Week 2

The EEG is NOT used for diagnostic purposes for which of the following medical issues?

READING 2 The encephalograph (EEG) produces very weak electrical signals (approximately 1/100 as strong as does the electrocardiograph); the signal must be amplified over 10,000,000 times in order to be recorded. The large current resulting from the amplification varies exactly as the original [4]. Brain waves are evoked primarily from the cerebral cortex and result from input from pathways projected through the thalamus. The wavelike patterns arise from synchronized cyclic activity of groups of neurons. One's EEG is as unique as one's fingerprints, but the EEG changes with the state of consciousness or emotion [5]. Often brain waves are irregular, but under some conditions distinct patterns can be recorded. Four main kinds of waves have been distinguished: the alpha, beta, delta and theta, waves [5]. These waves originate in specific regions of the brain and are evoked during certain states of mental activity. The alpha-wave rhythm (a slow-frequency, synchronized wave pattern) is evoked when a person is relaxed and resting with the eyes closed. This rhythm is most prominent in the occipital regions (the backmost region of the cerebral cortex responsible for receiving information in order to formulate an appropriate response). Beta waves are characteristic of states of heightened mental activity, such as information processing or problem solving. The beta-wave pattern is a fast-frequency rhythm most prominent in frontal and parietal regions, and is referred to as a desynchronized pattern. Delta waves are slow, large waves associated with normal sleeping [5]; they originate in the central and pariental regions [4] (the areas responsible for receiving information from sensory receptors and joints.) Theta waves occur primarily in the pariental and temporal regions in children, but are also evoked in some adults under emotional stress [5]. The EEG has proven to be valuable for diagnostic purposes. Abnormalities in the brain are recorded by the EEG because these regions produce radically different potentials than normal brain tissue. Tumor tissue is electrically inactive, but the immediately surrounding tissue may give abnormal brain wave readings [4]. In epilepsy, brain neurons discharge in an uncontrolled and excessive manner. The EEG can help locate lesions in the brain responsible for the discharge of one type of epilepsy called focal epilepsy [5]. Subdural hematoma and some types of drug poisoning may be diagnosed by electroencephalography. The EEG is also used to study patients who manifest various psychic disturbances. Their EEG shows a "brainstorm" with exaggerated peaks or spikes. The part of the brain that seems to be involved is the amygdala, an almond-shaped structure in the inner part of the brain. By passing a weak current through a wire into the amygdala, a neurosurgeon can stimulate the brian electrically to reproduce such violent anger or rage [4]. Which type of brain wave originates in the central and pariental regions of the brain and is associated with normal sleeping?

Oscilloscope channel addition problem. Read the following oscilloscope settings carefully. Suppose you display a 1 kHz, 2 V sine wave in channel A and a DC, 1V signal in channel B with sensitivities set on 1V/div. You select 'Add' so that the two signals are combined and you readjust the position so the trace is in the middle of the screen. If you switch the signal in channel B from DC to AC, what will happen? (Draw a picture if it helps.)

Which component of the EKG causes the heart to contract?

Oscilloscope time base problem. You are asked to display a 1 kHz sine wave signal on the screen. Since the period T is related to the frequency f by the formula T = 1 / f, the period of this wave is 1/(103/s)=1ms, so a reasonable setting for the "Time/div" dial is 1 ms/div. But suppose you select an inappropriate setting for the Time/div dial by mistake. What would you see if you set the dial on 1 μ s/div? 100 ms/div?

INTRODUCTION: THE EKG AND THE EEG In medical practice, two instruments that use the oscilloscope are the ELECTROCARDIOGRAM (EKG) and the ELECTROENCEPHALOGRAM (EEG). The EKG records the electrical potential differences that occur within the heart every time it beats. The EEG records the total electrical output of the brain cells. READING 1 The electrocardiogram is a record of potential differences at the surface of the body. However, the heart is not the only source of potentials at the body surface, and therefore it is necessary to distinguish between those potentials coming from the heart and those from other organs [1]. Every muscle within the body undergoes potential changes as its fibers contract. The magnitude of the action potentials for all nerves and all muscle fibers is about 120 mV. The motion of any skeletal muscle can give rise to body surface potential differences comparable to EKG potentials. To limit this source of distortion, the EKG is often recorded with the subject lying down [1]. Every time the heart beats, electrical potential changes occur within it. These potentials spread to the surface of the body. Electrodes at almost any pair of points on the surface of the body will show potential differences related in time to the heartbeat [1]. These potentials result only from electrical charges within the body [2]. Normal heart EKG's have three principle components: the P wave, the QRS complex and the T wave. Together, these components form an EKG with the repeating "wave" shown in Figure 2.9 [3]. These waves correspond to specific times within the heart's contraction cycle. The depolarization pulse (P wave), which causes the heart muscle to contract, originates in a special group of cells called the sinoatrial (SA) node [3] and results from the depolarization of the atrial muscle [2]. The pulse travels through the atrium to the atrioventricular (AV) node. The AV node acts as a time delay, but branches into fast conducting fibers (bundle branches and Purkinje fibers) [3]; this is the time interval between the P wave and the QRS complex. This provides the depolarization pulse for the ventricles, which produces the QRS complex. The final wave, the T wave, results from the return of the ventricular muscle to the polarized or resting state. At times the T wave is followed by a small U wave which seems to be produced by ventricular repolarization, like the T wave [2]. Which of the following is NOT a source of potential at the body surface?

Oscilloscope trigger problem. The 'trigger' settings determine when the trace begins. Suppose you have displayed a sine wave with V = Vosin(ωt) on the screen, where V o =5V and ω/2π=5kHz. Assume further that the signal triggers the scope when V=2.5V and ωt=30 degrees. EXAMPLE: Suppose you double the amplitude of the wave without changing any oscilloscope settings. What are V and ωt now when the scope is triggered? [Answer: the level hasn't changed so the scope still triggers when V=2.5V. V = 2.5 = 10sin (ωt); ωt=sin−1(2.5/10)=14.4775121851511 degrees.] Assume that you reduce V o back to 5V and switch the trigger slope polarity from '+' to '-' without changing the trigger level dial. What are V (express your answer with 1 decimal place) and ωt (express your answer without decimal places) when the scope is triggered now?

Oscilloscope amplitude and frequency problem. Study the above graph. The "volts/div" dial is set to 2 volts/div and the "time/div" dial is set to 5 msec/div. What is the peak-to-peak amplitude of the displayed signal (express your answer without decimal places)? What is the frequency in kHz (express your answer with 3 decimal places, write the leading zero before the decimal point)?