Dr. C. George Boeree
It is clear that most of what we think of as our mental life involves
the activities of the nervous system, especially the brain. This
nervous system is composed of billions of cells, the most essential
being the nerve cells or neurons. There are estimated to be as
many as 100 billion
neurons in our nervous system!
spinal cord neuron
A typical neuron has all the parts that any cell would have, and a few
specialized structures that set it apart. The main portion of the
cell is called the soma
or cell body
. It contains
, which in turn contains the genetic material in the
form of chromosomes.
Neurons have a large number of extensions called dendrites
spikes extending out from the cell
body. It is primarily the surfaces of the dendrites that receive
chemical messages from other neurons.
One extension is different from all the others, and is called the axon
hard to distinguish from the dendrites,
in others it is easily distinguished by its length. The purpose
of the axon is to transmit an electro-chemical signal to other neurons,
sometimes over a considerable distance. In the neurons that make
up the nerves running from the spinal cord to your toes, the axons can
be as long as three feet!
Longer axons are usually covered with a myelin sheath
, a series
of fatty cells which have wrapped around an axon many times.
These make the axon look like a necklace of sausage-shaped beads.
They serve a similar function as the insulation around electrical wire.
At the very end of the axon is the axon ending
, which goes by a
variety of names such as the bouton, the synaptic knob,
the axon foot, and so on (I do not know why no one has settled
on a consistent term!). It is there that the electro-chemical
signal that has travelled the length of the axon is converted into a
chemical message that travels to
the next neuron.
Between the axon ending and the dendrite of the next neuron is a very
tiny gap called the synapse
(or synaptic gap, or synaptic
cleft), which we will discuss in a little bit. For every neuron,
there are between 1000 and 10,000 synapses.
The action potential
When chemicals contact the surface of a neuron, they change the balance
(electrically charged atoms) between the inside and
outside of the cell membrane. When this change reaches a
threshold level, this effect runs across the cell's membrane to the
axon. When it reaches the axon, it initiates the action
, which is a rapidly moving exchange of ions.
The surface of the axon contains hundreds of thousands of miniscule
mechanisms called ion channels
. When the charge enters the
axon, the ion channels at the base of the axon allow positively charged
enter the axon,
the electrical balance between inside and outside. This causes
next group of ion channels to do the same, while other channels return
to the outside, and so on all the way down the axon.
little diagram, the red represents the positive ions going into the
axon, while the orange represents positive ions going out. The
action potential travels at a rate of 1.2 to 250 miles per hour!
This is, of course, over-simplified, but enough for
our purposes. But if you are interested in a little more detail, click
When the action potential reaches the axon ending, it causes tiny
bubbles of chemicals called vesicles to release their contents
into the synaptic gap. These chemicals are called neurotransmitters.
the next neuron, where they find special
places on the cell membrane of the next neuron called receptor sites.
The neurotransmitter acts like a little key, and the receptor site like
a little lock. When they meet, they open a passage way for ions,
which then change the balance of ions on the outside and the inside of
the next neuron. And the whole process starts all over again.
While most neurotransmitters are excitatory -- i.e. they excite
the next neuron -- there are also inhibitory
neurotransmitters. These make it more difficult for the
excitatory neurotransmitters to have their effect.
Types of Neurons
While there are many different kinds of neurons, there are three broad
based on function:
1. Sensory neurons are sensitive to various non-neural
There are sensory neurons in the skin, muscles, joints, and organs that
pressure, temperature, and pain. There are more specialized
in the nose and tongue that are sensitive to the molecular shapes we
as tastes and smells. Neurons in the inner ear are sensitive to
vibration, and provide us with
about sound. And the rods and cones of the retina are sensitive
to light, and allow us to see.
2. Motor neurons are able to stimulate muscle cells
the body, including the muscles of the heart, diaphragm, intestines,
3. Interneurons are the neurons that provide connections
sensory and motor neurons, as well as between themselves. The
of the central nervous system, including the brain, are all
Most neurons are collected into "packages" of one sort or another,
visible to the naked eye. A clump of neuron cell bodies, for
is called a ganglion (plural: ganglia) or a nucleus
(plural: nuclei). A fiber made up of many axons is called
nerve. In the brain and spinal cord, areas that are mostly
are called white matter, and it is possible to differentiate pathways
or tracts of these axons. Areas that include large number
cell bodies are called gray matter.
© Copyright 2003, 2009 C. George Boeree