Analytical investigation of the properties and behavior of the SFAP in the simplified model.

a: Normalized sensitivity function from a bipolar electrode in a realistic nerve geometry (blue trace) and a Gaussian approximation of a bipolar sensitivity function at the same location (black trace). b, c: Action potential shapes from an unmyelinated (left) and myelinated (right) fiber model (blue traces) and Gaussian approximations of the action potential shapes (black traces). d-i: Results of the analytical investigation. d, e: The SFAP amplitude increases with diameter for unmyelinated fibers (left) but plateaus for the myelinated fibers (right). Light blue bars indicate realistic fiber diameter range. f, g: Distinct trimodal waveforms are apparent for sufficiently separated contacts in the bipolar recordings of an unmyelinated fiber (f; diameter: 1 μ m, spacing: 1 cm) and a myelinated fiber (g; diameter: 10 μ m, spacing: 1.5 cm). h,i: With decreasing contact separation, the two waveforms merge and a single tetramodal one results (h: unmyelinated fiber, 0.1 cm spacing; i: myelinated fiber, 0.5 cm spacing). As expected, it resembles the derivative of the original waveform (dashed).