A Biphasic Compound Action Potential

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Biphasic CAP. For the recording of a biphasic compound action potential, both the positive and negative recording electrodes were used, with the negative recording electrode at position ‘D’ and the positive recording electrode at position ‘E’ (refer to Figure 1). The extracellular bipolar recording takes the difference between what the negative electrode picked up and what the positive electrode picked up. The first, positive deflection of the CAP was caused by the extracellular negative charge of the action potential recorded by the negative electrode subtracted from the positive resting charge at the positive electrode—a positive value minus a negative value yields a positive value, hence the positive, upward deflection of the
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The compound action potential adds up all the action potentials that each individual neuron experiences in the sciatic nerve. Different stimulus amplitudes cause different neurons to fire an action potential; this is due to the fact that each neuron has a different threshold potential, or the minimum voltage the neuron needs to fire an action potential. The individual neuron action potential is an ‘all-or-nothing’ event, but the CAP, as a summation of different individual neurons, is not. The CAP amplitude will increase with larger stimulus potentials because more neurons with higher individual thresholds will be recruited. For this frog sciatic nerve, there are three fiber types, A, B, and C. A fibers are further divided, in the order of decreasing diameter, into α, β, γ, and δ fibers. There is an inverse relationship between the diameter of the nerve fiber and the threshold potential: the larger the diameter, the lower the threshold. Thus, as the largest fibers, the Aα neurons will be the first to be stimulated at a low stimulus potential, and the Aδ neuron fibers will be the last to be recruited. Because the sciatic nerve is mostly composed of A fibers, the recruitment of A-subtype nerve fibers are more readily distinguishable from the data. The minimum potential required to stimulate the Aα fibers was between 75 mV and 80 mV. Once the stimulus potential reached 90 mV, Aβ neurons were recruited and contributed to the increase in amplitude of the CAP. At a stimulus

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