|MadSci Network: Neuroscience|
This is somewhat of a philosophical question. There are many isolated studies that can be used to look at individual aspects of this question. Letís first start with the simplistic situation. When neurons are placed in culture, the axons can sprout and if the sprouts contact a proper target they can form synapses. Letís use a simple invertebrate example, the leech. All the neuron cell bodies within the ganglion are named and their locations are identifiable. When particular neurons are placed in culture, a synapse may form to relay activity from cell 1 to cell 2 but not the other way around. Chemical synapses may form first, later followed by an electrical synapse formation, which can be bi-directional or uni-directional. But if a different target cell is used, no synapses will form. So there is some specificity of synapse (i.e., sites of communication between neurons) formation. So this observation alone demonstrates that synapses occur without the neurons being stimulated electrically. Since they are isolated from the body and placed in a culture dish there are no extrinsic cues except for the presence of the other neurons and factors in the culture dish. This approach is a reductionist approach but you can see the advantages since variables can be controlled. In fact, leech neurons will grow when just placed in saline without growth factors added. This example of course is not testing development directly but examining repair and regeneration. However, repair and regeneration is believed, in part, to replicate development. In development, it is known that cues from tissue such as surface molecules and secreted local factors can direct neuronal growth and promote connections. So even without neuronal activity, growth and connections can be made. There are studies that address which connections stay and which ones are pruned back. This is the real issue of maintaining synaptic connections. It appears the synaptic connections that are active remain and the ones that are not active will die back or look for a different target. This is most directly related to your questions. Even when sleeping, as you mention, in babies, auditory sounds, touch, and likely light that can go through the eyelids will stimulate primary sensory neurons and alter synaptic connections. In sleep, motor commands are somewhat suppressed by regions of the reticular region of the brain. This function is thought to happen so one will not act out dreams. Possibly this also can suppress some types of stimuli from eliciting a motor action while the brain still receives sensory information while asleep. Donít forget the body is sensing not just external signals but internal ones as well. So while the baby is sleeping, pH, oxygen content, blood sugar, osmotic values, blood pressure etc. are continuously being monitored. Such vital functions may occur even in the fetus, I am not sure. The point is that awake or asleep many neural responses are ongoing. Other such neural responses are known to develop with use or activity, such as with vision and motor coordination. Classic examples have to do with the Nobel prize winners Hubel and Wiesel (1981) that had shown visual sensory information is needed to correctly develop the visual centers in the brain. The work with monkeys and studies of human babies from Romania (during president Nicolae Ceausescu's regime) in which these animals were sensory deprived of play objects in the surroundings as babies, parts of the cerebellar cortex showed differences as compared to control animals that have had many objects to play with (i.e. an enriched environment). So here is a direct answer to why the use of nervous system while awake is important. I am not sure anyone has an answer of why the brain needs to sleep. There are a lot of ideas and postulations but I am not aware of any direct studies that address this problem. There are very rare conditions that can occur in human adults at some point in time in which, for some reason, they can not sleep even with medication. It results over time in a fatal situation. So in some ways we are preprogrammed but the nervous system is plastic in that the connections can be strengthened or weakened with use during development. I guess the old saying ďuse it or lose itĒ applies at various times during development. Well I hope I answered some of your questions, even though they may not appear to be directly answering your question about 90 % sleep vs. 10 % being awake as a baby.
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