Anatomy of a peripheral nerve root

It's embarrassing to admit that even though I've been a therapist for over 40 years I still have to go back to review the basics on a regular basis.

I'm a visual learner when it comes to learning the nervous system. If I can't "see" it, I can't make a memory for it.

Most images out there are so complicated and cluttered they don't really help.

I decided to make my own, post them around on the internet, as files, and here, as a sort of comic strip. The images and any content they contain are a compilation from many sources, greatly simplified. I made these images, and I'm showing them to you. My hope is that they can help a viewer learn the wiring diagrams more easily.
The images are not copyrighted in any formal way - I would hope and trust that no one would stoop to stealing them or not attributing them or trying to sell them...
I made them myself. Feel free to use them, but please link back to this blogpost, at least..



1: The basic plan

The junction between CNS and PNS is a foreign land to most manual therapy practitioners. They know it exists but don't understand it very well. So they default to imagining they can affect tissue directly with their hands.
Even more alarming perhaps, in the long ago past, a certain cargo cult mentality emerged in which human primate social groomers imagined they could affect this junction directly by bouncing around on the spine forcing noise to emerge from it. The spine houses this intricate system, but since when did a computer ever work better by banging around on the housing of it, as if it were a reluctant coke machine?

The fact is, this junction houses very long neurons that talk to each other and to the CNS/brain at many levels. These long neurons come all the way out to skin. You can touch them there. You don't have to try to bang the housing around like some kind of home renovator/carpenter. Instead, just hack into the nervous system itself - metaphorically of course. So much easier and kinder.


2. Spinal nerve components

There are a few names to learn. Do not confuse them.

Posterior (dorsal) root
Anterior (ventral) root

Those are the bits that plug straight into the spinal cord, before the peripheral spinal nerve is even a nerve. They are closest to CNS. There are no postganglionic ANS fibres in these. Our peripheral nerve isn't ready for distribution quite yet.
.......

Dorsal ramus
Ventral ramus

These are the two divisions that occur right after all the circuitry is in place, after post ganglionic fibres have added themselves, after all the fibres have sorted themselves out and are ready for the long trip to wherever they are going to end up.
The dorsal ramus heads for the back. It innervates paraspinal muscles and skin on the back of you, from the top of the head down to the tip of tailbone.
The ventral ramus innervates everything else in the whole body (below the head), all four limbs, muscles and skin. It innervates a body wall that surrounds the whole body, including the back of the body.
Wait a minute: didn't I just say the dorsal ramus innervates the muscles of the back? Yes, but not the neck and limb muscles that cover the back and enclose it, shutting it off from the world.. only a little of them, close to the spine.
Trapezius covers the back of the neck all the way down to T12.
Latissimus covers the back all the way from T6 down to the sacrum.
The cutaneous portions of the dorsal rami must pierce through both these huge flat muscular sheets to reach the skin organ. By then all they have are sensory and autonomic fibres. I think this anatomical arrangement poses a dilemma for them, could account for some portion of back pain in the population.
But I digress.


3. Ganglia

There are a whole bunch of these located just outside the spinal cord (DRGs), inside the body (chain ganglia), some of them in front of the aorta (preverebral or preaortic).
They are so interconnected that when you look at an anatomical image of them, it's bewildering, and the eye can't take in the information the first million times you try to look at it and understand it visually - there is just too much information - it looks like a big net covering up stuff. And it is. It's the body's internet. Sort of. It's necessary to slow way down, make it really simple, visually, in order to understand its workings, first. That's what this series is. A step-by-step breakdown of a cluttery looking system. It might help to visualize this horizontally, the way it is oriented in fish, in quadrupeds.

What's kind of awesome is the fact that these ganglia, this system, predates the CNS, predates the spinal cord. Fish invented the spinal cord a half billion years ago - up to then, it is thought that this nerve net was all there was. In fact, invertebrates are distinguished from vertebrates in that they do not have a CNS (spinal cord/brain).. which doesn't mean they aren't smart - octopi are quite smart according to most accounts.
Apparently a PNS suffices for a very large percent of the population on the planet.

Please note that the DRG is not to scale in this picture. In reality it's way smaller than it looks here. And it's not right up against the anterior root.

We will find out what these ganglia are for. But first...


4. Some basic spinal cord areas

As vertebrates, however, we do have a CNS. As humans, we are inordinately proud of ours, mostly of our brains. This is where the PNS, the oldest critter brain portion of our nervous system, joins the CNS, in the spinal cord, the oldest part of the CNS, or what I like to think of as the fish brain.

I always think of the spinal cord as a land spit stretching out into the ocean of the body, festooned with little docks on each side. It seems weird that the spit was there before the continent formed at one end. But it was.

5. Peripheral outflow (efferent)

A lot of people confuse efferent and afferent. Personally, this has never been a problem, but I can understand the problem. The words are pretty close in how they look.
You just have to memorize them. Ponder them until the meanings and the distinction sink in. They are opposites.

There are two kinds of "Efferent" from the spinal cord. One is from the critter brain further upstairs, and the other from the human brain (well, at least we'd like to think we are... human I mean.. )


6. Peripheral outflow (efferent)

Here are the two kinds. Basically, one is for striate muscle and the other for smooth.

The kind for smooth muscle basically runs all the physiology for the body. The brain needs to be able to intervene and change things in a big hurry, sometimes. That's what the sympathetic NS is all about.


7. Peripheral outflow: Voluntary movement

We will tackle the easy stuff first, get it out of the way. Unfortunately this is usually all that we learn in PT school. When I try to remember PT school, I'm also pretty sure this is all that was ever taught, too. 

Maybe it's different nowadays.

8. Peripheral outflow: Voluntary movement

This system lets you execute everything from the playing of Chopin, to knitting, or breakdancing, or winning the 100 meter dash in under 10 seconds. You were a lot more aware of your voluntary movement when you learned to walk, learned your first lessons about gravity. You were a lot more aware of your voluntary movement when you learned to talk, and learned what sort of effect vocalizing at various decibel levels had on those in your social environment.

Striate muscle is amazing, but it's good to remember it's just an effector tool at the end of an executive central nervous system that wants to output something to its environment through its anterior horn neurons.


9. Peripheral outflow: Autonomic

Let's take a look at the autonomic output system now.  Yeah, I know you've been dreading this. But it's not that bad. Not really. Baby steps now, baby steps...
The CNS outflow neurons for the ANS live in the lateral horn of the spinal cord.

(Yes, there are parasympathetic neurons too. They are in the top end [brain] and tail end of the spinal cord. None of them end up in the skin we ordinarily contact in manual therapy, so I'm more interested in sympathetic outflow.)


10. Peripheral outflow: Autonomic

So, first we trace the preganglionic neurons to their destinations. These are CNS neurons, and they go to ganglia. Period. Full stop. Only a short way. Not that they don't travel through a few ganglia up and down the chain until they decide to hook up with a post ganglionic fibre.. they certainly do.

They are a little bit myelinated. Therefore the communicating ramus between the anterior root and the chain ganglion is called the white communicating ramus, or white rami communicantes if we're speaking Latin, and in plural.


11. Peripheral outflow: Autonomic

Eventually they synapse somewhere, be it in a chain ganglion or a prevertebral ganglion, and then the PNS takes over, taking the info from the brain further out and down into the ocean, oops - I mean body.


12. Peripheral outflow: Autonomic

Here's the thing: if a preganglionic fibre synapses with a postganglionic fibre in a chain ganglion, it's going to head off down a familiar peripheral nerve for the soma. This is the musculoskeletal system and all the smooth muscle cells there, including all the glands and immune cells and blood vessels. This includes running all the thermodynamically obedient, heat-regulatory, heat-dissipatory layers and layers of vasculature in the thick skin organ/blubber layer that vertebrates evolved.

If a preganglionic fibre does not stop in a chain ganglion, projects further, and synapses with a post ganglionic fibre in a prevertebral ganglia, it will send info to the smooth muscle of viscera instead. Not that viscera doesn't have its own nervous system. It certainly does. It's called the enteric nervous system, and it works quite well all by itself without any help from the sympathetic nervous system, thank you very much. However, if a bear is coming at you, the sympathetic NS will intervene to stop the enteric NS so that blood flow can go into muscle instead, so you can run away from the bear. Cool eh?

It will also divert a bunch of blood from the skin organ into the muscle layer, for the same reason.
Got to escape.
Don't worry, we can cool you down later. Right now it's more important to get outta here.

But I digress.
The grey communicating ramus, or grey rami communicantes, contains only unmyelinated neurons. No myelin, no white, more grey.


13. Peripheral inflow (Afferent)

This is my favourite part of the PNS. My bias is because this is the part I can send kinesthetic messages to somebody else's CNS through, by touching their skin in the course of treatment.

People can give themselves feedback through this system, through proprioception from striate muscle. This is how you learned to play Chopin, and breakdance, in the first place, remember?

This is a very very old old old part of the PNS. It takes info to the CNS along a wide variety of sizes of fibre. The bigger and thicker and fatter and more myelinated the fibre, the faster will be the input.

Without feedback the system does not have a clue what to do. You wouldn't know where your limbs are with your eyes closed. You wouldn't know your butt is tired from having been sat on for too long.

The visual system can be hijacked into overthrowing this input, to a certain extent, through rubber hand illusions, etc.

Mostly though, I like accessing peoples' brains through their skin.
I like remembering that somatosensory fibres are so long, and so accessible, all the way out to skin, that there are only six cells between my brain and the consciously aware brain of somebody I'm treating. Three on their side and three on mine.


14. Peripheral inflow (sensory): meningeal nerve

This is a nerve we all hope never becomes sensitized.
It gives the CNS information about its own three-layer overcoat.
It's pretty short. One can't get their hands directly on it. I don't think manipulating spines in a high-velocity fashion does this nerve (or any other nerve for that matter) any favour, long term.


15. Peripheral inflow (sensory): somatosensory nerve

This is my favourite input channel - especially those fat fibres that go all the way, from skin contact, up to dorsal column nuclei in the medulla before they terminate, synapse with another neuron. There are other cool neurons in there, however - C-tactiles, thin C's in skin that transduce only pleasant sensation, or what I like to call yes-ciception, a term coined by Jason Erickson, a massage therapist.

One should remember, though, that fibres in these nerves are sending info in from everywhere inside the body, including all the nerves themselves, and the back of the head. Here is the last slide, and it shows that.


16. Peripheral inflow (sensory): from everywhere

That's it. That was easy, right? 

............

Happy holidays

Dec. 23/2014: Edit
1. Picture of meningeal nerve corrected to depict fibres arriving to more superficial area of dorsal horn

2. Picture of viscerosensory neurons corrected to depict their arrival through white communicating ramus instead of grey, and destination in more distinct areas of dorsal horn

(See? I worked really hard to get it right, and I still didn't get it quite right the first time. It's at least improved this time.)