Fig. 1.

Measurement of interpotential interval (IPI). A: normal motor unit (MU). The motoneuron (MN) innervates two fibers (a, b) at possibly different endplate locations (solid rectangles). Potentials are recorded from each fiber at possibly different electrode locations (inverted triangles), giving MU action potential (MUAP) spikes a and b. The IPI was measured between the spikes. l_a and l_b, Distances from the fiber endplates. B: doubly innervated fiber. Fiber b is innervated by MN₁ at endplate b₁ and by MN₂ at endplate b₂. MUAP₁ and MUAP₂ each contain spikes from singly innervated fibers (a₁, a₂) and from the doubly innervated fiber (b₁, b₂). The b spikes may be blocked from reaching the electrode if they collide with an action potential from the other endplate. The IPIs are measured separately for each MUAP and are normalized with respect to the latency of the b spike from the MUAP onset (T_b). The top trace shows the unfiltered MUAP₁ with the initial part expanded vertically (dotted line) to better see the onset.