiment is an attempt to familiarize you with the general logic used by a psychologist who is interested in measuring the speed of a psychological even that cannot be directly observed. To accomplish this, you must first understand the experiment that Helmholtz performed and also how his experimental technique can be applied to the measurement of the speed of the nerve impulse in humans.

Helmholtz' technique was quite simple. He first cut out a muscle and an attached nerve fiber from a frog's leg. The experiment then consisted of stimulating the nerve at various distances from the muscle and measuring the length of time between nerve stimulation and muscle contraction. First, he electrically stimulated the nerve close to the point at which it attaches to the muscle, then he stimulated the nerve farther from this point of attachment. He found that the second reaction time (that is, the time between stimulation and contraction) was longer than the first. To obtain an estimate of the nerve impulse speed, he used a simple bit of reasoning: The difference in time between the two measurements must correspond to the time it takes the nerve impulse to travel the distance between the two points of stimulation (see Fig. 1). Hence, the distance between the point of stimulation divided by the time difference between the conditions of stimulating close to the muscle versus stimulating farther away should yield an estimate of nerve impulse speed. This is how he obtained his estimate of fifty to one hundred meters per second.

Let A and B be two points of stimulation, M the point at which the nerve connects to the muscle, TA the time to contraction from stimulation at A, and TB the time to contract from stimulation at B. Then:

(Figure 1): nerve

                    A                                             B                         M
_____________________/____________________/___________________

                    ((A to M)-(B to M))/(TA-TB)= speed of the nerve

Happily, Helmholtz' estimate can be demonstrated in humans without having to resort to dissection. You might suppose that the simplest way to do this would be to perform the following sort of experiment: Stimulate someone on the ankle (by pinching them, for example) and have them respond by pushing a button as soon as they feel the stimulation. With a good timer, you could then measure the time between stimulation and depression of the button. To estimate nerve conduction time, you would then measure the total distance between ankle and brain, and between brain and finger and divide this number by the subject's reaction time. But there are complications which make this procedure unsuitable. Part of the reaction time, for example, would be due to the time it took the subject to decide to press the button, a figure which is obviously more than the speed of the nerve impluse time. From the point of view of processes in the nervous system, the reaction time includes the time to cross synapses as well as axon conduction time. In short, total reaction time is a confounded measure.

Thus, the experiment must be made more complicated. Using Helmholtz' logic, one could measure not only the time between ankle stimulation and response, but also between, say, upper arm stimulation and response. 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 ankle and the finger, and upper arm and 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.

In practice, 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 probably 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. Perform the experiment in the following way:

Methods:

Materials:

Each team of 6 students needs a stopwatch or watch with a second hand indicator, and a a ruler or tape measure

Procedure:

Decide who among you will be the measurer. This person will time all of the others. the remaining 5 people should form a circle with each person very loosely clasping the ankle of his or her neighbor to the right.

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

The measurer starts the study by squeezing one person's ankle and simultaneously noting the time on a second hand of a watch or starting a stopwatch. Now watch the ankle that was initially squeezed. When the measurer sees it squeezed for the fifth time (excluding the initial squeeze) note the time that elapsed. 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 (you can also download the data sheet here).

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.

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).

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.

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.

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.

Reference:

Jonides, J., & Rozin, P., (1983). Study Guide for Gleitman's Basic Psychology. New York, NY: W. W. Norton & Company.