Rheobase

Force development in skeletal muscle is achieved through the orderly recruitment of motor units. According to Henneman et al. s (1965) size principle, smaller motoneurons with smaller axons and slower conduction velocities have lower membrane capacitance, higher input resistance and lower rheobase, and thus are recruited first for a given synaptic input. For example, phrenic motoneurons with slower axonal conduction velocities are recruited first during inspiratory efforts, and type S and FR motor units are recruited first and more often, whereas type FInt and FF motor units are recruited later and less infrequently (Burke, 1981 Sieck and Fournier, 1989). 

Figure 5.17 Current level versus pulse duration, according to the hyperbolic rheobase model.

The most used current waveform is rectangular monophasic or biphasic. With defined electrode/tissue geometry, there is a relationship between the pulse duration and the current level necessary for nerve excitation. Figure 5.17 shows an example as the pulse duration increases, the necessary current approaches asymptotically a baseline current called the rheobase. The chronaxie is the pulse duration with the double rheobase current. 

Figure 5.17 shows a calculated current-duration graph with linear scales based on the hyperbolic model with normalized values rheobase current 1 mA and chronaxie 1 ms. 

The afferent pain nerves have a higher threshold and rheobase than sensory and motor nerves. Thus it is possible to stimulate sensory and motor nerves without ehciting pain. Very short pulses of 10—400 ps duration are used, with constant amplitude current up to about 50 mA and treatment duration of 15 min or more. The skin electrodes may be bipolar or monopolar. The position is in the pain region an electrode pair may, for example, be positioned on the skin on the back of the patient, or implanted with thin leads out through the skin. The electrode pair may also be positioned outside the pain area (e.g., at regions of high afferent nerve fiber densities in the hand). 

Figure 10.31 shows the minimum stimulus current to an efferent nerve fiber as a function of pulse duration for obtaining a certain muscle response. The coupling (synapses) between the nerve axon end plates and the muscle cells is an important part of this signal transmission line. It is not possible to lower the current under a certain minimum level, the rheobase value. The pulse length with current amplitude two x rheobase value is called the chronaxie. 

A single pulse or a repetitive square wave may give both DC and AC effects. In both cases, the duration of the pulse or square wave is an important variable (see rheobase and chronaxie). 

The other reference point is the pulse duration time, called the chronaxie, which is derived from the rheobase. By definition, the chronaxie is the threshold pulse duration at twice the rheobase voltage or current. The chronaxie can also be referred to as apparent chronaxie because the rheobase cannot be accurately determined. 

Fig. 1 Strength-duration (a) and charge-duration (b) plots for nerve stimulation [1], The current amplitude required to create a propagated action potential has been experimentally found to increase as the duration of the current pulse decreases. The actual magnitude depends on the separation between the electrode and nerve, which shifts the plot shown in a up or down. To accommodate for the shift and to make the result from one laboratory useful to another laboratory, the rheobase current is defined as the minimum current for a very long pulse,...

It has been known since 1935 that hydrostatic pressure strongly affects the physiology of whole nerve and muscle (3,4). These effects were later attributed to changes in the behavior of single nerve fibers (5,6), as characterized by a number of parameters, such as rheobase, threshold membrane potential, conduction velocity and action potential duration - quantities not easily related to the molecular events underlying excitation. 

The smallest current amplitude required to cause excitation is known as the rheobase current (/rh). T is the membrane time constant of the axon if the axon is stimulated intracellularly. For extracellular stimulation, T is a time constant which takes into account the extracellular space resistance. The relationship between current amplitude and pulse width can also be derived theoretically using the cable equation by assuming that total charge on the cable for excitation is constant [Jack et al., 1983]. 

Fig. 13 Strength-Duration and Charge-Duration Curves for Initiation of an Action Potential Rheobase current /rh is the current required when using an infinitely long pulse width. Chronaxie time is the pulse width corresponding to two times the rheobase current...

---