The pain system involves a set of ascending pathways that convey nociceptive information from peripheral nociceptors via neuronal tracts of the spinal cord to higher levels of the CNS. When free nerve endings are excited, their membrane potential changes (transduction) and is converted into an action potential (transformation).

Afferent (i.e. ascending) A-delta and C fibres of the periphery transmit the pain stimulus via action potentials to the dorsal horn of the spinal cord. The transmission of nociceptive information from the first to the second neuron takes place at the synapsis by means of excitatory neurotransmitters. The neurotransmitters bind post-synaptically to specific receptors and produce an action potential, which is transmitted to the brain.