Biophysics Lab 2

• Signal


  Any physical change that conveys information
• Types of signals


  • Non-electric (e.g. blood pressure, body temperature)
  • Electric (e.g. ECG)
• Non-electric signals are often converted to electric signals to make them easier to process
• Electric signals alternate in time, sensitively following changes in life processes
• Oscilloscope


  An instrument used to examine rapid electrical changes, indispensable for medical diagnostics
• Arterial pressure catheter


  Used to monitor periodic changes in blood pressure with a piezoelectric pressure transducer
• Electric signals can be decomposed as the sum of sine waves (sinusoidal signals), this is the Fourier theorem
• All signals can be built from sine signals, so the main goal is to examine the sine signal
• Alternating current (AC)


  When both the magnitude and the direction of the current and the voltage change periodically, in contrast to direct current (DC)
• Period (T)


  The time required to complete one whole cycle
• Frequency (f)


  The reciprocal of the period, giving the number of cycles per unit time
• Sinusoidal alternating current


  The most general alternating current where the voltage alternates in a sinusoidal pattern
• Effective voltage (RMS voltage)

  The voltage of a DC that performs the same amount of work (produces the same amount of heat) in the same conductor with resistance R over the same amount of time as a given alternating current
• In Hungary, the frequency of mains supply is 50 Hz and its voltage is 220-230 V (RMS value), with a peak value of 311 V
• Oscilloscope


  An electronic instrument that can examine alternating current, plotting voltage change as a function of time
• Cathode-ray tube


  • Airtight glass tube where electrons leaving the incandescent cathode are formed into thin beams and accelerated by an electron gun
  • Electron beam is deflected by horizontal and vertical deflection plates based on the input signal and a sawtooth oscillation
• Resonance circuit


  An electric circuit that can be vibrated by a single external energy source and produce sinusoidal AC vibration
• Resonance circuit components


  • Resistor (R)
  • Coil (L)
  • Capacitor (C)
• Resonance circuits periodically exchange energy between the coil and capacitor, with resistance causing damping and energy loss
• Damping, dissipative effect


  Sinusoidal vibration gradually loses its energy and decays due to continuous heat loss
• Resonance circuits


  • Contain a resistor (R), a coil (L), and a capacitor (C)
• Resistance


  Ohmic resistance in Direct Current Measurements
• Coil


  Helical electrical conductor that acts as an inductive circuit element
• Capacitor


  Circuit element that can store electric charge by creating an electric field, capacitance is measured in farad (F)
• Circuit elements


  • Can be connected in series or parallel
• Producing resonance


  Capacitor is charged using a DC voltage source, then voltage source is turned off and capacitor discharges through the coil, inducing voltage in the coil which recharges the capacitor, continuing the cycle until amplitude decreases to zero due to resistance and heat loss
• Resting membrane potential


  Potential difference (voltage) that can be measured between the outer and inner surfaces of cells, ranging from -30 to -100 mV
• Stimulus from the environment


  Converted into an electrical signal through the change of the resting membrane potential, which is transmitted as an information signal
• Membrane potential changes below threshold

  • Can be modeled by a parallel RC circuit, where R is the ohmic resistance and C is the capacitance
• Cell membrane is a large-surface insulator with charges accumulating on both sides, which can be characterized by a capacity of a few nF/mm²s</b>
• Ions (charge) diffuse between the two sides of the membrane (leakage) even at resting potential, which can be described by selective conductivity</b>
• Membrane models (RC circuits)


  • One resistor and one capacitor for one type of ion, more complex RC-circuit for several types of ions
• Pacemakers


  Control the contraction of the cardiac muscle by delivering electrical signals when the pacing system of the heart does not work or does not work properly
• Pacemaker structure


  • Battery in a small metal compartment, pulse generator, and electrodes implanted into the cardiac muscle
• Pacemaker operation


  Electrodes continuously detect the electrical condition of the cardiac muscle cells and transmit information to the computer, if heart rate is inadequate the generator starts a pulse by discharging capacitors, the electric charges reach the electrodes through the wires causing the myocardium to contract
• Modern pacemakers detect physical activity and adjust the pulse frequency accordingly
• Alternating current flowing through the right arm and right leg


  If it flows from the right arm through the heart to the left arm, ventricular fibrillation (abnormal heart rhythm) can occur, which may be fatal
• 15 mA AC is quite painful, but a 100 mA electric shock is usually fatal