Session 2 [More Axon, NIA2]

Session 2 [More Axon, NIA2] - BIO335 WEEK 2 LAB...

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BIO335 WEEK 2 1 LAB INSTRUCTIONS – week 2 Part 1: Earthworm giant axon continued... In this first part of today’s lab you will continue last week’s work on the earthworm giant axon. As soon as you arrive, please obtain an earthworm and set your rig up for recording action potentials, (pages 12-13 of last week’s instructions). As you are working with a new preparation, which will likely have different threshold and conduction velocity properties, you will first need to estimate these values again. Please write them on the whiteboard table, as before. The continue your work from where you left off last week, so as to be able to complete the remaining questions on last week’s worksheet. In addition, consider these two advanced problems. (This week’s worksheet will echo these questions). 1. As you increase stimulus intensity from near threshold to well above threshold, you may see the action potential shift position along the x axis, from one stimulus event to the next. (If you do not see this occurring in your own preparation, look for another group that does). In which direction does the shift occur? Is it a gradual shift or a jump? What is the size of the shift? Can you propose a possible explanation for this observation? (If nothing comes to mind you may want to revisit this question after observing the effects of increasing stimulus intensity in your NIA axon simulations, below. Remember also that each giant “axon” in the worm is in fact chain of cylindrical cells that are connected by gap junction proteins, which form open electrical connections between the cells.) Which time position of the action potential should be used to estimate most accurately the action potential conduction velocity? Why? 2. The aquatic worm Lumbriculus variegatus (blackworm - an oligochaete annelid, like the earthworm) also has medial and lateral giant fibers. Cross sections at different positions along the worm show that they are tapered in opposite directions: the MGF decreases in diameter from head to tail, and the LGFs increase in diameter from head to tail. What action potential properties are likely to be affected by the change in fiber diameter? How? Design an experiment to test if the earthworms ( Lumbricus terrestris ) you are using also have tapered giant fibers, or if they are of uniform diameter. If you have time, carry out your experiment. If needed, you may request a new worm to do this. Part 2: Modeling nerve excitability: Neurons in Action I. BACKGROUND The earthworm giant fiber and frog sciatic nerve experiments that you will do in the first weeks reveal much about how action potentials travel along nerve fibers. But they do have constraints that limit our insight into the electrical events underlying nervous system action. Extracellular electrodes cannot record the actual electrical potentials inside cells, nor the currents that flow into and out of a cell during an action potential. We can’t measure or control ion concentrations inside the cells, and have not yet tried to change extracellular ion concentrations.
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