hello everyone this is dr fox let's talk about the structure of the sympathetic
division of the autonomic nervous system in this video we'll discuss how the sympathetic division
of the autonomic nervous system is structured so recall that the sympathetic division is oftentimes
referred to as the thoracolumbar output or outflow and it gets that name because it is arising
from spinal levels t1 through l2 and that signal is carried out through portions of spinal nerves
and conducted to the sympathetic trunk from levels t1 through l2 the connection between the ventral
primary rami of spinal nerves and the sympathetic trunks are known as white rami communicantes or
white communicating branches so the overall distribution of white rami communicates
starts with t1 and moves distally along the length of the spinal cord
until l2 so above the level of t1 there is no connection between the central nervous system
and the sympathetic trunk below the level of l2 there is no connection between the
sympathetic nervous system and the spinal cord all of that communication is between t1 through l2 so
it's the preganglionic fibers which are originating in the spine and coming out
into the sympathetic trunk where most of them are going to synapse in ganglia which constitute quite
a bit of the trunk we'll discuss the specific route by which they reach to those ganglia but
the connections of the white rami communicantes are between the ventral primary rami or the vpr of
the spinal nerve and the sympathetic trunk there are other connections between the sympathetic
trunks and spinal nerves and these are called gray ray my communicantes so great rami communicantes
occur at every level of the vertebral column and so from the superior cervical ganglion
just inferior to the skull all the way down to the ganglion impar just anterior to the coccyx
we have a means by which signal from the sympathetic trunk can either go back to individual
spinal nerve levels or move out to viscera and it's through these gray rami communicantes which
are going to connect the sympathetic trunk to the ventral primary rami
of spinal nerves that postganglionic fibers or most postganglionic fibers are going to travel
so preganglionic fibers via white rami communications postganglionic fibers
through gray raymi communicates now let's take a look at the pathway by which
these fibers are going to travel so here we can see the spinal cord this is the anterior
side of the cord and this is the posterior side of the cord and so within the gray matter here
in there we can see two lateral horns uh within these lateral horns are the
intermedia lateral nuclei of the spinal cord or the iml for short so these preganglionic
sympathetic fibers originate in the iml or the lateral horns and they are going to
exit from the cord via ventral rootlets so we can see the ventral rootlets here
the ventral rootlets are going to coalesce into the ventral root of the
spinal nerve the ventral root meets up with the dorsal root of the spinal nerve to form the trunk
of the spinal nerve that trunk is going to be mixed so both afferent and efferent fibers or
sensory and motor fibers and it can be divided into the dorsal primary ramus or the dpr
the ventral primary ramus or the v pr so from the ventral primary ramus
we can see that there are two connections to the sympathetic trunk
the first which is along the pathway from the iml through the ventral rootlets through
the ventral root through the trunk through the ventral primary ramus is going to be the white
ramus communicans so those white rami communicates carry the preganglionic
fibers into the sympathetic trunk which is represented here by that ganglion and that fiber
once those fibers get into the trunk they may do several different things probably the most obvious
of all the things that they may do and here let's let's repeat this root so we start in the iml
we exit the spinal cord via the ventral rootlets through the ventral root through the
trunk of the spinal nerve through the ventral primary ramus out the white ramus communicans
and we're into the sympathetic trunk and here we are seeing that the first of all the possible
things that may happen is that that preganglionic sympathetic fiber can synapse
once it synapses we have a postganglionic fiber which is exiting the trunk via a gray
ramus communicans and going back to the ventral primary ramus of the spinal nerve and it can
distribute itself through the dorsal primary ramus or the ventral primary ramus either way
another option is that um the ganglionic
fiber can enter into the here let me erase this that preganglionic fiber can enter into the
trunk via the white ramus communicans and that it may ascend
or extend the trunk it can go either way
and then it may synapse in this case it's descending synapses and then it returns back to
another spinal nerve at a lower level or if it ascends it can go back at a higher level
the fourth option is that the pre-ganglionic sympathetic fiber can leave the spinal nerve
via the white ramus communicans and it can exit the trunk without ever synapsing
and in this case it's going to contribute to something called a splanchnic nerve a splanchnic
nerve is a nerve that serves viscera that's what splanchnic means and so we'll see later on
in the in the m1 experience how these splenic nerves are going to exit the
thoracic cavity and enter into the abdominal cavity to participate in various
perivascular plexuses in synapse in pre-aortic ganglia but that story is for a later date for now
simply know that pre-ganglionic fibers can when they enter the trunk synapse
descend the trunk then synapse ascend the trunk then synapse and then finally return back to
spinal nerves via gray grey rami communications or leave the spinal nerve as a fiber
or they may pass through the ganglia without synapsing as a part of a spine
so post-ganglionic fibers as we've discussed have a couple of options for what they'll do
once they synapse they can return to the ventral primary ramus or a ventral primary ramus
at any particular level and they can go out along the primary rami either the dorsal or the ventral
to serve various regions of the body they may also leave as a fiber from the ganglion or from
the nerve and participate in perivascular plexuses a perivascular plexus is a meshwork of autonomic
fibers that are transmitted along a vascular pathway or an arterial pathway out to a target
organ so there are different types of fibers depending on the target organs
that they affect so there are we can categorize these visceral motor fibers as vasomotor fibers
uh vasomotor fibers are going to target the smooth muscle of blood vessels
so when they are stimulated there is vasoconstriction when they are not stimulated
there is vasodilation there are also sudomotor fibers or secretomotor fibers which are going to
stimulate sudoriferous glands or sweat glands
these postganglionic fibers oftentimes are going to especially in the abdominal pelvic cavity
participate as well as in the thoracic cavity in autonomic plexuses these autonomic plexuses
are going to be in meshworks of postganglionic sympathetic fibers and oftentimes
preganglionic parasympathetic fibers that serve target organs and in block three we'll see how
splanchnic nerves that are arising from the thoracic sympathetic trunks
can leave the the trunks and descend down into the abdominal cavity to serve viscera there so
we've discussed the overall structure of the sympathetic nervous system in the various pathways
that both pre and post ganglionic fibers are going to take from the lateral horns of the spine
out to the target tissues this is your summary slide thank you very much for your time