Generation of Unidirectionally Propagated Action Potentials - 2

The experimental preparation is shown in the top of the Figure was designed to test the hypothesis that unidirectionally propagating action potentials were actually generated. R1 and R2 are electro-myogram and electro- neurogram recording sites respectively. S1 and S2 are hook electrodes used to apply currents to the nerve trunk for test stimuli. The tripolar nerve cuff is in the center, around the nerve trunk. Action Potentials (APs) generated under the central contact ('cathode') are arrested at the contact on the left, which carries more current than the one on the right and allowed to propagate to the right, because the contact on the right carries less current than the one on the left but just enough current to suppress the development of a virtual cathode on the left side.
Response to a test stimulus at S1 is shown on the bottom of figure. A maximum Gastrocnemius EMG response is recorded at R1 and a Compound Action Potential from the Sciatic nerve at R2.

Above a certain current amplitude, the cuff electrode causes arrest of APs towards the muscle (left) while allowing them to escape to the right.
Stimuli applied to tripolar nerve cuff: 350 µsec plateau phase, 350 µsec falling phase time constant, of increasing amplitude from 0.50 to 6.00 mA. Gastrocnemius EMG recorded at R1 and Sciatic ENG recorded at R2. As the current amplitude is increased, the maximum EMG response (a-e) diminishes (f, g) and then disappears (i,j). The sciatic ENG shows compound action potentials at all amplitudes.

If unidirectionally propagating (moving to the right) were actually generated at the cuff electrode, the nerve fibers carrying these APs would be refractory for a period of time after the APs had passed the region of the S2 electrode. The experimental data shown in the figure support the hypothesis. Stimuli from the tripolar electrode 'arrests' APs propagating towards the Gastrocnemius, while allowing them to escape antidromically. Only when the S2 stimuli were applied at delays greater than 2.4 msec was the S2 stimulus capable of exciting the nerve to cause muscle contraction. This implies that a unidirctionally propagating pulse was generated and the refractory period was greater than 2.18 msec.

van den Honert, C. and J. T. Mortimer (1979). “Generation of unidirectionally propagated action potentials in a peripheral nerve by brief stimuli.” Science 206(4424): 1311-2.