Evokedpotentialsareusedclinicallytoassesstheintegrityofasensorypathway,atleasttotheleveloftheprimarysensoryreceivingarea.Thesepotentialscanberecordedincomatoseindividuals,aswellasininfantstooyoungtoundergoasensoryexamination.Theinitialpartsoftheauditoryevokedpotentialactuallyreflectactivityinthebrainstem;therefore,thisevokedpotentialcanbeusedtoassessthefunctionofbrainstemstructures.
Sleep and wakefulness are among the many functions of the body that show circadian (about 1-day) periodicity. Characteristic changes in the EEG can be correlated with changes in the behavioral state during the sleep-wake cycle. Beta wave activity dominates in an awake, aroused individual. The EEG is said to be desynchronized; it displays low-voltage, high-frequency activity. In relaxed individuals with their eyes closed, the EEG is dominated by alpha waves (see
Fig. 10.5 ). A person falling asleep passes sequentially through four stages of slow-wave sleep (called stages 1 through 4) over a period of 30 to 45 minutes (see
Fig. 10.5 ). In stage 1, alpha waves are interspersed with lower frequency waves called theta waves. In stage 2, the waves slow further, but the slow-wave activity is interrupted by sleep spindles, which are bursts of activity at 12 to 14 Hz, and by large K complexes (large, slow potentials). Stage 3 sleep is associated with delta waves and with occasional sleep spindles. Stage 4 is characterized by delta waves without spindles.
During slow-wave sleep, the muscles of the body relax, but the posture is adjusted intermittently. The heart rate and blood pressure decrease, and gastrointestinal motility increases. The ease with which individuals can be awakened decreases progressively as they pass through these sleep stages. As individuals awaken, they pass through the sleep stages in reverse order.
About every 90 minutes, slow-wave sleep changes to a different form of sleep, called rapid eye movement (REM) sleep. In REM sleep, the EEG again becomes desynchronized. The low-voltage, fast activity of REM sleep resembles that seen in the EEG from an aroused subject (see
Fig.
10.5 , bottom trace). Because of the similarity of the EEG to that of an awake individual and the difficulty awaking the person, the term paradoxical sleep characterizes this type of sleep. Muscle tone is completely lost, but phasic contractions occur in a number of muscles, most notably the eye muscles. The resulting rapid eye movements are basis of the name for this type of sleep. Many autonomic changes also take place. Temperature regulation is lost, and meiosis occurs. Penile erection may occur during this type of sleep. Heart rate, blood pressure, and respiration change intermittently. Several episodes of REM sleep occur each night. Although it is difficult to arouse a person from REM sleep, internal arousal is common. Most dreaming occurs during REM sleep.
Thesleep-wakecyclehasanendogenousperiodicityofabout25hours,butitnormallybecomesentrainedtotheday-nightcycle.Thesourceofcircadianperiodicityappearstobethesuprachiasmaticnucleusofthehypothalamus.Thisnucleusreceivesprojectionsfromtheretina,anditsneuronsseemtoformabiologicalclockthatadaptstothelight-darkcycle.However,theentrainmentcanbedisruptedwhenthesubjectisisolatedfromtheenvironmentorchangestimezones(jetlag).Destructionofthesuprachiasmaticnucleusdisruptsanumberofbiologicalrhythms,includingthesleep-wakecycle.
The proportion of slow-wave (non-REM) sleep to REM sleep varies with age. Newborns spend about half of their sleep time in REM sleep, whereas elderly people have little REM sleep. About 20% to 25% of the sleep of young adults is REM sleep.