March 05, 2008

Pacemaker Electronics

In electronic systems, circuits capable of generating automatic impulses is often required. They serve important roles as clock circuits in computers, electronic clocks, timers and many others. Learning the way they act, may shed some intuition in the working of our own physiological signal generators; be they in the pacemaker node of the heart or the cells of Cajal in the intestine or the pulsatile circuit in the hypothalamus!

Tank circuit oscillator animationThe animated picture/schematic on the left portrays an oscillator circuit, called the L C tank circuit (L= inductor, C= capacitor). The coil represents the inductor, while the two parallel horizontal lines represent the capacitor. No voltage source is shown (for simplicity) as such but let us assume that the capacitor is already starting with an initial voltage.

Lets assume that the capacitor plates are fully charged from the beginning (and there's no other activity anywhere in the circuit). The charged plates of the condenser will discharge through the inductor coil. Like all other electrical conductors these coils are resistive, obeying the Ohm's Law. But in addition to the resistive load, they resist the passage of electricity in another way called 'inductance'. As electrons move through these coils, a magnetic flux (change of field) is created; this magnetic flux, in turn, create electricity that oppose the primary current (didn't we know that electricity and magnetism were related?). Once the flow of electrons have ceased, the magnetic field collapses. This collapsing (=changing) magnetic field then induces a voltage of opposite polarity in the condenser. This voltage is again discharged through the coil, which opposes it as described, produces a back EMF as per Lenz's law and charges the capacitor back. This cycle goes on repeatedly, producing a oscillating waveform.

Electronic circuits also employ RC circuits (resistor capacitor), Crystal controlled oscillators, dedicated waveform generator ICs (such as the ubiquitous 555 type, ICL 8038, XR 2206) and various others.equivalent circuit of a quartz crystal Nowadays quartz crystals are being increasingly used for reasons of accuracy, among others. They also operate in a similar way, as an equivalent circuit of a crystal is shown on the left will demonstrate. These crystals also bear an uncanny similarity with muscular elements in our bodies: resistive and elastic elements.

Our pacemakers too work in an analogous fashion. Though Mr Henry (inductors are measured in henry/ microhenry etc) is probably not there; there is depolarization (upstroke) caused by leaking calcium ions, followed by repolarization (downstroke) caused by closing of calcium currents and opening of potassium channels leading to efflux of positive charges from the cells' interior. Electronic waveform generators are integral part of artificial pacemaker devices. They may even help us understand how in physiology autorhythmicity is produced. Our knowledge about biological mechanisms are far from complete and still evolving.

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