Re: Concerning the brain growth of infants

This is somewhat of a philosophical question. There are many  isolated 
studies that can be used to look at individual aspects of this question.

Let’s 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. Let’s 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. Don’t 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 “use it or lose it” 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.