By Alan Macy, BIOPAC Systems, Inc.
Any physiological measurement is concerned with the subject of stimulus and response, because all physiological processes are subject to change with applied stimuli. The processes of life encapsulate the idea that environmental change precipitates sensory activity that results in stimulation. Because sensory activity is mediated by the flow of ions in the nervous system, highly controlled electrical stimulation capability can provide a unique tool for the life science researcher to discern sensory function and associated physiological responses in living subjects.
Computer-based systems can be used to define very exacting electrical stimuli and those signals can be directed to a subject via electrodes. The level of stimuli can be extremely wide and the shape of the stimuli can be finely controlled. Signals can be introduced directly into nerves or muscle fibers to evoke a specific response.
There are two types of electrical stimulators, voltage and current. A voltage stimulator behaves like a voltage source, in that it outputs a variable voltage with low output impedance. A current stimulator behaves like a current source, in that it outputs a variable current with high output impedance.
The primary physical expression that characterizes the behavior of electrical stimulators is:
Vs = Is * Zs
Vs – stimulation voltage from stimulator
Is – stimulation current from stimulator
Zs – impedance of stimulation loop
Zs is the collective impedance of the complete stimulation loop. From a simplified view, Zs consists of the series combination of two electrode / skin junction impedances and the stimulated tissue volume. Zs has resistive and capacitive components, so the capacitive components charge in accordance to the expression:
Ics = Cs * (dVcs/dt)
Ics – stimulation current through Cs
Cs – capacitive component associated with Zs
dVcs/dT – change in stimulus voltage, across Cs, as a function of time
Accordingly, electrical stimulation is associated with the charging of the stimulation loop capacitance. If fast electrical stimulation is required, then the stimulator should have high current level drive capability. For a given Cs, the speed of charging (dVcs/dt) will be higher (faster) as Ics is increased.
There are two major sub-impedances in the stimulation loop that define the portion of the stimulator output voltage (Vs) that is directly across the tissue volume (Vt):
Ze – electrode to site impedance (total of both sites)
Zt – stimulated tissue volume impedance
Vs = Is * Zs
Vs = Is * (Ze + Zt)
Vt = Vs * (Zt / (Ze + Zt))
Because of this relationship, the value of Vt might not be reliably specified, given a certain Vs.For any given tissue volume, electrical stimulation can be specified in terms of voltage across the tissue volume (Vt) or current through the tissue volume (It). In practice, because two terminal stimulation is simple to apply, current stimulation is often used because the current through the tissue volume (It) is the same as the sourced stimulation current (Is). However, if the stimulation current (It) is specified then Vt may be indeterminate. Sometimes, both (It) and (Vt) are required to be known, such as with bioimpedance measurements. In this case, voltage monitoring electrodes can be placed on the tissue volume so both (It) and (Vt) can be identified.
Compliance, an essential characteristic of electrical stimulators, is addressed in Electrical Stimulators for Physiological Research: Compliance.
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