In this tutorial, we're going to look at the branches of the brachial plexus. This follows
on from the first tutorial I did which outlined the location and the anatomical relations
for brachial plexus.
So you can see here, I've got a dissected brachial plexus. We've got the roots, C5,
C6, C7, C8, T1 here. We've got the trunks, the divisions lying under the clavicle. And
then you've got the cords distally.
So we've got this nice diagram here. We've got the roots, C5-T1 here. We've got the trunks
-- the superior, the middle and the inferior trunk. We've got the divisions. So each trunk
divides into an anterior and a posterior division. And then we've got the cords.
So coming off the roots, we've got two nerves that you need to know. The one up here is
called the dorsal scapular nerve. You can see its origin here on the C5 root. So the
dorsal scapular nerve innervates the rhomboid muscles. You can see the rhomboid muscles
here, the minor and major (rhomboid minor and major) attaching from the spinous processes
to the medial border of the scapula. That's innervated by the dorsal scapular nerve.
The other nerve that you need to know that comes from the roots is called the long thoracic
nerve. This originates from three roots -- C5, C6 and C7. It innervates the serratus anterior
muscle. This is the serratus anterior muscle, which attaches to the ribs and the costal
surface of the scapula.
So from the roots, you've got the dorsal scapular nerve innervating the rhomboids and you've
got the long thoracic nerve innervating the serratus anterior muscle.
So next, we come onto the branches of the trunks of the brachial plexus. So just like
the roots, there's only two nerves you need to remember and also, they come from the upper
part of the brachial plexus. The two muscles you need to know both begin with S and they
both come from the superior trunk. I just remember them as the S-nerves. So you've got
'superior trunk' and you've got two nerves beginning with S. You've got the suprascapular
nerve and you've got the nerve to the subclavius. So subclavius and suprascapular nerve.
So the nerve to the subclavius comes from the superior trunk and it innervates the subclavius
muscle as the name suggests. The subclavius muscle is this little muscle here which sits
between the first rib and the clavicle. It originates on the 1st rib and inserts onto
the lower part of the clavicle.
So the other nerve is the suprascapular nerve which you can see coming off here. This nerve
innervates the supraspinatus and infraspinatus. So the suprascapular nerve is called the suprascapular
nerve because it supplies muscles which sits on top of the scapula, so they're supra+scapula.
So you can see here you've got the supraspinatus above the spine of the scapula and the infraspinatus
sitting below the spine of the scapula. These muscles are innervated by the suprascapular
nerve and these two muscles are part of the rotator cuff group of muscles.
So next, we've got the divisions of the brachial plexus. So you've got the trunks dividing
into anterior and posterior divisions and the divisions don't have any branches coming
off them.
So after the divisions, you've got the cords which are formed from the divisions joining
together. So the superior trunk gives off an anterior division which becomes the lateral
cord when combined with the anterior division of the middle trunk.
So the posterior cord is quite easy to remember because it's right at the back, so it receives
all the posterior divisions of the three trunks. So you can see this division here coming off
the posterior division of the superior trunk, the posterior division of the middle trunk
and the posterior division of the inferior trunk coming together to form the posterior
cord.
So the medial cord is formed by the anterior division of the inferior trunk. The posterior
cord is formed by the contribution of the posterior branches from all three trunks.
And the lateral cord is formed by the anterior branches, anterior divisions of the superior
and middle trunk.
So it's useful to get your hand around these anterior and posterior divisions because the
anterior divisions give rise to cords which are associated with the anterior compartment
and the posterior divisions are associated with the posterior cord which is associated
with muscles of the posterior compartment.
Because the lateral and medial cords are formed by anterior divisions, they're associated
with muscles to do with the anterior compartment. And because the posterior cord is formed from
posterior divisions, it's associated with muscles of the posterior compartment.
So what I'm going to do now is before talking about the nerves which come off the sides
of the cords, I'm going to talk about the terminal nerves. You've got five terminal
nerves, which is where these cords terminate. There's a simple way of remembering these
terminal nerves with a mnemonic, which is fairly memorable. It's 'my auntie raped my
uncle'. It's not the cleanest mnemonic, but it helps to remember things.
The first nerve is the musculocutaneous nerve, which comes off the lateral cord. Then you've
got the axillary and radial nerve, which come off the posterior cord. You've got the median
nerve, which is formed by two branches which comes off the medial and lateral cords. They
unite to form the median nerve. And then you've got the ulnar nerve. So 'my auntie raped my
uncle' -- musculocutaneous, axillary, radial, median, ulnar.
So now you know the terminal nerves of the cords. We can now look at some of the branches
that come off the sides of the cords.
We'll start with the lateral cord. You know from the mnemonic that the first terminal
nerve is the musculocutaneous nerve. You also know that the median nerve is formed by branches
that come off the lateral and the medial cords. So as well as the musculocutaneous nerve,
you've got the lateral root of the median nerve.
And then you've got this little nerve here coming off the lateral cord. This is the lateral
pectoral nerve. This innervates the pectoralis major muscle. The pectoralis major muscle
is this large muscle here.
So now we'll talk about the branches coming off the medial cord. So again, you know that
the median nerve is formed by the union of branches that come off the medial and lateral
cords. The lateral cord provides the lateral root of the median nerve, so the medial cord
therefore provides the medial root of the median nerve.
You also know from the mnemonic that the ulnar nerve comes is the terminal nerve of the medial
cord. So you already know that we've got the medial root of the median nerve and we've
got the terminal ulnar nerve.
So we've got a few other nerves that you can see. We've got three other nerves. These are
pretty straightforward to remember because they're the M-nerves. Remember the S-nerves
on the superior trunk? Now we've got the M-nerves on the medial cord. It's quite easy because
you know that there was a pectoral nerve on the lateral cord, so the lateral pectoral
nerve. We've got the medial pectoral nerve on the medial cord. So the medial pectoral
nerve is the most proximal nerve coming off the medial cord.
And then you've got two nerves to do with sensation to the skin. So you've got cutaneous
nerves. You've got a cutaneous nerve to the upper arm and you've got a cutaneous nerve
to the forearm.
So the arm comes before the forearm. It's more proximal than the forearm, so the more
proximal nerve is the medial cutaneous nerve of the arm. And then after that, you've got
the medial cutaneous nerve of the forearm. So you might also hear these nerves referred
to as the medial brachial cutaneous nerve and the medial antebrachial cutaneous nerve.
So 'brachial' just refers to arm and 'antebrachial' means forearm.
So this diagram here just shows the sensory distribution of these nerves. So the medial
cutaneous nerve of the arm supplies the skin over the medial side of the arm. So you can
see this thin strip of innervation here in yellow. The medial cutaneous nerve of the
forearm has a larger distribution where it innervates the skin.
So that should be quite straightforward. From the mnemonic, you know you've got the medial
root of the median nerve and you've got the ulnar nerve, which are the terminal nerves.
And then you've got the three M-nerves. So you've got nerves to do with the pectoral
muscle and you've got nerves to do with the supply to the skin, the sensory supply to
the skin, the cutaneous nerves.
With the posterior cord, there's a nice mnemonic for remembering the five branches. From the
mnemonic before, 'my auntie raped my uncle', you know that the axillary and the radial
nerves come off the posterior cord. So you've got the axillary nerve here and the radial
nerve here.
The other mnemonic for remembering nerves from the posterior cord is ULTRA. So you've
got the upper subscapular nerve, the lower subscapular nerve, the thoracodorsal nerve
and then you've got the radial and axillary nerves.
So first, we've got the upper subscapular nerve or the superior subscapular nerve. Then
we've got the thoracodorsal nerve. Then distally here, we've got the lower subscapular nerve.
And then the axillary and the radial nerves.
So remember we saw the suprascapular nerve coming off the superior trunk here. Now, we've
got the subscapular nerves. The suprascapular nerves innervated muscles which sat on top
of the scapula, so now the subscapular nerves innervate muscles to do with the underside
of the scapula.
So the upper subscapular nerve supplies these subscapularis muscle. The subscapularis muscle
is this muscle which sits underneath the scapula and internally rotates the humerus. And then
you've got the teres major muscle, which also medially rotates the humerus.
So the upper subscapular nerves supplies the subscapularis and the lower subscapular nerves
innervates the subscapularis and the teres major muscle.
So the thoracodorsal nerve innervates this large muscle, which I've just added on to
the model. This is the latissimus dorsi muscle. This adducts, extends and medially rotates
the humerus.
So that's the branch of the posterior cord. Remember the mnemonic, ULTRA. U is upper subscapular
(that's the most proximal branch of the posterior cord); L is lower subscapular; that comes
after the thoracodorsal, which is the T part (T is thoracodorsal); and then you've got
RA, so radial and axillary. That's ULTRA.