The central nervous system, which consists of the central and peripheral nervous system, can be considered as a network via which the pain stimulus is conveyed within the body.
The following structures are involved:
The pain process has four elements:
In contrast to pain, which is defined as a subjective sensorial or emotional event, nociception means the reception, transmission and central nervous processing of noxious (tissue-damaging or potentially tissue-damaging) stimuli.
The relevant structures of the CNS in perception are the cerebral cortex, brain stem and the dorsal horn of the spinal cord.
The cortex is the “thinking” part of the brain where the unpleasant pain sensation is generated to cause awareness, while the limbic system generates emotional responses, such as sadness, tears and anger.
Concerning pain, the following structures are of special interest:
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).
Centrally located grey substance of the midbrain, the periaquaeductal grey (PAG), receives a cortical and subcortical response and initiates inhibitory nerve impulses which descend the CNS in two different tract systems.
By sending responses back to the periphery, the CNS can induce the release of neurotransmitters which reduce the transmission of pain signals (feedback loop).
Cortical and subcortical response
Neuronal centres of the cortex and subcortical areas of the brain respond to incoming (ascended) pain signals and can modulate pain signals by activating inhibitory descending efferent pathways.
Inhibitory impulses descend to the dorsal horn of the spinal cord segment where respective painful stimuli are transmitted to the second order neuron. Activated inhibitory interneurons release inhibitory neurotransmitters such as endorphins, noradrenaline and serotonin.