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Political Science
POLS 1090
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Iworx, Wayne 2009 Compound Action Potential in the Frog Sciatic Nerve Purpose: This exercise is designed to familiarize the student with the basic principles of nerve conduction. Performance Objectives: At the end of the exercise the student should be able to: 1. Determine threshold and maximal stimulus. 2. Measure the conduction velocity of the frog sciatic nerve. 3. Determine if a nerve is able to conduct electrical signals in two directions. 4. Demonstrate recruitment using the equipment provided. 5. Define resting potential, action potential, compound action potential. 6. Explain what is meant by compound action potential. 7. Draw a picture of the laboratory setup used for this exercise. 8. Find and dissect the frog sciatic nerve for placement in a nerve chamber. 9. Describe the dissection of the frog sciatic nerve 10. Explain the procedure and significance of a single and double pith of the frog. Introduction The interior of a neuron at rest is negatively charged with respect to its exterior. The Resting Membrane Potential (RMP) is a measure of this difference in charge across a cell’s membrane. In most organisms, the RMP is between -50 and -80 mV (charge inside with respect to outside) and is a consequence of negatively charged proteins in the cytoplasm and the membrane’s permeability to potassium. Stimulation, like synaptic activity coming from other nerve cells, can transiently reverse the membrane potential of a neuron (i.e. the interior of the cell goes from negative to positive and back to negative, again). This event is called an action potential (AP) and takes place in a matter of milliseconds. At the onset of an action potential, the inside of a cell becomes more positive due to an increase in sodium permeability. More specifically, when a neuron is stimulated to its threshold or beyond, voltage gated Na+ channels open, allowing Na+ to enter the cell. As Na+ enters, the interior of the cell becomes more positive (depolarized), particularly in the regions adjacent to these open channels. Since depolarization acts as a stimulus to open more v-gated Na+ channels, new action potentials are generated along the length of the axon. Na+ channels do not remain open indefinitely, however, and the cell never reaches the equilibrium potential of sodium. This is due to the fact that voltage-gated sodium channels close shortly after they open 1 Since there can be problems with the software and other issues, collect your data as you go. 1 Periodically save your data to the desktop. Iworx, Wayne 2009 and voltage-sensitive potassium channels open. With Na+ channels closed and K+ channels open, potassium ions exit the cell and cause the membrane to repolarize (hyperpolarize) towards its resting level. Voltage-gated potassium channels close and then sodium channels are ready to open once more. In this laboratory, you will record action potentials from the Sciatic Nerve of a frog. The Sciatic Nerve is composed of thousands of individual axons grouped together and enclosed in a connective sheath. The nerve contains sensory, motor and autonomic axons that individually vary in diameter, myelination, threshold and speed of conduction. Large, myelinated axons with the fastest conduction velocities are known as Type A fibers. Type B fibers are also myelinated, but have smaller diameters and slower conduction velocities. Type C fibers are very small, unmyelinated axons. When a large stimulus is delivered to the nerve, many axons in the nerve respond and the recorded potential is the summation of these axons firing simultaneously. This potential is known as the compound action potential (CAP). Equipment Required Laptop computer, iWorx/214 orr 204 and USB cable Nerve Chamber AAMI cable and nerve chamber leads (red and black) Glass hooks Stimulator cable Grounding adapter or cable Frog Ringer's solution at two temperatures: 25ml per station chilled on ice 100 ml per station at room temperature Start the Software 1. Click the Windows Start menu, move the cursor to Programs and then to the iWorx folder and select LabScribe; or click on the LabScribe icon on the Desktop 2. When the program opens, select Load Group from the Settings menu. 3. When the dialog box appears, select IP ------ and then click Load. 4. Click on the Settings menu again and select Animal Nerve, Compound Action Potential. . 5. After a short time, LabScribe will appear on the computer screen as configured by Compound Action Potential setting in Scope mode. 6. Press the mode button on the Channel 1&2 section of the iWorx box until the EMG light is illuminated. 7. The equipment is ready. Refer to the picture at the end of the lab if the cables are not connected. 1 Since there can be problems with the software and other issues, collect your data as you go. 2 Periodically save your data to the desktop. Iworx, Wayne 2009 The Dissection 1. Review the movie provided by the instructor. 2. To begin dissection, retrieve a frog from your instructor and place it in a dissecting tray. 3. Remove the skin from the legs by making an incision through the skin and around the entire lower abdomen. Cut the connections between the skin and the body-- especially around the base of the pelvic girdle. 4. Use stout forceps to pull the skin off the frog in one piece (like a pair of pants). 5. Place the frog with its dorsal side up. Moisten the exposed tissue (legs) with Ringer's solution and place a wet paper towel (saturated with Ringers solution) over one of the legs of the frog so that it is completely covered and wet. 6. Use forceps to separate the muscles of the thigh (the leg not covered with the paper towel). The muscles are surrounded by connective tissue called fascia, and the large medial and lateral muscles on the dorsal side of the upper leg are joined to each other by a fusion of their fascia along a thin "white line". Grab the muscle groups on either side of the "white line" with forceps, and firmly pull the muscle groups apart. The fascia will tear. 7. Pin the muscles apart so that more underlying muscle is visible. This should also expose the cream-colored Sciatic nerve lying deeply between the muscles. The Sciatic nerve is covered with fascia, which also includes some blood vessels. 8. Use a glass hook to separate the nerve from the fascia and the vessels. If possible, avoid cutting the blood vessels. If bleeding does occur, rinse away the blood with lots of Ringer's solution. Free the nerve from the knee joint to the pelvis. 9. Use the glass hook to place a suture thread under the nerve. Move the thread as close to the knee joint as possible. Ligate (tie off) the nerve; you may observe calf muscle fibrillation or foot movement as the knot is tied off. Be sure the knot is tied tightly. Cut the nerve between the knot and the knee joint. Keep the exposed nerve moist at all times with Ringer's solution. 10. Carefully separate the muscles of the pelvis to expose the Sciatic nerve. Remember to rinse any blood away with Ringer's solution. The Sciatic nerve enters the abdomen of the frog through an opening at the end of the urostyle, a bone that forms part of the pelvis. 11. Carefully expose the remainder of the nerve through an opening along the lateral side of the urostyle. To avoid cutting the nerve, lift the end of the urostyle with forceps as you cut the muscle away from the urostyle with blunt scissors. Cut along the urostyle from its tip to the vertebral column. 12. Deflect the muscle away from the urostyle to expose the Sciatic nerve. Use a glass hook to separate connective tissue from the nerve and to place a piece of suture thread under the nerve. Move the thread as high as possible on the nerve to obtain as large a section as possible. Ligate (tie off) the nerve; the leg may jump again as the knot is tied tightly. 1 Since there can be problems with the software and other issues, collect your data as you go. 3 Periodically save your data to the desktop. Iworx, Wayne 2009 13. Cut the nerve between the knot and the vertebral column. Keep the exposed nerve moist at all times. 14. Use forceps to grasp the suture thread at the proximal end (end closest to head) and lift the nerve out of the body cavity. Do not pinch or stretch the nerve. Remove any connective tissues, blood vessels, or nerve branches that may still keep the nerve attached to the frog. Continue to grasp the suture to lift the nerve until it is clear of the abdomen, the pelvis, and the thigh. 15. Grasp the suture at either end to remove the nerve from the body entirely. Place the nerve across the gold-colored electrode pins in the nerve bath. Add a small quantity of Cold Frog Ringers to the bottom of the chamber. The Frog Ringers should not touch the gold-plated electrode pins. 16. Cover the chamber with a glass slide. The Nerve Chamber 1. The proximal end of the nerve (end connected to the spinal column) should be over the stimulating electrodes, and the distal end (from knee region) should be over the recording electrodes. 2. The ligature (knot) on the distal end should be located between the two recording electrodes in the nerve bath. Make sure that the nerve is in contact with the stimulus and recording electrodes. 3. Place a few drops of frog ringer's solution into the chamber. The level of the frog ringers must not contact the electrodes. Cover the chamber with glass microscope slides. The picture below show the nerve chamber and the proper connectionof the leads. Exercise 1: The Compound Action Potential Goal: To apply a brief stimulus at the proximal end of the nerve and record a compound action potential from the distal end. 1Since there can be problems with the software and other issues, collect your data as you go. 4 Periodically save your data to the desktop. Iworx, Wayne 2009 Procedure 1. Select from the menu bar. . The stimulus amplitude should be 0.25 V and the pulse width should be 0.1ms. 2. Click Record to stimulate and record from the nerve. LabScribe is set to use Scope mode and to display Repetitive sweeps. This means that the nerve will be stimulated again, after the preceding sweep is completed, A new recording of the nerve response replaces the previous sweep on the Main window. Scope will continue to stimulate the nerve and display new compound action potentials until the Stop button is clicked. Click the Stop button to preserve the latest sweep displayed in the window. 3. A mark line appears on the left side of the screen to indicate the point in time when the stimulus was delivered to the nerve. There may be a stimulus artifact at the
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