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
the
nucleus, which in turn contains the genetic material in the
form of chromosomes.
Neurons have a large number of extensions called
dendrites.
They
often
look
likes
branches
or
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.
Although
in
some
neurons,
it
is
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.
axon endings
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
of
ions (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
potential, 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
ions to
enter the axon,
changing
the electrical balance between inside and outside. This causes
the
next group of ion channels to do the same, while other channels return
positive ions
to the outside, and so on all the way down the axon.
In this
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!
Types of Neurons
While there are many different kinds of neurons, there are three broad
categories
based on function:
1. Sensory neurons are sensitive to various non-neural
stimuli.
There are sensory neurons in the skin, muscles, joints, and organs that
indicate
pressure, temperature, and pain. There are more specialized
neurons
in the nose and tongue that are sensitive to the molecular shapes we
perceive
as tastes and smells. Neurons in the inner ear are sensitive to
vibration, and provide us with
information
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
throughout
the body, including the muscles of the heart, diaphragm, intestines,
bladder,
and glands.
3. Interneurons are the neurons that provide connections
between
sensory and motor neurons, as well as between themselves. The
neurons
of the central nervous system, including the brain, are all
interneurons.
Most neurons are collected into "packages" of one sort or another,
sometimes
visible to the naked eye. A clump of neuron cell bodies, for
example,
is called a ganglion (plural: ganglia) or a nucleus
(plural: nuclei). A fiber made up of many axons is called
a
nerve. In the brain and spinal cord, areas that are mostly
axons
are called white matter, and it is possible to differentiate pathways
or tracts of these axons. Areas that include large number
of
cell bodies are called gray matter.
