Monophasic voltage stimulation

Monophasic voltage stimulation is the most basic neuro-stimulation scheme. During stimulation, the circuit outputs a constant-voltage pulse (Fig. 12a). Current is injected through the output capacitor and to the electrode to stimulate the tissue. The current level is limited by the tissue resistance. After the pulse, the circuit output returns to ground. The voltage stored across the output capacitor and electrode interface drives the reaction in reverse and helps to recover the charge. 

Contractions are usually evoked by applying 0.5-s trains of pulses of 110% maximal voltage (train frequency 0.1 Hz pulse frequency 5 Hz pulse duration 0.5 ms) (see Note 4 and Table 2 for more details). The amplitude of each monophasic contraction so induced appears to be determined more by released ATP than noradrenaline (Subheading 1.), as we have found that twitch amplitude is attenuated to a significantly greater extent by PPADS (a P2 receptor antagonist) than by prazosin (an ai-adrenoceptor antagonist) (12). Different stimulation conditions have been described by another laboratory (see Note 5). 

FIGURE 30.5 Action potential initiation by extracellular stimulation of CNS neurons by cathodic and anodic stimuli. Each trace shows transmembrane voltage as a function of time for different sections of the neuron, (a) Stimulation with a monophasic cathodic stimulus pulse from an electrode positioned f mm over a node of Ranvier of the axon. Depolarization occurs in the node directly beneath the elertrode (solid arrowhead) and hyperpolarization occurs in the adjacent nodes of Ranvier (open arrowhead). Action potential initiation occurs in the node of Ranvier directly under the electrode (arrow) and the action potential propagates in both directions, (b) During threshold stimulation with an electrode positioned 1 mm over the cell body, action potential initiation occurs at a node of Ranvier of the axon. With cathodic stimuli (duration 0.1 msec) action potential initiation occurred at the second node of Ranvier from the cell body (arrow), (c) With anodic stimuli (duration 0.1 msec) action potential initiation occurred in the third node of Ranvier from the cell body (arrow). 

The pulses may be of unidireetional eurrent (interrupted DC, monophasic), which implies that the current has a DC component. High-voltage pulsed galvanic stimulation is also used, with pulse currents up to some amperes, but pulse duration only a few microseconds. If DC effects are to be avoided (e.g., to reduce electrolytic effects or electrode metal corrosion), the current is biphasic. Faradic currents are biphasic currents of the type generated by an induction coil. If the pulses are slowly increased in amplitude, then reduced, and after a pause again increased, we have a ramp or surged current. Because many effects are current controlled, it is often better to use a constant amplitude current mode than a constant amplitude voltage mode of the stimulator output. 

To guarantee safety, a capacitor is placed at the output terminal of the electronics. In the event of electronics failure, in which the output is shorted to one of the supply voltages, the output capacitor is crucial in preventing a continuous DC charge injection. The output capacitor is also useful in implementing charge balance, as can be seen in the monophasic stimulation example (Section 4.3). 

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