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a. Consult Figure 2, to see how an action potential creates a moving charge distribution that looks a lot like a dipole moving along the direction of the propagating action potential. Draw a new picture of a dipole, but this time put the positive and negative charge very close to one another, like a very zoomed out perspective from your electric potential lab setup. Your body does create a dipole charge separation, except the charge separation is on a much smaller scale. The dipole moves quickly in time, but for the purposes of this question, consider a snapshot in time, and draw in the equipotential lines around this dipole. b. For this case, you can think of the depolarizing cell action potential direction as the direction down your forearm toward your finger, and the dipole is created in your forearm as an EMG V(t) signal. This means you need to put both voltage measurement leads on your forearm. State where you think is a good place to put the sensor stickers such that you can measure a large 3 Spring 2022 5C Pre-Lab 7 UCLA Department of Physics and Astronomy Input answers in supplied pre-lab Google Slides template. Sketches may be added as pictures in your submission. potential difference when the action potential dipole is moving down your forearm. Explain why you choose to arrange the sensors like that. (you can flip your palm and inspect your own forearm above your wrist, move your finger, and see where the muscle fibers are. There are nerve fibers right around the same area).
Consult Figure 3 to reason how the moving dipole creates the changing voltage signal, even when you measure with fixed voltage measurement positions. Provide some explanation, in your own words, of how you might explain this to a classmate who asks, “I understand how | can change my potential difference measurements when I move my leads around on the dipole setup from your electric potential lab, but how can I get difference potential measurements when my measurement leads are fixed and not moving? Shouldn't I just measure the potential difference of the dipole in the body?" If you are struggling with this question, just reason as best you can, or explain why you are confused, and then you can be the one to ask the clarification questions in lab! Direction of propagation + negative pole fiber positive pole 2 ECG Jual CLAN Figure 3: Schematic to show how the moving electric dipole in the body creates a changing voltage vs. time trace, depending on whether the action potential is before both voltage sensors, in between the voltage sensors, or after both voltage sensors. The ECG or EMG potential difference measurement shows a large positive potential difference when the action potential is between the sensors (assuming you have measurement leads oriented the same way as the dipole). Source: Catherine H. Crouch, Swarthmore College, 2018.
Sketches may be added as pictures in your submission. (a) resting cell (ion channels closed) ++++++ +++++++++++++++++++ ++++++ (b) depolarizing cell (ion channels open at boundary) 1++++ + + + + + T++ (c) equivalent moving dipole Figure 2: (a) shows a realistic charge setup for a resting cell, like a neuron or muscle cell where an action potential can propagate down the cell axon or muscle fiber. (b) shows a realistic cell model, with an action potential propagating down the cell and causing a charge separation wavefront that moves with the action potential. (c) shows a simplified model of how this charge separation is equivalent to a dipole moving down the cell, where the distance between the (+) and (-) in the dipole is very small. This dipole is much smaller than the one created in your previous lab, but it has the same electric potential distribution! This means that we can measure potential differences coming from this dipole potential with our body as the electrical source. Source: Catherine Crouch, Swarthmore College, 2018. RE RED VEN