1. Pain Modulation Pain Transmission
Nociceptor (primary pain receptor) stimulates a secondary nerve with a NT called “substance P”
The secondary nerve sends stimulus to the thalamus
The thalamus relays stimulus to the cerebral cortex (where pain is perceived)

2. Pain Modulation Pain blocking
Stimulus from hypothalamus and cerebral cortex are sent into the gray matter of the mid brain
The mid brain sends signal to the medulla oblongata
The medulla stimulate serotonin-secreting fibers in the spinal cord

3. Pain Modulation
The fibers stimulate interneurons to release an inhibitor into the synapse that is with the secondary nerve (postsynaptic inhibition)
Some serotonin-secreting fibers act on the primary nerve to inhibit (presynaptic inhibition)