1 EXPERIMENT 8 Bio-Electric Measurements Objectives 1) Determine the amplitude of some electrical signals in the body. 2) Observe and measure the characteristics and amplitudes of muscle potentials due to the heart muscles (EKG). Introduction Many biological systems, ranging from the single cell to the human body, produce electrical signals that can be detected and recorded by sensitive electronic equipment. In recent years, the study of these signals has played an increasingly important role in the biological sciences, particularly in human medicine. Recently, there has been much interest in the electrical characteristics of plants. Even though research in this area is still in its infancy, there seems to be some evidence that plants change their electrical characteristics in response to changes in the environment. While a complete explanation of the origins of electric phenomena in biological systems is not possible here, we will introduce the very basic concept of electricity produced by ionic diffusion. The weak electrical signals measured in this experiment are typical of those encountered in animal and plant cells. Through this experiment you will gain the basic knowledge of bio-electric measurements and the precautions necessary for obtaining meaningful data from biological systems. When the heart is at rest, the inside of the heart muscle cells are negatively charged and the exterior of the cells are positively charged. The cells are said to be polarized. Depolarization and repolarization of the heart muscle cells causes the heart to contract and blood to be pumped throughout your system. Depolarization is accomplished when some of the positively charged ions move through the cell membrane, resulting in a lower potential difference between the exterior and interior of the heart muscle cells. Shortly after depolarization, positive ions move back to their original location and the heart cells are repolarized. Figure 1 is an electrocardiogram (EKG) two successive heartbeats. The P-wave represents the depolarization of the two atrium chambers of the heart. The Q, R and S waves represent the depolarization of the two ventricle chambers of the heart. The T wave represents the repolarization of the two ventricle chambers. The atria are repolarized at the same time as the ventricles are depolarizing and are therefore obscured by the much larger ventricle depolarization. The EKG can be measured by placing electrodes on the surface of your body. However, the resistance of dry skin is fairly high and it is necessary to reduce this resistance in order toBio-electric Measurements (Version 4.0, 6/18/2008) 2 obtain any measurements. This can be done by applying a conducting paste or gel to the skin. In this lab you will use adhesive, disposable foam, single use EKG electrodes which contain a hydrogel to reduce the resistance of your skin. Figure 1 In addition to measuring cardiac signals in this experiment, you will observe AC noise which your body picks up because it acts as an antenna and measure DC muscle potentials produced when you flex your muscle. You will use an oscilloscope and a differential amplifier to measure these signals. Safety Precautions Any time electronic equipment is connected to a human or animal subject, the matter of electrical shock must be considered. The severity of shock depends on the amount of current flowing through the body and the frequency of that current. See figure 1 and 2. The amount of current which will flow through the body is determined by Ohm's Law, I = V/R, where the voltage is fixed, and the current is determined by the body resistance. The arm-to-arm resistance with contacts on dry skin is of the order of 105 Ω. Sticking your fingers in the 120-volt wall outlet would let a current of 1-2 mA flow through your body -- definitely painful. With dry skin, the maximum voltage you should even consider touching is 30 volts.Bio-electric Measurements (Version 4.0, 6/18/2008) 3 Figure 2 Figure 3 Effects of current (60 Hz) Using the disposable electrodes can reduce your skin resistance as low asΩ×−3105. Such a reduction in body resistance significantly raises the possibility of severe injury from an electric shock. Electronic instruments used to amplify and measure voltages have no potential difference across their inputs, and therefore present no risk of shock. However, if some malfunction of the equipment were to transpire and allow a high voltage to be present at the inputs, a severeBio-electric Measurements (Version 4.0, 6/18/2008) 4 shock to the subject could result. Although the probability of such a malfunction is very small, even one incident of shock in thousands of subjects would be unfortunate. Therefore, the system you will use completely precludes the possibility of large voltages being present at the inputs of the differential amplifier. The safety device used is known as an optical coupler. Essentially what the optical coupler does is convert the output of the differential amplifier to an optical signal; this optical signal is then detected and converted back to an electrical signal which can be displayed on the oscilloscope. Therefore, no electrical path exists between the differential amplifier and the measuring device.Bio-electric Measurements (Version 4.0, 6/18/2008) 5 WORKSHEET Bio-Electric Measurements Experiment 8 Name_________________________ Lab Partner____________________ Section________________________ Date__________________________ Procedure You will not use an Excel spreadsheet to record your data. In its place, you will use this worksheet. 1. AC noise signal First, we shall view AC noise voltages, which the body picks up from the surrounding power lines and cables. This forms a large portion of the signal which you would detect if you were to connect a set of electrodes from your body to the oscilloscope. a) To observe the AC noise signal, touch the red end of the cable connected to the oscilloscope with your finger. b) Adjust the voltage sensitivity and the time base on the scope to get a reasonable view of the signal. c) Sketch the noise signal that you see on the scope: (You may want to freeze the trace by pressing the LOCK button on the scope.) d) Measure the peak-to-peak voltage and the frequency of the noise signal. Remember: the differential amplifier has a gain, so you must take that into account when