Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Anxiety, Sleep, and Electrical Activity of the Brain
Electroencephalogram

The primary electrical activity of the Brain in unanesthetized animals was first described in the 19th century. Later, it was systematically analyzed by the German psychiatrist Hans Berger, who coined the term electroencephalogram (EEG) to describe the recording of brain potential fluctuations. An EEG can be recorded using cranial electrodes placed on the intact Skull or via electrodes implanted directly within the brain. The term electrocorticogram (ECoG) is sometimes used to designate recordings obtained from electrodes placed directly on the pia mater of the Cerebral Cortex.

EEG recordings can be either bipolar or monopolar. Bipolar recordings reflect potential differences between two cortical electrodes, whereas monopolar recordings measure the difference between a cortical electrode and a theoretically indifferent reference electrode placed on a distant part of the body away from the cortex.

Alpha Rhythm

When an EEG is recorded from the scalp of an awake adult human who is resting with eyes closed and mind unfocused, the predominant feature is a stable wave pattern with a frequency of 8-12 Hz and an amplitude of 50-100 mV, known as the alpha rhythm. It is most prominent over the parieto-occipital region, although it can occasionally be detected elsewhere. A similar rhythm has been observed in many mammalian species (Fig. 11-4). For instance, in cats, it is only slightly faster than in humans. Despite minor interspecies variations, the alpha rhythm is remarkably similar across all mammals.

Other Rhythms

In addition to the dominant rhythm, other patterns with frequencies of 18-30 Hz and lower amplitudes are recorded over the frontal regions. For example, the beta rhythm may serve as a harmonic wave of the alpha rhythm. Gamma oscillations, ranging from 30 to 80 Hz, typically occur when an individual is aroused and intensely focused on a task. This activity is sometimes interrupted by irregular fast activity during the initiation of motor responses to stimuli. High-amplitude regular waves at 4-7 Hz, termed theta rhythm, are common in children, while in experimental animals, they are generated by the hippocampus (see below). Large slow waves of up to 4 Hz are sometimes referred to as delta waves.

EEG Variations and Abnormalities

As humans age, the frequency of the dominant resting EEG rhythm changes. In infants, beta-like bioelectrical activity is rapid, whereas the occipital rhythm is slow, ranging from 0.5 to 2.0 Hz. In children, this rhythm accelerates, and an adult-like alpha rhythm gradually emerges during Puberty. The frequency of the alpha rhythm decreases during hypoglycemia, hypothermia, reduced glucocorticoid hormone levels, and elevated arterial partial pressure of CO₂ (Pa_CO2), while opposite conditions lead to an increased frequency. In clinical practice, forced hyperventilation is sometimes used to unmask latent EEG abnormalities.

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Fig. 11-4. EEG recordings illustrating alpha rhythms in various species.

Alpha Blockade

When visual attention is focused, the alpha rhythm is replaced by rapid, somewhat irregular, low-voltage activity—a phenomenon known as alpha blockade. The disruption of the alpha rhythm can also be triggered by any form of sensory stimulation (Fig. 11-5) or mental concentration, such as solving arithmetic problems. The general term used to describe the replacement of the regular alpha rhythm with irregular low-voltage bioelectrical activity is arousal, or the alarm reaction (associated with alertness and anxiety). It is also referred to as desynchronization, because it disrupts the synchronized neuronal activity required to generate regular waves. However, the enhanced EEG activity observed during heightened states of arousal is also synchronized, albeit at a higher frequency level; therefore, the term desynchronization is somewhat imprecise.

Sleep Rhythms

There are two distinct types of sleep: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep, also known as slow-wave sleep. NREM sleep is divided into four stages. As a person falls asleep, they enter stage 1, which is characterized by low-amplitude, high-frequency EEG activity (Fig. 11-6). Stage 2 is marked by the appearance of sleep spindles—brief bursts of alpha-like waves (10-14 Hz, 50 mV). Stage 3 features lower-frequency rhythms with increased EEG wave amplitude, while stage 4 exhibits maximal slowing accompanied by large-amplitude delta waves. Thus, deep sleep is characterized by rhythmic slow waves with pronounced synchronization.

REM Sleep

The high-amplitude slow waves observed on the EEG during sleep are occasionally interrupted by rapid, low-voltage EEG activity that, in primates and humans, resembles stage 1 sleep. However, sleep is not interrupted during this state; the arousal threshold to sensory stimuli and reticular formation activation is actually elevated. This state is called paradoxical sleep because the EEG activity resembles that of wakefulness. Throughout paradoxical sleep, rapid roving Eye Movements occur, hence the term REM sleep. These movements are absent during slow-wave sleep, which is consequently termed NREM sleep. Another characteristic feature of REM sleep is large-amplitude phasic potentials (occurring in groups of three to five) originating in the Pons and rapidly propagating to the lateral geniculate Nucleus and subsequently to the occipital cortex. Accordingly, these are termed ponto-geniculo-occipital (PGO) spikes. During this phase of sleep, Skeletal Muscle tone in the neck region drops significantly (see Fig. 11-6). Other Muscles retain their tone, though a relative paralysis of voluntary motor activity occurs, regulated by the locus coeruleus. In cats with lesions in the locus coeruleus, REM sleep is associated with thrashing movements, as if they are physically acting out their dreams.

* From the English term spike, denoting a sharp peak in bioelectrical activity.