Pain Signal Transmission
The signals from your cut hand travel into the spinal cord through the dorsal roots. There, they make synapses on neurons within the dorsal horn (the top half of the butterfly-shaped gray matter). They synapse on neurons within the spinal cord segment that they entered and also on neurons one to two segments above and below their segment of entry. These multiple connections relate to a broad area of the body -- this explains why it's sometimes difficult to determine the exact location of pain, especially internal pain.
The secondary neurons send their signals upward through an area of the spinal cord's white matter called the spinothalamic tract. This area is like a superhighway where traffic from all of the lower segments rides up the spinal cord. The signals of the spinothalamic tract travel up the spinal cord through the medulla (brain stem) and synapse on neurons in the thalamus, the brain's relay center. Some neurons also synapse in the medulla's reticular formation, which controls physical behaviors.
Nerves from the thalamus then relay the signal to various areas of the brain's somatosensory cortex -- there is no single pain center in the brain.
Pain signals travel along pathways through the body. On the next page we'll learn about them.