In this lecture, I would like to cover the
structure of peripheral nerves and the brain.
At the end of the lecture, I would like to
have some understanding of the structure of
the peripheral nerve, of the basic histology
of two very important components of the brain.
I am going to describe how nerve cells are
supported by glial cells and also how the
brain and spinal cord is protected
The main function of the nervous system is
communication and we have special sensory
neurons that receive information from the
periphery, from our internal organs and send
their information into the brain to be processed.
We have motor neurons that send information
to our skeletal muscles and our smooth muscles
to allow us to move individuals and for some
about internal organs to change in dimension
and, therefore, alter their function.
All these important occurrences have happened
as a result of the neurons in the nervous
system. Let us now look at the structure of a peripheral
nerve. Here is a diagram showing you a section
through the spinal cord. The spinal cord is
in the middle of the diagram. It has two components.
One is colored yellow that is the outer white
matter and the internal butterfly or H-shaped
structure is called the gray matter.
Concentrate on the gray matter. It has two components.
It has a ventral horn and a dorsal horn
on either side. The ventral horn contains
the cell body of a motor neuron that is going
to pass out through the ventral root and form
the spinal nerve. In the dorsal horn, and in
the dorsal root, are axons projecting into
the central nervous system from sensory neurons.
And the cell body of these sensory neurons are
located in the dorsal root ganglion.
Now that cell body also has an axon that projects out
through the spinal nerve to the periphery and that
axon receives information from the periphery
in this case about the pain after a finger
prick. And that information is traveled from
the area that is being pricked by the finger
through the axon all the way up through the
dorsal root, through the axons associated
with this sensory neuron who again as I mentioned
again, the cell body is located in the dorsal
root ganglion. So this is the basic structure
of a spinal nerve. It consists of sensory
axons carrying information into the spinal
cord, and motor neurons carrying information
into the skeletal muscles in the periphery.
If we look across to the right-hand side
of the diagram, the other half of the spinal
cord there represents the visceral components.
The visceral afferent sensory neurons and
the visceral efferent motor neurons, associated
with the autonomic nervous system. Have a
look at the tube drawn down the bottom.
It represents perhaps the wall of the gut or
may be or the wall of a blood vessel.
That information is received from that structure,
that internal visceral organ and the information
passes through the spinal nerve into the dorsal
root and into the central nervous system.
Again, the cell body of this visceral afferent
sensory neuron is located in the dorsal root
ganglion. All sensory neurons cell bodies are
located in these dorsal root sensory ganglions,
unless they are in cranial nerves. Now focus
on the visceral motor components of the autonomic
nervous system, and make sure you understand
there are two neurons involved whereas in
the somatic motor, the skeletal muscles there
is only the one neuron. The preganglionic neuron
originates in the lateral horn of the
spinal cord. That is where the preganglionic
neuron cell body is located and it then projects
the axon out through the ventral root to then
pass on to a ganglion and then it synapses
with the postganglionic neuron in that ganglion.
And that postganglionic neuron then passes
down, usually following blood vessels, to where
those neurons are going to do their job where
they are going to stimulates smooth muscles
perhaps to contract around blood vessels or
around parts of the gut. Now in this diagram,
a sympathetic pathway is shown because the
postganglionic cell bodies are located in
here. They are located in the prevertebral
ganglion, but there is also the paravertebral.
There is also the paravertebral ganglion also
indicated. In sympathetic pathways, the postganglionic
cell is always located in these two ganglions
either released to ganglion close to the spinal
cord. If this was a representation of a parasympathetic
pathway, the postganglionic fibre would not
be located in these ganglia, but in ganglia
wire in the periphery next to the visceral
organs to which they are going to send axons
to innervate. And it's in those peripheral ganglia
that the postganglionic fibre will originate
and only travel a short distance to the site
at which the activity is going to take place.