Well, it is now important to really have a
look at the division of the autonomic nervous
system, the parasympathetic and the sympathetic
division. In this diagram, it looks rather
complicated but is just a simple story that
I want to try and explain. And that is let
us first of all look at the sympathetic division.
Sympathetic division comes from only certain
spinal cord segments. They come from thoracic
and lumbar areas. The thoracolumbar components
make up the sympathetic vision of the autonomic
nervous system. And those thoracolumbar preganglionic
neurons pass to ganglia just outside the spinal
cord. You can see them labelled there in a
long chain running down next to the spinal
cord, chain of ganglia. You can see little
blue-covered neuron exiting the thorax and
the lumbar region of the spinal cord.
These represent the preganglionic sympathetic neurons
and they travel into this paravertebral or
prevertebral chain of ganglia or other little
ganglia you see in circles there and then
the second order neuron or the postganglionic
neuron then passes to the tissue where it
does its activity where it innervates the
smooth muscle or whatever the target tissue is.
Parasympathetic division is different. Neurons,
the preganglionic neurons of the parasympathetic
division of the nervous system exit from the
cranium or from the sacral region, craniosacral.
They do not exit the spinal cord at any other
location. And these preganglionic neuron travel
a long way, some distance out into the organs,
the viscera and there they locate the postganglionic
neurons and those postganglionic neurons are
located very close to the organ in which the
postganglionic neuron is then going to innervate.
So in simple summary, the sympathetic preganglionic
neurons are short, but the postsympathetic
neurons are long. In the case with the parasympathetic
division, the preganglionic fibers are very
long because they contact ganglion and postganglionic
cells in those ganglia are very close to the
organ in which they are going to innervate.
Sometimes the transmitterrs used vary in each of these
types of neurons. Some can use acetylcholine
as a neurotransmitter. Some can use adrenalin
and therefore they are often referred to as
being cholinergic and adrenergic neurons. And
I only say that because I might use that
term in later lectures.