Electrical stimulation of tissues. The pulse and their characteristics. Biophysical bases of electrostimulation

Lecture 6

electrical stimulation of tissues

1. The pulse and their characteristics

The electrical pulse is a voltage (or current) change of short duration.

In technology two groups of pulses are distinguished. They are video pulses and radio pulses. Video pulses are electrical pulses having constant nonzero component.

Video pulse can be square pulse, saw-tooth pulse, trapezoidal pulse, exponential pulse, Gaussian pulse and so on.

Radio pulses are modulated electromagnetic oscillations.

In physiology only video pulses are used.

Let us consider an ideal electrical pulse.

We can distinguish four stages of electrical pulse (see Fig.1): the time interval 1-2 is called leading edge; 2-3 is called top; 3-4 is called descending part; 4-5 is called trailing edge.

 

The real pulse has diffusive boundaries between pulse stages. Let us draw the scheme of such pulse (see Fig. 1, b). Mark the values of voltage being equal 0.9U_max and 0.1U_max. So, time interval when the voltage value is higher than 0.1U_max represents pulse duration (t_pulse). The time interval to the left from the pulse top represents leading edge and to the right one represents trailing edge.

One of the important characteristics of electrical pulse is a rate of pulse rise.

The rate of pulse rise acan be calculated using the following formula:

a=   

Repeated pulse are called pulsing current.

The characteristics of pulsating current are:

1.  Period T (That is average duration between neighboring pulse start);

2.  Frequency f (f=1/T);

3.   Off-duty factor (or porousness) Q=,

4.     Mark-to-space ratio K (the quantity is inversely proportional to off-duty factor)                   

               K=

2.  electrostimulation

The effect of alternating current on the organism depends on its frequency. The alternating current with frequency up to 200 kiloHz (pulsating current) it causes irritant action on tissues. The irritant action of pulsating current depends on pulse shape, pulse duration and its amplitude.

The irritant action of current on a tissue is caused by an acceleration of the motion of tissue ions (d²v/dt²). The irritant action of pulsing current increases with the increase of rate of pulse rise (a). The most stimulating effect is caused by square pulses. 

The stimulating effect of square pulses depends on their duration:

	where ithr is threshold current, a, b are constants and tpulse is  pulse duration.

 

The minimal value of current which causes stimulating effect on tissue is called threshold current. According to the equation, the very short pulses do not exert an irritant action on tissue.

3. Biophysical bases of electrostimulation

The operating principle of electrostimulation is very simple and faithfully reproduction of the processes involved, for example, in muscle contraction, under the control of our brain. When we decide to contract a muscle, the brain sends the order in the form of electrical currents that travel at high speed along the nerve fibers. At the end of the run, these electrical currents excite the motor nerve, which then passes the information to the immediate surroundings of the muscle, triggering a muscle contraction.

The pulsing current passed along nerve fibers and other conducting fibers is used by the organism in many vital processes. There are muscle activity (including heart), reception (reception of sound, light and so on) and regulatory processes.    

The pulses of the different shapes are used for electrostimulation of various organs and tissues.

There are different types of electrostimulation:  an electrostimulation of central nervous system (electrosleep, electric anesthesia), an electrostimulation of neuromuscular system (electromyostimulation (EMS) for muscle strengthening of athletes and for the treatment of atrophies), an electrostimulation of cardiovascular system (cardiostimulator orpacemaker, defibrillator).

For example, square current with t_pulse=0.1-1 ms and f=5-150 Hz is used for treatment by electrosleep;

with t_pulse=0.8-3 ms and f=1-1.2 Hz is used for cardiostimulation;

saw-tooth current with t_pulse=1-1.5 ms and f=100 Hz and

exponential current with t_pulse=3-60 ms and f=8-80 Hz is used for treatment by electromyostimulation.

The most important stimulation parameters are the pulse waveform (shape), the pulse duration, the intensity of stimulation (depending on patient tolerance), the frequency of stimulation.

4. electrostimulators

The devices for electrostimulation are called electrostimulators.

They are complex devices. Briefly electrostimulator is composed of a generator of pulsing current and a pulse shaper.

There are two basic generator of pulsing current. They are multivibrator (or flop) and blocking generator.

Each generator is composed of an amplifying element (triode (vacuum lamp) or transistor), a constant-current source and a positive feedback.

The RC circuits used to be use as pulse shapers.

Pulse generators generate of square or saw-tooth pulses. We can shape pulse of necessary one by RC circuits.

                   

Important aspect of RC circuits is how they affect a voltage pulse. The voltage V_m at the input is equal to the sum of the voltage drops across R and C, i.e.

V_in = V_R + V_c = IR +Q/C.

At times less than zero V_in = 0 and V_R = V_c = 0. At the leading edge of the pulse, t = 0, the voltage V_in suddenly increases to V. At this instant the charge on the capacitor is zero so that V_c = 0. At time greater than zero, charges + Q and — Q accumulate on the two plates of the capacitor so that V_c= Q/C increases. It can be shown that during the charging of the capacitor, with constant input voltage V, the voltage V_c is

                                                V_c= V'(1- e^-t/τ),                                                     

where τ = RC is called the time constant of the circuit

 

At the trailing edge of the pulse (t =T₁) the voltage suddenly drops to zero. The positive charge, +Q, flows from the positive plate through R and the external circuit to the negative plate and neutralizes the negative charge, – Q, so that the voltage V_c decreases. This is called the discharge of the capacitor and

V_c=V'e^-t/τ.                                                         

The value of voltage across the resistor is equal to V_R = V_in— V_c.

Using differentiating circuit or integrating circuit we can change pulse shape and pulse duration.

Electrostimulators can be stationary,

                                         carried and implantable.

An example of stationary electrostumulator is a defibrillator. This is a generator of high-voltage electromagnetic oscillations for the treatment of serious heart rhythm disturbance.

An example of carried or implantable electrostumulator is a cardiostimulator for controlling heart rate at arrhythmias (a pacemaker).

Special type of electrostimulators is stimulators for transferring information that usually is perceived with sense organs. That is a cochlear prosthesis. It transforms sound to electrical signal which is transmitted to nerve fiber. It replaces cochlea of internal ear.