From Diffusion MRI to Brain Connectomics - Modeling in Computational Biology and Biomedicine - page 187

6.1.1 The Central Nervous System

The human nervous system can be divided into the CNS, which consists of the
brain and spinal cord and the peripheral nervous system (PNS) which consists of the
cranial and spinal nerves and their ganglia. The CNS is the largest part of the nervous
system and is composed of the spinal cord and the brain which comprises a lower
part, the brainstem, and an upper part, the prosencephalon or forebrain composed
of two main units. One is known as the diencephalon. It is located in the midline of
the brain and contains the thalamus and the hypothalamus. The other is called the
telencephalon or cerebrum and holds the lateral ventricles, the basal ganglia and the
cerebral cortex. The brainstem is also composed of two units: the mesencephalon or
midbrain and the rhombencephalon or hindbrain which connects the forebrain and
midbrain to the spinal cord.
The nervous system is made of about 100 billion nerve cells, or neurons, able to
generate and propagate electrical signals to process and transmit neural information.
Neurons can receive electrical stimulation from other neurons on their soma,
through their multiple dentrites. They can integrate this information and propagate
it to more or less distant locations of the cerebrum by an extension called an axon.
Nerve signal communication is performed at specialized loci called synapses. Each
neuron has on average 1,000 synaptic connections with other neurons. This yields
about 100 trillion connections within a human brain. All these synapses result in an
impressively dense and complex network between functional areas, which can be
understood as aggregates of nerve cells' soma and dendrites. They are essentially
located in the grey matter while the underlying wiring constitutes the white matter.
According to the connectionist point of view, the human brain is organized into
distinct processing regions interconnected by a network of anatomical relays.
Processing units handle the execution of primary cognitive functions, and higher
cognitive tasks arise from a global coordination between these processing units.
Neural signal is processed in the cerebral cortex and transmitted to various regions
of the brain through the white matter. So we distinguish:

￿ Grey matter essentially forms the outer part of the cerebrum, some nuclei within

the brain, as well as the deeper part of the spinal cord. It is made of neurons
and their unmyelinated fibers. The cerebral cortex is the most important structure
of the grey matter and plays a major role in various functions such as memory,
attention and language.

￿ White matter is composed of axonal nerve fibers, covered by a myelin sheath

giving its distinctive colour in MRI. It is found in the inner layer of the cortex,
the optic nerves, the central and lower areas of the brain and surrounding the
central shaft of grey matter in the spinal cord.

The CNS white matter axons can be distributed diffusely or concentrated in
bundles, also referred to as tracts or fiber pathways. The brain's white matter
pathways are generally categorized into commissural, association and projection
fibers, depending on the areas they connect (Fig. 6.1 ).