Bioelectric Potentials and Electrodes Questions & Answers

Q1. What do you understand by bioelectric potentials? List the different types of biopotentials.

Bioelectric potentials are ionic voltages produced when special types of cells perform electrochemical activities. These bioelectric potentials can be measured and the measurement results provide meaningful information to physician to diagnose and treat the patient. The devices that convert ionic potentials into electronic potentials are called electrodes.

The most common form of potentials generated in the different parts of the body are measured for diagnosis purposes with the help of electrodes which are inserted into body parts such as muscle nerve or some part of the brain. The biopotentials generated by the muscles of the heart give electrocardiogram (ECG). The biopotentials generated by the neuronal activity of the brain give us the electroencephalogram (EEG). The biopotentials obtained from the retina of the eye is referred to as the electroretinogram (ERG). Electrooculogram (EOG) is a measure of the variations in the corneal-retinal potential. Electrogastrogram (EGG) is associated with the peristatic movement of the gastro intestinal tract.

Q2. Name 3 types of biopotential electrodes/Bioelectrodes

• Microelectrodes –these electrodes are designed to measure bioelectric potentials near or within a single cell.
• Needle electrodes –these electrodes are designed to penetrate the skin so that it can record EEG potentials from a particular region of the brain or EMG potentials from a specific group of muscles.
• Skin surface electrodes – these Bioelectrodes are designed to measure ECG, EEG and EMG potentials from the surface of the skin hence this type of electrodes is least traumatic.

Q3. How does Electrocardiogram (ECG) help physicians in the diagnosis of the heart?

The biopotentials generated by the muscles of the heart with time is referred to as the electrocardiogram. The action potentials starts from a point called the pacemaker or Sino-atrial (SA) node which is located near the top of the right atrium. The action waveform propagates in all directions along the surface of both atria. The waveform reaches junction of the atria and the ventricles. This waveform now terminates and stops for some time at a point on this junction which is called atrio-ventricular (AV) node. This delay in the propagation of excitation at AV node provides time to the ventricles so that they can be filled with blood from the atria. Once the period of delay is over, the excitation restarts and it spreads to all parts of the ventricles with the assistance of the bundle of HIS. The fibres in HIS bundle (called purkinje fibres) branch out into two parts to initiate action potentials simultaneously in the myocardium of the ventricles and action potentials ultimately terminate at the end tip of the heart, thereby completing the depolarization of all cells up to the end tip. After a period of 0.2 to 0.4 sec from depolarization, a wave of repolarization starts and each cell of the heart returns to its resting potential independently.

If this potential is recorded from the surface of the heart, then the curve obtained is called Electrocardiogram (ECG). A typical ECG is shown below:

The alphabetic designations in the ECG curve helps in explaining the events related to the action potential propagation. The horizontal portion to the right of the point P is considered the baseline or equal potential line. The wave P represents depolarization of the atria myocardium. The depolarization of the ventricles and the repolarization of the atria takes place simultaneously which is indicated by QRS part of the curve. The wave T represents the repolarization of the ventricles. The after potential in the ventricles is given by U wave. The P-Q part of the curve shows the period of the delay imposed on the excitation wave by the AV node.

The ECG helps in the diagnosis of malfunctioning of the heart. Longer cycle time or slow propagation of waveform is called bradycardia while shorter cycle time or fast propagation of waveform is called tachycardia. The cycle must be evenly spaced otherwise it indicates the patient has arrhythmia. If duration P and R is greater than 0.2 second, then it suggests the blockage of propagation at AV node. If any feature of the curve is missing, it indicates a heart block. Electrocardiography is the instrument used to record ECG. The cardiac disorders specially those involving the heart valves cannot be diagnosed by the ECG.

Q4. Define EEC

The recorded representation of bioelectric potentials generated by the activity of the brain (neuronal activity) is called the electroencephalogram (EEG).

Q5. What are the sources of bioelectric potentials in the body and why are they present?

Physiological systems of the body generate their own monitoring signals while performing life sustenance functions. These signals carry useful information about the state of functioning of the physiological systems. These monitoring signals are known as ionic voltages or bioelectric potentials.

Bioelectric potentials are generated due to:

• Nerve conduction
• Brain activity
• Heartbeat
• Muscle activity

The special cells in nerve, brain, and muscles are encased in a semipermeable membrane. This membrane has a special property that permits certain substances to pass through it while other substances are not permitted to pass through it. In normal conditions, the sodium ions (Na⁺) cannot enter the cell through semi-permeable membrane and they remain outside of the cell. However ions like potassium (K⁺) and chloride (Cl^–) can pass easily through this membrane. The distribution of positively ions (Na⁺) outside and negatively charged chloride ions (Cl^–) inside the cells results into the presence of negative potential within a cell (-90 mV). The cell in such a condition is said to be polarised. However, when the membrane is stimulated, all positive sodium ions (Na⁺) rush into the cell while negative ions (Cl^–) are pumped out of the cell. This process is called depolarization and now the cell has positive charge (+20 mV). However, the cell regains its normal state in a short time by the process known as repolarization.

The cell attains its negative potential (-90 mV) after, repolarization. The process for the change of potential in the cell is shown in the figure above.

The discharging and recharging of the cell produces the voltage waveform shown below:

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