• tissue damage by thermal, mechanical or chemical forces results in activation of free nerve endings called nociceptors (pain receptors)
• sensory pathways transmit information from peripheral pain receptors to CNS
• 1st order afferent neuron synapses with 2nd order afferent neuron in the dorsal horn of spinal cord-> 2nd order synapses with 3rd order in the thalamus-> 3rd order sends axonal projections into the sensory cortex
• substance P is a key neurotransmitter involved in the propagation of nociceptive signals across the synapses between 1st and 2nd order sensory neurons
• sensory pathways can be modulated by the higher centres in the CNS to limit the amount of afferent nociceptive stimulation which can be perceived as pain
• the most important site of modulation is the synapse between 1st and 2nd order sensory neurons in the outer layers of the dorsal horn where efferent modulatory neurons descend from the brainstem
• modulatory neurons release neurotransmitter substances (endorphins, enkaphlins, norepinephrine, serotonin, GABA) which dampen the nociceptive transmission by impairing the release of substance P or by making the post-synaptic membrane more difficult to depolarize
It's hard to determine a good layout that will depict the course of the pain signal (ascending pathway) from the source (nociceptors) to the destination (sensory cortex) , including the resultant modulation (descending pathway), while labeling all the relevant anatomical features. Any feedback on this one would be especially appreciated!