The electroencephalogram (EEG)
The electroencephalogram (EEG) records electrical activity of the cerebral cortex via electrodes placed on the skull. The EEG waves originate from alternating excitatory and inhibitory synaptic potentials that produce sufficient extracellular current flow across the cortex to be detected by surface electrodes. In normal healthy people, most waves in the EEG can be classified as alpha, beta, theta, and delta waves with variable frequency (Hz, or cycles per second) and amplitude(uV):Alpha waves are rhythmical waves that occur at frequencies between 8 and 13 cycles per second and amplitude of 50-100 uv found in the EEGs of quiet, resting state of cerebration with closed eyes. During deep sleep, the alpha waves disappear. When attention is focused on something the Alpha waves replaced by rapid slow voltage irregular wave this is called alpha blockage.
Beta waves occur at frequencies greater than 14 cycles per second and as high as 80 cycles per second. Recorded while a person is experiencing visual and mental activity. They are recorded mainly from the parietal and frontal regions.
Theta waves have frequencies between 4 and 7 cycles per second. They occur normally in the parietal and temporal regions in children, but they also occur during emotional stress in some adults, particularly during disappointment and frustration. Theta waves also occur in many brain disorders, often in degenerative brain states.
Delta waves include all the waves of the EEG with frequencies less than 3.5 cycles per second, and they often have voltages two to four times greater than most other types of brain waves. They occur in very deep sleep, in infancy, and in serious organic brain disease.
Figure:The electroencephalogram (EEG)
SleepSleep is defined as unconsciousness from which the person can be aroused by sensory or other stimuli. It is to be distinguished from coma, which is unconsciousness from which the person cannot be aroused. There are two types of sleep: rapid eye movement (REM) sleep and non-
REM (NREM) sleep. The normal adult generally sleeps 7–8 hours per night; each sleep cycle typically is about 90–120 minutes, with approximately 25% of the time spent in REM sleep. Young children sleep for longer periods and have a higher proportion of REM sleep, whereas elderly persons have reduced sleep time and an absent stage 4, often leaving them with the complaint that they do not feel as rested as they used to feel.
Non- REM (NREM) sleep
NREM sleep is characterized by a reduction in physiological activity. Assleep gets deeper, the brain waves as measured by EEG get slower and
have greater amplitude, breathing and heart rate slow down, and blood
pressure drops. The NREM phase consists of four stages ,the four NREM stages (stages 1, 2, 3, and 4) roughly parallel a depth-of sleep continuum, with arousal thresholds generally lowest in stage 1 and highest in stage 4 sleep.:
• Stage 1: is a time of drowsiness or transition from being awake to falling asleep. Brain waves and muscle activity begin to slow down during this stage. People in stage 1 sleep may experience sudden muscle
jerks, preceded by a falling sensation.
• Stage 2 is a period of light sleep during which eye movements stop. Brain waves become slower, with occasional bursts of rapid waves called sleep spindles, coupled with spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows and body temperature decreases.
• Stages 3 and 4 (which together are called slow wave sleep), are characterized by the presence of slow brain waves called delta waves interspersed with smaller, faster waves. Blood pressure falls, breathing slows, and body temperature drops even lower.
Fig. EEG waveforms in different Non- REM (NREM) sleep stages.
Rapid eye movement sleep (REM sleep),REM sleep is characterised by a fast mixed frequency low voltage EEG with saw-tooth waves and rapid eye movements. REM sleep, occurs in episodes that occupy about 25% of the sleep time in young adults; each episode normally recurs about every 90 minutes. Each REM cycle divide into tonic and phasic phases. REM Sleep marked by intense brain activity. Brain waves are fast and desynchronized, similar to those in the waking state. Breathing becomes more rapid, irregular, and shallow; eyes move rapidly in various directions and limb muscles become temporarily paralyzed. Heart rate increases and blood pressure rises. This also is the sleep stage in which most dreams occur.
Fig. EEG waveforms in REM sleep
Physiological changes during sleep1. Respiratory system
During NREM sleep, there is a decrease in respiratory drive and a reduction in the muscle tone of the upper airway leading to a 25% decrease in minute volume and alveolar ventilation and a doubling of airway resistance accompanied by a small increase in PaCO2 and decrease in PaO2. The breathing pattern is irregular especially during phasic REM sleep.
2. Cardiovascular system
Blood pressure decreases during NREM and tonic REM sleep but may increase above waking values during phasic REM sleep. Cardiac output is generally decreased during all sleep phases. Systemic vascular resistance (SVR) and the heart rate are both reduced during NREM and tonic REM sleep and increased during phasic REM sleep.
3. Central nervous system
Cerebral blood flow (CBF) increases by 50–100% above the level of resting wakefulness during tonic REM sleep and is even greater during phasic REM sleep. Cerebral metabolic rate, oxygen consumption and neuronal discharge rate are reduced during NREM sleep but increased above resting values during REM sleep. The autonomic nervous system shows a general decrease in sympathetic tone and an increase in parasympathetic tone, except in phasic REM sleep.
4.Renal system,
The glomerular filtration rate and filtration fraction are reduced and ADH secretion is increased resulting in a low volume concentrated urine.
5.Endocrine system
Growth hormone is mostly secreted during the first episode of SWS, particularly during puberty. Prolactin concentrations also increase shortly
after sleep onset and decrease with wakefulness. The secretion of cortisol decreases with the onset of sleep and reaches a trough in the early hours of the morning and a peak just after waking.
6.Temperature control
Thermoregulation is maintained during sleep.
Basic Theories of Sleep
The passive theory , is an earlier theory of sleep it depend on fact that Wakefulness,is due to the activity of the excitatory areas of the reticular activating system, this area is simply fatigued during the waking day and became inactive "tired” toward the end of the day as a result the subject go into sleep.Recent theories
Recent insight into the physiological patterns of sleep and wakefulness has shown that different brain-processing networks and neurochemical systems are involved in both states. The ascending arousal system, play important role in sleep-wake cycle together with many transmitters, and receptors that centered in the hypothalamus. One of these centers is the orexin neurons in the lateral hypothalamus. Indeed, recent work using selective stimulation of orexin neurons by artificially inserted receptors sensitive to fiberoptic light pulses, produces arousal. This arousal is mediated by orexin activation of norepinephrine neurons in the locus coeruleus. Orexin activation plays a critical role in preventing abnormal transitions into REM sleep during the day, as occurs in narcolepsy. In experiments with mice, in which the gene for the neurotransmitter orexin was experimentally removed, the animals became narcoleptic. In humans with narcolepsy, the orexin levels in the brain and spinal fluid are abnormally low.
The circadian timing system
The circadian timing of sleep and wakefulness system is regulated by the suprachiasmatic nucleus, a small group of nerve cells in the hypothalamus that acts as a master clock. These cells express clock proteins, which go through a biochemical cycle of about 24 hours, setting the pace for daily cycles of activity, sleep, hormone release, and other bodily functions. The suprachiasmatic nucleus receives input directly from the retina, and the clock can be reset by light so that it remains linked to the outside world’s day-night cycle.Sleep Disorder
Sleep Apnea, a serious, potentially life-threatening disorder characterized by episodes of interrupted breathing during sleep. There are two types of sleep apnea- central and obstructive. Obstructive sleep apnea, the more common type, occurs when air cannot flow into or out of the person’s airway despite efforts to breathe. Central sleep apnea occurs when the system that controls breathing is abnormal and there are decreased efforts to breathe during sleep. Frequent arousals from apneic events during the night prevent the person from getting enough deep, restorative sleep. people with sleep apnea often feel very sleepy, causing problems with daytime concentration and performance. It recently has been shown that sleep apnea contributes to high blood pressure. Risks for heart attack, irregular heartbeat, and stroke also are increased in those with sleep apnea. Other consequences of sleep apnea include depression, irritability, sexual dysfunction, learning and memory difficulties, and falling asleep while at work, on the phone, or driving. Successful treatment, usually with nasal continuous positive airway pressure or CPAP, may reduce sleepiness, motivating patients to effectively lose weight and, in turn, help alleviate the breathing obstruction. People with sleep apnea should not take sedatives or sleeping pills, because they may prevent them from awakening to breathe.Narcolepsy, a chronic neurological disorder that causes the sufferer to fall asleep at times when he or she wants to be awake. In addition to an overwhelming and recurring need to sleep at inappropriate times.
Insomnia, insomnia is the complaint of difficulty initiating or maintaining sleep, waking too early and not being able to get back to sleep, or waking feeling unrefreshed and lethargic. Insomnia occurs more often among women, the elderly, individuals of low socioeconomic status, and those who are widowed, divorced, or separated than in other individuals. It may be primary, or not directly associated with any other health condition or problem, or secondary to some underlying health condition, such as depression, heartburn, cancer, asthma, or arthritis, or as a result of medications or drugs, including alcohol and caffeine.
