Peripheral signal transduction
When free nerve endings are excited by damaging stimuli, their membrane potential changes (transduction) and is converted into an action potential (transformation). Afferent (i.e. ascending) A-δ and C fibres of the periphery transmit the pain stimulus to the dorsal horn of the spinal cord.
Synaptic excitation, 2nd order neuron, spinothalamic tract
Transmission of nociceptive information from the first to the second neuron takes place by means of excitatory neurotransmitters. Neurotransmitters bind post-synaptically to various receptors and produce an action potential, which is transmitted to the brain by so-called nociceptive projection neurons. On each segmental level, these neurons cross the spinal cord to the contra-lateral side where they form the ascending spinothalamic tract.
Processing in higher levels of the CNS
Some ascending fibres of the spinothalamic tract induce vegetative reactions by activating the reticular formation and areas of the upper spinal cord (medulla oblongata). They affect consciousness (mild pain increases concentration, severe pain leads to unconsciousness), and lead to cardiovascular and respiratory response to pain stimuli.
Other ascending fibres reach the hypothalamus where endocrine response is triggered (e.g., endorphin release from the pituitary gland).