Overwhelming fatigue; pins and needles; numbness or dead spots on hands/arms or feet; twitches or loss of power in limbs (especially if one-sided rather than symmetrical). Ringing in the ears; eye twitching (nystagmus) or unable to focus. Persistent cough.
They are all conditions connected with the peripheral nervous system (PNS).
The PNS has two parts - sensory and motor. The sensory part lets you feel things - if you hold something in your hand then nerve endings on the palm of your hand send signals up your arm, up your spine, to your brain which your brain can interpret (each joint in your fingers also sends a signal to say what angle it’s at, the muscles in your forearm say how stretched they are and how hard they’re working, and so on).
The motor part lets you do things. Your brain sends a signal to each individual muscle, for the muscles that operate your hand, to tell them how much to pull or relax (muscles can’t push) to get the perfect grip on the thing you’re holding (or the piano you are playing).
The rest of the nervous system is the central nervous system (CNS), which is essentially the brain and spine. Your ear and eye, even though they’re inside your skull, are actually part of the PNS because they aren’t “brain” (ear served by vestibulocochlear nerve = Cranial VIII; eye served by optic (I) for sensory, and oculomotor (III), trochlear (IV) and abducen (VI)).
B12 is vital for manufacturing cell membranes - without B12 the membrane can form badly. Nerves have a lot of cell membrane - the Schwann cell which surrounds each axon (the part of the nerve cell that transmits the signal) is essentially tens or hundreds of layers of cell membrane as insulation. Just like the wires in a mobile phone, if they aren’t insulated from each other then the nerve doesn’t work.
It either does nothing (signal doesn’t get through), or sends a signal when it’s not supposed to (picks up a signal from a nearby axon and sends that on by mistake).
So when you feel pins and needles, it could be because the nerves that serve your skin are picking up signals from other nerves on the way up your arm, or in your neck. Your stretch receptors are trying to say “20% stretch” which is an “on” signal on the nerve that detects 20% stretch. But the “on” signal jumps across to a skin nerve cell (it might go up both). So your brain gets a signal from a skin nerve cell, and assumes there’s pain on the skin at that point. The stretch signal keeps jumping onto different skin nerve cells (they’re all in a bundle of nerve axons together), and you get the pins-and-needles effect. Whereas if the “on” signal jumps to a motor nerve axon, it may cause a muscle to twitch which makes your arm or leg or finger twitch and you can’t control it.
Alternatively the “on” signal going from the skin nerve cell may not get through at all. You get a numb patch - even if you stick a pin in you can’t feel it! Or if it’s a motor nerve that can’t get through, you could have weakness (eg foot drop) or paralysis of one or a few muscles.
You can have the same effect in one of the cranial nerves, making you see dark patches
It isn’t always B12 deficiency, but it might be if you can’t find another cause. It’s the same. For a motor nerve, it either causes twitching (eg eye movements, funny focus) or you can’t make a muscle move (similar to lazy eye). For sensory nerves you might get flashing lights or dark patches, or tinnitis. And the persistent dry cough could be if a throat surface nerve keeps sending a “something stuck here” signal to the brain.
Yes. Lack of B12 means that the cell membrane can’t form properly (it shows up on the nerves because they have so many layers of membrane). But supplementing with B12 lets the nerves repair themselves. Don’t expect instant results, but expect some things to get better over time.
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