Figure 1.
The constant-current source (FET) draws current from the battery-powered supply. A waveform generator provides the shape of the cathodic and anodic waveform, driving a transistor output stage. Transistor switches are used to select the waveform polarity (cathodic and anodic).
Figure 2.
(A) Stimulation of the sciatic nerve induces contraction of the gastrocnemius muscle. Force is quantified by a force transducer. (B) Rectangular stimulus waveform. Generation of a rectangular stimulation waveform arises from a rectangular command signal that opens a FET to current. (C) Triangular stimulus waveform. A centered-triangular stimulation waveform can be generated from a rectangular command signal that charges a capacitor of an integrator circuit (upward ramp), followed by a symmetric, negative command that discharges the capacitor (downward ramp).
Figure 3.
Adjustable compliance stimulator.
(A) Experimental stimulator design. (B–D) Voltage/Current traces during a single trial of stimulation with a 200 µs rectangular waveform at threshold. Stimulation with the minimum calculated compliance voltage (2.8 V). (B) Voltage across the FET. (C) Voltage across the tissue and electrode load. (D) Stimulation current.
Figure 4.
Stimulus amplitude for both the rectangular and triangular waveforms was set to the threshold amplitude for maximal contractile force in the gastrocnemius muscle. (A) The threshold energy dissipated across the combined electrode-tissue load was determined for both waveforms, with pulse-widths ranging from 10 to 500 µs. (B) The mean voltage during stimulation for both waveforms. (C) The peak current during stimulation. (D) Charge injected across the load. (E) Direct comparison of charge injection and energy dissipation. Error bars represent standard error. Significance indicated with * for p<0.05, ** for p<0.01, and *** for p<0.001.
Figure 5.
Fixed versus adjustable compliance voltage.
(A) Stimulator energy consumed to maximally activate a rat sciatic nerve. Stimulator set to a compliance voltage of 5 V, 10 V, 20 V and the minimum compliance voltage. Stimulation tested across a range of pulse-widths. Incomplete datasets (compliance voltage was insufficient to activate nerve in some animals) were not included in statistical analyses. (B) For reference only, the stimulator energy consumed to activate rat sciatic nerve, with incomplete datasets included. (C) Energy consumed by the FET and by the load across a range of pulse-widths, with compliance voltage minimized. Error bars represent standard error.
Figure 6.
Normalized energy required to recruit 50% of the axons of different diameters in a simulated nerve bundle, across a range of pulse-widths (10–1000 µs; 10 µs resolution).