Journal of Pain & Relief
Open Access

Reaction of pain is observed by dilation of the pupil or any other involuntary act occurring in response to a stimulus causing sharp pain anywhere.

This is a long pathway, in which neurons make connections in both the brain and the spinal cord. Explain what happens when one slams a door on one's finger. First, nerve endings in the finger sense the injury to the finger (sensory neurons) and they send impulses along axons to the spinal cord (magenta pathway). The incoming axons form a synapse with neurons that project up to the brain. The neurons that travel up the spinal cord then form synapses with neurons in the thalamus, which is a part of the midbrain (magenta circle). The thalamus organizes this information and sends it to the sensory cortex (blue), which interprets the information as pain and directs the nearby motor cortex (orange) to send information back to the thalamus (green pathway). Again, the thalamus organizes this incoming information and sends signals down the spinal cord, which direct motor neurons to the finger and other parts of the body to react to the pain (e.g., shaking the finger or screaming "ouch!").

Some of the most obvious reactions to pain can be seen in the body. People tense up and hold their breath. It is common for them to get restless and keep changing their position. However, some people do the opposite and freeze.

Reaction to Pain