This is not an exhaustive overview of the muscles of the arm and forearm, but it demonstrates some of the tricky relationships that often catch people up. Some of the key points of compression are also noted and the muscles are coloured per their innervation. One thing not mentioned in the doodle that is important to notice is the way that the aponeurosis of the biceps veers medially (ulnarly) this is why it is also a supinator and not just an elbow flexor.
The branches of the three main terminal branches of the brachial plexus can be difficult to remember. Even worse is trying to remember where all of those pesky compression points are and why it is that you get some symptoms with some and not others.
This diagram attempts to clarify the branches of the radial, median, and ulnar nerves and where they can get squished along the way. There are of course, slight anatomic variations, but this is a good starting point. I’ve even included where the famed Martin-Gruber anastomosis and the Riche-Cannieu anastomosis are, since they can make an otherwise (totally not) straightforward examination of a median or ulnar nerve palsy more muddied since both carry motor fibers between the two nerves.
Most interestingly is John Struthers, whose namesake structures compress the median nerve as a ligament and the ulnar nerve as an arcade.
I’ve drawn the brachial plexus before showing more of its anatomical relationships (which is actually why the trunks and cords are named as they are). As I’m gearing up studying, I created this more schematic diagram of the plexus, including the distal targets (mostly the muscles but some sensory too).
Hopefully this will help you figure out “where is the lesion?” when you are faced with a brachial plexus question on your exams (and in life) as well.
I’ve also included a printable version for your printing and pasting-up-to-the-wall-to-passively-absorb pleasure.
Long thoracic: serratus anterior
Dorsal scapular: rhomboids, levator scapulae
Suprascapular: supraspinatus, infraspinatus, sensory to the AC & GH joints
Nerve to subclavius: subclavius
Lateral pectoral: pec major (clavicular head), sensation to pec
Superior subscapular: subscapularis (upper part)
Thoracodorsal (aka middle subscapular): lat dorsi
Inferior subscapular: subscapularis (lower part), teres major
Medial pectoral: pec minor, pec major (sternocostal head)
Medial cutaneous n. of arm: sensory to medial surface of arm (tiny area)
Medial cutaneous n. of forearm (antebrachial cutaneous): sensory to skin over biceps and medial forearm
Amyotrophic Lateral Sclerosis (ALS) is a degenerative disease of the motor neurons in the brain and spinal cord. It progressively affects all the muscles in the body but there is no known cause and no treatment. Only about 5-10% of cases are inherited while the rest are sporadic.
The neurons ALS affects are primarily the upper motor neurons. These are the ones that originate in the brain and travel down the spinal cord. These neurons then synapse with the lower motor neurons in the ventral horn, and it is the lower motor neurons that go directly to the muscles.
In ALS there are both upper motor neuron and lower motor neuron symptoms. As the neurons die, a constellation of symptoms including numbness, weakness and paralysis emerge. Eventually the paralysis progresses leading to inability to speak, swallow and breath. There is no cure for ALS and treatments only help with the symptoms, they do not slow the progression of the disease.
So you may have seen a lot of ice bucket challenges over the last few weeks but please support this cause as it is a horrible disease that up until now had almost no recognition or support. So please donate to The ALS Association (alas.org).
And in case you get tired or jaded seeing your social media full of these videos, watch this one of my father doing it. He’s not an emotional guy, but he has lost more than his fair share of friends to this disease.
The brachial plexus is the bane of many med students’ existence during any sort of neuro block. So many nerves, so many connections, so many seemingly arbitrary names of different sections. It’s just a woven mess of misery. (especially when they start getting into the “where is the lesion” questions)
Thus I’ve decided to have a couple posts about the brachial plexus, hopefully demystifying it to some extent. This first doodle is about the brachial plexus and its anatomical relationship to some of the structures that show why anatomists who named the parts weren’t as crazy as they seem.
Important structures to remember because they explain why parts are named the way they are:
- Anterior and posterior scalene muscles
- Subclavian artery
- The arm (in its anatomical position)
Vertebrae: There are 7 cervical vertebrae and 12 thoracic vertebrae. To make things confusing the cervical spinal nerves exit ABOVE their named vertebrae (except for C8) while the thoracic, lumbar and sacral exit BELOW. This messes up the whole numbering system because there are SEVEN cervical vertebrae but there are EIGHT cervical spinal nerve roots. The brachial plexus generally includes the nerve roots C5-T1*
* I say generally because there’s are anatomical variations such as a “prefixed” plexus that goes from C4-C8 and a “postfixed” plexus that goes from C6-T2
Scalene Muscles: The brachial plexus is nestled between the scalenes in the neck. At this point the plexus is oriented up and down and therefore the trunks are superior (closest to your noggin), middle, and inferior.
Subclavian Artery/Anatomical Position: The artery is in front of the plexus at the level of the trunks and then the plexus starts to wrap around it (or at least seems to because we don’t keep our arms straight out to our sides in “anatomical position” at all times). The cords are named for their relationship to the artery. One is lateral (again, if the arm was held out to the side), one is posterior and one is medial (think closest to armpit).
Subdivisions of the Brachial Plexus
The parts are: Roots/Trunks/Divisions/Cords/Branches or, as I remember them being a classy east coast Canadian: Real/Truckers/Drink/Cold/Beer
Then you might think, “But how do I remember which of the terminal branches comes off where?” For that I think of the two “M” branches being on the M: Musculocutaneous, Median and (M)Ulnar and that the whole thing together can just be said as “MARMU” Pick the mnemonics you want, the brachial plexus is rife with them. I personally just like the sound of the word marmu.
A prolapsed (slipped) disc is when the squishy innards of the disc (nucleus pulposus) bulge out past the stiffer wall of the disc (annulus fibrosis). The problem is that sometimes when this happens, the bulge can impinge the spinal cord or the spinal nerve root. This could result in an anterior cord syndrome (remember this doodle) or it could just knock out the nerve root, resulting in a specific radiculopathy (check out this doodle for where to check for numbness and weakness).
The tricky thing to remember is that though, for example, the L3 root exits at L3, if the L3,4 disc herniates, it doesn’t hit the L3 root but the L4.
Slipped L3,4 disc = L4 nerve injury
The disc hits the nerve after it has branched off the spinal cord, but before it has exited the vertebral canal.
The lower leg (and especially the foot) have a pretty fancy pattern of skin innervation by the terminal branches. For example, the skin of the foot is innervated by 7 separate nerves:
- Superficial peroneal nerve
- Deep peroneal nerve
- Sural nerve
- Saphenous nerve
- Calcaneal branch of the tibial nerve
- Medial branch of plantar nerve
- Lateral branch of plantar nerve
Also good to keep in mind that the anterior compartment is innervated by the deep peroneal nerve, the lateral compartment by the superficial peroneal nerve and the posterior compartment by the tibial nerve.
The femoral triangle is a convenient triangle where the femoral nerve, artery and vein pass from the abdomen to the leg. The best part about this is that they’re all quite superficial, making it a great place to stick things in (place catheters, nerve blocks, etc).
Because the femoral triangle is often getting poked at for various reasons, it’s important to know what’s where because you don’t want to be hitting the nerve when you meant for the artery (or vice versa).
The triangle is made up by the sartorius, adductor longus and inguinal ligament and if you just remember NAVVAN.
The danger zone on the face is a little triangle from the corners of the mouth up to the bridge of the nose. The reason it has such an epic name is because due to its venous drainage (from the facial veins and pterygoid plexus) there’s the possibility of infection traveling from that area into the cavernous sinus.
The cavernous sinuses (there’s one on each side) is an area posterior to the maxillary sinuses and lateral to the pituitary. It receives blood from the superior and inferior ophthalmic veins, superficial cortical veins and the basilar plexus. The blood then drains into the petrosal sinuses (you guessed it, there’s a superior and inferior one of those too) and then those drain into the internal jugular vein.
The thing about the cavernous sinuses a whole lot of important stuff passes through it.
- CN III (occulomotor)
- CN IV (trochlear)
- CN V1 (ophthalmic branch of trigeminal)
- CN V2 (maxillary branch of trigeminal)
- CN VI (abducens)
- Internal carotid (and the sympathetic fibres on the carotid)
This means that if you are so unfortunate as to have infection tract back into it, there can be some nasty consequences like meningitis and cavernous sinus thrombosis which will generally present as problems involving those nerves.
The abducens and carotid are more medial and thought to be more bathed in the warm loving venous drainage meaning these are generally the first to show signs of a problem a-brewin’.
The arm has too many muscles. It also has too many nerves.
The problem is that someone comes in with weakness or numbness and you need to think, “Where in the brain/spine/nerve root/bits of brachial plexus/terminal branch is the actual problem?!”
One of the ways to help suss this out is by testing the deep tendon reflexes.
- Only biceps reflex absent – might be problem with musculocutaneous nerve or C5
- Both brachioradialis and triceps absent – problem with the radial nerve
- Only triceps absent – potential problem with C7
Of course you should correlate the reflex findings with the sensory findings and motor strength (remember the good old ASIA exam for testing specific nerve roots) to help determine if it is a terminal branch issue or something higher up.
And don’t forget to grade those reflexes!
|4+||Very brisk +/- Clonus|
Generally upper motor neuron (UMN) lesions result in hyperreflexia while lower motor neuron (LMN) lesions result in hyporeflexia.