Exercise 3: Neurophysiology of Nerve Impulses: Activity 6: The Action Potential: Coding for Stimulus Intensity Lab Report
Pre-lab Quiz Results
You scored 100% by answering 4 out of 4 questions correctly.
1. The time after an action potential when a second action potential cannot be generated no matter how intense the
stimulus is called the
You correctly answered: b. absolute refractory period.
2. The time after an action potential when a second action potential can be generated only if the stimulus intensity is
increased is called the
You correctly answered: c. relative refractory period.
3. The term frequency refers to
You correctly answered: c. the number of action potentials per second.
4. The purpose of this activity is to explore
You correctly answered: b. the relationship between stimulus intensity and the frequency of action potentials.
Experiment Results
Predict Question:
Predict Question: What effect will the increased stimulus intensity have on the frequency of action potentials?
Your answer : b. The frequency of action potentials will remain the same.
Stop & Think Questions:
4. At the site of stimulation, the stimulus keeps the membrane of the axon at threshold for a long time, but this depolarization
does not spread to the recording electrode. After one action potential has been generated and the axon has fully recovered
from its absolute and relative refractory periods, the stimulus is still present to generate another action potential
Measure the time (in milliseconds) between action potentials. This interval should be a bit longer than the relative refractory
period (measured in Activity 5).
Click Measure to help determine the time between action potentials. A thin, vertical yellow line appears at the first action
potential. You can move the line in 10 millisecond increments by clicking the + and - buttons beside the time display, which
shows the time at the line. Subtract the time at the first action potential from the time at the second action potential to
determine the interval between them.
Enter the interval between action potentials in the field below and then click Submit Data to display your answer in the data
table.
You answered: .10 milliseconds
5. The interval between action potentials is sometimes called the interspike interval (ISI). Action potentials are sometimes
referred to as spikes because of their rapid time course.
From the ISI, you can calculate the action potential frequency. The frequency is the reciprocal of the interval and is usually
expressed in hertz (Hz), which is events (action potentials) per second. From the ISI you entered, calculate the frequency of
action potentials with a prolonged (500 msec) threshold stimulus intensity. Frequency = 1/ISI. (Convert the ISI to seconds
before calculating the frequency.)
Enter the frequency in the field below and then click Submit Data to display your answer in the data table.
You answered: .100 Hz
7. Enter the interval between action potentials (the ISI) in the field below and then click Submit Data to display your answer
in the data table.
Click Measure to help determine the time between action potentials. A thin, vertical yellow line appears at the far left side of
the oscilloscope screen. You can move the line in 10 millisecond increments by clicking the + and - buttons beside the time
display, which shows the time at the line.
You answered: .10 milliseconds
8. From the ISI you entered, calculate the frequency of action potentials with a prolonged (500 msec) 30 mV stimulus
intensity. Frequency = 1/ISI.
Enter the frequency in the field below and then click Submit Data to display your answer in the data table.
You answered: .100 Hz
11. Enter the interval between action potentials (the ISI) in the field below and then click Submit Data to display your answer in the data table.
Click Measure to help determine the time between action potentials. A thin, vertical yellow line appears at the far left side of
the oscilloscope screen. You can move the line in 10 millisecond increments by clicking the + and - buttons beside the time
display, which shows the time at the line.
You answered: .10 milliseconds
12. From the ISI you entered, calculate the frequency of action potentials with a prolonged (500 msec) 45 mV stimulus
intensity. Frequency = 1/ISI.
Enter the frequency in the field below and then click Submit Data to display your answer in the data table.
You answered: .100 Hz
Experiment Data:
Stimulus Voltage (mV) Stimulus Duration (msec) ISI (msec) Action Potential Frequency (Hz)
20 0.5 --- ---
20 500 .10 .100
30 500 .10 .100
45 500 .10 .100
Post-lab Quiz Results
You scored 100% by answering 4 out of 4 questions correctly.
1. If the interval between action potentials (the interspike interval) is 0.1 (1/10) seconds, what frequency of action potentials
would be observed?
You correctly answered: c. 10 Hz
2. With a prolonged stimulus that is just above (more depolarized than) threshold, you would expect to get additional action
potentials when the membrane has completed
You correctly answered: b. the absolute and relative refractory periods.
3. Which of the following changes occurs when you increase the stimulus intensity?
You correctly answered: c. The frequency of action potentials increases.
4. The absolute refractory period is about 3.75 msec. What intensity stimulus would produce action potentials with this
interspike interval?
You correctly answered: d. None of these stimuli would produce action potentials at this high frequency.
Review Sheet Results
1. Why are multiple action potentials generated in response to a long stimulus that is above threshold?
Your answer:
The longer the stimuli allow for time of recovery and the above threshold allows the action potential to occur after the
relative refractory period.
2. Why does the frequency of action potentials increase when the stimulus intensity increases? How well did the results
compare with your prediction?
Your answer:
As the intensity of a stimulus increases, there is more time for the sensory neuron to generate an action potential, recover,
and generate a second or more action potentials.
3. How does threshold change during the relative refractory period?
Your answer:
During the relative refractory period, the true threshold voltage stays the same, but because the neuron is hyperpolarized
during that period, only a larger depolarization can result in an action potential.
4. What is the relationship between the interspike interval and the frequency of action potentials?
Your answer:
The frequency of the action potentials is the reciprocal of the interspike interval with a conversion from milliseconds to
seconds.
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