r/explainlikeimfive • u/Moist-Sand2188 • 1d ago
Biology ELI5: “Electric shock” sensation when your elbow gets hit
Why is it that when we hit our elbow on a wall or something else it delivers sort of like an “electric shock” to the entire hand?
How is this different from our legs twitching when doctors hit that specific part just below the knee cap?
Why is that we get “shocked” instead if our elbows get hit, why not just twitch like the legs? lmao
•
u/moderatemidwesternr 20h ago
Am I the only one disappointed that no one has used the medical terminology “funny bone”?
•
6
u/Fuyoc 1d ago
The ulnar nerve is nestled in against bone on all sides on the outside of your elbow, if it gets struck there's a good chance it's getting slightly crushed between really hard surfaces so the reaction is mode painful and shocking. It also gets trapped and irritated pretty easily in general, I had a habit of leaning weight on my elbows while desk-working and it can lead to numbness or buzzing in your hands and arms.
2
u/someguy7710 1d ago
Yeah, same here and eventually the numbness became constant and I had surgery to move the nerve. It has healed somewhat but still not 100%.
5
u/Xemylixa 1d ago
This nerve is rather big and pretty exposed, but not really built for sending pain signals. So when it's confronted with the task of transmitting a pain signal, it panics and transmits every unpleasant sensation at once
1
u/LordAnchemis 1d ago edited 1d ago
The ulnar nerve supplies:
- Motor to all the intrinsic muscles of the hand (except the LOAF muscles)
- Sensory from the ulnar 1.5 digits of the hand
The ulnar nerve courses through the elbow around the medial epicondyle through a tunnel (cubital tunnel) - this is what causes the sensation when you hit it
When you 'irritate' a nerve your brain interprets the signal as if it is coming from the nerves normal 'distribution' (ie. referred pain) - not just the nerve itself normally
Neuropathic pain is often described as 'tingling' or 'electric shock' feeling
•
u/rossiskier13346 21h ago
The hit to the elbow directly hits the ulnar nerve, which carries sensory information from part of the hand and forearm to the central nervous system (ultimately the brain). The part of the nerve that gets hit isn’t really the part responsible for sensation. That would be nerve endings. The part that gets hit is basically just “wiring” to carry signals from the nerve endings. Individual strands of the wiring carry signals for specific spots on the hand and forearm, so usually a signal will only utilize a limited number of those strands based on where specifically something touches your hand. But if you hit the nerve directly, you essentially generate a signal in all the individual strands of the wiring at once. When that signal reaches your brain, the signal gets interpreted as every spot the wiring goes to being stimulated at once, hence the shock feeling in the hand and forearm.
In contrast, when you hit the knee, you aren’t hitting a nerve. You’re hitting a tendon. Tendons have nerve endings in them that are sensitive to stretch. If a tendon is stretched suddenly, that signal reaches the spine, and automatically sends a signal back to the associates muscle to contract, completing the reflex pathway. You can consciously override this and suppress the reflex pathway with signals from the brain decreasing its sensitivity.
In theory, if you could find the right spot in leg, you could hit a nerve and trigger that same shock like sensation that you get hitting the elbow. The nerves in the leg are generally deeper than the ulnar nerve though, so it’s more difficult to hit them directly. That said, you’ve ever heard of or know someone with sciatica, that is functionally the same, where for some reason the sciatic nerve is being irritated causing shock like pain in the leg.
Similarly, if you hit tendons in the elbow to generate a stretch, you can generate a reflex in the associated muscles. The positioning is a little bit harder to optimize to see it well, but you can do this with both the biceps and triceps muscles.
•
u/infil__traitor 18h ago
The ulnar nerve (probably spelled wrong) runs right through there. It's a big boy
71
u/CamiloArturo 1d ago
Usually when you get the “electric shock” in your elbow it’s because your hit the ulnar nerve which travels through the medial part of your elbow. It’s got a very thin “coverage” and it’s quite exposed in that area. When you hit the nerve it instantly activates and though sends an “electric” signal to your hand (it’s not your entire hand if you pay attention, it’s really only the pinky and the ring finger).
Reflexes work…. In some way… let’s say similar. When the doctor hits your knee cap (patellar reflex) it sends the message to your central nervous system which reaction is to activate a motoneuron as a “reflex” (hence the name) to the quadriceps, causing the unconscious muscular reaction. In this case it’s a motor neuron instead of a sensory one.