the first stimulators

The First Stimulators

timulators for exciting nerve and mus- S cle are readily available commercially and assume a variety of configurations. Prior to the 1940s, investigators built their own electronic stimulators or used the induction coil (inductorium). The modem stimulator dates from the 1940s and it is the objective of this report to chronicle our knowledge of stimulation and the development of stimulators.

By the late 1700s, an object of scientific study was the Galvani (179 1,92) frog nerve-muscle preparation which was inac- tive and required stimulation so that its properties could be investigated. Aldini, Galvani’s nephew, championed the use of direct-current stimulation. In a remarkable demonstration in England, he twitched the muscles in the decapitated head of an exe- cuted criminal. This event linked electricity and life and it was believed that electricity could restore life, a process called reanimation.

Soon, all kinds of excitable tissues became the focus of attention and there arose the need for controllable electrical stimuli to induce activity. Only the heart and the diaphragm were accessible muscles that exhibited spontaneous activity and provided the opportunity to investigate their naturally arising bioelectric signals. How- ever there were no rapidly responding bioelectric recorders until the late 1800s; the cathode-ray oscilloscope did not appear until the late 1920s. Nonetheless, it became possible to rob the heart of its rhythmicity (by Stannius ligatures) and use rhythmic stimuli to evoke ventricular contractions, the electrical activity of which could be sampled and reconstructed with the Bemstein (1868) rheotome (current-slicer) and galvanometer. The same device was used to chart the fractional- millisecond duration nerve action potential as well as that of skeletal and cardiac muscle.

The f i s t stimulator was the capacitor. Then came the electrochemical cell connected to a switch to initiate and arrest current flow. Some remarkable switches

532

Reviewing the history of electrical stimulation and the devices

crucial to its development

L. A. Geddes Potter Engineering Center

Purdue University

were developed to control the repetition and duration of current flow; such devices were called rheotomes. However, it was the discovery of magnetic induction by Faraday in 1832 that paved the way for creation of the most controllable stimulator, the inductorium. A description of all of these stimulators follow.

Capacitor-Discharge Stimulator The f i s t electrical stimulator was the

static electricity machine, of which there were many. The most successful of these was the rotating-disk type (Fig. 1) due to Ramsden (c. 1768). In fact, it was while using one of these machines that Galvani (1791) made his discoveries that ushered in the study of bioelectricity. The combi- nation of a static-electricity machine and the capacitor (Leyden jar), which could store the charge produced by a static-electricity machine, ushered in the quantitative study of electricity as well as providing the opportunity of delivering single stimuli of controlled intensity and

1. Static electricity machine of the Ramsden type (c. 1768) used by Galvani (1791).

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duration to enable discovery of the law of stimulation.

Originally the Leyden jar consisted of a phial filled with water into which an electrode dipped (Fig. 2a). The other con- ductor consisted of the hand of the subject holding it. Discovery of the Leyden jar is accorded jointly to Van Musschenbroek and Von Kleist in 1743. According to Burnham (1963), later models (Fig. 2b) consisted of tin foil on both sides of aglass phial; this version was introduced by John Bevis in 1746.

I

2. One of the earliest Leyden jars (Kruger 1746) (a); and the traditional Leyden jar which appeared before the turn of the 18th century (b); this model was used by Wheatstone (1843).

Almost immediately, the Leyden jar was put to use as a stimulator for entertainment purposes. A favorite diversion in- volved arranging subjects in a semi-circle and having them join hands. Then a charged Leyden jar was connected to the free hand of the first and last subject. Immediately they all jumped, providing great entertainment for the spectators.

So attractive was the medical potential of the Leyden jar that, according to Heil- born (1979), the number of medical pub- lications devoted to it (40) in 1752 was almost equal to those published on all other aspects of its use (55). By 1789, the number of papers describing medical ap- plications had risen to 70, with only 30

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. The first strength-duration curves ob-

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tained by Hoorweg (1892), showing the voltage, charge (quantitat) and energy (ergs) required to stimulate excitable tissue using capacitor-discharge current. The ordinate and abscissa are in rela- tive units, the latter being expressed in capacitance units which are linearly pro- portional to the stimulus duration (Re- drawn from Hoorweg 1892).

devoted to the physical aspects of its electricity.

Empirical Strength-Duration Curve A plot of the inverse relationship be-

tween the stimulus current pulse and its duration is known as the strength-duration curve. Elucidation of this relationship was slow in coming because of difficulties in creating stimuli with known parameters, as there were no measuring instruments for millsecond pulses. When the electrochemical cell was developed by Volta (1800) and improved by his successors, it was used to charge a capacitor to the desired voltage. Then the capacitor was connected to electrodes on the excitable tissue to stimulate it. However, the gradations in intensity (voltage) could only be in terms of the number of cells joined in series. Finer control of the voltage was provided by use of the rheocord or current string, which was the original potentiometer developed by Wheatstone (1843). It was d’Arsonva1 (1881) who proposed that a capacitor could be used as a quantitative stimulator. Soon thereafter, Hoorweg (1892) conducted the first quantitative studies of stimulation, using capacitor- discharge pulses. Figure 3 is a reproduc- tion of his results in which capacitor voltage, delivered charge, and energy are plotted versus the capacitance. This is re- ally the first strength-duration curve because the abscissa is a measure of the stimulus duration (time constant). At the turn of the century, George Weiss (1901)

August/September 1994

enunciated his law of stimulation, derived from the use of capacitor-discharge current to stimulate frog, toad and turtle muscle. His paper was in French and the title translates as follows: “On the possibility of rendering comparable, between them- selves, the apparatus used to provide electrical excitation.” He reported that the charge (Q) necessary for stimulation increased linearly with stimulus-pulse duration (d). His law of stimulation is Q = a + bd, where a and b are constants. Weiss stated “The properties of a tissue, from the point of view of its electric excitability, will be known when one deter- mines the constants a and b. This determination can be obtained easily by the procedure that I employed in my research~ it suffices to perform two experiments corresponding to two different durations ... to permit calculation of a and b” (Author’s translation). He then stated that the stimulus duration d depended on the time constant (RC) of the circuit. Weiss was therefore the f is t to provide a general expression for the requirements of a stimulus.

There is a relatively unknown sidelight in the history of discovery of the current strength-duration curve attributed to Louis Lapicque in 1909. In fact, in 1905 his wife Marcelle presented her thesis to the Faculty of Sciences at the University of Paris. In her thesis, she described the law of excitation using frog, crab, and aplysia muscle, stimulated with capacitor- discharge current. The empirical expres-

k C

sion that she obtained was V = - + b R ,

where V is the voltage on the capacitor C; R is the circuit resistance, and k and b are constants. Recall that current is voltage (V) divided by resistance (R), and stimulus duration (t) is the product of capacitance (C) and resistance (R), i.e., t = R C . Therefore, her expression for current would be I = KIRC + b, where RC is the duration of the stimulus, expressed as the time constant. This expression was presented later following research with her husband, Louis, (1909) who formalized this relationship by defining the tissue-dependent excitability quantity, the chronaxie (c). Chronaxie is the duration (time constant) of the stimulus that is twice rheobasic current, the latter being defined as the long-duration current asymptote (b).

A typical capacitive stimulator of the day, developed by Lapicque, is shown in Fig. 4. The capacitor C was charged to e, a fraction of the voltage E of the battery; e depended on the position of the slider (S) on the potentiometer (P). The capacitor was then discharged by moving the switch (SW) from A to B. The duration of the discharge current was made relatively in- sensitive to the variable (and unknown) resistance of the electrode-subject circuit (Res) by the resistor R’, which was made high with respect to the assumed resistance (Res) of the electrode-subject circuit. In this way, any variation in the resistance of the electrode-subject circuit would not appreciably alter the delivered

- OUT ’

(a) LAPICQUE CHRONAXIE APPARATUS

7KR 1 OKR

(b) EQUIVALENT CIRCUIT I

4. A typical capacitive stimulator. (a) illustrates the Lapicque stimulator and B is the equivalent circuit (Courtesy Grass Instrument Co., Quincy, MA).

IEEE ENGINEERING IN MEDICINE AND BIOLOGY 533

~


I ‘{entiimrs de Srconde

. 0 s ( 0

5. The Lapicque strength-duration curve (a), showing the definition of chronaxie (c), the duration where the stimulus intensity is twice the rheobasic (long duration as- ymptotic value). In (b) are strength-duration curves for different tissues (log-log plot from Lapicque 1922).

current which resulted from e’, the voltage across the 3K ohm resistor (r). The resistance r was made low with respect to R’+ Res. Therefore, variations in Res would not appreciably alter r. The time constant of discharge was therefore equal to (Rr+r) C. Thus, a capacitor value C could be selected, charged to e volts, and discharged through Rr and r. The stimulating current was very close to e’/R’, where e’ equals erl(r + Rr) . In this way, it was possible to determine the current intensity versus stimulus-duration (time constant = RC) relationship for an excitable tissue. With this stimulator, Lapicque (1909 and 1922) enunciated the fundamental law of excitability of irritable tissues; namely, as the duration of the stimulus pulse is decreased, the current required for stimulation is increased; he also introduced the terms chronaxie and rheobase. Figure 5a shows the definition of rheobase (b) and chronaxie (c) and Figure 5b shows strength-duration curves for a variety of excitable tissues (log-log plot). The chronaxie (c) is a property of each type of excitable tissue. The rheobase (b) is the long-duration current asymptote which depends on electrode geometry. Lapicque stated his law of stimulation mathemati- cally as I = b ( 1 + c/d), but gave no reason for defining the chronaxie as the duration for twice rheobasic current. However, his choice is of more than passing interest

534

because it represents the duration for minimum energy in the stimulus pulse. This fact was pointed out much later by Geddes and Bourland (1985).

In Lapicque’s time, it was known that cell membranes exhibited capacitive properties; however, the intimate details were a subject of speculation. Nonethe- less, Lapicque (1907) derived an exponential expression for the strength-duration curve, based on a resistive-capacitive model, as was done later by Blair (1932). Lapicque (1907) first gave credit to Weiss for the charge-duration expression; then he stated: “But I have arrived at the neces- sity of replacing it by another. The electricity appears to stimulate nerve by producing a polarization; the physiologic researches on this point are varied and converge toward this concept. The inter- nal polarization of tissues, explicitly envi- sioned by du Bois Reymond a half-century ago, can be precise, as demonstrated by Ostwald by the well-known, semipermeable membrane:

“This polarization of the membrane can be treated, in the first analysis, no more, no less as a charged capacitor that has a leak.”

“According to a capacitor C of which the plates are constituted by conductors of resistance p: of which the sum of R of the external resistors. When there will be in the system, a known potential difference,


we can create a physical problem entirely defined and we can treat the problem by mathematical analysis completely.”

“If, for example, one specifies the charge that will exist in the capacitor at time t, one finds for the electric force V constituted by

e being the base of natural logarithms.” “Let us now consider the case of exci-

tation which will be attained when the hypothetical capacitor becomes charged to a certain potential, we can (by means of the equations below) determine the voltage V necessary to attain the result in the given time t; this relationship is the most direct form of the law of excitation.

“The formula, using the global constants will be:

“This logarithmic formula is very different, by itself, from that of Weiss, V = ah + b (hyperbolic). But for the order of durations most frequently employed for research on the frog nerve (3 to 30 ten thousandths of a second), one can, with one or the other law, obtain curves almost superimposable.” (Author’s translation).

It is probably because the exponential expression provided the same fit for ex- permental data as Lapicque’s hyperbolic expression, little attention was paid to the former. Although derived using single capacitor-discharge stimuli and expressing duration as the time constant, subsequent studies using constant-current rectangular pulses showed that the Lapicque hyperbolic expression was applicable.

Theoretical Strength-Duration Curve Also recognizing that the membranes

of excitable cells can be equated to a par- allel capacitance (C) and resistance (R), Blair (1935) developed the following expression for the strength-duration curve for a rectangular current pulse of intensity I and duration d:

I = h / ( l -e-d’T)

where b is the rheobasic current T and is the membrane time constant, which is specific for each type of excitable tissue. Fig- ure 6 shows log-log representations of the Blair strength-duration curves for tissues with different membrane time constants.

Although there is still no accurate expression for the strength-duration curve, both the Lapicque hyperbolic and Blair



b I=-

‘t = Membrane

d = Duration

time constant

I I 0.001 0.003 0.01 0.03 0.1 0.3 1.0 3.0 10 30 100

d = DURATION (msec)

6. The Blair strength-duration curve in which is the membrane time constant, being specific for each type of tissue.

i*\ ‘ I r \

I Propagated \Action

I \,Potential

TP - TP =Threshold potential

0

NODE RESPONSE

7. The mechanism of stimulation, showing the changes in membrane potential with cathodal and anodal stimuli. (Re- drawn from Hodgkin, A.L. 1939).

exponential expressions provide a good basis for understanding the excitability characteristics of all excitable tissues; Ay- ers et al., 1986 and Mouchawar et al., 1989 showed, as did Lapicque, that both provide a reasonably good fit for exper- mentally obtained data.

Basis for Stimulation A knowledge of the process by which

excitable tissue is stimulated had to await the availability of electrodes small enough to be placed inside a single cell without damage to its enveloping membrane. Such electrodes were made by many investigators starting around 1930. With such a microelectrode inside a cell, paired with an external electrode, the resting (trans)- membrane potential could be measured, as well as its excursion, which is the action potential. It was Hodgkin in 1939 who

examined the change in membrane potential in response to single cathodal and anodal pulses of different intensities. The remarkable discovery was that with a subthreshold stimulus, the membrane potential decreased (hypopolarization), then recovered, as shown in Fig. 7. As the strength of the cathodal stimulus was increased incrementally, the transient reduction in membrane potential became larger. Finally, when a critical intensity was reached, the membrane potential decreased rapidly and propa-

gated excitation, i.e., stimulation occurred. Interestingly, the transmem- brane potential had to be reduced by only a fraction of its value to the threshold potential (TP) for excitation to occur, as shown in Fig. 7. When the experiment was repeated with anodal (positive) pulses, the membrane hyper- polarized and no excitation occurred.

Galvanic (Direct-Current) Stimulator The first galvanic (direct-current)

stimulator consisted of joined dissimilar metals in contact with electrolytic tissue fluids which, in the hands of Galvani (1791), resulted in contraction of muscles in the frog’s leg. The first practical source of direct current was Volta’s pile (1800), which consisted of a stack of dissimilar metals interspersed with thick paper soaked in an electrolyte. This was the first battery and ushered in the scientific study of direct-current electricity.

One of the most important galvanic stimulator for physiologists was that developed by Bernard (1 858), with which he discovered the action of curare, a drug that blocks transmission of nerve impulses that cause muscle contraction. Figure 8 illustrates what he called the pincer stimulator, which consisted of a voltaic pile of zinc and copper electrodes fashioned on two wooden arms connected together by a spring-metal member at the apex. Be- tween the zinc and copper disks were ab- sorbing pads. The two points of the pincer

8. Bernard’s electric-pincer stimulator. The smaller unit on the right was not ade- quately strong to twitch a muscle directly; although it could evoke a muscular contraction by nerve stimulation. To excite the muscle by direct stimulation, Bernard constructed the larger pincer stimulator (left) which consisted of zinc (Z) and copper (C) electrodes, the electrolyte used was vinegar. The pincer stimulator, was, in reality, two voltaic piles, one on each arm of the pincer. The voltage available from the larger pincer stimulator was about 16. (From Bernard C, Lecons Sur la Physiolo- gie et La Pathologie du Systeme Nerveux. Paris, 1858. J-B. Balliere et Fils. Vol. 1, 520 PP (P. 144)).

Augusiheptember 1994 IEEE ENGINEERING IN MEDICINE AND BIOLOGY 535


P

-

9. Various types of cells and batteries. Wollaston’s version of the voltaic pile (a). Daniell (b), Grove (c), Bunsen (a), and Leclanche (e) cells. -

ipP

10. Current interrupters. (a) and (b) are hand operated (the latter including a Vol- taic pile); (c) Du Bois Reymond’s cam-type rheotorne; (d) Page’s interrupter (including a generator); (e) and (0: the electromagnetic interrupters of Neff and Wagner.

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constituted the stimulating electrodes. To activate the voltaic cells, Bernard soaked the assembly in vinegar, a weak acid.

In describing the use of his pincer stimulator, Bernard wrote:

“It was for my experiments on curare and to show that this substance had the property that I discovered of not destroy- ing the nerve, but respecting the muscular irritability. But the little pincer, very ade- quate to excite the nerve since it needs a single arc of copper and zinc, demonstrated well that the nerve had not lost its excitability; but it could not demonstrate that the muscle had retained its irritability, because it was not strong enough to excite a contraction by touching it against the muscle itself.

“It was for this reason that I made a large electric pincer, in which the branches are made of many couples which considerably augments the tension [voltage] .”

Figure 8 illustrates Bernard’s small and large pincer galvanic stimulators.

Many physicists and chemists developed electrochemical cells that were superior to Volta’s pile or his series (crown) of cups containing dissimilar metals and an electrolyte. The goal was to obtain a stable current for a prolonged period. Wollaston constructed a horizontal version of Volta’s pile, as shown in Figure 9a. Daniell (Fig 9b), Grove (Fig. Sc), Bunsen (Fig. 9d) and Leclanche (Fig. 9e) all constructed their own cells. The following describes the composition of each. Each inventor claimed superior performance (i.e., sustained current flow) for his cell.

Daniell Cell The Daniell cell consists of a copper

electrode in a saturated solution of copper sulfate. In this solution is a porous cup containing a solution of zinc sulfate into which the other electrode of amalgamated zinc is placed. The potential of the Daniell cell is 1.07 volts. If sulfuric acid is added to the zinc sulfate, the potential can reach 1.14 volts.

Grove (Bunsen) Cell The Grove cell consists of a zinc plate

(+) in contact with a sulfuric acid solution. In this solution is a porous cup containing strong nitric acid into which is dipped a carbon rod (Grove) or platinum electrode (Bunsen). The potential of these cells is about 1.9 to 2.0 volts.

Leclanche Cell The Leclanche cell is the forerunner of


SC E short circuit OC = open circuit

i = e/R

Pendulum \ R

+vv- F Excitable Tissue

‘ITd I t I

11. The pendulum rheotome. When released, the pendulum opened the first switch (SC), thereby allowing current to flow through the excitable tissue. Then the pendulum opened a second switch (OC) which arrested current flow through the tissue. The duration (t) of current flow depended on the length of the pendulum and the distance (d) between the two switches (SC and OC).

the modem “dry” cell. In its original form, a carbon rod (+) was contained in a porous cup packed with crushed carbon and man- ganesq dioxide. The electrolyte employed was sqlrqnmoniac (ammonium chloride) in contact with a zinc electrode (-). The potentih bf the Leclanche cell is 1.5 volts.

Interrupted Direct-Current Stimulators

The use of a cell or group of cells (battery) provides current flow as long as the source is connected to the electrodes on the excitable tissue. However, stimulation occurrs at the onset of the current flow. With a simple hand-operated switch, current flow was relatively long and there arose the need to produce current pulses of known and selectable durations. After du Bois Reymond (1843) discovered that a train of stimuli is needed to produce a tetanic (sustained) skeletal muscle contraction, there arose the need to produce this type of stimulus. A remarkable variety of ingenious current interrupters (rheotomes) were created to meet these needs; Fig. 10 illustrates a few of the popular types of that time.

In 1850, Helmholtz solved the problem of creating a single current pulse of controllable duration by modifying the Pouil-

I


let ballistic pendulum; the principle employed is shown in Fig. 11. The typical pendulum rheotome consisted of a hinged rod with a heavy steel weight. When raised and released, the pendulum opened a first (short-circuiting) pair of contacts (SC in Fig. 11) and near the bottom of its decent (i.e., where the speed was maximum), it opened a second pair of contacts (OC) along its path. The first pair constituted a short circuit across the stimulating electrodes, thereby preventing current flow. When these contacts opened, current flowed through the electrodes and excitable tissue. The second pair of contacts was in series with the direct-current source and when these opened, current flow ceased. The duration (t) of the current flow was selected by a micrometer which set the distance (d) between the first and second pair of contacts. Durations down to a fraction of a millisecond were easily at- tainable.

An electromagnetically driven current interrupter was used by Du Bois Reymond (1 850) to deliver a train of current pulses to his induction coil stimulator. The music metronome was also used as a rhythmic current interrupter. By the turn of the century, a wide variety of keys, switches, breakers, interrupters, undulators, trem- bleurs, rheotomes, etc., were available.

An interesting use of the galvanic stimulator and current interrupter was reported by Steiner (187 l), who pointed out that cardiac arrest was a frequent compli- cation of chloroform anesthesia. He stated that the heart could be made to beat by galvanopuncture, i.e., electrical stimulation with direct current. To demonstrate this fact, he overanesthetized horses, dogs, cats, and rabbits to produce cardiac arrest. Heart action was witnessed by observing the movement of a long needle thrust through the left thorax into the ventricles. Each time a heart beat occurred, the needle moved visibly. Car- diac action was also monitored with a stethoscope and by pal- pating a peripheral artery. When cardiac arrest occurred, Steiner applied intermittent galvanic current to the needle to evoke rhythmic contractions.

Figure 12 illustrates the author’s concept of the tech- nique. Steiner recommended applying the positive pole to the needle in the heart, the negative pole in the pit of the stomach or on the left side of the chest in the 7th intercostal space. Although his recommended polarity was

12. Steiner’s method of pacing the chloroform-arrested heart by galvanopuncture.

incorrect, it is quite clear Steiner had paced chloroform-arrested animal hearts. This was the first study in which inten- tional cardiac pacing was accomplished. Sadly, in his paper, Steiner stated that the method was applied to a human subject without success.

Weiss (1901), a contemporary of Lapicque, also devised a stimulator that provided single rectangular current pulses. His stimulator was truly ingenious and although he never published a picture of it, the details provided allow the composition of a sketch that shows the essen- tial features. Figure 13 (author’s sketch) shows that the Weiss stimulator operated on the principle of cutting wires with a

a

i = e/R

‘Td I t I

I

13. The principle employed in the Weiss ballistic rheotome. Current flow is initiated by firing a projectile, which cuts the short circuit (SC) wire. Current flow ceases when the projectile cuts the open-circuit (OC) wire. The duration (t) of current flow is dependent on the distance, d, and the velocity of the projectile, which is measured with a ballistic pendulum.

IEEE ENGINEERING IN MEDICINE AND BIOLOGY 537


14. Michael Faraday and the coil that he used to discover electromagnetic induction.

15. A Du Bois-Reymond induction coil. The large coil on the right could be moved over the smaller (primary) coil by turning the knob (K) to control the output. A separate two-solenoid (s) current interrupter (I) was included. (Courtesy Bakken Li- brary of Electricity in Life, Minneapolis, MN).

projectile fired from a carbon-dioxide powered carbine. When fired, the projectile cut the first wire (SC) which removed a short circuit from the two stimulating electrodes, thereby allowing current to flow through the tissue. When the projectile cut the second wire (OC), the current flow was interrupted. By knowing the velocity of the projectile and the distance (d) between the two wires, Weiss could cal- culate the duration (t) of the current pulse. In those days, the velocity of a projectile was measured accurately by allowing it to strike and become embedded in a wooden block suspended from the end of a string, thereby creating a ballistic pendulum. By

538

measuring the maximum displacement of the pendulum, and knowing the mass of the bullet and pendulum, it is easy to cal- culate the velocity of the projectile at im- pact. Weiss claimed that a stimulus pulse as short as 77 microseconds was easily obtained with his rheotome. Use of rheotomes to produce current pulses persisted in one form or another until the late 1920s. It must be recognized that Gasser and Erlanger (1927), in their Nobel-prize win- ning studies on the velocity of the nerve impulse using the cathode-ray tube, employed a mechanical rheotome to deliver the stimulus and initiate the oscilloscope time base.


Inductorium Primitive as it was by modem stand-

ards, the induction-coil stimulator (inductorium) was very popular because it could provide single, as well as repetitive stimuli of easily controlled intensity (but not duration). A remarkable variety of discoveries resulted from its use (see Geddes 1989). For example, inhibition by nerve stimulation was demonstrated by cardiac slowing and arrest in response to stimulation of the vagus nerves. The caliber of small blood vessels was controlled by nervous action, as shown by applying induction-coil shocks to sympathetic nerves. The secretory function of glands was demonstrated by induction-coil shocks delivered to nerve. The motor area in the brain was also demonstrated with the inductorium. In addition to being able to twitch and tetanize skeletal muslce, the inductorium permitted identifying motor points, i.e., sites on the skin surface where a low stimulus intensity produced muscle contractions. One important application was the production of inspiration by stimulation of the motor point for the phrenic nerve in the neck.

The inductorium (induction coil stimulator) became possible because of Fara- day’s discovery of the law of magnetic induction in 1831 (Fig. 14). He showed that a changing current in one coil (the primary), induced a voltage in a second coil (the secondary), which lay in the magnetic field produced by current in the primary. Du Bois Reymond’s contribution (1850) was the addition of an electromagnetically driven current interrupter to the primary coil. In this way, a train of make and break stimuli could be generated.

Figure 15 shows the du Bois-Reymond inductorium with the electromagnetically driven current interrupter (S). The output was derived from the (secondary) coil, K, which could be slid over the primary coil, P, to control the stimulus intensity.

There were two methods of operating the inductorium to deliver short-duration stimuli. With the first (Fig. 16), a battery (E) was placed in series with a switch (ON-OFF) and the primary coil of the inductorium. With the second method (Fig. 17), an electromagnetic current interrupter (Ai) was placed in series with the battery (E) and primary coil, as shown.

In the first mode of operation (Fig. 16), when the OFF-ON switch is closed, direct current is applied to the primary and a magnetic field of increasing intensity is produced immediately. This changing magnetic field, due to the rise in the primary current, induces a voltage (eo) in the secondary. When the primary current



PRIMARY SECONDARY

iron A wires

L eo

Iff voltage

I I M B

16. The make (M) and break (B) shocks provided by the inductorium. C is the time that the current I flows in the primary coil. (Redrawn from Geddes et al. Med. Instr 1989, By permission.)

PRIMARY SECONDARY

7- E I --output voltage

17. The inductorium with a magnetically-driven current interrupter (Ai) in series with the primary coil. This arrangement provided delivery of a train of make (m)- break (b) stimuli, e,. Such a train was called a faradic stimulus. (Redrawn from Ged- des, L.A. The Physiologist 1984,27(1):Supp. Sl-S47). (From Geddes et al. Med. Instr. 1989, by permission.)

reaches its steady value, the magnetic field is no longer changing and no voltage is induced in the secondary. When the primary current is interrupted (B for break the current flow), the magnetic field col- lapses instantly and a very high voltage of the opposite polarity is induced in the secondary. Thus a small voltage pulse appears when the current is made to flow in the primary (make shock); a much larger voltage pulse is delivered when the primary circuit is broken (break shock). The time ( C ) between the make (M) and break (B) shocks is determined by the time of making and breaking current flow in the primary coil.

When it was desired to deliver only the strong break shock:, a short circuit was first placed across the :secondary coil and then the current was applied to the primary coil. The short circuit was then removed from the secondary and only the break shock was delivered when current flow in the primary circuit was interrupted. The Har- vard inductorium (Fig. 18), incorporated this feature from the outset; L is the short- circuiting switch. Another important feature of the Harvard inductorium made use of the magnetic field produced by the primary coil to operate the current interrupter. Another valuable asset was the ability to place the secondary coil (when

fully withdrawn) at any desired angle with respect to the primary coil. It thus provided a fine control of stimulus voltage to zero, the condition when the secondary coil is at right angles to the primary coil.

The second method of using the inductorium consisted of placing an automatic current interrupter (Ai) in series with the primary coil. Figure 17 illustrates this arrangement with the current interrupter contact on the armature (Ai) opposite the iron-wire core of the primary coil. When a battery, E, (direct current) is applied to the primary, current flows through the contact on the armature and establishes a magnetic field in the core. This also produces the make shock across the secondary. However, the magnetic field attracts the armature (Ai) and the primary current is interrupted, producing the break shock across the terminals of the secondary. A spring returned the interrupter to the make position. When the induction coil is operated in this way, the vibrating current interrupter causes a train of make (M) and stronger break (B) shocks of opposite polarity to appear across the terminals of the secondary. The frequency of the train of pulses depends inversely on the mass of the armature (Ai) and directly on the stiff- ness of the leaf spring (L in Fig. 17) that restores its position when current flow ceases. The frequency of the current interrupter was typically 30- 150/sec.

It is of more than passing interest to use an oscilloscope to illustrate the output current waveforms from the du Bois-Rey- mond and Harvard inductoriums. With nothing connected across the terminals of the secondary windings, the open-circuit voltage waveforms for both are highly underdamped sine waves. However, when stimulating electrodes are connected, the waveform is quite different because the damping is increased. The output voltage waveforms of the du Bois-Reymond (Fig. 14) and the Harvard (Fig. 17) inductoriums were recorded with a 1000-ohm resistor across the output terminals. This value of resistance is a good approximation to the impedance of typical stimulating electrodes applied to excitable tissue. Figure 19 shows the make (M) and break (B) shocks with the du Bois-Reymond inductorium. Figure 19 presents the same information for the Harvard inductorium. It is customary to define the duration of such waveforms in terms of their time constants, being defined as the time for the current (or voltage) to fall from 100 percent to 37 percent amplitude. For the du Bois-Reymond coil, this time is 0.8 msec (Fig. 19a) and for the Harvard coil, it is 0.9 msec (Fig. 19b).

Augustheptember 1994 IEEE ENGINEERING IN MEDICINE AND BIOLOGY 539


18. Harvard Inductorium. The larger coil on the right could be advanced over the smaller (primary) coil to control the output. The current in the primary coil activated the current interrupter (I). The lever (L) was lowered to short-circuit the output during the make shock and raised to deliver only the break shock. In the lower illustrations is shown the secondary rotated at an angle to the primary to reduce the output to a minimum. (Courtesy Bakken Library of Electricity in Life, Minneapolis, MN).

Many different types of induction stimulators developed as alternating current generators started to appear in the mid 1800s. Figure 20 illustrates two of these devices. From about 1850, the inductorium was the stimulator of choice in research and teaching physiology laboratories. It entered clinical medicine in the late 1800s and its use in both fields continued until just after the end of World War 2 (1945).

Galvanic-Faradic Stimuli to Diagnose Nerve Injury

Because of the difference in chronaxie of motor nerve (0.1 msec) and skeletal muscle (3 msec), it became possible to assess motor-nerve injury and reinnervation using galvanic and induction-coil (faradic) shocks. From the strength-duration curve, it is clear that as the stimulus duration is reduced, the threshold current required for stimulation rises at longer durations for a tissue with larger chronaxie than for a tissue with a smaller chronaxie.

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19. Make (M) and break (B) shocks from the du Bois-Reymond (a) and Harvard (b) inductoriums. (From Geddes et al. Med. Lnstr. 1990, by permission.)

20. Various induction generators that were used as stimulators; Pixii’s (a), Clark’s (b) and Stohroer’s (c).

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consuming and was replaced by elec- tromyography in the early 1940s.

Modern Stimulators Vacuum-tube stimulators started to

make their appearance just before the start of World War 2 in Europe (1939). Figure 21 shows the Rahm, thyratron (capacitor- discharge stimulator) used by Penfield to map the human brain cortex in the early 1940s. The circular meter in the center of the front panel indicated each stimulus, but did not display its intensity, the pointer movements indicating that the stimulator was functioning. The frequency range was 1-100/sec, and a narrow range of capacitor-discharge pulse durations was available. Figure 22 illustrates the first research stimulator, built by Albert Grass in the early 1940s. This stimulator permitted the independent selection of pulse duration and frequency, the pulses being rectangular. It also provided direct (galvanic) current and synchronizing pulses for an oscilloscope. With it, high-quality

21. One of the early electronic stimulators: the Rahm, capacitor-discharge device. This stimulator was used by Penfield in the early 1940s to map the human cortex (Photo courtesy C. Hodge, Montreal Neurological Institute, McGill University, Montreal, Canada.)

22. The first research stimulator, built by Albert Grass (mid 194Os), which provided independent control of stimulus frequency and pulse duration. Rectangular pulses and galvanic (direct) current were provided along with a synchronizing pulse for an oscilloscope (Courtesy Grass Instruments, Quincy MA).

Therefore, an innervated skeletal muscle can be stimulated easily with a short-duration pulse. However, if the muscle is denervated, the chronaxie is now that of muscle which requires a long-duration stimulus. Therefore, if an induction coil shock fails to stimulate a paralyzed muscle, but a long-duration (galvanic) stimulus produces a twitch, it means that there

paralysis is nerve damage. As reinnervation occurs, the muscle begins to respond to the short-duration induction-coil shocks, indicating progressive reinnervation. Later, Ritchie (1944) plotted strength-duration curves for denervated muscle and as reinnervation was occur- ring, dramatically showing the decrease in chronaxie as innervation was restored.

strength-duration curves could be obtained and, with an oscilloscope, nerve conduction times easily measured.

L. A. Geddes is the Showalter Distin- guished Professor Emeritus of Bioengi- neering at Purdue University. Professor Geddes may be reached at the William A. Hillenbrand Center for Biomedical Engineering, Purdue University, A.A. Potter Engineering Center, West La- fayette, IN 47907-1293.

is viable muscle present and the reason for Elegant as this method was, it was time 188.

References 1. Ayers GM, Aronson SW and Geddes LA: Comparison of the ability of the Lapicque and exponential strength-duration curves to fit experimentally obtained data. Australasian Journ. Phys- ics & Eng. in Med., 1986,9(3): 1 1 1-1 16. 2. Bernard C: &cons sur la Physiologie du Sys- feme Nervewr. Paris 1858, Balliere (2 Vols.). 3. Bernstein J: Ueber den zeitlichen Verlauf der negativen Schwankung des Nervenstroms. ArchJd. ges. Physiol. 1868, 1:173-207. 4. Blair AA: On the intensity-time relations for stimulation by electric current. Journ. Gen. Physiol. 1932,15:731-755. (seealsopp 177.189). 5. Burnham, J: Capacitor. Encyclopedia Brifan- nica. 1963, 32312. Chicago, William Benton. 6. d’Arsonval A: Nouvelle methode &excitation electrique des nerfs et des muscles. Compfes Ren- dus Acad. Sci. 1881,92(1):1520-1522. 7. Du Bois-Reymond E R Untersuchungen Ue- ber Thierische Electricitat. Berlin 1848. G. Re- imer. (2 vols.). 8. Du Bois-Reymond, E.R. Sur la loi qui preside a I’irritation electrique des nerfs et sur la modifi- cation du courant musculaire par I’effet de la contraction. Ann. Chim. Phys. 1850, 3S, 30178-

August/September 1994 IEEE ENGINEERING IN MEDICINE AND BIOLOGY 541


9. Faraday M: Effects on the production of electricity from magnetism (1 83 1). In: Michael Fara- day by Williams, L.P. New York 1965 (Basic Books) Chapman and Hall, 531 pp. 10. Franklin B: Experiments and Observations on Electricity Made at Philadelphia by Benjamin Franklin, L.L.D. andF.R.S. Letter to John Pringle, Dec. 21,1757. pp. 359-361. London 1769. Printed for David Henry and sold by Francis Newberry. 496 pp. 11. Galvani A: De Viribus Electricitatis in Motu Musculari. Bonoiae 1791, Typographia Instuti Scientarium 58 pp + 4pl. 12. Galvani A: DeViribus Electricitatis in Mus- culari cum Joannis Aldini. Mutinae 1792. So&- tatem Typographicam. 80 pp. 3pl. 13. Gasser HS and Erlanger J: A study of the action currents of nerve with the cathode ray os- cillograph. Amer. Journ. Physiol. 1922, 62:496- 524. 14. Gasser HS and Erlanger J: The role played by the sizes of the constituent fibers of a nerve trunk in determining the form of the action potential. Amer. Journ. Physiol. 1927,80:522-547. 15. Geddes LA and Bourland JD: The strength- duration curve. IEEE Trans. on Biomed. Eng., 1985, BME32(6):458-459. 16. Geddes LA, Foster KS, Senior JE and Kuf- feld A: The inductorium: the stimulator associated

with discovery. Med. Instr. Technol. 1989, 23:308-3 13. 17. Heilborn JL: Electricity in the 17th and 18th Centuries. Berkeley 1979, University of Califor- nia Press. 606 pp. 18. Helmholtz H: Note sur la vitesse de propaga- tion de l’agent nerveux dans les nerfs rachidiens. Comptes Rendus Acad. Sci. 1850,30:204-206. 19. Hodgkin AL: The subthreshold potentials in crustacean nerve fibre. Journ. Physiol. 1939, 126:87-121. 20. Hoorweg JL: Ueber die elecktrische Nerven- erregung. Arch. t d . ges. Physiol., 1892, 52237- 109. 2 1. Kruger J: Beschichte der Erde. Lubetvatbis- chen Buchhanblung. 1746. (NP). 22. Lapicque Mme. M: Recherches sur L’Exci- tabilite (These). Lille 1905, Laroche-Delattore

23. Lapicque L: Premiere approximation d’une loi nouvelle de I’excitation electrique basee sur un conception physique du phenomene. Comptes Rendus Soc. Biol. 1907, 1(62):615-619. 24. Lapicque L: Definition experimentale de I’excitation. Compres Rendus Acad. Sci. (Paris)

25. Lapicque L: L’Excitabilite en Function du Temps. Paris 1926. Presses Universitaires de France. 371 pp. (also Libraire Gilbert 1926).

1 18pp.

1909,67(2):280-283.

26. Mouchawar G, Geddes LA, Bourland JD, and Pearce JA: Ability of the Lapicque and Blair strength-duration curves to fit experimentally obtained data from the dog heart. IEEE Trans. on Biomedical Engineering, 36(9):971-974, 1989. 27. Mottelay PF: Bibliographical History of Electricity and Magnetism. New York, 1975. Chas. Griffin Co. 673 pp. 28. Ramsden (see Mottelay 1975) 29. Ritchie A: The electrical diagnosis of peripheral nerve injury. Brain 1944; 67:314-330. 30. Steiner F Ueber die Electropunctur des Her- zens als Wiederbelebungsmittel in der Chloro- formsyncope. Archiv. f. klin. Chir. 187 1,

3 1. Volta A: On the electricity excited by the mere contact of conductors of different kinds. Phil. Trans. Royal Soc. (London) 1800,403-43 1 pp. 32. Volta A: Account of some discoveries made by Mr. Galvani of Bologna Phil. Trans. Roy. Soc. London 1793,83:285-291. 33. Weiss G: Sur la possibilite de rendre compa- rables entre eux les appareils servant a l’excitation. Arch. Ital. de Biol. 1901, 35:413-446. 34. Wheatstone C: An account of several new instruments and processes for determining the constants of a voltaic circuit. Bakerian Lecture. Phil. Trans. Royal Soc. (London) 1843, 133:303- 375.

12:748-780.

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the first stimulators

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