the

Phrenic nerve

I was doing some teaching the other week

and we were in a cadaver talk about the

pericardium and there was some confusion

about the phrenic nerve so I thought

I'll do a quick video for fiber phrenic

nerve in front of nervous it's really

really important there are two of them

there's one on either side the reason

they're so important is that they

innervate the major muscle of

respiration so let's have a talk about

what the fronting nerve does it has

sensory roles as well and there's a bit

about referred pain that's important and

we can talk about where it appears in

the neck and how it gets down to the

diaphragm go back from Amsterdam in the

early hours of the other morning that

students tell me I have a little bit

less energy than usual than if that's a

good thing or a bad thing

anywho so the functions of the front

nerve I've said that it innovates the

diaphragm so it has motor innervation to

the diaphragm the major muscle the

changes of volume within the thorax and

helps us breathe in and breathe out but

it's also sensory from the pericardium

you remember we looked at the

pericardium there's the fibrous

pericardium the thick tough connective

tissue sheet surrounding light but the

heart and holding it in place and it's

also sensory from the diaphragm

certainly from the central diaphragm

around the outside here of course we've

got intercostal nerves and the subcostal

nerve and those are also going to

innervate the bits of the diaphragm

around the outside and it's also sensory

from the other structures that it passes

nearby so as we go down have a think

about that but it's not just sensory

from the pericardium is also sensory

from the mediastinal pleura the parietal

pleura that's next to it of course the

inferior surface of the diaphragm is

covered with peritoneum

so it's also sensory from areas of the

peritoneum so what happens if you injure

the funny nerve then right whereas as I

said there are two finders

either side so you can imagine them as

as innovating half of the diaphragm each

so obviously if you cut a funny nerve

you lose the sensory functions that we

talked about but the the most important

thing you lose is motor function - half

of the diaphragm so if half of the

diaphragm is then weak the muscle is

weak and it won't contract this means

that when you breathe in normally the

diaphragm would be pulled down the

muscle would contract the volume inside

the thorax would increase and you would

breathe air in when you breathe out the

diaphragm relaxes returns to a dome

position the volume inside the thorax

decreases and air is pushed out right so

that's what normally happens but if one

side is weak when you when you breathe

in and you're trying to increase the

volume in the thorax the the paralyzed

part of the diaphragm they're paralyzed

or weak heavy diaphragm is actually

going to get pulled upwards by that

change in pressure so you so you're

working in the opposite direction to

instance to expect it so if the

hemidiaphragm is paralyzed when you

breathe in it'll go up the other side

will go down and when you breathe out

the working side will go back up again

and the the weak or paralyzed side will

kind of resettle back to his neutral

position paradoxical weird huh

how could this nerve be damaged well

it's visible in the neck here so it

could be damaged by trauma to the neck

but of course once it's passed into the

thoracic cage it's fairly well protected

so it could be damaged by trauma through

surgery if a surgeon is working in this

area it could be damaged by you know

neuro pathological conditions of the you

know conditions that damaged nerves

generally can damage the phrenic nerves

but also you've got to consider tumors

inside the thoracic cage so a lung tumor

if it's medial in the lung it could

press on the phrenic nerve and cause for

any nerve dysfunction and in itself

damage to the phrenic phrenic nerve

isn't often immediately obvious in a

patient you would have to look quite

carefully for it because there are so

many other muscles involved in in

respiration in breathing in and out that

the term it's kind of compensated for

right okay so an interesting note about

sensory innervation and referred pain

something that you might have

experienced yourself the frilly nerves

are going to pass through the diaphragm

and innervate the diaphragm from the

inferior surface and the sensory

information carried back is is pain and

proprioceptive and stuff like that so

when the diaphragm is irritated the

friendly nerve is carrying that

innovation back to the roots of the

phrenic nerve which it in the neck so

when you're running on the right side

you've got the liver which is suspended

from the diaphragm so the liver may well

be bouncing up and down and irritating

the diaphragm causing an unpleasant

sensation called a stitch right usually

tend to get a stitch on the right side

if you get a stitch on the left side is

possibly because you just fill your

stomach with water and it's doing the

same thing the stomach is irritating the

diaphragm so because of these

intercostal and sub costal nerves that

are innervated in the diaphragm here you

feel the pain localized down here but if

you keep going you tend to find the pain

moves up into the shoulder and this is

referred pain so the pain is referred to

the shoulder because the the

supraclavicular nerves are also coming

out of the neck at the same levels of

the phrenic nerve and they're innovating

the skin around here and it seems to be

because those nerves are entering the

spinal cord at the same level the brain

perceives that pain is coming from up

here whereas really the pain is in the

diaphragm down here and this is

something we often see with viscera

because we're we're not used to feeling

pain from organs from viscera but we're

used to feeling sensation from the skin

we have grown up learning that then pain

often gets referred to places where the

pain actually isn't and you can use this

in clinical testing

there are a number of things you can do

to to see if there is blood in the

peritoneum and that sort of things and

if someone's to see if someone's

diaphragm is irritated causing shoulder

pain so what is the root of the phrenic

nerve then see three four five keeps the

diaphragm alive so spinal nerves c3 c4

and c5 mostly c4 form the phrenic nerve

and we have a number of muscles here so

here's sternocleidomastoid we have to

look d2 sternocleidomastoid then we can

see there's scalene muscles the scalene

muscles there are three of them anterior

middle and posterior and between the

anterior and middle scalene muscles we

see the brachial plexus appearing that's

this big puddle of yellow here but

between the anterior and middle scalene

muscles we also see the phrenic nerve

appearing this is where it pops out from

those three roots it then moves on to

the anterior surface of the anterior

scalene muscle and this is how I find

the phrenic nerve what I'm just saying I

look for the muscle and then I look for

the nerve around here and it's going to

descend and as it descends it's going to

come across the subclavian vein but um I

am tired

it's gonna come it's gonna come across

the subclavian vein and the subclavian

artery and we can see on this model and

we see in in the body that the phrenic

nerve on both sides will pass posterior

to this to the subclavian vein and

anterior to the subclavian artery and

then at that point then it is

disappearing deep to the clavicles and

is moving into the thoracic cage do I

want to take you apart we man not really

let's get an easier model to take apart

the the front nerves on both sides

continue down into the thorax and here

we have the structures at the root of

the lung so the funny nerve is going to

run anterior to the structures of the

root of the lung and now we're getting

to the heart the heart remember is

covered in the pericardium so the

friendly nerve is going to travel kind

of between the Parekh

and the the mediastinal pleura I mean

they're like they're like a a fashio

they're serous membranes they're you

know that we're talking about two layers

like two layers two sheets of paper and

the friendly nervous kind of between

them but really the phrenic nerve blends

with them as much as anything else when

we look inside the body want me to set

the funny nerve out as it runs down here

it disappears into the pericardium and

we kind of have to tease it out of the

pericardium that's where it really runs

to and then it continues to descend and

on the right side we'll take the heart

eight of course we've got this very

convenient hole here this is the hole

for the inferior vena cava so the right

front nerve is going to go through the

hole for the inferior vena cava and then

it passes to the inferior surface of the

diaphragm whereas the right funny nerve

is going to pierce through the diaphragm

and innovate the inferior surface of the

diaphragm and in that way the friendly

nerve is going to innervate the muscle

of the diaphragm sensory innervation

from the diaphragm and is going to carry

sensory innervation back from the

parietal well from the from the fibrous

pericardium and the parietal mediastinal

pleura and that's it that's the course

of the phrenic nerve now if you can

imagine that inside your minds then you

can imagine how of course remember

everything is very closely packed

together inside this space just as it is

another part of the body so you can

imagine how a mass forming in the lung

on the medial side could push into that

for any nerve it's an important nerve

the anatomy is quite straight forward is

well worth you knowing about it all

right the next time you get a pain in

the shoulder you can think about your

diaphragm especially if you're out

running if you do go running and you get

a stitch just one more everything seems

to firm up and get stronger and stitches

go away trust me if you get a stitch on

the left side when you're running well

just drop drinking water before you go

running you don't need to drink water

when you're money you'll be fine it's a

little bit of thirsty it's fine for like

30 minutes yeah anyway see you guys next

week okay

so she still talk about running

[Music]

you

[Music]