Page 20 - Handbook of Electronic Assistive Technology

Chapter 1 • Basic Neurosciences With Relevance to Electronic Assistive Technology 7

periventricular areas are predominantly associated with the long motor pathways – the
corticospinal tracts, which relay messages from the brain motor cortex down to specific
levels of the spinal cord and from there to the musculoskeletal system. The closer any
bleed (or haemorrhage) followed by subsequent necrosis of the white matter (periven-
tricular leukomalacia (PVL)) is to the lateral ventricle, the further down the body is the
involvement.
By 40 weeks of foetal development – full term – most of the brain, especially the cor-
tex, copes relatively well with the transient challenges to blood flow, oxygenation and
energy supply that can occur at the time of delivery. At this stage it is the deep grey
matter structures of the brain (the basal ganglia) which are most active metabolically
and have the greatest energy need. They therefore become prone to damage if starved
of oxygen and/or energy for a relatively short period of say 10–15 minutes, resulting in
hypoxic/ischaemic encephalopathy (low oxygen, poor blood supply-associated brain
damage).

Basic Structural Anatomy and Physiology
Basic Neurophysiology

The main cells of the nervous system are called neurons; they are key to the input, inte-
gration and transmission of electrical signals. The cell body receives electrical input from
branches called dendrites and outputs signal via a single elongated axon that transmits to
another neuron or a specific end ‘effector’ organ such as a muscle or gland.
Different types of neurons have different neural functions throughout all nervous sys-
tems and have a variety of microscopic structures, sizes, speeds and types of transmission.
The transmitting cells of the CNS are supported and nourished by a variety of other cell
types called neuroglia, such as astrocytes, ependymal cells, microglia and oligodendro-
cytes. This last cell type creates the Swiss roll of insulating material around the axon that
works as insulation to markedly speed up the rate of signal transmission.
Electrical signals are transmitted between segments of the axon by successive open-
ing of channels down the axonal membrane, with negative or positive charge fluxing on
either side of the membrane through selectively permeable chlorine, sodium or potas-
sium channels. Changes in charge are called action potentials and are created by this
flux in membrane permeability in response to stimulus or impulse. This action potential
travels down to an end point that links with end structures called synapses. Here stimula-
tion of the axonal terminal end plate causes a release of chemical signal across the syn-
aptic cleft, which in turn excites or inhibits electrical signals at the distal postsynaptic
membrane.
There are a wide variety of chemical substances within the nervous system that have
specific roles in up and down stimulating end organs and other neurons via these syn-
apses. Each neuron has a single neurotransmitter transmitting a signal across synapses,
such as acetylcholine, serotonin, dopamine or gamma-aminobutyric acid.