An Electroencephalogram Records Electrical Activity of the Brains Surface

The influence of the ascending reticular activating system on the brain's activity can be monitored via electroen-cephalography. The electroencephalograph is a sensitive recording device for picking up the electrical activity of the brain's surface through electrodes placed on designated sites on the scalp. This noninvasive tool measures simultaneously, via multiple leads, the electrical activity of the major areas of the cerebral cortex. It is also the best diagnostic tool available for detecting abnormalities in electrical activity, such as in epilepsy, and for diagnosing sleep disorders.

The detected electrical activity reflects the extracellular recording of the myriad postsynaptic potentials in cortical neurons underlying the electrode. The summated electrical potentials recorded from moment to moment in each lead are influenced greatly by the input of sensory information from the thalamus via specific and nonspecific projections to the cortical cells, as well as inputs that course laterally from other regions of the cortex.

EEG Waves. The waves recorded on an electroencephalogram (EEG) are described in terms of frequency, which usually ranges from less than 1 to about 30 Hz, and amplitude or height of the wave, which usually ranges from 20 to 100 |xV. Since the waves are a summation of activity in a complex network of neuronal processes, they are highly variable. However, during various states of consciousness, EEG waves have certain characteristic patterns. At the highest state of alertness, when sensory input is greatest, the waves are of high frequency and low amplitude, as many

Delta

Seizure spike

Seizure spike

Spike-and-wave

Spike-and-wave

Abnormal Eeg Waves

Patterns of brain waves recorded on an EEG. Wave patterns are designated alpha, beta, theta, or delta waves, based on frequency and relative amplitude. In epilepsy, abnormal spikes and large summated waves appear as many neurons are activated simultaneously.

Patterns of brain waves recorded on an EEG. Wave patterns are designated alpha, beta, theta, or delta waves, based on frequency and relative amplitude. In epilepsy, abnormal spikes and large summated waves appear as many neurons are activated simultaneously.

units discharge asynchronously. At the opposite end of the alertness scale, when sensory input is at its lowest, in deep sleep, a synchronized EEG has the characteristics of low frequency and high amplitude. An absence of EEG activity is the legal criterion for death in the United States.

EEG wave patterns are classified according to their frequency (Fig. 7.5). Alpha waves, a rhythm ranging from 8 to 13 Hz, are observed when the person is awake but relaxed with the eyes closed. When the eyes are open, the added visual input to the cortex imparts a faster rhythm to the EEG, ranging from 13 to 30 Hz and designated beta waves. The slowest waves recorded occur during sleep: theta waves at 4 to 7 Hz and delta waves at 0.5 to 4 Hz, in deepest sleep.

Abnormal wave patterns are seen in epilepsy, a neurological disorder of the brain characterized by spontaneous discharges of electrical activity, resulting in abnormalities ranging from momentary lapses of attention, to seizures of varying severity, to loss of consciousness if both brain hemispheres participate in the electrical abnormality. The characteristic waveform signifying seizure activity is the appearance of spikes or sharp peaks, as abnormally large numbers of units fire simultaneously. Examples of spike activity occurring singly and in a spike-and-wave pattern are shown in Figure 7.5.

Sleep and the EEG. Sleep is regulated by the reticular formation. The ascending reticular activating system is periodically shut down by influences from other regions of the reticular formation. The EEG recorded during sleep reveals a persistently changing pattern of wave amplitudes and frequencies, indicating that the brain remains continually active even in the deepest stages of sleep. The EEG pattern recorded during sleep varies in a cyclic fashion that repeats approximately every 90 minutes, starting from the time of falling asleep to awakening 7 to 8 hours later (Fig. 7.6). These cycles are associated with two different forms of sleep, which follow each other sequentially:

1. Slow-wave sleep: four stages of progressively deepening sleep (i.e., it becomes harder to wake the subject)

2. Rapid eye movement (REM) sleep: back-and-forth movements of the eyes under closed lids, accompanied by autonomic excitation

EEG recordings of sleeping subjects in laboratory settings reveal that the brain's electrical activity varies as the

Sleep Cycle Hypertension

The brain wave patterns during a normal sleep cycle. (See text for details.) (Modified from Kandel ER, Schwartz JH, Jessel TM. Principles of Neural Science. 3rd Ed. New York: Elsevier, 1991.)

The brain wave patterns during a normal sleep cycle. (See text for details.) (Modified from Kandel ER, Schwartz JH, Jessel TM. Principles of Neural Science. 3rd Ed. New York: Elsevier, 1991.)

subject passes through cycles of slow-wave sleep, then REM sleep, on through the night.

A normal sleep cycle begins with slow-wave sleep, four stages of increasingly deep sleep during which the EEG becomes progressively slower in frequency and higher in amplitude. Stage 4 is reached at the end of about an hour, when delta waves are observed (see Fig. 7.6). The subject then passes through the same stages in reverse order, approaching stage 1 by about 90 minutes, when a REM period begins, followed by a new cycle of slow-wave sleep. Slow-wave sleep is characterized by decreased heart rate and blood pressure, slow and regular breathing, and relaxed muscle tone. Stages 3 and 4 occur only in the first few sleep cycles of the night. In contrast, REM periods increase in duration with each successive cycle, so that the last few cycles consist of approximately equal periods of REM sleep and stage 2 slow-wave sleep.

REM sleep is also known as paradoxical sleep, because of the seeming contradictions in its characteristics. First, the EEG exhibits unsynchronized, high-frequency, low-amplitude waves (i.e., a beta rhythm), which is more typical of the awake state than sleep, yet the subject is as difficult to arouse as when in stage 4 slow-wave sleep. Second, the autonomic nervous system is in a state of excitation; blood pressure and heart rate are increased and breathing is irregular. In males, autonomic excitation in REM sleep includes penile erection. This reflex is used in diagnosing impotence, to determine whether erectile failure is based on a neurological or a vascular defect (in which case, erection does not accompany REM sleep).

When subjects are awakened during a REM period, they usually report dreaming. Accordingly, it is customary to consider REM sleep as dream sleep. Another curious characteristic of REM sleep is that most voluntary muscles are temporarily paralyzed. Two exceptions, in addition to the muscles of respiration, include the extraocular muscles, which contract rhythmically to produce the rapid eye movements, and the muscles of the middle ear, which protect the inner ear (see Chapter 4). Muscle paralysis is caused by an active inhibition of motor neurons mediated by a group of neurons located close to the locus ceruleus in the brainstem. Many of us have experienced this muscle paralysis on waking from a bad dream, feeling momentarily incapable of running from danger. In certain sleep disorders in which skeletal muscle contraction is not temporarily paralyzed in REM sleep, subjects act out dream sequences with disturbing results, with no conscious awareness of this happening.

Sleep in humans varies with developmental stage. New-borns sleep approximately 16 hours per day, of which about 50% is spent in REM sleep. Normal adults sleep 7 to 8 hours per day, of which about 25% is spent in REM sleep. The percentage of REM sleep declines further with age, together with a loss of the ability to achieve stages 3 and 4 of slow-wave sleep.

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