Sleep Paralysis: Awake But Still Asleep
A person may wake up and find himself unable to move or speak as if he is frozen. He also may hear footsteps, see a ghost-like creature, or feel someone sitting on his chest. Throughout the history, people considered this phenomenon as work done by evil spirits. However, the modern science can explain the terrifying event as a Sleep Paralysis. A Sleep Paralysis is possibly a hereditary disorder in which one experiences very frightening seconds or minutes of total body paralysis with little respiration and eye movements (1). A victim in this state feels awake, but he cannot move or speak . In addition to the immobility, the common symptoms include feeling choked or suffocated, hearing strange noises like footsteps and voices, seeing beings or dark shadows, and feeling an existance of someone in the room (1). Although these symptoms often direct the victims to believe in ghosts, mistransmission of neural signals in the brain causes Sleep Paralysis. When a person sleeps, his brain sends signals to inhibit any muscle contraction . If he comes into consciousness before the brain sends signals to activate muscle contraction, he cannot move his body, and consequently, become "paralyzed". In order to understand how a body becomes paralyzed while the person is awake, it is necessary to understand sleep cycles. In a mammalian sleep, the brain activity undergoes two different states called non-REM (NREM) sleep and REM sleep, which differ very much from wakefulness . NREM and REM sleep alternate cyclically through the night; in human, about 80 minutes of NREM sleep starts a night of sleep, about 10 minutes of REM sleep follows, and this 90 minute cycle is repeated about 3 to 6 times during the night . During NREM sleep, a body produces few movement, but the body has capability of tossing about in bed and producing some other motor events, such as sleepwalking and sleeptalking . The cardiac-muscle contraction and breathing occur at a uniform rate, and the eyes move slowly . During REM sleep, on the other hand, heart rate, respiration rate, and blood pressure vary . The eyes move rapidly because most dreaming takes place in this period, and the sleeper probably "look" at the moving objects in a dream . The brain's control over muscles during REM sleep points out that in this period, a body is normally in the state of total paralysis, called a "nonreciprocal flaccid paralysis" . Probably to prevent a person from "acting out" a dream, the brain sends signals to inhibit any muscle contractions . Although some peripheral muscles, such as the muscles of the fingers and face, still twitch, the large skeletal muscles become relaxed, or "paralyzed" as a result . Some evidence supports that the motor paralysis of REM sleep protect against the acting out of one's dreams. A patient who suffers from rare syndrome called REM Sleep Behavior Disorder lacks the normal nonreciprocal flaccid paralysis, and he acts out violent dreams during REM sleep, often with injurious consequences . For example, a 60-year-old surgeon dreamt that he was attacked "by criminals, terrorists, and monsters who always tried to kill [him]" and fighting against them in the nightmare, he was actually punching and kicking his wife who slept in the same bed .
A nonreciprocal flaccid paralysis during REM sleep is accomplished actively by postsynaptic inhibition of motorneurons . Although the exact process of motor inhibition is not clear, some neurotransmitters and hormones are known to generate the many components of REM sleep. Aministering physostigmine, an inhibitor of the catabolic enzyme, increases the concentration of acetylcholine within the neurons in the pons, making it possible to artificially generate and start REM sleep in the middle of NREM sleep . Carbachol, the cholinergic agonist, produces a period of REM sleep in cat when directly injected into the pontine tegmentum . The hormone melatonin, a "master hormone" that...
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