of the Neuron
Silvia Helena Cardoso, PhD
(Back to the First
The structure of the neuron. A typical neuron fas
four morphologically defined regions: cell body, dendrites, axon, and presynaptic
terminals. Neurons receive nerve signals from axons of other neurons. Most
signals are delivered to dendrites (1). The signals generated
by a neuron are carried away from its cell body (2), which
contains the nucleus (2a), the storehouse of genetic information.
Axons (3) are the main conducting unit of the neuron. The
axon hillock is the site at which the cell's signs are initiated. Schwann
cells (6), which are not a part of a nerve cell, but one of
the types of glial cells, perform the important
function of insulating axons by wrapping their membranous processes around
the axon in a thight spiral, forming a myelin sheath (7),
a fatty, white substance which helps axons transmit messages faster than
unmyelinated ones. The myelin is broken at various points by the nodes
of Ranvier (4), so that in cross-section it looks rather like
a string of sausages. Branches of the axon of one neuron (the presynaptic
neuron) transmit signals to another neuron (the postsynaptical cell) at
a site called the synapse (5). The branches of a single axon
may form synapses with as many as 1000 other neurons.
The brain is grey and white. Why?
Maybe you have heard the term "grey matter"
for the brain; there is also "white matter". In a section made through
the brain, it is easy to see both grey and white areas. The cortex and
other nerve centres are grey, the regions in between, white. The grey coloration
is produced by the aggregation of thousands of cell bodies, while
the white is the color of myelin. The white color reveals the presence
of bundles of axons passing through the brain, rather than areas in which
connections are being made. No neuron has direct conection with any other.
At the far end of the axon are a number of terminal filaments, and these
run up close to other neurons. They may be close to the dendrites of the
other neuron (sometimes to special structures called dendritic spines,
or close to the cell body itself. Where the first neuron comes close to
the second neuron, a synapse is formed, a space acreoss which the first
neuron communicate with the second.
of the cell and their function