00:03
Well, when we look at a nerve fibre in more detail,
we can see that there are other structures
that we need to describe and recognize. Here
is a section again of some neurons found in
the brain region. The left-hand image,
you can see long elongated structures and
on the right-hand side, you can see a very
high magnification of another neuron.
00:31
So I would describe each of these. They are motor
neurons. Motor neurons are those that bring
about movement of skeletal muscle in the somatic
division of the nervous system or they bring
about movement of smooth muscle in the autonomic
division of the nervous system. On the left-hand
side, the brown stained neurons you see are
called pyramidal cells. They consist of a
cell body. We call the cell body a soma when
we refer to the cell body of a neuron.
01:09
And if we have a look at this cell body in high
magnification, as we can see on the right-hand
image, we see details of the cell body in
more recognizable position, a more stained illustration
than you see on the left-hand side and so
is also a high magnification. Now this ventral
horn cell that you see labelled on the right-hand
side has got lots and lots of dark stained material
in the cell body, in the soma. It is called
nissil substance and that reflects all the granula
or rough endoplasmic reticulum inside the
cell body, the soma. Because neurons are not
just like electrical cables and transmit nerve
impulses, they manufacture lots of substances,
lots of proteins and lots of other structures.
And here is an example where this cell, the
ventral horn cell which I will describe in
more detail in a minute is making a lot of
protein. Some of that is going to be the neurotransmitter
substances that enable the impulse to transfer
from one neuron across to another at a synapse,
or from the motor endplate to the muscle.
02:32
So they are very busy cells manufacturing
those sorts of components. And the axon extends
a very long way into other parts of the body.
But before the axon carries an impulse down
through the nerve fibre, a lot of information
is received by the cell body through a dendritic
branch, a whole of little branches from the
cell body called dendrites and then the transmission
goes down the long process you see labelled
as the axon. Now those axons are extremely
long in some cases. These two cells communicate
with each other. The pyramidal cell is located
in the cortex of the cerebrum, in the motor
cortex. So you are looking really here, on
the left hand side, of an image of the cortex,
the cerebral cortex of the brain, the motor
cortex. And those pyramidal cells, those axons
you see labelled project all the way down
the spinal cord from the cerebral cortex.
So that can be extremely long. They can go
all the way down to the lumbar or sacral part
of our spinal cord. And there they communicate
with these ventral horn cells you see on the
right-hand side. Those ventral horn cells
sitting what we will term later on and describe
later on as the ventral horn of the spinal
cord. And these pyramidal cells then stimulate
these ventral horn cells to then innervate
skeletal muscles. And therefore those neuron
processes are also very long. They can extend
from the spinal cord all the way down to our
toes. So these axons are extremely long and
these axons from these ventral horn cells
will form part of what we call a peripheral
nerve. Now, I mentioned earlier that these cells are
busy making neurotransmitter substance.
04:52
So within these long processes, these long axons
there are certain components that allow the
transport of these neurotransmitter substances
and other substances down the length of the
axon. Sometimes slow transport, sometimes
very fast transport. And those structures are
the microtubules you will see inside the axons
if you look at them under the electron microscope.
05:17
Lots and lots of microtubules, they're like a railway
line, a railway track that carries packages
of neurotransmitter substances all the way
down to the axon terminals. So it's a very
important function of nerve cells to have
that transport mechanism. And the transport
of the cell body down towards the axonal processes
at the end, the endplates or the synapses
is called antegrade flow. Now things can also
flow backwards from the end terminals of the
axons all the way back to the cell body. That
is called retrograde flow. And in fact, that's
how the early neurobiologists traced where
different cell bodies were located that control
different parts of the body. They injected
dyes etc into say peripheral muscles or peripheral
regions of the body and those dyes were taken
up by the axon terminals. And through this
retrograde flow, those dyes or whatever markers
these early neurobiologists used was taken
back up into the brain to the surface of the
nerve cell bodies. And there the neurobiologists
were able to identify where these cell bodies
were located and therefore, map out parts
of the brain and regions of the brain that
control various parts of the body. And that
is also a bad story about that as well because
some viruses can therefore, get into the end
terminals of nerve fibres and travel into
the central nervous system. Sometimes the
rabies virus resulting from the bite, from
a dog for instance can replicate in skeletal
muscle over a period of 10 or 12 weeks and
then finally find its way into the axon terminals
and move all the way up into the central nervous
system. And there the virus can move throughout
the central nervous system and even travel
down either axons by antegrade flow to other
parts of the body and cause all sorts of problems.