The axon, which is much narrower than the width of a human hair, extends out
from the cell body. Axons transmit messages from neuron to neuron. Sometimes,
signal transmissions—like those from head to toe—have to travel over very
long distances. Axons are covered with an insulating layer called myelin
(also called white matter because of its whitish color). Myelin, which is made
by a particular kind of glial cell, increases the speed of nerve signal
transmissions through the brain.
Dendrites also branch out from the cell body. They receive messages from the
axons of other neurons. Each neuron is connected to thousands of other nerve
cells through its axon and dendrites.
Groups of neurons in the brain have special jobs. For example, some are
involved with thinking, learning, and memory. Others are responsible for
receiving information from the sensory organs (such as the eyes and ears) or the
skin. Still others communicate with muscles, stimulating them into action.
Several processes all have to work smoothly together for neurons, and the
whole organism, to survive and stay healthy. These processes are communication,
metabolism, and repair.
COMMUNICATION
Imagine the many miles of fiber-optic cables that run under our streets. Day and
night, millions of televised and telephonic messages flash at incredible speeds,
letting people strike deals, give instructions, share a laugh, or learn some
news. Miniaturize it, multiply it many-fold, make it much more complex, and you
have the brain. Neurons are the great communicators, always in touch with their
neighbors.
Neurons communicate with each other through their axons and dendrites. When a
dendrite receives an incoming signal (electrical or chemical), an “action
potential,” or nerve impulse, can be generated in the cell body. The action
potential travels to the end of the axon and once there, the passage of either
electrical current or, more typically, the release of chemical messengers,
called neurotransmitters, can be triggered. The neurotransmitters are released
from the axon terminal and move across a tiny gap, or synapse,
to specific receptor sites on the receiving, or post-synaptic, end of dendrites
of nearby neurons. A typical neuron has thousands of synaptic connections,
mostly on its many dendrites, with other neurons. Cell bodies also have receptor
sites for neurotransmitters.
Once the post-synaptic receptors are activated, they open channels through
the cell membrane into the receiving nerve cell’s interior or start other
processes that determine what the receiving nerve cell will do. Some
neurotransmitters inhibit nerve cell function (that is, they make it less likely
that the nerve cell will send an electrical signal down its axon). Other
neurotransmitters stimulate nerve cells, priming the receiving cell to become
active or send an electrical signal down the axon to more neurons in the
pathway. A neuron receives signals from many other neurons simultaneously, and
the sum of a neuron’s neurotransmitter inputs at any one instant will
determine whether it sends a signal down its axon to activate or inhibit the
action of other neighboring neurons.
During
any one moment, millions of these signals are speeding through pathways in the
brain, allowing the brain to receive and process information, make adjustments,
and send out instructions to various parts of the body.
METABOLISM
All cells break down chemicals and nutrients to generate energy and form
building blocks that make new cellular molecules such as proteins. This process
is called metabolism. To maintain metabolism, the brain needs plenty of blood
constantly circulating through its billions of capillaries to supply neurons and
other brain cells with oxygen and glucose. Without oxygen and glucose, neurons
will quickly die.
REPAIR
Nerve cells are formed during fetal life and for a short time after birth.
Unlike most cells, which have a fairly short lifespan, neurons in the brain live
a long time. These cells can live for up to 100 years or longer. To stay
healthy, living neurons must constantly maintain and repair themselves. In an
adult, when neurons die because of disease or injury, they are not usually
replaced. Research, however, shows that in a few brain regions, new neurons can
be generated, even in the old brain.
