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Which part of the brain is the first place (from top to bottom) where all 31 pairs of spinal nerves (on each side) meet? Or if they all start at one place - where is that?

Nominally, the beginning of the spinal cord is the medulla oblongata, but perhaps the spinal nerves or some sort of "root" of every nerve start at the Thalamus?

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2 Answers 2

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They don’t meet.

Some framework:

  • Spinal nerves contain motor, sensory, and autonomic fibers. Each of these have different pathways.
  • Spinal nerves don’t go to the brain. Rather, they synapse in the spinal cord with other neurons which in turn go to the brain (sometimes requiring one more synapse).
  • In the case of motor neurons, we talk about upper and lower motor neurons. In the sensory system, we talk about first order and second order neurons. Explanation below.

Regarding sensory (afferent) neurons:

  • Touch and limb position sense are mediated by the dorsal column-medial lemniscus system. The first order (primary sensory) neurons from the spinal nerves, synapse on a second order neuron in the dorsal column in the medulla. There it decussates (i.e. courses to the contralateral side) and ascends via the medial lemniscus to the thalamus. There it synapses with a third neuron that courses via the internal capsule to the primary somatic sensory cortex.
  • Pain, itch, and temperature sense are mediated by the anterolateral system. These first order (primary sensory) neurons synapse at the spinal level they enter. The second order neuron decussates at that level and courses rostrally via the anterolateral column, ending up in the thalamus. There it synapses with a third neuron that courses via the internal capsule to the primary somatic sensory cortex.

Conclusion: It takes three neurons (inclusive) to get from the spinal nerve to the primary sensory cortex in the postcentral gyrus of the parietal lobe.

Regarding motor (efferent) neurons:

  • Limb muscles are innervated by circuits beginning in the primary motor cortex. These upper motor neurons descend to the medulla, decussate here, and continue caudally via the lateral corticospinal tract. At the level of the spinal nerve, they synapse with neurons in the lateral ventral horn. These are the lower motor neurons that become the spinal nerve.
  • Axial/girdle muscles are innervated by circuits that also begin in the primary motor cortex. These upper motor neurons synapse in the midbrain, descend, and decussate at the appropriate spinal level. They synapse there with the lower motor neurons that exit the spinal cord and end up in the muscles.

Conclusion: It takes two neurons (inclusive) to get from the primary motor cortex in the pre-central gyrus of the frontal lobe to a spinal nerve.

In the cortex, these circuits also don’t all come together. They are actually quite diffusely organized across the sensory and motor cortices. Here is a representation of the sensory cortex.* It is a schematic of the post-central gyrus with pictures to indicate where the signals from various body parts end up. The motor cortex is similar. (Google homunculus if interested.)

sensory

In addition the pathways described for cognitive perception and control, there are others that help with sub-conscious control of movement (e.g. those involving the cerebellum and basal ganglia), sub-cortical perception of pain signals (e.g. amygdala), and even spinal reflex arcs that never reach the brain. Not all circuits that begin or end in the spinal nerves are destined for or originate from the cortex

This is already considerably over-simplified, and the autonomic system is more complex yet and is not confined to spinal nerves, so I won’t detail it. These signals originate in and feed back to an entirely different area of the brain (mostly hypothalamus). Similar to the motor and sensory systems, at least one synapse is required to connect the brain to the peripheral nerve.

Conclusion: they don’t meet.


Reference: Martin, John. Neuroanatomy: Text and Atlas. McGraw-Hill Medical; 3 edition (March 27, 2003)
*The source of this image is: http://www.mindtrippingshow.com/mind-trip-of-the-week-17-how-do-we-feel-the-world

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Thank you very much. This is exactly the type of answer I was looking for. And though technically they don't meet - what I meant was that (according to your answer) they meet at the somatic sensory cortex / primary motor cortex. Do I understand correctly that the muscles aren't affected by the Thalamus, and the limb muscles - not even by the midbrain? And that sensory perception is not affected by the midbrain? –  user133943 Sep 15 at 18:49
    
If I understand correctly - Though the cerebellum doesn't act as a relay for these messages (both afferent and efferent) - doesn't it communicate with the rest of the body through the spinal cord (again, both afferent and efferent)? –  user133943 Sep 15 at 18:51
    
@user133943 I have edited the answer to address your second comment. Regarding the first, the nerves don't really meet, but the circuits (partially) head to the same area, so I'm glad I was able to provide some useful info. As for the thalamus, the midbrain....everything is affected by everything, and the sub-circuits are really too complex to describe here (or for me to remember - I'd be copying from my neuroanatomy text; I suggest you just buy a copy for yourself if interested!). –  Susan Sep 16 at 3:03

I am not sure whether I grasped the gist of your question appropriately, but let me provide some clarifications.

$\text{Nerve}$ is simply a collection of axons. Therefore, all the spinal nerves are just bundles of assorted nerve axons.

Now, the origin of nerve is slightly ambiguous as far as its meaning is considered. It can mean the physical origin, the place where the particular branch of nerve starts off from a main trunk or some other nervous organ. It can also mean the location of the cell bodies the axons of which comprise the nerve, the functional origin.

Now coming back to the question, the physical origin of the spinal nerves is not common. Each pair of spinal nerves originates from a specific segment of the spinal cord via two roots, the dorsal and the ventral. Therefore, each nerve originates separately and different spinal nerves cannot be traced back to a common physical origin. (The cauda equina, is actually a collection of several spinal nerves which move dorsally and periodically gives of the nerves out of the vertebral column. Although the nerves comprising the Cauda Equina, second lumbar nerve onwards, appear to have a common origin, they actually originate from their own specific segment and then move caudally as a bundle to overcome the developmental constraint of spinal cord being shorter than the vertebral column, necessitating the caudal movement of nerves within the column to emerge at appropriate points)

The functional origin is a bit more complicated. This is because each spinal nerve has a variety of nerve fibres. There are some sensory fibres with cell bodies in the dorsal root ganglion, there are motor fibres with their cell bodies in spinal cord, there are sympathetic fibres with cell bodies in the series of collateral or the sympathetic ganglia, connected to the spinal nerve via gray and white ramus communicans. There can also be cell bodies located directly in medulla which passes nerve fibres in cranial nerves as well as through its branches into the sympathetic ganglia and ultimately into the spinal nerves. Sacral nerves also contain parasympathetic nerve fibres functionally originating in the CNS. Thus, considering the functional origin, though each spinal nerve has a lot of origins, there is a slight convergence of origins for different nerves. The sympathetic ganglia and the spinal cord are two places where the origins of different spinal nerves may coincide or be structurally and functionally integrated. But it must be kept in mind that this is a very limited convergence. There is an appreciable uniqueness to the functional origin of each spinal nerve and hence we cannot say that they "meet".
IMAGE

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Thank you for your answer. However, it seems that you understood the question differently than how I meant it. For example, you wrote: "Each pair of spinal nerves originates from a specific segment of the spinal cord via two roots" (yes, I have read your whole answer. This is just an example). I'm asking "Which part of the brain...". and "Nominally, the beginning of the spinal cord is the medulla oblongata..." - I want to know which part of the brain "decides" to send a "message" through the spinal cord. (or receives, of course...). But thanks again. –  user133943 Aug 7 at 18:03

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