STIMULATION OF EXCITABLE TISSUES - Design and Development of Medical Electronic Instrumentation - page 310

Figure 7.4 It is possible to see from this stylized strength-duration curve that stimulus current and duration can be mutually traded off over
a certain range. The strength-duration curve was characterized by Lapicque by the value of the rheobase (in volts or milliamperes) and the
chronaxie, which is measured along the time axis and defined as the stimulus duration (in milliseconds) that yields excitation of the tissue
when stimulated at twice the rheobase strength. In this example, rheobase 3.5 mA and chronaxie 0.22 ms.

rheobase is the minimum stimulus strength that will produce a response, which is the point
at which the curve asymptotes, about 3.5 mA. To determine the chronaxie, simply look for
the stimulus duration that yields a response when the stimulus strength is set to exactly
twice rheobase, or 7 mA. In this example, the chronaxie is 0.22 ms.
The strength-duration curve is highly dependent on the type of tissue being stimulated.
For example, the chronaxie of human motor nerve is approximately 0.01 ms, about 0.25 ms
for pain receptors, and approximately 2 ms for mammalian cardiac muscle. That is why
there is rarely a need for pulses longer than 2 ms in nerve stimulation, whereas a pulse
width as long as 10 ms is often necessary for direct stimulation of certain smooth muscles.
The empirical equations for the threshold current, charge, and energy for a rectangular
stimulation pulse are

chron
t

axie

I threshold rheobase 1

chron
t

axie

Q threshold rheobase t 1