Çarşamba, Aralık 30, 2009

sleepy in the womb

There is no question that the new-born baby is awake. Its eyes are wide open, it wriggles and grimaces, and, most important, it cries. But all that is not the same as being conscious, of experiencing pain, seeing red or smelling Mom’s milk.

New-born baby processes complex visual stimuli and attends to sounds and sights, preferentially looking at faces. The infant’s visual acuity permits it to see only blobs. Her sight will improve in months.

Exposure to maternal speech sounds in the womb enables the fetus to pick up statistical regularities so that the newborn can distinguish its mother’s voice and even her language from others.

The Road to Awareness

The thalamo-cortical complex that provides consciousness with its highly elaborate content, begins to be in place between the 24th and 28th week of gestation. Roughly two months later synchrony of the electroencephalographic (EEG) rhythm marks the onset of global neuronal integration. Thus, many of the elements necessary for consciousness are in place by the third trimester.

Third-trimester fetus is almost always in one of two sleep states. Called active and quiet sleep, these states can be distinguished using electroencephalography. Their different EEG signatures go hand in hand with distinct behaviors: breathing, swallowing, licking, and moving the eyes but no large-scale body movements in active sleep; no breathing, no eye movements and tonic muscle activity in quiet sleep. These stages correspond to rapid-eye-movement (REM) and slow-wave sleep common to all mammals. In late gestation the fetus is in one of these two sleep states 95 percent of the time, separated by brief transitions.

Fetus is actively sedated by the low oxygen pressure (equivalent to that at the top of Mount Everest), and neuroinhibitory and sleep-inducing substances produced by the placenta and the fetus itself such as adenosine; two steroidal anesthetics, allopregnanolone and pregnanolone, one potent hormone, prostaglandin D2 and others. Neonatologists conclude that the fetus is asleep while its brain matures.

The dramatic events attending delivery by natural (vaginal) means cause the brain to abruptly wake up. The fetus is forced from the protected, aqueous and warm womb into a hostile, aerial and cold world that assaults its senses with utterly foreign sounds, smells and sights, a highly stressful event.

Massive surge of norepinephrine as well as the release from anesthesia and sedation that occurs when the fetus disconnects from the maternal placenta, arouses the baby so that it can deal with its new circumstances. It draws its first breath, wakes up and begins to experience life.

excerps from
"When Does Consciousness Arise?"
Christof Koch
Scientific American
http://www.scientif icamerican. com/article. cfm?id=when- does-consciousne ss-arise

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