WHAT HAPPENS WHEN YOU BURN YOURSELF? WHAT TELLS YOU THAT IT HURTS AND MAKES YOU REMOVE YOUR HAND, FOR EXAMPLE?

First, you have to know that in your skin, you have sensory neurons, more specifically nociceptor neurons.

Sensory neurons have receptors that react when temperature increases (like a burn).

Activated  by the heat, cationic channels open themselves and let pass Ca2+ cations.

There is a depolarization of the membrane and an action potential is created.

The action potential propagates along sensory neuron axon until sensory ganglia. These ganglia are located on each side of the spinal cord. That’s where we find the cell bodies of the nociceptor neurons. (fig. 1)

Then, the action potential goes into the posterior gray columns of the spinal cord. It is relayed to another neuron  by the first synapse of pain transmission.

The message is relayed to a projection neuron which goes from the spinal cord to the brain. This transmission is regulated by interneurons which can increase or decrease it. (fig. 1)

Projection neurons can use two different ways of fibers to go up to the brain:

• By the first pathway, they go up straight to the thalamus, second synapse, and then to the cortex. 
• By the second way, axons project to the parabrachial nucleus, where they establish new synapses. This nucleus send projections to the thalamus, but also to the hypothalamus.

The first pathway leads to a sensory discriminative perception of pain. The second pathway regulates the autonomic nervous system, through the hypothalamus, which leads to an unconscious response, beyond one's control. The heartbeat are an example. (fig. 2)