Chapter 4 Synaptic Integration in Bistable Motoneurons

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Several types of motoneurons exhibit bistable behavior in which brief periods of excitation and inhibition can toggle self-sustained firing on and off. In spinal motoneurons in the adult cat, bistable behavior is especially strong, but recent studies have shown that this bistable behavior varies systematically across the motoneuron pool. Fully bistable cells can maintain self-sustained firing for many seconds, whereas partially bistable cells can generate at most 1–2 seconds of self-sustained firing. It is important to realize that fully bistable motoneurons tend to have slow axonal conduction velocities and low input conductances. In contrast, partially bistable cells tend to have fast conduction velocities and high input conductances. Thus, fully bistable cells are likely to innervate fatigue-resistant muscle fibers and to be either of type slow contracting (S) or fast, fatigue resistant (FR) in the type classification scheme of Burke and colleagues. Using the single electrode voltage clamp technique, this chapter measures the total persistent inward current (I_PT) underlying bistable behavior. Although I_PT is of similar amplitude in fully and partially bistable cells, it tends to slowly decay with time in the partially bistable group while exhibiting little or no decay in fully bistable cells. The chapter briefly reviews the evidence that I_PT and the plateau potential largely originate in dendritic regions and then considers the implications of a large and persistent inward current in dendrites for synaptic integration. Finally, it presents some guidelines for detecting the signs of plateau potentials in single motor unit firing patterns in human subjects.