![[image of network of neurons]](images/neurnet.gif)
Nerve cells, called neurons,
are the fundamental elements of the central nervous system. The
central nervous system is made up of about 100 billion neurons (10
to the power 11). Neurons are much like other cells of the body in
their general organization and their biochemical systems. However they
also possess unique features which are crucial to the functioning of
the central nervous system. In essence, a given neuron may both
receive and send out signals to neighboring neurons in the form of
electrical pulses. A neuron is built up of three parts: the cell
body, the dendrites and the axon as shown in the
figure. The body of the cell contains the nucleus of the cell and carries the biochemical transformations necessary to synthesize enzymes and other molecules necessary to the life of the neuron. It is roughly spherical or pyramidal in shape - the precise shape depending on position and function in the brain. It is typically several microns in diameter (a micron is a millionth of a meter).
Each
neuron has a hair-like structure surrounding it - these are the
dendrites. Dendrites are some tens of microns in length. The
branch out into a tree-like form around the cell body. The dendrites
are like electrical cables which serve to conduct incoming signals to
the cell. The axon or nerve fiber is the outgoing connection
for signals emitted by the neuron. It differs from the dendrites in
its shape and by the properties of its external membrane. It is
usually much longer than the dendrites, varying from a millimeter (one
thousandth of a meter) to one meter. At its end it branches into
smaller structures which communicate with other neurons. The branching
of the dendrites, in contrast, takes place much closer to the cell
body. Neurons are connected together at these extremities in a complex
spatial arrangement. Typically a given neuron is connected to about
ten thousand other neurons. The specific point of contact between the
axon of one cell and a dendrite of another is called a
synapse.
Organizations |
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Large Scale Features