REVIEW SHEET EXERCISE 3 Neurophysiology of Nerve Impulses Name Lab Time/Date ACTIVITY 1 The Resting Membrane Potential 1. Explain why increasing extracellular K + reduces the net diffusion of K + out of the neuron through the K + leak channels. Increasing the extracellular potassium reduces the steepness of the concentration gradient and so less potassium diffuses out of the neuron. 2. Explain why increasing extracellular K + causes the membrane potential to change to a less negative value. How well did the results compare with your prediction? Dr. Crowther says: This question is nonsensical. In fact, increasing the concentration of positive ions outside the cell membrane means that the inside has effectively become MORE negative relative to the outside, so the membrane potential has become more negative. 3. Explain why a change in extracellular Na + did not alter the membrane potential in the resting neuron. Dr. Crowther says: This is another terrible question. Just as in the previous question, increasing the concentration of positive ions outside the cell WOULD affect the membrane potential it would make it more negative. 4. Discuss the relative permeability of the membrane to Na + and K + in a resting neuron. The resting neuron is (4 5) times more permeable to potassium because of the increased number of leakage channels. 446
5. Discuss how a change in Na + or K + conductance would affect the resting membrane potential. Dr. Crowther says: The answer given by the answer key is poor, but it is hard to provide an alternative that is correct, yet simple to understand. Let s just skip this one. ACTIVITY 2 Receptor Potential 1. Sensory neurons have a resting potential based on the efflux of potassium ions (as demonstrated in Activity 1). What passive channels are likely found in the membrane of the olfactory receptor, in the membrane of the Pacinian corpuscle, and in the membrane of the free nerve ending? The efflux of potassium ions is maintained by passive potassium channels. 2. What is meant by the term graded potential? Graded potentials are brief, localized changes in the membrane potential that can be either depolarizing or hyperpolarizing. 3. Identify which of the stimulus modalities induced the largest amplitude receptor potential in the Pacinian corpuscle. How well did the results compare with your prediction? The moderate intensity pressure modality induced a receptor potential of the largest amplitude in the Pacinian corpuscle. 4. Identify which of the stimulus modalities induced the largest-amplitude receptor potential in the olfactory receptors. How well did the results compare with your prediction? The moderate intensity chemical modality induced a receptor potential of the largest amplitude in the olfactory receptor. 5. The olfactory receptor also contains a membrane protein that recognizes isoamyl acetate and, via several other molecules, transduces the odor stimulus into a receptor potential. 447
Does the Pacinian corpuscle likely have this isoamyl acetate receptor protein? Does the free nerve ending likely have this isoamyl acetate receptor protein? The pacinian corpuscle and the free nerve ending are not likely to have the isoamyl acetate receptor because they did not respond to chemical stimuli. 6. What type of sensory neuron would likely respond to a green light? Photosensory neurons would respond to green light. ACTIVITY 3 The Action Potential: Threshold 1. Define the term threshold as it applies to an action potential. Threshold is the voltage that must be reached in order to generate an action potential. 2. What change in membrane potential (depolarization or hyperpolarization) triggers an action potential? A depolarization in the membrane potential results in an action potential. The membrane potential must become less negative to generate an action potential. 3. How did the action potential at R1 (or R2) change as you increased the stimulus voltage above the threshold voltage? How well did the results compare with your prediction? The action potential didn't change as the stimulus voltage increased. This is because once threshold is met, the event is all or none, not graded. 4. An action potential is an all-or-nothing event. Explain what is meant by this phrase. This means that once threshold is met an action potential occurs. If the stimulus is too small an action potential does not occur. 5. What part of a neuron was investigated in this activity? The trigger zone was investigated. This is where the axon hillock and the initial segment come together. 448
ACTIVITY 4 The Action Potential: Importance of Voltage-Gated Na + Channels 1. What does TTX do to voltage-gated Na + channels? through the voltage-gated sodium channels. TTX blocks the diffusion of sodium 2. What does lidocaine do to voltage-gated Na + channels? How does the effect of lidocaine differ from the effect of TTX? Lidocaine blocks the diffusion of sodium through the voltage-gated sodium channels. 3. A nerve is a bundle of axons, and some nerves are less sensitive to lidocaine. If a nerve, rather than an axon, had been used in the lidocaine experiment, the responses recorded at R1 and R2 would be the sum of all the action potentials (called a compound action potential). Would the response at R2 after lidocaine application necessarily be zero? Why or why not? With a compound action potential, the results would not necessarily be zero because some axons could remain unaffected. 4. Why are fewer action potentials recorded at R2 when TTX is applied between R1 and R2? How well did the results compare with your prediction? TTX blocked the sodium channels, preventing the propagation of the action potential from R1 to R2. 5. Why are fewer action potentials recorded at R2 when lidocaine is applied between R1 and R2? How well did the results compare with your prediction? Lidocaine blocked the sodium channels, preventing the propagation of the action potential from R1 to R2. 449
6. Pain-sensitive neurons (called nociceptors) conduct action potentials from the skin or teeth to sites in the brain involved in pain perception. Where should a dentist inject the lidocaine to block pain perception? Lidocaine should be applied to the receptors to prevent the generation of an action potential that would lead to the perception of pain. ACTIVITY 5 The Action Potential: Measuring Its Absolute and Relative Refractory Periods 1. Define inactivation as it applies to a voltage-gated sodium channel. Voltage-gated sodium channels are inactivated when they no longer allow sodium to diffuse through. 2. Define the absolute refractory period. The absolute refractory period is the time in which no action potential can be generated regardless of the strength of the stimulus. 3. How did the threshold for the second action potential change as you further decreased the interval between the stimuli? How well did the results compare with your prediction? The threshold for the second action potential increased as the interval between the stimuli decreased as predicted. 4. Why is it harder to generate a second action potential during the relative refractory period? A greater stimulus is required because voltage-gated potassium channels that oppose depolarization are open during this time. ACTIVITY 6 The Action Potential: Coding for Stimulus Intensity 1. Why are multiple action potentials generated in response to a long stimulus that is above threshold? The longer stimuli allow time for recovery and the above threshold allows the action potential to occur after the relative refractory period. 450
2. Why does the frequency of action potentials increase when the stimulus intensity increases? How well did the results compare with your prediction? Action potentials can occur more frequently if there is a constant source of stimulation as long as the relative refractory period is reached. 3. How does threshold change during the relative refractory period? The threshold that must be achieved is higher than the original stimulus intensity during the relative refractory period. 4. What is the relationship between the interspike interval and the frequency of action potentials? The frequency of the action potentials is the reciprocal of the interspike interval with a conversion from milliseconds to seconds. ACTIVITY 7 The Action Potential: Conduction Velocity 1. How did the conduction velocity in the B fiber compare with that in the A fiber? How well did the results compare with your prediction? The velocity of the B fiber was slower because it had a smaller diameter and was less myelinated. 2. How did the conduction velocity in the C fiber compare with that in the B fiber? How well did the results compare with your prediction? The conduction velocity of the C fiber was slower because it has no myelination and a smaller diameter. 451
3. What is the effect of axon diameter on conduction velocity? diameter, the greater the conduction velocity. The larger the axon 4. What is the effect of the amount of myelination on conduction velocity? myelination, the greater the conduction velocity. The greater the 5. Why did the time between the stimulation and the action potential at R1 differ for each axon? The time between the stimulation and the action potential at R1 differed for each axon because the diameter and the degree of myelination varied. 6. Why did you need to change the timescale on the oscilloscope for each axon? This is necessary in order to see the action potentials. The velocity changes so when it gets very slow you need a longer time scale. ACTIVITY 8 Chemical Synaptic Transmission and Neurotransmitter Release 1. When the stimulus intensity is increased, what changes: the number of synaptic vesicles released or the amount of neurotransmitter per vesicle? The number of synaptic vesicles released increases when the stimulus intensity increases. 2. What happened to the amount of neurotransmitter release when you switched from the control extracellular fluid to the extracellular fluid with no Ca 2+? How well did the results compare with your prediction? Without calcium present, no neurotransmitter was released because the exocytosis of the synaptic vesicles is dependent upon calcium. 452
3. What happened to the amount of neurotransmitter release when you switched from the extracellular fluid with no Ca 2+ to the extracellular fluid with low Ca 2+? How well did the results compare with your prediction? When a small amount of calcium is added back, a small amount of synaptic vesicles are released. 4. How did neurotransmitter release in the Mg 2+ extracellular fluid compare to that in the control extracellular fluid? How well did the result compare with your prediction? The neurotransmitter release was less when magnesium was added. 5. How does Mg 2+ block the effect of extracellular calcium on neurotransmitter release? When magnesium is added to the extracellular fluid it blocks the calcium channels and inhibits the release of neurotransmitter. ACTIVITY 9 The Action Potential: Putting It All Together 1. Why is the resting membrane potential the same value in both the sensory neuron and the interneuron? The resting membrane potential is the same value because this is the typical resting membrane potential regardless of the type of neuron. 2. Describe what happened when you applied a very weak stimulus to the sensory receptor. How well did the results compare with your prediction? When you applied a very weak stimulus to the sensory receptor, a small, depolarizing response occurred at R1, and no responses occurred at R2, R3, and R4. 453
3. Describe what happened when you applied a moderate stimulus to the sensory receptor. How well did the results compare with your prediction? When you applied a moderate stimulus to the sensory receptor, a larger, depolarizing response occurred at R1, and an action potential was generated at R2 and at R4. 4. Identify the type of membrane potential (graded receptor potential or action potential) that occurred at R1, R2, R3, and R4 when you applied a moderate stimulus. (View the response to the stimulus.) Action potentials occurred at R2 and R4 and graded receptor potentials occurred at R1 and R3. 5. Describe what happened when you applied a strong stimulus to the sensory receptor. How well did the results compare with your prediction? When you applied a strong stimulus to the sensory receptor, a large, depolarizing response occurred at R1 and R3, and action potentials occurred at R2 and R4. 454