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EXERCISE 3: Neurophysiology of Nerve Impulses Student Name: _____SOLUTION______ Student ID#: _____________________ Student instructions: Follow the step-by-step instructions for this exercise found in your lab manual and record your answers in the spaces below. Submit this completed document by the assignment due date found in the Syllabus. Rename this document to include your first and last name prior to submitting, e.g. Exercise3_JohnSmith.doc. Please make sure that your answers are typed in RED. (You may delete these instructions before submission.) ACTIVITY 1: The Resting Membrane Potential Answers The nervous system contains two general types of cells: neuroglia cells and nerves. cell bodies. neurons. nephrons. C The resting membrane potential of the neuron in this lab under the control conditions was _______ mV. -70 Mv True or False: For most neurons, the concentration of Na+ and K+ ions inside and outside the cell are the primary factors that determine the resting membrane potential. TRUE Explain why increasing the extracellular K+ causes the membrane potential to change to a less negative value. The concentration of K+ ions is highest inside the cell. If the extracellular concentration of K+ is increased, then the net diffusion of the K+ would decrease and the resting membrane potential (RMP) would become less negative. Discuss the relative permeability of the membrane to Na+ and to K+ in a resting neuron. In a resting neuron, the permeability to K+ ions is higher than that of Na+ ions. Discuss how a change in Na+ or K+ conductance would affect the resting membrane potential. The resting membrane potential mostly is established by the relative concentrations of the Na+ and the K+ ions between the inside and outside of the cell. If conductance of these ions is changed, the RMP will change also. ACTIVITY 2: Receptor Potential The “receiving end” of a sensory neuron is called the sensory receptor. It has proteins that can generate a signal called the _________ potential when the sensory neuron is stimulated. receptor Which of the following is not a sensory modality? taste smell touch height d The maximum amplitude of response of the Pacinian corpuscle to pressure in this simulation was ____ mV. 40 mV Sensory neurons have a resting membrane 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? potassium (leak) channels What is meant by the term graded potential? With an appropriate stimulus of a sensory receptor, the amplitude of the receptor potential increases with the intensity of the stimulus. A potential change such as this, which depends on the intensity of the stimulus, is called a graded potential. Identify which of the stimulus modalities induced the largest amplitude receptor potential in the Pacinian corpuscle. pressure Identify which of the stimulus modalities induced the largest amplitude receptor potential in the olfactory receptors. chemical What type of sensory receptor would likely respond to a green light? A free nerve ending ACTIVITY 3: The Action Potential: Threshold In this lab simulation, what stimulus voltage first caused voltage to be seen at R1 and R2? _______ mV 20 mV True or False: A neuron must be polarized to the threshold voltage before an action potential is generated. False…it must be depolarized. If the resting membrane potential of a neuron is -70mV and the threshold voltage of that neuron is -50 mV, then the neuron must be depolarized by a minimum of ______ mV before an action potential is generated. 10mV 15mV 20mV 30mV c Define the term threshold as it applies to an action potential. The threshold voltage is the voltage at which you first observe an action potential. What change in membrane potential (depolarization or hyperpolarization) triggers an action potential? depolarization How did the action potential at R1 (or at R2) change as you increased the stimulus voltage above the threshold voltage? When the stimulus voltage was increased above the threshold voltage, the peak voltages at R1 and at R2 stayed at 100 µV. An action potential is an “all-or-nothing” event. What does this mean? If a neuron is depolarized to its threshold voltage (or above), then an action potential results…the all-or-none event. And, once this action potential occurs, it is conducted down the axon. ACTIVITY 4: The Action Potential: Importance of Voltage-Gated Na+ Channels According to your lab manual, “voltage-gated” channels open when the cell membrane of the neuron ___________. depolarizes When Na+ ions flow through open channels into the neuron, then the membrane potential becomes more negative less negative closer to, at, or above the threshold voltage both b. and c. d True or False: When a voltage-gated Na+ channel is closed, then the Na+ ions actually flow in the opposite direction than if the channel is open. FALSE What does TTX do to voltage-gated Na+ channels? Tetrodotoxin (TTX) blocks voltage-gated sodium channels so that they will not open. What does lidocaine do to voltage-gated Na+ channels? How does its effect differ from the effect of TTX? Lidocaine also blocks voltage-gated sodium channels so that they will not open. Why are fewer action potentials recorded at R2 when TTX is applied between R1 and R2? When the TTX was applied, the action potential (nerve impulse) could not be conducted between R1 and R2. Why are fewer action potentials recorded at R2 when lidocaine is applied between R1 and R2? When lidocaine was applied, it had the same result as TTX. 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? The lidocaine should be injected very near the sensory nerve receptors and/or the sensory nerve supplying the skin or tooth. ACTIVITY 5: The Action Potential: Measuring Absolute and Relative Refractory Periods According to your lab manual, voltage-gated Na+ channels inactivate (close) about _______ milliseconds after they open. 1-2 msec During the absolute refractory period, a neuron would need to be depolarized by _____ mV before another action potential could be generated. 50 mV 75 mV 100 mV Another action potential cannot be generated d True or False: In this lab simulation, when the interval between stimuli was 7.5 msec and the stimulus voltage was 60 mV, a second action potential was seen. TRUE Define the absolute refractory period. The absolute refractory period is the time period during which a neuron cannot fire a second AP, no matter how intense the stimulus. How did the threshold for the second action potential change as you further decreased the interval between the stimuli? As the interval between stimuli was decreased, the threshold for the second AP will increase (requiring a larger depolarization.) Why is it harder to generate a second action potential during the relative refractory period? As in #34, a larger depolarization (a higher intensity stimulus) must occur during the relative refractory period, because the neuron is hyperpolarized (more negative). ACTIVITY 6: The Action Potential: Coding for Stimulus Intensity In the previous activities, you should have noticed that the amplitude (height) of the action potential is always the same. is an all-or-none event. depends on the intensity of the stimulus. Both a. and b. are true. d True or False: It is important for the body to be able to determine the intensity of a stimulus. TRUE In this lab simulation, when the stimulus voltage was 30 Mv, the ISI was 62msec. This codes for an action potential frequency of _______ Hz. 1/62 Hz or 0.0161 Hz Why does the frequency of action potentials increase when the stimulus intensity increases? As the intensity of a stimulus increases, there is more time for the sensory neuron to generate an AP, recover, and generate a second (or third or….) AP. How does the threshold voltage change during the relative refractory period? 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 AP. ACTIVITY 7: The Action Potential: Conduction Velocity True or False: All of the axons in the human body conduct the nerve impulse (action potential) at the same velocity. FALSE Which of these three axons was able to conduct the action potential the fastest? The A fiber The B fiber The C fiber All velocities were the same a In this simulation, the conduction velocity of the A fiber was ______ m/sec; the conduction velocity of the B fiber was ______ m/sec; the conduction velocity of the C fiber was ______ m/sec. 50 m/sec; 10 m/sec; 1 m/sec What is the effect of axon diameter on the conduction velocity? As the diameter of an axon increases, the conduction velocity also increases. What is the effect of the amount of myelination on conduction velocity? As the amount of myelination of an axon increases, the conduction velocity also increases. Why did the time between stimulation and the action potential at R1 differ for each axon? The time between stimulation and the recording of an AP depends on the conduction velocity of that fiber. ACTIVITY 8: Chemical Synaptic Transmission and Neurotransmitter Release The synapse is the location where the axon of one neuron communicates with another neuron. a muscle fiber. a sensory receptor. any or all of the above. d Neurotransmitter is released into the synaptic gap by a process called secretion. excretion. exocytosis. diffusion. c The name of the specific neurotransmitter released at a synapse between a motor neuron and a muscle cell is ____________. acetylcholine (ACh) 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 as the stimulus intensity increases. What happened to the amount of neurotransmitter released when you switched from the control extracellular fluid to the extracellular fluid with no Ca2+? Because the exocytosis of the synaptic vesicles is Ca2+- dependent, when the extracellular fluid contained no Ca2+, no neurotransmitter was released. What happened to the amount of neurotransmitter released when you switched from the extracellular fluid with no Ca2+ to the extracellular fluid with low Ca2+? When Ca2+ ions were added to the extracellular fluid, a small amount of neurotransmitter was released. ACTIVITY 9: The Action Potential…Putting It All Together True or False: The amplitude of the depolarization that can result at the postsynaptic receptor is always the same. FALSE In this simulation, when the sensory neuron membrane potential at the receptor was -40mV, the interneuron membrane potential at the “receiving end” was ______ mV. -50 mV In this simulation, with a “strong stimulus”, _____ sensory neuron vesicles were released from the axon terminal. many, many 15 10 6 d…but many, many molecules of neurotransmitter were released! Describe what happened when you applied a very weak stimulus to the sensory receptor, i.e. was there an action potential? How many vesicles were released? With a very weak stimulus, no AP resulted and no neurotransmitter was released. Describe what happened when you applied a moderate stimulus to the sensory receptor, i.e. was there an action potential? How many vesicles were released? With a moderate stimulus, an AP did result and 4 vesicles were released from the axon terminal. 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. (Compare/view the response to the stimulus.) A graded stimulus was recorded at R1 and at R3; an AP was recorded at R2 and at R4. Describe what happened when you applied a strong stimulus to the sensory receptor. What type(s) of membrane potential were caused? Were the responses at R1 through R4 all the same? With a strong stimulus, a graded stimulus was recorded at R1 and at R3; an AP was recorded at R2 and at R4; 6 vesicles were released from the axon terminal.
