HOW TO
SURVIVE
Structure:
Review of Nervous System
Image taken from: http://www.mirm.pitt.edu/people/bios/LBS-neurons2.jpg
SPRING
2008
A
SDSG
PUBLICATION
“How to Survive” Guide – Structure- Review of Nervous System
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TABLE OF CONTENTS
Note- For the following guide, please follow along with your syllabus pictures or atlas for a full
understanding of the stuff. This survival guide is meant to highlight the key aspects of the
nervous system, but just in case this guide is lacking in some information, please also read the
relevant sections of your notes or textbook.
Topic
Introduction to the Nervous System
Pages
3-6
Somatic Nervous System
7-8
Sympathetic Nervous System
9-10
Parasympathetic Nervous System
11
One Last Note: Learning Patterns
12
How to Survive: Structure – Review of Nervous System
Samuel Anandan (5th year)
Ayanna Lewis (4th year)
Kathy Zhang (4th year)
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INTRODUCTION TO THE NERVOUS SYSTEM
The nervous system is responsible for carrying signals to your brain from the rest of your body,
such as when you make your arm move, or if you’re digesting a tasty meal. It is also responsible
for sending signals from the rest of your body to the brain, such as if you slice your finger open
with your anatomy scalpel.
The nervous system has two main divisions. These are the central nervous system and the
peripheral nervous system. The central nervous system contains your brain and your spinal
cord. Think of it as being in the center of your body. The peripheral nervous system is
everything that leaves the spinal cord, including the nerves innervating your muscles and your
organs.
The peripheral nervous system is further subdivided into the somatic and autonomic nervous
systems. The somatic nervous system is what innervates your muscles and causes voluntary
motion, and it also lets you feel pain and sensation from your skin. The autonomic nervous
system controls involuntary things, like your heart, lungs, eyes, and other organs. The autonomic
nervous system is further subdivided into the sympathetic and parasympathetic nervous systems.
The sympathetic nervous system, which basically causes your organs to respond to “fight or
flight” reactions, is organized differently from the parasympathetic nervous system, which
causes you to “rest and digest.”
To understand the functions of the sympathetic nervous system, imagine yourself running away
from a villain. You want to run fast, so more blood is pumped to your muscles. You want to
breathe faster, so your airways open wider, called dilation. Your pupils dilate. You don’t want to
release waste, so your bladder and colon relax. Your heart also pumps faster. These are the
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functions of the sympathetic nervous system. You can remember this because the sympathetic
nervous system is used for “fight or flight.”
The parasympathetic nervous system typically has effects opposite to that of the sympathetic
system. It constricts your airways, lets your stomach digest and your intestines move food down,
lets you release waste, and slows down your heart. You can remember this because the
parasympathetic nervous system “rests and digests.”
If you understand the functions of these systems, you’re in good shape so far, because you’ll
delve further into the functions of these two systems and how to block them in classes like
neuroscience, physiology, and pharmacology.
As we said previously, the CNS consists of the brain and spinal cord. All the nerves come from
here and go to their various locations throughout the body. The sympathetic nerves come from
the thoracic and lumbar sections of the spinal cord, thus they are said to come from the
thoracolumbar region. The parasympathetics come from the cranial region in the form of
cranial nerves (CN), or from the sacral region, and thus are said to come from the craniosacral
region.
The nerves you see throughout the body are composed of nerve cells. A nerve cell has a cell
body, a myelinated axon, and dendrites. Now, there are three types of nerve fibers. This may be a
bit detailed, but as it’s in your syllabus and could show up on a miniboard, it’s best to know it.
The three types of fibers are unipolar, pseudounipolar, and multipolar.
A unipolar cell is basically a cell just like in the picture below. A pseudounipolar neuron has one
long axon, basically, with a cell body in the middle. And a multipolar neuron has one cell body,
but an axon that splits into many parts to go to many different targets. This plays a role once we
discuss more about the sympathetic and parasympathetic nervous systems.
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Image taken from http://www.alongnaturestrail.com/images/nerve_cell.jpg
Take a look at the picture below, which is a section of a spinal cord, most likely thoracic. See the
black area? That’s called gray matter, and in general it contains cell bodies. The whitish area
surrounding it is called white matter, and it mostly consists of axons. There are three areas of
importance in the gray horn. See how it looks almost like an “H”? The long part is called the
dorsal horn, and this is where all the nerves that make you feel stuff are from. The shorter part
is called the ventral horn, and it’s where all the stuff that makes you move your muscles come
from. And see that little spike in the middle of the H on both sides? That’s the lateral horn, also
known as the IML column, and this is where the sympathetic nerves come from. So, if the
sympathetic nerves only come from the lateral horn, in which sections of the spinal cord do you
find the lateral horn? Only in the thoracic and lumbar segments!
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Image Taken From http://medstudy.webmd.idv.tw/histo/display.php?ID=NjQ=
There are different fiber types that you have to know about. The first is called General Somatic
Efferent (GSE). Let’s take a look at what the individual words mean. “General” distinguishes
this nerve type from the special type, which you’ll learn more about in neuroscience. “Somatic”
means body, and in the case of nerve fibers, it basically means that this type of nerve is
innervating a muscle. Finally, “efferent” means that it’s going to innervate something and make
it move, rather than receive a signal such as temperature, pain, etc. Signals going from the
outside to your brain, like pain and stuff, are called “afferent” fibers. General somatic afferent
(GSA) fibers receive signals from your skin and stuff, like if you stab yourself with your pencil.
General visceral efferent (GVE) fibers go to organs, since viscera pretty much means organs.
GVE fibers are both sympathetic and parasympathetic. Finally, the last fiber type that you really
have to know for now are general visceral afferent (GVA) fibers, and they collect sensations
from your organs, like if your stomach stretches, and brings them back to your brain. There’s
debate over whether GVA fibers are technically part of the sympathetic or parasympathetic
systems, so don’t worry about that for now.
Fiber Type
GSE
GSA
GVE
GVA
Location of Cell Bodies
ventral or anterior horn
dorsal root ganglia (DRG)
lateral horn aka IML
dorsal root ganglia (DRG)
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THE SOMATIC NERVOUS SYSTEM
Let’s talk about the somatic system first, as it’s probably the least complicated one. The
somatic system consists of GSA and GSE fibers. The GSA fibers come from the dorsal horn,
while the GSE fibers come from the ventral horn. Let’s first talk about the GSE fibers. Please
look at the pictures in your syllabus or textbook as we discuss this. Now, the GSE fibers start in
the ventral horn, and they exit the spinal cord as a ventral root. They combine with the dorsal
root to form a mixed spinal nerve. Since the afferent and efferent fibers mixed, every fiber that
leaves from this point will have some afferent and some efferent activity, although they probably
have a majority of one or the other. After becoming a mixed spinal nerve, the fibers will break up
into dorsal and ventral rami. The dorsal rami goes to your back to innverate the skin there, as
well as some muscles like the erector spinae. The ventral rami will innervate the vast majority of
skin and muscles in your body. Now, these nerves typically have two functions. The GSA fibers
innervate your skin, called cutaneous innervations, and the GSE fibers innervate muscles. The
nerves that innervate your skin usually come from a single spinal level, like C8 or T2. However,
the nerves that innervate your muscles are a collection of nerves, like C5, C6, and C7, as you’d
find in the brachial plexus.
Now, recall that each ventral and dorsal root came from a specific spinal cord level. The
ventral roots of these nerves travel into what you call plexuses, such as the brachial plexus,
sacral plexus, and so on. Once these nerves enter the plexus, they mix to form nerves like the
axillary nerve, radial nerve, and ulnar nerve. These nerves go on to innervate the muscles, like
the pronator teres and brachioradialis. The cutaneous nerves, which as you may recall come from
the individual spinal cord levels, are also in these mixed nerves, but they leave at their right point
to innervate their area of skin, called a dermatome. You can think of the mixed spinal nerve sort
of like a train. Passengers, like the cutaneous nerves, get off at certain stops, and the train keeps
moving on. It’s important to remember the direction these fibers travel in. Those innervating the
muscles go form the spinal cord to the muscles. But those innervating the skin actually go from
the skin, into the nerve, and back to the spinal cord, since they are transmitting messages from
the outside world back to your brain.
Let’s talk a bit more about the GSA fibers. Those start at their point of origin, such as the
skin, and carry sensations like pain, touch, and so on back to your CNS for processing. We said
before that the GSE cell bodies are located in the anterior or ventral horn. Where are the GSA
cell bodies located? You would think it’s in the posterior or dorsal horn, but in actuality, there’s
a little structure outside the spinal cord called the dorsal root ganglion (see picture below), and
that’s where these cell bodies are located. It also happens to be where the GVA cell bodies are
located. So, the cells that carry afferent fibers travel from their point of origin on the skin, organ,
etc. and travel back to the dorsal horn without synapsing, and they have their cell bodies in the
DRG. Now, let’s move on to the sympathetic nervous system.
“How to Survive” Guide – Structure- Review of Nervous System
Image taken from http://www.laesieworks.com/spinal/pict/SpinalCord.jpg
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THE SYMPATHETIC NERVOUS SYSTEM
The sympathetic nervous system is a bit more complex in organization than the somatic
nervous system. The sympathetic nervous system is located in the thoracolumbar region,
typically between T1 and L2 to L3, although you may see different values in different books. So,
remember we said that the sympathetic GVE fibers have their cell bodies in the lateral horn, also
called the IML? From there, how do they get to their target? They can actually do this in four
different ways, but they all start out the same way. First, they leave the IML through the anterior
horn. After this, they also leave via the ventral root, but instead of going along through the rami,
they go through something called the white communicating ramus. This structure is connected
to the sympathetic trunk, which is sort of like an elevator for the sympathetic nerves, in that
they allow the sympathetic nerves to travel up, down, etc. So, these nerves enter the white rami,
and in MOST cases they synapse in the sympathetic trunk with another sympathetic cell. After
this, they can travel up, down, or stay at the same level, and then leave through the gray
communicating ramus to go to their target organ, like the blood vessels, erector pillae muscles
on the hair (they make your hair stand up), and some sweat glands, like those on your palm.
Don’t worry about the really specific areas they innervate, because these will be highlighted in
neuroscience, physiology, and pharmacology (yeah, this stuff pretty much stays with you for a
while). Please note that the white rami are myelinated while the gray are not; this is a major
difference between the two. So, the first three things sympathetic nerves can do is1.) Synapse in the sympathetic trunk and leave at the same level
2.) Synapse in the sympathetic trunk and travel up and leave at a higher level
3.) Synapse in the sympathetic trunk and travel down and leave at a lower level
But we said there were four things that a sympathetic nerve could do, right? A nerve fiber
can also enter the sympathetic trunk, through the white rami, and leave through the gray rami
without synapsing. From here, it goes to what’s called a plexus. Now, don’t confuse this with
the somatic nerve plexuses, like the brachical plexus. This is a autonomic plexus, such as the
celiac plexus and superior mesenteric plexus in the abdominal region. Once it travels here, it
synapses in the plexus, and the next nerve cell carries the signal to the target organ, like the
appendix or stomach. There’s always an exception to the rule, and there is one specific nerve
fiber that travels without synapsing anywhere, not in the sympathetic trunk, not in the plexus, but
actually goes straight to the target organ itself. This is the nerve that innervates the adrenal gland,
which are on top of your kidney. The adrenal gland has special cells that act receive these signals
directly from the nerve that came straight from the IML, with no synapses whatsoever. So, I
guess you can say there’s actually five things that sympathetic can do, the three we listed above,
and the following4.) Enter and leave the sympathetic trunk and synapse in an autonomic plexus, and then go to
their target organs
5.) Don’t synapse anywhere at all and go straight to the adrenal gland
Now, about those splanchnic nerves. The sympathetic GVE fibers that synapse in the
plexus and go to their target organs are called abdominal splanchinc nerves, which you’ve seen
as the greater, lesser, and least splanchnic nerves in the thorax and abdomen. However, there are
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also cardiopulmonary splanchnic nerves, and these act like regular sympathic nerves in that
they do synapse in the sympathetic trunk, leave out the gray ramus, and go to the esophagus,
heart, etc. to innervate those structures. So please remember, there are two types of sympathetic
splanchnic fibers, the abdominal and cardiopulmonary types, but the main difference between
these two are that the abdominal ones DON’T synapse in the sympathic trunk, but the
cardiopulmonary ones do. I believe that sympathetic nerves are also multipolar neurons, but
please confirm that in your notes or textbook.
Now, once the non-splanchnic sympathetic nerves leave the gray ramus, they travel with
the mixed spinal nerves. But they’re like the cutaneous nerves, in that they don’t mix in the
somatic plexuses (like the brachial plexus), but travel with the nerve and leave at different points
to innervate their target structures.
So, if the sympathetic system is only from T1 to L2, at which spinal cord levels can you
find the white rami? From T1 to L2 only, because that’s the only place sympathetic nerves enter
from! What about the gray rami? That would be the entire spinal column, since sympathetic exit
all over the body. And that also means that the sympathetic trunk runs the entire spinal cord. The
highest part of the sympathetic trunk contains the superior cervical ganglion (that’s just the name
for the highest place where the sympathetic nerve can synapse), and this is important for
innervating stuff in your head, like your eyes.
This should cover what you need to know about the sympathetic nervous system at this
point, so let’s jump to the last system, the parasympathetic nervous system.
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THE PARASYMPATHETIC NERVOUS SYSTEM
The parasympathetic nervous system is craniosacral, so it’s carried by cranial nerves and
pelvic splanchnic nerves. Don’t confuse these splanchnic nerves with the cardiopulmonary and
abdominal splanchnic nerves, which are sympathetic; these are parasympathetic splanchnic
nerves. We’ll get to that in a bit, but first, the cranial nerves. You’ll encounter these in the head
and neck unit of anatomy, and they are really important for the organs in your head and more.
But all cranial nerves don’t carry parasympathetic fibers. The ones that do are cranial nerves 3, 7,
9, and 10. The major one, which you’ve seen or will see, is the vagus nerve, CN 10, which is
also called the “wanderer” because it travels all over the body.
The pelvic splanchnic nerves come from S2-S4, and they provide parasympathetic
innervations to everything from part of the transverse colon down, including the bladder. The
parasympathetic nerves are also involved in penile erection, while the sympathetic nerves are
involved in ejaculation; you can remember this by the mnemonic “point and shoot” for
parasympathetic and sympathetic. Now, please remember that the parasympathetics don’t deal
with the sympathetic trunk, that’s only for sympathetic nerves. Parasympathetic nerves don’t
synapse in the sympathetic trunk, but they synapse in those autonomic plexuses we mentioned
before. The ganglion is where two nerves synapse. The preganglionic fiber for parasympathetic
nerves are long, since they have to travel all the way from the spinal cord or your head to a
plexus. The postganglionic fibers are short. In contrast, since the sympathetic nerves synapse
early on in the sympathetic trunk (in most cases, that is), they have short preganglionic fibers,
but long postganglionic fibers. Please remember this! Pop question- do the abdominal splanchnic
nerves have longer pre- or post-ganglionic neurons? They have longer preganglionic, since they
go to the plexuses. What about the cardiopulmonary splanchnic nerves? Longer postganglionic,
since they actually synapse in the trunk.
Finally, we get to the GVA fibers, which may or may not be part of the autonomic
nervous system, as there’s debate as to whether they are or not. But the GVA fibers are pretty
much like your GSA fibers. They travel from their target organ, like the heart or stomach, and go
back through the dorsal root ganglion into the dorsal horn and back up to your brain for
processing. In case your wondering in more detail how they travel back, they will enter back into
the spinal nerve through the white ramus, and travel back to the dorsal horn, but this is a bit of
extra detail, so don’t worry too much about it.
A quick note- because of the way the nervous system is structured, if you feel pain in an
organ like the heart, sometimes you can feel it in other places, like your arm. This is called
referred pain.
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ONE LAST NOTE: LEARNING PATTERNS
An important thing to notice in the nervous system is that there are certain patterns that
you can identify that will make learning the nervous system less difficult. For example, in the
head and neck, there are 4 important ganglia that you will find in the autonomics system. They
are the otic ganglion, the cilliary ganglion, the pterygopalatine ganglion, and the submandibular
ganglion. What is special about these ganglia is that they all have a similar structure. Within each
ganglion, you will find 3 types of fibers passing through them: sensory fibers, parasympathetic
fibers and sympathetic fibers. The sensory fibers suspend the ganglion and are always come from
the trigeminal nerve (Cranial Nerve 5). The sympathetic fibers arise from the superior cervical
ganglion and always wrap around blood vessels nearby the ganglion. Lastly the parasympathetics
always come from a Cranial Nerve that carries sympathetics, either Cranial Nerve 3, 7, 9 or 10.
This is just one example of the patterning that you see in the nervous system. Once you begin to
recognize that such patterns exist, learn the rules and apply them, learning the nervous system
becomes far easier.
This should be pretty much all the things you have to know, in this course, concerning the
autonomic nervous system. This guide should help you understand your textbook and syllabus
readings a bit better, so you can hopefully ace these questions on the exam and the miniboard!
Good luck!