Figure 134

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The EEG record of a person (a) passing from the awake state to deep sleep (stage 4) and (b) during REM sleep.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Awake

Stage 2

Stage 3

FIGURE 13-5

A typical night's sleep in an average young adult. The lavender colored lines indicate periods of REM sleep.

Adapted from Nicholi.

Awake

Stage 2

Stage 3

FIGURE 13-5

A typical night's sleep in an average young adult. The lavender colored lines indicate periods of REM sleep.

Adapted from Nicholi.

low-voltage, high-frequency, asynchronous pattern characteristic of the alert, awake state (Figure 13-4b). Instead of the person waking, however, the behavioral characteristics of sleep continue at this time.

REM sleep is also called paradoxical sleep because the sleeper is difficult to arouse despite having an EEG that is characteristic of the alert, awake state. When awakened during REM sleep, subjects generally report that they have been dreaming. This is true even in people who do not remember dreaming when they awaken later spontaneously.

If uninterrupted, sleep continues in this cyclical fashion, moving from stages 1, 2, and 3, to 4 then back up from 4 to 3, 2, and 1, where NREM sleep is punctuated by an episode of REM sleep. Continuous recordings of adults show that the average total sleep period comprises four or five such cycles, each lasting 90 to 100 min (Figure 13-5). NREM sleep constitutes 75 to 80 percent of the total sleeping time in young adults; the remainder is spent in REM sleep. The time spent in REM sleep increases toward the end of an undisturbed night. Initially, as one moves from drowsiness to stage 1 sleep, there is a considerable tension in the postural muscles, but the muscles become progressively more relaxed as NREM sleep progresses. Sleepers awakened during NREM sleep rarely report dreaming.

With several exceptions, skeletal-muscle tension, already decreased during NREM sleep, is markedly inhibited during REM sleep. Exceptions are the eye muscles, which cause the rapid bursts of saccade-like eye movements (Chapter 9), and the motor neurons to the muscles of respiration. In the disease known as sleep apnea, however, stimulation of the respiratory muscles temporarily ceases, sometimes hundreds of times during a night. This disease is associated with excessive— and sometimes dangerous—sleepiness during the day.

During the sleep cycle, there are many changes throughout the body, in addition to altered muscle tension. During NREM sleep, for example, there are pulsatile releases of growth hormone (Chapter 10) and the gonadotropic hormones from the anterior pituitary, as well as decreases in blood pressure, heart rate, and respiratory rate. REM sleep is associated with an increase and irregularity in blood pressure, heart rate, and respiratory rate. Moreover, twitches of the facial muscles or limb muscles occur (despite the generalized lack of skeletal-muscle tone), as may erection of the penis and engorgement of the clitoris.

Although adults spend about one-third of their time sleeping, the functions of sleep are not known. One hypothesis is that the neural mechanisms of REM sleep facilitate the chemical and structural changes that the brain undergoes during learning and memory and, through dreams, may provide for the expression of concerns in the "subconscious."

The sleep states are summarized in Table 13-1.

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