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EXERCISE 3: Neurophysiology of Nerve Impulses Student Name: 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.) Grading: True/False, Multiple-Choice, and Fill-in-the-blank type questions will be worth 1 point each whereas Short-answer type questions will be worth 2 points each. This lab will be worth a total of 91 points but will be converted to a percentage grade when registered in your “Gradebook”. 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. neurons. Pg. 33 PhysioEx 9.0 The resting membrane potential of the neuron in this lab under the control conditions was _______ mV. -70 Pg. 35 PhysioEx 9.0 Chart 1 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 Pg. 34 PhysioEx 9.0 Explain why increasing the extracellular K+ causes the membrane potential to change to a less negative value. If the membrane is permeable to a particular ion, that ion will diffuse down its concentration gradient from a region of higher concentration to a region of lower concentration. Potassium ions diffuse out across the membrane, leaving behind a net negative charge—large anions that cannot cross the membrane. Pg. 34 PhysioEx 9.0 Discuss the relative permeability of the membrane to Na+ and to K+ in a resting neuron. Membrane permeability to Na+ is very low because there are only a few sodium leakage channels. Nevertheless, sodium ions do slowly diffuse inward, down their concentration gradient. Left unchecked, such inward leakage of Na+ would eventually destroy the resting membrane potential. The small inward sodium leak and outward leak of potassium leak are offset by the sodium potassium pumps. These pumps help maintain the resting membrane potential by pumping out Na+ as fast as it leaks in. At the same time, the sodium-potassium pumps bring in K+ Pg. 260 Discuss how a change in Na+ or K+ conductance would affect the resting membrane potential. The resting membrane potential is a potential difference between the inside of the cell and the outside of the cell across the membrane. It is a steady-state condition that depends on the resting permeability of the membrane to ions and on the intracellular and extracellular concentration of those ions to which the membrane is permeable. Pg 34 PhysioEx 9.0 and Pg. 259 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 Pg. 37 PhysioEx 9.0 Which of the following is not a sensory modality? taste smell touch height d. height Pg. 254 The maximum amplitude of response of the Pacinian corpuscle to pressure in this simulation was ____ mV. 40 Pg. 38 PhysioEx 9.0 Chart 2 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 Pg. 259 also PhysioEx 9.0 chart 2 What is meant by the term graded potential? The term graded potentials are brief occurrences of depolarizations or hyper- polarizations of an area of a membrane Pg. 260 and Pg. 37 PhysioEx 9.0 Identify which of the stimulus modalities induced the largest amplitude receptor potential in the Pacinian corpuscle. High intensity produced the largest amplitude of response in the Pacinian corpuscle. Pg 38 PhysioEx Chart 2 Identify which of the stimulus modalities induced the largest amplitude receptor potential in the olfactory receptors. Modality was chemical and intensity was High. This produced the largest amplitude of 25 mv receptor potential in the olfactory receptors. Pg. 38 PhysioEx 9.0 Chart 2 What type of sensory receptor would likely respond to a green light? Photoreceptors Pg. 327 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 Pg. 40 PhysioEx 9.0 Chart 3 True or False: A neuron must be polarized to the threshold voltage before an action potential is generated. True Pg. 39 PhysioEx 9.0 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. 20mV Define the term threshold as it applies to an action potential. The threshold applies to an action potential if a stimulus causes the membrane to depolarize to a critical level, this is called threshold, and then an action potential arises. An action potential has two main phases: a depolarizing phase and a repolarizing phase. The membrane voltage must be reached to trigger an action potential. Pg. 260 and Pg. 39 PhysioEx 9.0 What change in membrane potential (depolarization or hyperpolarization) triggers an action potential? Action potentials are triggered when stimulus is strong enough to cause depolarization to the threshold and an action potential occurs. Pg. 261 How did the action potential at R1 (or at R2) change as you increased the stimulus voltage above the threshold voltage? The action potential did not change as I increased the stimulus voltage above the threshold voltage. The stimulus voltage was increased and the R1 and R2 stayed at 100. An action potential is an “all-or-nothing” event. What does this mean? An action potential is an “all-or-nothing” event because it is either going to be strong enough to cause stimulation or not, It either happens or it doesn’t. Pg. 261 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 Pg. 40 PhysioEx 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. both b. and c. Pg. 40 PhysioEx 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 Pg. 261 What does TTX do to voltage-gated Na+ channels? Voltage-gated Na+ channels get blocked with chemicals such as TTX and lidocaine. Pg. 40 PhysioEx 9.0 What does lidocaine do to voltage-gated Na+ channels? How does its effect differ from the effect of TTX? The Lidocaine will bind to the voltage-gated Na+ channels and prevent the flow of Na+. The effects differ from the effect of TTX because TTX irreversibly blocks voltage-gated sodium channels; this is why lidocaine is typically used to block pain in dentistry and minor surgery. . Pg. 40-41 PhysioEx 9. Why are fewer action potentials recorded at R2 when TTX is applied between R1 and R2? Because the TTX blocks the propagations of the action potential from R1 to R2. Why are fewer action potentials recorded at R2 when lidocaine is applied between R1 and R2? Because the Lidocaine blocked the propagation of the action potential from R1 to R2. 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? A dentist should inject lidocaine into a bundle of axons where the pain is or where the work is going to be done. Pg. 42 PhysioEx 9.0 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 Pg. 42 PhysioEx 9.0 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. Another action potential cannot be generated Pg. 42 PhysioEx 9.0 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. It is the period immediately following the firing of a nerve fiber when it cannot be stimulated no matter how great a stimulus is applied—called also absolute refractory phase. Merriam-Webster How did the threshold for the second action potential change as you further decreased the interval between the stimuli? The threshold for the second action potential change increased when I decreased the interval. Pg. 43 PhysioEx Chart 5 Why is it harder to generate a second action potential during the relative refractory period? It is harder to generate a second action potential during the relative refractory period because during the refractory period, another action potential cannot be generated. The all-or-none principle would need to kick in. Pg. 267 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. Both a. and b. are true. Pg. 44 PhysioEx 9.0 True or False: It is important for the body to be able to determine the intensity of a stimulus. True Pg. 44 PhysioEx 9.0 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? The frequency of action potentials increases when the stimulus intensity increases because the frequency of action potential increases. The size of the action potential does not become larger. Pg. 44 PhysioEx Chart 6 How does the threshold voltage change during the relative refractory period? By increasing in the relative refractory period. When a second action potential can be generated if the stimulus intensity is increased. Pg 44 PhysioEx 9.0 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. The A fiber 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, 10 and 1 What is the effect of axon diameter on the conduction velocity? Propagatioin/conduction occurs because there are voltage-gated sodium and potassium channels located along the axon The action potential easily brings the next region of the axon to threshold. The conduction velocity Pg. 46 PhysioEx 9.0 What is the effect of the amount of myelination on conduction velocity? Myelination refers to a special wrapping of the membrane from glial cells around the axon. The more myelination the better on the conduction velocity. Pg. 46 PhysioEx 9.0 Why did the time between stimulation and the action potential at R1 differ for each axon? Because of the amount of myelination. Pg. 46 PhysioEx Chart 7 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. any or all of the above. Pg. 47 PhysioEx 9.0 Neurotransmitter is released into the synaptic gap by a process called secretion. excretion. exocytosis. diffusion. c. exocytosis. Pg. 47 PhysioEx 9.0 The name of the specific neurotransmitter released at a synapse between a motor neuron and a muscle cell is ____________. Acetylcholine (ACh) Pg.265 When the stimulus intensity is increased, what changes: the number of synaptic vesicles released or the amount of neurotransmitter per vesicle? When the stimulus intensity was increased the number of synaptic vesicles released directly affected the amount of calcium entering the axon. The synaptic vesicles caused more synaptic vesicles to undergo Exocytosis. Pg. 47 PhysioEx 9.0 What happened to the amount of neurotransmitter released when you switched from the control extracellular fluid to the extracellular fluid with no Ca2+? Exocytosis of the synaptic vesicles is calcium dependent so The neurotransmitter did not even move, it didn’t release PhysioEx 9.0 Pg.47-48 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 I switched from the extracellular fluid with no Ca2+ to the extracellular fluid with low Ca2+ ions only a small amount of neurotransmitter were released. Pg. 47 PhysioEx 9.0 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 Pg. 50 PhysioEx 9.0 Chart 9 In this simulation, with a “strong stimulus”, _____ sensory neuron vesicles were released from the axon terminal. many, many 15 10 6 d. 6 Pg. 50 PhysioEx 9.0 Chart 9 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? When I applied a weak stimulus to the sensory receptor the stimulus does not depolarize the axon of the sensory neuron to threshold. There was no action potential and 0 sensory neuron vesicles were released from the axon terminal. Pg. 50 PhysioEx 9.0 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? When I applied a moderate stimulus to the sensory receptor there was an action potential, 4 sensory neuron vesicles were released from the axon terminal. Pg. 50 PhysioEx 9.0 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.) The stimulus induces a graded action potential at R1. 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? When I applied a strong stimulus to the sensory receptor it caused the threshold to be reached and an action potential was generated and propagated. The responses at R1 through R4 were not all the same.
