00:00
So let's now have a look at the
arrangement and the location of
the motor and sensory neurons in the spinal
cord. So now it is important for us to make
sure we know clearly the locations of sensory
and motor neurons. On this diagram, you can
see the gray matter and the white matter of
the spinal cord. So just get used to this
diagram and just have a little view of what
sort of information it is going to illustrate.
00:37
I want you to first concentrate on the left-hand
part of the figure, the bit that talks about somatic
reflexes and we'll talk about the visceral
reflexes later on. Picture the peripheral
nerve on the outside, and also picture the the
ventral horn in the gray matter. And now have
a look at the ventral horn. A branch passes
from the spinal cord. It is called the ventral
root and if you look up top, at the dorsal
horn, you can see that this is also a dorsal
root and these combine together to form the
peripheral nerve. Now, the ventral horn if you
look very carefully houses the cell body of
the motor neuron, the somatic motor neuron.
01:43
And that neuron has an axon that passes out
of the ventral root, travels down the peripheral
nerve and innervates the muscle. In this case,
it is innervating a finger muscle in response
to a finger prick. So that really summarizes
the location of a somatic motor neuron, an
efferent neuron, exiting the spinal cord. And
notice there's only one neuron involved.
02:14
Now picture also on this image the right hand
side, the visceral reflex and we will describe
now the somatic efferent visceral neuron pathway,
the autonomic pathway. Motor neurons that
are going to innervate smooth muscle and components
that we are not conscious about and again
remember that I said two neurons were involved.
Well remember that I described the lateral horn when
we looked at the section through the spinal
cord before. So we have labelled it here on
this diagram. Make sure you can recognize
that lateral horn. And in that lateral horn
is going to be the very first neuron, neuron
number 1 and its called the preganglionic neuron.
03:13
Preganglionic because it travels to a
ganglion, a group of cell bodies and interacts
with the second order neuron, the postganglionic
neuron. And in this case if you look very carefully
you can see the axon in the diagram moving
its way out of the ventral root as well, travelling
out of the spinal cord and it travels to this
structure labelled called a prevertebral ganglion.
03:44
This prevertebral ganglion is really just
outside the spinal cord and I will talk about
this in more detail later on. Well, that is where
the postganglionic neuron is located and it
then travels or its axon then travels and
innervates the smooth muscle. So it is important
that you understand these two neuron pathway
to do with the autonomic nervous system.
04:17
Sympathetic or parasympathetic, follows the same sort
of pathway. It is just as we will see later on
that location of the ganglion that has the
postganglionic neuron is located in the different
place, whether it is sympathetic or whether
it is parasympathetic. And there is a paraventral
or paravertebral ganglion shown there. It does not
matter at this stage. What the difference
between these two ganglia are? Well let's have
a look now at sensory neurons. Where they are
located? Have a look again at the peripheral
nerve, familiarize yourself again on the somatic
reflex side of this diagram and have a look
at the dorsal root ganglion labelled there.
05:10
That is where the cell body of the sensory
neuron is located, and notice its axon travels
from the periphery, from the fingertip picking
up the sensation of pain probably from the
finger prick, and that pain travels along the
axon into the dorsal root ganglion, and travels
further on past the cell body of that sensory
neuron, into the dorsal root and into the dorsal
horn. And there it can interact to the motor
neuron, in the ventral horn, by simple interneuron
and create the reflex out. So when you prick
your finger, sensory information goes in through
the dorsal root via the sensory neuron, interacts
very quickly with the motor neuron from the
ventral horn, and then that motor neuron innervates
skeletal muscles to remove your finger.
06:12
Well let's have a look at the visceral sensory
neurons. Only one neuron is involved in this
situation. Again the dorsal root ganglion
shown here helds its sensory neurons.
06:30
All sensory neurons are held in these dorsal root ganglia
or in some ganglia in the head to do with
sensory neurons carrying in with cranial nerves.
And have a look at the visceral sensory neuron,
it is detecting information about perhaps
the contraction of the wall of the gut, and
carrying that information all the way in, through
the ganglia. It does not synapse with any
neuron. They are all the way in through the
dorsal root ganglion and then through the
dorsal root just like any other sensory or
somatic sensory neuron. That is the pathway
then of the visceral sensory neuron.