1
$\begingroup$

My understanding of the "sensory pathway" is that its a linear, directional pipeline as follows:

  1. Nerves (fire various signals depending on the type of sensors they are)
  2. Fibers (transmit signals from nerves to spinal cord)
  3. Spinal cord (transmits signals from fibers to brainstem)
  4. Brainstem (transmits signals from spinal cord to brain)
  5. Brain (the somatosensory cortex register signals as pain/pressure/temperature/etc.)

So again, my understanding is that sensory nerves are 1-way sensors, and that once they fire a signal, its 1 and only ultimate destination is the brain.

So, as a precursor to my question, if anything I have stated is incorrect, please begin by correcting me!


I read that pinched nerves in the back and neck can cause neuropathy ("pins and needles") all over the body: hands, feet, face, etc.

But how can this be? If my understanding of the sensory pathway is correct, then a pinched nerve in one's neck should only send "pins and needles" signals directly to the brain; it should not at any point forward/relay signals on to any other areas of the body.

To state that a pinched nerve in one's neck could possibly cause neuropathy in their arm, then that insinuates there is some connection between the pinched "neck nerve" and the nerve(s) in the arm that are experiencing the pins and needles sensation.

In fact, unless I completely misunderstand the entire sensory pathway and how pain signals transmit, this implies that a pinched nerve sends signals out to the spinal cord, down the spinal cord and into, say, an arm, and that the sensory receptors in the arm subsequently react to these signals.

How is this possible?!? Is there some kind of feedback mechanism at play where nerves in the neck and back can relay signals on to other areas of the body, instead of just feeding directly into the "spinal cord => brainstem => cortex" pipeline?

$\endgroup$
  • 1
    $\begingroup$ Individual nerves are one-way, but there are plenty of nerves running both ways along the "pipeline" $\endgroup$ – Luigi Dec 17 '14 at 1:23
  • $\begingroup$ Thanks @Luigi (+1) - however I'm not following you. First you say that individual nerves are 1-way, but then you say nerves run both ways...which is it?!? Are there certain types of nerves that are bidirectional? If so, what are they called? How would they fit in to my scenario of pinched neck nerves causing neuropathy in an arm or other area? Physiologically, how does all this map out? Thanks again! $\endgroup$ – smeeb Dec 17 '14 at 1:32
  • 1
    $\begingroup$ By "plenty" I mean that there are more than one. Think of a four-lane highway. Four run north, four south. Each individual lane goes in only one direction, but the highway carries cars in both directions. $\endgroup$ – Luigi Dec 17 '14 at 1:35
  • $\begingroup$ Thanks again @Luigi (+1 again) - I feel like you're holding back! Don't be afraid to hit me with some science here! I get your highway anology...but does that mean that the 4 lanes going back are for motor nerves? Or can a sensory nerve handle "traffic" going both ways? $\endgroup$ – smeeb Dec 17 '14 at 1:36
  • 1
    $\begingroup$ Motor nerves control your movement (brain->muscle) and sensory nerves send info to the brain (extremities->brain). No single nerve handles two-way "traffic" $\endgroup$ – Luigi Dec 17 '14 at 1:38
2
$\begingroup$

Reverse signals (dendrite -> axon) do occur in neurons, and are called back propagating action potentials (bAPs). However, whatever role bAPs play in the nervous system at large is subtle/small enough that we don't really understand them at all.

In any case, as @luigi points out, pinched nerves don't have anything to do with bAPs. The reason why a pinch in one place (the neck) feels like it has an effect in another place (the arm) is because some neurons trace long (on the order of several feet) pathways through the body. Ultimately, the sensory input of everything below your head passes through neurons in your neck at some point. If there is a pinch in the neuron in your neck that relays sense information from your arm, that neuron in your neck can activate spuriously, even in the absence of any stimulus acting on your arm. In that case your brain will interpret the spurious activation of that neck neuron as being the result of some kind of strange arm stimulus, and so you feel pins and needles.

See here for more information on the sensory cortex in your brain and the "sense image" it maintains of your body.

Edit

To clarify what I mean by a "pathway", enter image description here

The part marked "proprioceptors or mechanoreceptors" is what's buried in your skin. If you poke a mechanoreceptor, it evokes a signal in the neuron marked "first-order". In some cases, that first-order neuron will reach all the way to brainstem (as in the picture above), and in some cases that first-order neuron will interface with an intermediate neuron in the spine, but the effect is the same in any case. A first-order neuron stimulates a second-order neuron, which in turn stimulates a third-order neuron, etc., all the way until the original signal arising from the mechanoreceptor reaches the somatosensory cortex. A pinch in any of those first, second, etc. order neurons will potentially send spurious signals to the somatosensory cortex.

Second edit:

Rereading your question, I think a lot of the trouble that you're having has to do with your understanding of how the brain assembles sense information. In order to feel pain in your arm, it is not necessary for anything to be happening to the arm itself. Your arm could be perfectly healthy and normal, but as long as your brain is receiving a signal equivalent to the one produced by a damaged arm you will still feel the same sensation of pain.

$\endgroup$
  • $\begingroup$ Thanks @tel (+1) - however your answer completely pummels my understanding of this "sensory pathway"! My understanding is that nerves connect to fibers, which connect to the spinal cord. The spinal cord connects to the brainstem and then the brain. When you say "Ultimately, the sensory input of everything below your head passes through neurons in your neck at some point.", this seems to imply that the spinal cord connects back to fibers/nerves and then the brainstem after that... Can you confirm all the major elements of this pipeline/pathway? $\endgroup$ – smeeb Dec 17 '14 at 3:12
  • $\begingroup$ ...And perhaps elaborate on how a signal sent from a neuron in my hand passes through a neuron in my neck? Again, as I understand the pathway, any neurons in my neck would connect to the spinal cord, then the brainstem, then the brain; and a nerve in my arm would never send signals that pass through a neuron in my neck. Thanks again! $\endgroup$ – smeeb Dec 17 '14 at 3:13
  • $\begingroup$ @smeeb tel means that your spinal cord passes through your neck, so you can pinch a nerve in your spinal cord (which happens to be in the part of your spinal cord that passes through your neck) and it could affect the sensation your brain feels in your arm. Please look at this discussion for more $\endgroup$ – Luigi Dec 17 '14 at 4:07
  • $\begingroup$ Pinching a nerve in a different part of your neck, i.e. trachea or throat, will not and can not have any effect in an unrelated area (arm) $\endgroup$ – Luigi Dec 17 '14 at 4:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.