The Action Potential Coding for Stimulus Intensity

2/9/2021 PhysioEx Exercise 3 Activity 6 PhysioEx Lab Report Exercise 3: Neurophysiology of Nerve Impulses Activity 6: The Action Potential: Coding for Stimulus Intensity Name: Date: 9 February 2021 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: 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: relative refractory period. 3 The term frequency refers to You correctly answered: the number of action potentials per second. 4 The purpose of this activity is to explore You correctly answered: the relationship between stimulus intensity and the frequency of action potentials. Experiment Results Predict Question 1 Predict Question: What effect will the increased stimulus intensity have on the frequency of action potentials? Your answer: The frequency of action potentials will increase. Stop & Think Questions 1 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. i 1/5 2/9/2021 PhysioEx Exercise 3 Activity 6 Measure the time (in milliseconds) between action potentials. This interval should be a bit longer than the relative refractory period (measured in Activity 5). Select 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 selecting 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. You answered: 100 milliseconds. 2 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. You answered: 0.01 Hz. 3 Enter the interval between action potentials (the ISI). You answered: 60 milliseconds. 4 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. You answered: 0.0167 Hz. 5 Enter the interval between action potentials (the ISI). You answered: 30 milliseconds. 6 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. You answered: 0.03 Hz. Experiment Data i 2/5 2/9/2021 PhysioEx Exercise 3 Activity 6 Stimulus Voltage (mV) Stimulus Duration (msec) ISI (msec) Action Potential Frequency (Hz) 20 0.5 --- --- 20 500 100 0.01 30 500 60 0.0167 45 500 30 0.03 i 3/5 2/9/2021 PhysioEx Exercise 3 Activity 6 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: 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: the absolute and relative refractory periods. 3 Which of the following changes occurs when you increase the stimulus intensity? You correctly answered: 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: 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: A long stimulus that is above threshold activates the action potential 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: The frequency of action potentials increases because the increasing stimulus intensity can trigger more action potentials within the relative refractory period. 3 How does threshold change during the relative refractory period? Your answer: Threshold increases. i 4/5 2/9/2021 PhysioEx Exercise 3 Activity 6 4 What is the relationship between the interspike interval and the frequency of action potentials? Your answer: they are reciprocals. i 5/5