When getting immunized, how does the needle go through the skin so easily and painlessly?

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This might be just me, this excludes the pain afterwards.

In: Biology
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Partially because it’s super sharp and partially because it’s really small. When it’s sharp, it slices a small hole rather than brute forcing one. That’s why mosquito bites don’t feel like somebody is stabbing you.

To answer that properly, we first have to ask “how do we feel pain?”. It turns out that we have specialized nerve endings in our skin for feeling pain. They are imbedded vertically in the skin waiting to be stimulated by a cut or impact. When you stimulate one, you feel a tiny prick of pain. When you stimulate many, you feel a lot of pain. Now these receptors aren’t evenly distributed throughout the body. There are more on the hands and other sensitive areas, and fewer elsewhere. That’s why a papercut to the finger hurts more than one on your leg, for example. So, when you get a shot, it’s in an area with fewer pain receptors. Since the needle is small and moving in the same orientation as the nerve endings, it only activates a few pain receptors, or sometimes none at all. If they had to give the shot into the palm of your hand at an angle, you’d probably not bother getting the second shot unless you were REALLY motivated.

The needles are tiny, very sharp, and also cut at an angle like this |/ which allows them to puncture the skin smoothly, and ideally without hitting many nerve endings.

The needle is very sharp, and also very small.

They choose a place where your pain receptors are relatively spread out. This makes the skin less sensitive to pain compared to, say, the tip of your fingers or your lips. You can measure your pain receptors on your arm if you like to get a sense of this.

Take a sharp pencil or pen and (carefully!) press it to the skin on the back of your forearm to the point of pain – without breaking the skin! Take another pencil and press it to your skin about ten centimetres away. You should feel both pencils. Move the second pencil a centimetre closer to the first and see if you can still feel it. At some point, it will feel like one dot of pain on your arm even though the pencils are a couple of centimetres apart.

Sharp clean cuts don’t hurt much. Paper cuts hurt because they essentially microscopically saw away the line that they “cut”. A cut from a knife doesn’t hurt nearly as bad (y’know, so long as the knife’s clean) because the damage is only one very thin line. A lot less pain sensors in your skin are activated in a sharp cut. Those needles are very sharp and hardly cause any damage to the skin, and therefore don’t cause much pain.