Why no referred pain?

[GreenLedbetter]

I looked in Snell and Moore and couldn't find answers. Internet search wasn't fruitful either.

With all of the nerves running into and separating from one another why are signals not jumbled very extensively? With respect to the brachial plexus in particular, why is sensation from a localized point on the upper limb not referred to the entire limb? How can dermatomes exist if their nerves become jumbled within the brachial plexus? Is it due to some sort of differential synapsing among the specific nerve fibers?

My prof answered by referring to axial line angles. I couldn't find those in the books either.

Thanks

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[osli]

The dendrites of a nerve cell for say a touch receptor in your finger will travel uninterrupted to the cell body of that neuron in the spinal cord. And for muscle control, the motor neuron in the spinal cord will send an axon uninterrupted all the way to that muscle fiber group. These axons and dendrites stay separate and distinct as tiny fibers the entire way along their journey, in this case from finger to spine.

Now, all of these individual fibers are bound together into a visible structure we call a "nerve." Think of "visible nerves" as hollow tunnels in which the little fibers run. In the brachial plexus, the tunnel system may have convergences and branches, but the fibers are only crossing from one tunnel to another... the fibers themselves do not merge or branch.

In the same way, the spinal cord is just a big tunnel with individual fibers running here and there along very specific paths. If the spinal cord was a true merging of nerves running into and out of it, you'd never be able to control a specific finger or toe.

Wait until you get to neuroscience/neuroanatomy. The name of the game is tracing an axon from a specific neuron in a specific area of the brain all the way through the brain, brainstem, spinal cord, and peripheral nerves. It may synapse once or twice along the way, but the path is largely discrete.

 

[bcat85]

I second the above.

 

[Hayden2102]

Nice answers.

Yeah, referred pain can basically only occur when there is a synapse that can be confused for another synapse... because nothing actually synapses within (for example) the brachial plexus, the axons move on and aren't confused for each other by the brain. Referred pain can occur in the spinal cord where visceral pain fibres synapse in a level of the spinal cord that also receives sensory fibres from a particular dermatome. The brain interprets the pain as from the dermatome by mistake.

Hope that helps.

 

[badasshairday]

Nice answers.

Yeah, referred pain can basically only occur when there is a synapse that can be confused for another synapse... because nothing actually synapses within (for example) the brachial plexus, the axons move on and aren't confused for each other by the brain. Referred pain can occur in the spinal cord where visceral pain fibres synapse in a level of the spinal cord that also receives sensory fibres from a particular dermatome. The brain interprets the pain as from the dermatome by mistake.

Hope that helps.

About the reffered pain.
I thought viceral afferent fibers do not synapse at all... Where do they synapse? They don't at the sympathetic ganglion. Also somatosensory fibers don't synapse either at the dorsal root ganglion. So where the heck are they synapsing causing this reffered pain confusion?

 

[ZonaRidicularis]

z

 

[abooodMD]

There are a bunch of receptors for sensation in the body such as: vibration (pacinian corpuscles), high discrimination touch (Meissner's), muscle tendon (Golgi tendon organ), etc.

The signals detected by these receptors are also transmitted through a bunch of different types of nerves; most of which can be differentiated based on their diameters. Example: type A, C, beta, alpha, gamma never fibers.

To make things more complicated... these signals are processed in their own ways once it reaches the brain.

About referred pain and dermatomes - usually referred pain involves the visceral organs. It's believed that visceral pain fibers synapse with neurons in the spinal cord that also receive pain signals from the skin. This is what causes referred pain.

Dude is having pain in his left arm and neck. What does this sound like? Heart attack of course. He's percieving heart pain as pain in his arm and neck because the visceral afferent signals from the heart converge on the same spial cord neurons that recieve input from his left arm/neck/jaw.

I have no idea if that answered your question or not but it was interesting for me to look this stuff up 😀

this is an answer. 😉

 

[TMP-SMX]

Yeah. Once you get into the reticular formation in the brain you start to feel visceral reflexes like nausea or malaise. It is part of the reason that people respond differently due to emotional effects of pain. Also involves awareness/consciousness.

But yes, even before that the spinal afferents synapse on common portions of the spinal cord and can jumble visceral-somatosensory inputs giving referred pain.

I thought neuro last year was really interesting.

 

[Hayden2102]

About the reffered pain.
I thought viceral afferent fibers do not synapse at all... Where do they synapse? They don't at the sympathetic ganglion. Also somatosensory fibers don't synapse either at the dorsal root ganglion. So where the heck are they synapsing causing this reffered pain confusion?

Within the spinal cord?

 

[badasshairday]

Within the spinal cord?

yeah. i was just being confusing. lol.

 

[ZonaRidicularis]

z

 

[SnowTown]

Why don't the signals in these wires get jumbled?

 

[ZonaRidicularis]

z