So now that we've discussed the
different branches of the spinal nerves,
let's talk about the axons from
the anterior rami that form networks
on both the left and
the right side of the body
by joining with axons from adjacent
nerves to form what we know as plexuses.
All of the anterior rami form plexuses with the
exception of the anterior rami of T2 through T12.
These nerves are referred to as
intercostal or thoracic nerves
and they go directly to the intercostal
spaces instead of forming these networks.
So the first of the plexuses that we
will discuss is the cervical plexus.
This plexus supplies the skin and
the muscles of the head, neck
as well as the superior portion of the
shoulders and chest and the diaphragm.
The cervical plexus extends from
the anterior rami of C1 through C5.
And of these, one of the nerves that eventually
comes from this plexus are the phrenic nerves.
The phrenic nerves are very important
because they supply the diaphragm.
Damage to the phrenic nerve
causes an inability to breathe.
The next plexus is the brachial plexus.
This plexus is a bit more complex.
It provides almost the entire nerve
supply to the shoulders and the upper limbs.
It is made up of roots, trunks,
cords, divisions and branches.
Because of the complexity
of the brachial complex,
let's take a closer look at
the different components.
First you have your roots
which are the anterior rami.
Next you have your trunks.
The roots or the anterior rami
are going to unite to form trunks.
There are three types of trunks: the
superior, the middle and inferior trunk.
After the trunks, you have divisions.
Posterior to the clavicle, the trunks are going to
diverge into the anterior and posterior division.
In the axilla under the arms, these
divisions are going to unite to form chords.
And finally, you have the branches which will
form the principal nerves of the plexus.
The next plexus is the lumbar plexus.
This is going to supply the
anterolateral abdominal wall,
the external genitalia
and parts of the lower limb.
This plexus is much less
complex than the brachial plexus.
This plexus includes nerves
that will go to the iliohypogastric,
lateral cutaneous nerve of t high,
and the lumbosacral trunk.
The last plexus is the sacral plexus.
This plexus is going to extend
from the rami of L4 through S5.
This is going to be divided into the
posterior and anterior division.
In the posterior division,
you have the superior gluteal,
the inferior gluteal
and the nerve of piriformis.
On the anterior division, you have the
tibial as well as the common fibular nerve.
These two nerves together are going to
form what's known as the sciatic nerve.
If you've ever had sciatica or nerve damage that
results in shooting pain going down your leg,
then you've had damage to the sciatic nerve.
So now let's talk about dermatomes.
A dermatome is an area of skin that provides
sensory input to the central nervous system
via a posterior root of one of the pairs of
spinal nerves or via the trigeminal nerve
which is not a spinal nerve
but instead is a cranial nerve.
The trigeminal nerve is specifically gonna serve
most of the skin of the face and the scalp.
So knowing which spinal cord
segment supplies each dermatome
makes it possible to locate
damaged areas of the spinal cord.
As well, we use dermatomes in order
to block pain using anesthetics.
The dermatomes however can
overlap and if the overlap is considerable,
then if you only damage one of the nerves,
you may have very little loss in sensation.
As well if you're trying to
block pain using anesthetics,
you may need to block multiple nerves
if there is considerable overlap as well.