As far as I’ve been able to ascertain “it’s just nerves”.
More specifically, it’s down to reflexes. Reptiles have tiny brains and, having evolved to deal with low body temperatures and the associated slow nerve conduction, they have retained a lot of the control of their body in the spinal cord, rather than in the head. Because the control resides in the spinal cord, those functions keep working just fine after decapitation and they work for a long time after death.
Humans also have reflexes based in the spine, but very few of them. the most well know of these is the knee jerk reflex that doctors use to rest reflexes. If you’ve ever had that test administered, you know that it is not accompanied by any sensation. The hammer hits then tendon in the knee, the tendon stretches and fires a message to the spine, the spine fires back a message to thee muscle sin the thigh. “You” don’t experience anything because the brain was never involved. You only become aware of the effects because your brain feels the leg move.
It’s the same with reptiles. A pain receptor fires in the skin of the back, and the whole body whips around to allow head to see what is causing the pain and dislodge it. The brain isn’t involved at all in the movement so there is no sensation for the animal. But the effect is that it looks like a decapitated snake is trying to bite the person skinning it using it’s bloody stump.
You can do similar freaky things with frogs. If you destroy the brain by passing a needle down the spinal cord, you can turn the frog on its back and the corpse will turn itself back upright. If you put a drop of acid on its back, the hind leg will come up and scratch at the spot where the acid is. And the corpse will continue to do those things for *days *after the animal is dead if you pithed it properly.
And just in case you think the animal may not be dead, you can actually bisect the animal just behind the front legs. And the hind legs will still try to scratch off any acid on the back, even when the front half of the animal is lying on a table in the next room.
This is all very zombie-like behaviour, but it’s not causing pain to the animal. The brain, the centre of awareness, is dead, or at least it sure ain’t connected to the area that is feeling the pain. They are just complex reflex arcs. The animal isn’t in any way aware of what the body is doing, any more than you are aware of a pain in your knee when it jerks. Because these are involuntary reflexes, there isn’t even any evolutionary *point *is having the animal aware of them. They are going to happen whether or not the animal uses processing power to monitor them.
The only caveat is what happens with the head. In humans severing the spinal cord mean instant loss of consciousness, and the loss of blood pressure means that you never regain consciousness. That may or may not be the case with animals. So while the moving body certainly can’t feel anything. I wouldn’t swear that the severed head is totally unaware. It won;t be aware of what is happening to the body, of course. But it might be able to see. :eek:
