hi everyone this is Dr Fox and let's kick off the learning objectives for
this session with an overview of autonomics
in this video we'll discuss the basic divisions of the autonomic nervous system and their structures
so the autonomic nervous system is the portion of the nervous system responsible for the innervation
of various viscera so a lot of responses that are automatic that are completely outside the
realm of conscious Sensation that we want to regulate such as heart rate rate of respiration
what we're dealing with glucose etc all fall under the purview of the autonomic nervous
system now this system is rather elegant there are two basic divisions the sympathetic division
in the parasympathetic division of the autonomic nervous system targets of the ANS
are nearly all antagonistically innervated by the sympathetic and parasympathetic divisions
so for instance the sympathetic division may increase heart rate in the parasympathetic
division may decrease heart rate now each of these divisions serve their various visceral
target tissues via visceral motor fibers these visceral motor fibers are generally arranged in
a chain so there are preganglionic fibers which synapse with postganglionic fibers within ganglia
and those postganglionic fibers then are going to innervate the target tissue
there are also visceral sensory fibers which aren't technically autonomic fibers but they're
going to transmit back afferent information to the central nervous system from the target
so over here we have a rather elegant schematic to kind of show how the visceral motor fibers may
work so in red we have the sympathetic and in blue we have the parasympathetic and so let's just take
a look at what's happening with the heart art is predominantly intrinsically regulated with
respect to heart rate but there is extrinsic control from the autonomic nervous system
so looking at the thoracic spine we can see several preganglionic fibers extending from
the thoracic spine to the sympathetic trunk and it's here in a series of sympathetic ganglia that
those short preganglionic fibers are going to synapse with longer postganglionic fibers
which will go out and affect heart rate extrinsically by acting upon the SA node
we also have contributions from the parasympathetic in this case from cranial
nerve 10 or the vagus nerve shown here in blue parasympathetics have very long preganglionic
fibers that synapse with postganglionic fibers generally within the walls of Target viscera so
they have very very short postganglionic fibers as we'll see now the sympathetic division we
oftentimes call the fight or flight response because a lot of the effects of sympathetic
stimulation are those which would be preparing an individual either for a flight fight or to flee
from a fight whereas the parasympathetic division is the rest and digest division you know doing all
those things that would be quite the opposite of engaging in or fleeing from a fight as we'll see
so let's take a look at these two divisions side by side just as a very high level overview so
the fight-or-flight response of this sympathetic division is often referred to as the adrenergic
response and that's because the postganglionic fibers secrete norepinephrine AKA adrenaline
um sympathetic division as I've mentioned has very short preganglionic fibers and very long
postganglionic fibers which are secreting that norepinephrine parasympathetic division that rest
and digest is often known as the cholinergic response because it's postganglionic fibers
secrete acetylcholine in fact all preganglionic fibers whether they are sympathetic or
parasympathetic secrete acetylcholine as well so that parasympathetic division has very very long
preganglionic fibers and these tend to synapse with short postganglionic fibers within the
walls of Target organs let's just quickly review some of the responses that would be sympathetic
versus parasympathetic so in terms of heart and respiratory rate sympathetic stimulation fight
or flight will increase heart and respiratory rate whereas parasympathetic will decrease heart
and respiratory rate sympathetic stimulation is going to relax the smooth muscle of the conductive
Pathways of the respiratory tract this is known as a bronchodilation whereas parasympathetic
stimulation is going to constrict that smooth muscle leading to Broncho constriction so if
someone has severe allergies they may carry an EpiPen epinephrine so as to mimic that
adrenergic response so as to bronchodilate in case of exposure to that that allergen
sympathetic stimulation is going to increase the concentration of blood glucose whereas
parasympathetic division is going to decrease the concentration of blood glucose by storing that
glucose as glycogen in the liver the sympathetic division is going to dilate the pupils so as to
let more light in so it innervates the dilator pupillae muscles which are intraocular muscles
whereas the parasympathetic division constricts the pupils because it innervates the constrictor
pupillae muscles so if you're looking within a person's eyes and their pupils are dilated
they're probably more agitated if they are constricted they're probably a little more relaxed
the sympathetic division is going to have a constriction effect on muscles of the arteries
so that's known as vasoconstriction whereas the parasympathetic stimulation will relax
that smooth muscle of the arteries which leads to vasodilation generally throughout the body
sympathetic division will relax the smooth muscle of the wall of the gut tube or the
alimentary canal whereas the parasympathetic stimulation will constrict the smooth muscle
of the wall of the gut causing for movement of materials through the gut tube or the
alimentary canal and then finally an example of imbalance between these two divisions the
sympathetic division is the only one of the two which innervates sudoriferous or sweat glands and
its stimulation causes these sudoriferous or sweat glands to secrete so when you're sympathetically
stimulated that leads to you more sweating that's very congruent with a fight-or-flight response
so let's take a look at these two divisions in terms of a very high
level structure so that parasympathetic or the rest and digest is also known as the
craniosacral outflow so cranial because the brain stem is going to give rise to
four specific cranial nerves cranial nerves III the oculomotor nerve cranial nerve VII,
IX the glossopharyngeal nerve and X the vagus nerve and of these four the vagus is probably
the most significant for our study of anatomy as we'll see so three of these four nerves three
seven and ten are all going to serve preganglionic parasympathetic fibers to ganglia in the head
um three is going to um ultimately serve uh intraocular muscles and intraocular vasculature
seven the facial nerve is going to serve the lacrimal glands as well as glands within
the nasal cavity and major salivary glands including the sublingual gland
in the submandibular gland whereas nine the glossopharyngeal nerve is going to serve the
other major salivary gland the parotid gland the vagus nerve serves no Targets in the head but it
does serve very many Targets parasympathetically in the neck and the thorax and the abdominal
cavity in addition to the cranial contributions of the craniosacral outflow there's also the sacral
contributions the sacral contributions are the pelvic splanchnic nerves splanchnic means
viscera and these are nerves which arise from Ventral Primary Rami or VPR from sacral nerves to
three four so that's the sacral side of the cranium sacral output
the sympathetic division the fight-or-flight response is oftentimes referred to as the
thoracolumbar output and we'll see why in a second this is very neatly organized in
bilaterally paired sympathetic trunks which largely consist of sympathetic ganglia strung
together by fibers which we can see located anterior laterally to the vertebral column
these trunks run the entire length of the spinal cord and vertebral column and Beyond as we'll
see so we can't see the cervical sympathetic trunks here but we can see the thoracic trunks
the lumbar trunks the sacral trunks all the way down to where they join at the ganglion Empire
just anterior to the coccyx now we say that this is the thoracolumbar output as opposed to
the craniosacral output because the preganglionic fibers are all traveling from the spinal cord to
the sympathetic trunk via white rami communicantes so rami are branches communicantes are
communicating so white communicating branches at the levels of T1 through L2 so that's about there
to there what that means is the only connection between the spinal cord and the
sympathetic trunks are between T1 to L2 and that's going to be significant because there will be no
inputs to the trunks above the level of T1 or below the level of L2 now at every
spinal nerve level there will be postganglionic fibers returning back to that spinal nerve from
the trunk via gray Rami communicates so from the superior cervical ganglion at the superior most
part of the sympathetic trunks to the ganglion and Par at the inferior most part of the sympathetic
trunk we have postganglionic fibers returning to each spinal nerve it's important to note that the
head is served exclusively from postganglionic fibers arising from the superior cervical ganglia
and they're going to distribute along major vascular Pathways so external and internal
carotid that are surrounded by these perivascular plexuses which are mesh works of fibers on the way
out to Target tissues so we've discussed the autonomic nervous system generally as well as
its constituent divisions the parasympathetic and sympathetic division will give you more details
in videos to follow this is your summary slide thank you so very much for your time