This is a brief tutorial on the lumbar plexus.
So we're going to look at the basic structure and the location of the lumbar plexus and
a few ways to remember some of the nerves and the basic organization.
So the lumbar plexus is formed from the anterior rami of L1 to L4.
It also receives some fibers from thoracic nerve, T12.
So there are six main nerves which come off the lumbar plexus and they can be remembered
using the following mnemonic -- so it's 'I twice get laid on Fridays'.
The nerves that you need to remember are the iliohypogastric nerve, the ilioinguinal, the
genitofemoral, the lateral femoral cutaneous, the obturator nerve and the femoral nerve.
So the mnemonic is 'I (twice) get laid on Fridays'.
So the lumbar plexus actually forms inside the psoas major muscle and the branches of
the lumbar plexus can be thought of in relation to the psoas major muscle and where they emerge
in relation to this muscle.
So I've just switched over to this model here.
What we're looking at is an anterior view with the muscles of the anterior abdominal
wall dissected away.
So we've got the psoas muscle sitting here, the psoas major muscle.
We've got the six branches of the lumbar plexus sitting around this muscle.
So what you need to remember is that all the muscles except the genitofemoral and obturator
nerve emerge from the lateral aspect of the psoas major muscle.
So emerging anteriorly from the psoas major muscle, we've got the genitofemoral nerve
coming here, which I'm just drawing quite crudely on.
And then it splits into the genital and femoral branches.
And medial to the psoas major muscle, we've got the obturator nerve.
So I'm just drawing this on in yellow, but it's already sitting there.
So the other four nerves emerge lateral to the psoas major muscle.
So we've got the iliohypogastric, we've got the ilioinguinal nerve, the lateral femoral
cutaneous nerve and we've got the femoral nerve.
So now that you know the names of the six main branches of the lumbar plexus and you
have an idea of their relationship with these nerves to the psoas major muscle, we can now
take a look at which roots these nerves emerge from.
So this diagram here might look a little bit complicated, but there's a nice little way
of remembering which roots the nerves come from.
The mnemonic is '2 from 1, 2 from 2, 2 from 3'.
I'll go on the explain what this means.
So it essentially means that there's two nerves that come from one root.
There's two nerves which come from two roots.
And there's two nerves that comes from three roots.
So let's start from the top and work our way down.
So remember our mnemonic, 'I (twice) get laid on Fridays'.
So the first two nerves are the "I" nerves.
So we've got the iliohypogastric and the ilioinguinal nerves.
So the iliohypogastric nerve is superior and the ilioinguinal nerve is inferior.
So the way I remember that is that iliohypogastric has the word 'gastric' in.
Gastric refers to stomach, so the stomach is actually higher than the inguinal region,
so the ilioinguinal nerve is inferior.
So these two nerves come from one root.
So they come from the L1 root.
So that's '2 from 1'.
So next in the mnemonic is '2 from 2'.
So we've got two nerves coming from two roots.
So if we go back to the first mnemonic, we know that after the iliohypogastric and the
ilioinguinal nerve, we've now got the genitofemoral nerve.
So this nerve here is the genitofemoral nerve and it comes from the anterior rami of L1
and L2.
So the next nerve in the mnemonic is the lateral cutaneous nerve of the thigh or the lateral
femoral cutaneous nerve.
So this nerve comes from the anterior rami of L2 and L3.
So now you understand the second part of that mnemonic.
We've got two nerves coming from two roots.
So we're combining all of our mnemonics together here and our little bits of knowledge of the
lumbar plexus to work out the organization of the branches and origins of the nerves
that come from the lumbar plexus.
So the first bit of knowledge is that the lumbar plexus is formed from the anterior
rami of L1 to L4.
Our second bit of knowledge is the mnemonic, 'I (twice) get laid on Fridays'.
So this gives us the order of the nerves, the six branches that come from the lumbar
plexus.
And our third bit of knowledge is the mnemonic, '2 from 1, 2 from 2, 2 from 3'.
So we know our first two nerves are the iliohypogastric and ilioinguinal nerves and we know that two
nerves come from one root, so these are the first two nerves, so they come from the 1st
root of the lumbar plexus, so L1.
We know that there are two nerves which come from two roots.
So we look to the mnemonic and we look at our next two nerves.
So we've got the genitofemoral nerve and we've got the lateral cutaneous nerve of the thigh.
So the genitofemoral nerve comes first in the mnemonic, so it takes the first two nerve
roots.
It comes from L1 and L2.
The lateral cutaneous nerve of the thigh is the next nerve in the mnemonic, so this comes
from the next two roots.
So it comes from the L2 and L3 roots.
So now we're left with two nerves from three roots.
So by process of elimination, we know that this must be the obturator nerve and the femoral
nerve.
So these are the principle nerves of the lumbar plexus.
They innervate the largest muscle groups.
So the obturator nerve innervates the muscles of the medial compartment of the thigh and
the femoral nerves innervates the muscles of the anterior compartment of the thigh.
So these are the principle large nerves.
They originate from the largest number of nerve roots, so three nerve roots.
So you can see the obturator nerve here and it originates from L2, L3 and L4.
And as you can see, this is the femoral nerve here and it also originates from the same
roots.
It originates from L2, L3 and L4.
So just to reiterate, we've got two nerves from one root.
We've got the iliohypogastric and the ilioinguinal nerve coming from the 1st lumbar nerve.
We've got two nerves coming from two roots.
So we've got genitofemoral nerve coming from the 1st lumbar nerve and the 2nd lumbar nerve.
And we've got the lateral cutaneous nerve of the thigh coming from the 2nd lumbar nerve
and the 3rd lumbar nerve.
And then we've got two nerves coming from three roots.
So we've got the femoral nerve coming from the 2nd, 3rd and 4th lumbar nerves.
And we've got the obturator nerve coming from the same roots, so L2, L3 and L4.
So just to point a few things out in this diagram which you might have noticed, up here,
we've got the subcostal nerve, which is a branch from the last thoracic nerve, so the
12th thoracic nerve.
This nerve contributes to the lumbar plexus as you can see here.
Over here, we've got these two branches coming off the 3rd and 4th lumbar nerve.
This is the accessory obturator nerve and it's not always present.
And then over here, we've got two little nerves coming off the femoral nerve.
These are muscular branches to the psoas muscle and the iliacus muscle.