Page 17 - Handbook of Electronic Assistive Technology

4 HANDBOOK OF ELECTRONIC ASSISTIVE TECHNOLOGY

neuroectoderm. This plate develops a groove and the pizza oval folds up into a calzone,
thereby creating a tube with internal neuroectoderm (central nervous system (CNS)) and
external ectoderm (skin). This closes from the middle so that complete internalisation
occurs at both ends by around day 24, with an obvious top rostral neuropore and bottom
caudal neuropore.
The head end then bends, flexes and wraps up on itself into what is by 11 weeks a fairly
recognisable fore-, mid- and hindbrain with a clear caudal spinal projection.
From there, there is lot of neuronal (nerve cell) and pathway specialisation that occurs
within the brain while the rest of our body development catches up. The brain cortex folds
in on itself to form a large surface area of grey matter within a relatively small volume; core
pockets of cells differentiate into a series of integrated central circuits – the basal ganglia;
central white matter pathways are constantly created and regress together with a vast dif-
ferentiation of supporting cell types forming and supporting the nascent system.
We used to think that the development of embryological pathways from the CNS out to
their specific peripheral effector organs was a carefully structured process. It seems there
is a lot more of a blunderbuss approach. The whole cortex sends early projections down
circuits to the terminal fields of projection, both on the same side (ipsilateral) and oppo-
site side (contralateral), not just to the areas that they end up innervating but pretty well
everywhere. The specific remodelling and restriction of circuits and tract development
from certain key areas of the brain is extremely dynamic. Pathways specialise much later
in humans than in other mammals, in comparison to the overall timing of foetal develop-
ment that allows us to increase the complexity of our circuits.
By 24/40 weeks of gestation the wiring (axons) from the cells has developed to the
lower end of the cord. Rhythmical patterns of movement at an early stage of foetal growth
modify innervation, tracts nip and regress, facilitating specialisation of the pathways. By
full term, 40/40, there is relatively complete innervation to the peripheries with much
more in the way of crossing of messages from one side of the brain to the opposite side
of the body.
With all the cellular organisation and specification, by the time we are born the brain
is more than 10% of our entire body weight, whereas by the time we’re adult it’s only 2%.
Though the period of most rapid growth and differentiation occurs in foetal stages, it con-
tinues markedly during infancy and early childhood. Ever-changing new cell types are
being made, and new pathways are created and subsequently altered. The wiring between
different areas of central and cortical grey matter becomes more differentiated, with devel-
opment of normal insulation of the nerve fibres in the central and peripheral nervous
systems increasing the potential speed of nerve signal transition by the laying down of
concentric fatty myelin sheathes. In this phase of rapid differentiation and specification
there is a considerably greater capacity for neuroplasticity or potential for pathway and
neuronal relearning in the stage before myelination is complete. By the time we are 4–5 years
of age, the process is pretty stuck; plasticity or pathway modification is much more difficult.
The brain normally weighs about 350–400 g at birth and 1 kg by 1 year, and by 2 years of
age its relative size, proportions and subdivisions are similar to that of an adult. It’s this