After studying this chapter, you should be able to:
Describe the primary types of rhythms that make up the electroencephalogram (EEG).
List the main clinical uses of the EEG.
Summarize the behavioral and EEG characteristics of each of the stages of nonrapid eye movement (NREM) and rapid eye movement (REM) sleep and the mechanisms responsible for their production.
Describe the pattern of normal nighttime sleep in adults and the variations in this pattern from birth to old age.
Describe the interplay between brainstem neurons that contain norepinephrine, serotonin, and acetylcholine as well as GABA and histamine in mediating transitions between sleep and wakefulness.
Discuss the circadian rhythm and the role of the suprachiasmatic nuclei (SCN) in its regulation.
The sensory pathways described in Chapters 8–11 relay impulses from sense organs to particular sites in the cerebral cortex. These impulses must be processed in the awake brain to be perceived. Behavioral states range from wakefulness through deep sleep. Discrete patterns of brain electrical activity correlate with each behavioral state. Changes in brain electrical activity also can signify pathologies (eg, seizures). This chapter reviews the neurophysiological basis for the electroencephalogram, changes in the sleep–wake states, and the circadian rhythms.
SLEEP–WAKE CYCLE: ALPHA, BETA, & GAMMA RHYTHMS
The electroencephalogram (EEG) recorded from the scalp is a measure of the summation of dendritic postsynaptic potentials rather than action potentials (Figure 14–1). Propagated potentials can be generated in dendrites, and recurrent axon collaterals end on dendrites in the superficial layers. When the sum of the dendritic activity is negative relative to the cell body, the neuron is depolarized and hyperexcitable; when it is positive, the neuron is hyperpolarized and less excitable.
In adult who are awake but at rest with the mind wandering and the eyes closed, the most prominent pattern of the EEG is an alpha rhythm that is a fairly regular sequence of waves at a frequency of 8–13 Hz and amplitude of 50–100 μV (Figure 14–1). It is most marked in the parietal and occipital lobes and is associated with decreased levels of attention. Clinical Box 14–1 describes some variations in the alpha rhythm. In an awake, alert individual with their eyes open, the alpha rhythm is replaced by an irregular 13–30 Hz low-voltage activity, the beta rhythm (arousal or alerting response) that can be produced by any form of sensory stimulation or mental concentration.
EEG records showing the alpha and beta rhythms. When attention is focused on something, the 8–13 Hz alpha rhythm is replaced by an irregular 13–30 Hz low-voltage activity, the beta rhythm. This phenomenon is referred to as alpha block, arousal, or the alerting response. (Reproduced with permission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology, 11th ed. New York, NY: McGraw-Hill; 2008.)