Confusion about dermatomes vs peripheral nerves

[V5RED]

I am very confused by the neuroanatomy intro we just had. My teacher has yet to respond to my email, so I figured I would try here.

We were just taught about dermatomes and peripheral nerves.

For the dermatome part, the teacher showed us this map and told us that the spinal nerve corresponding to a particular place on that map will provide the innervation for that area of the map. She said if you impinge a spinal root, the area of its dermatome is where numbness will be.

She then told us about peripheral nerves formed from a bunch of spinal nerves which had their own map of areas they innervated and it looks like the spinal nerves spread out and innervate all over the place, not just at the dermatome as shown on the previous map. She told us that if you impinge the spinal root, it will cause numbness to many areas.

My question is how can these two jive? It looks like they are contradictory since she first tells us that each of these regions is innervated by a given spinal nerve and then tells us that the spinal nerves actually converge into plexi and then branch out into various peripheral nerves that are each a combination of spinal nerves.

Based on the dermatome info, impinging a given spinal root should make its dermatome and only its dermatome numb. Based on the peripheral nerve map, impinging a given spinal root should make parts of a bunch of dermatomes numb and it should be extremely difficult to figure out which nerve is impinged since all the parts it is innervating are innervated by a bunch of spinal nerves.

Thanks for your help.

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[alamo4]

The short answer is that both things are happening. A neuron can start in a nerve root, merge into a plexus and then branch out again in such a way that it joins with nerves from other spinal roots and then travel together as a peripheral nerve to a particular region and then divide up again in such a way that it innervates the particular stripe of skin that corresponds to a dermatome. Again, the neurons of particular roots can split up in a plexus and join with neurons from other from other roots, and then go out again as peripheral nerves, but they will be arranged in such a way that they respect the dermatome boundaries.

I got a very nice book which I saw the neurology residents and fellows carrying around that is a useful reference. "Aids to the Examination of the Peripheral Nervous System". This will show you how to basically test every single nerve and root for sensory and motor innervation.

http://www.amazon.com/gp/product/0702034479/

I know it seems totally weird and illogical at first, but when you see how it fits together, and you know the circuits, it actually is quite logical and interesting. Someone who knows all this stuff can diagnose weird patterns of peripheral nerve injury just by clever exam very quickly, and basically predict the EMG study results.

Did your instructor show the dermatome map of a quadruped, like a dog or a horse? It actually makes the human innervation more understandable if you look at a different mammal.

It is confusing stuff.

 

[alamo4]

Based on the dermatome info, impinging a given spinal root should make its dermatome and only its dermatome numb. Based on the peripheral nerve map, impinging a given spinal root should make parts of a bunch of dermatomes numb and it should be extremely difficult to figure out which nerve is impinged since all the parts it is innervating are innervated by a bunch of spinal nerves.
Thanks for your help.

To answer more clearly, impinging on a spinal root can give numbness or pain in a dermatomal distribution. Damage to a more peripheral nerve can cause a problem in the sensory distribution of that peripheral nerve. What you professor might have been saying is that you can use this help localize the problem. If something spans dermatomes, but is in the sensory distribution of a particular peripheral nerve, than you can localize the damage to some point in the peripheral nerve, likely distal to the plexus. If the patient has a problem that is along a dermatome, but one that is covered by two different peripheral nerves, then you know that the problem is likely to be proximal to the plexus, perhaps at the spine.

 

[V5RED]

Okay, so let's see if I am getting this correct by putting it in my own words. I think I am getting what you are saying.

The dermatome represents an area of the skin that any particular spinal nerve is generally restricted to innervating. Ie the L2 nerve will be innervating the L2 dermatome and not other dermatomes like the L5 or T8. It might be part of a peripheral nerve that spans these regions, but it's activity will be restricted to its own dermatome. Thus it will provide the innervation to the area of that peripheral nerve that coincides with its own dermatome.

If I were to have loss of function and it were restricted to a dermatome, then it is highly likely I have suffered damage at the root of that dermatome. If I were to suffer loss of function at portions of multiple dermatomes(but not any whole dermatome), then it is likely that I have suffered damage to a peripheral nerve as opposed to suffering damage at the spinal root of the nerves.

Is that about the size and shape of it?

I really appreciate the help.

Btw, she did not show anything but a human dermatome map.

 

[alicealicealice]

Just a point, dermatomes are sensory, hence remember when talking about dermatomes you are talking about sensation, not all functions of the spinal nerve at that level (though obvi an impinged root may also have motor deficits etc but that is not why is being referred to by the word dermatomes or the dermatomes map)

A good correlation is herpes zoster (shingles). It sets up shop in the dorsal root ganglion of the spinal nerve (you'll learn about this, it's where the cell bodies of the sensory neurons of the spinal nerve are located). When activated, it crawls down the axons of those sensory neruorns that originate from that spinal level ( which may in turn contribute to several different peripheral nerves) and therefore causes pain in a very typical dermatomal distribution of that particular spinal nerve.

 

[178624]

http://antranik.org/peripheral-nervous-system-spinal-nerves-and-plexuses/

scroll to second image. and don't make this more difficult than it is. read her comment above. very good stuff about shingles. you'll understand soon...

 

[178624]

if you had damage at the dorsal root ganglion then yes, you will lose the dermatome. damaging the actual spinal nerve will cause loss of function and dermatome. loss of function will affect all the peripheral nerves associated with that spinal nerve. these can regenerate though cause they're outside the CNS. I think the actual dorsal root ganglion can regenerate too as long as it's outside the cord.

 

[178624]

this is why you can do a nerve graft from another part of the body and it will grow to make muscles have function restored.

 

[178624]

btw, perfect opportunity for you to learn about lumbar disc herniation. go look it up, you'll be able to put the pieces of the puzzle together.