Understanding pain systems
Persistent pain is a tricky condition to explain to anyone. There are many changes in the pain nerve systems (neural processing systems) and their function in persistent pain.
Nociceptors detect pressure, heat and chemical changes in the skin, muscles, joints and other areas. They generate an electrical signal that travels to the spinal cord and then to the brain. The brain assesses the level of ‘threat or danger’ linked to the signal and decides if it requires attention and/or protective action. One of the responses to draw attention to whatever is generating the signal is pain. Nociceptors appear to become more sensitive in persistent pain. When this happens, more electrical signals are sent, more quickly than ‘normal’. This can cause the brain to become over-reactive to the incoming signals, perceiving more danger or threat than is actually the case. This process is called sensitisation.
Damaged myelinated nerves, for example in nerve root compression due to spinal disc protrusion, are more excitable. The nerve fibres where the myelin is damaged communicate as they are now in direct physical contact with each other. This is called “cross talk” and is similar to exposed electrical wires shorting across contact points with each other. This explains the electric shock type pains that people with neuropathic pain describe.
‘Windup’ is a neuroplastic change that occurs predominantly in the dorsal horn of the spinal cord. In basic terms, there is a multiplication of the incoming signals within the spinal cord, leading to increased transmission to the brain. This can lead to a relatively minor stimulus causing a significant, painful response and for repeated stimuli to lead to increasing levels of pain.
Loss of synchronisation of neural processing within the brain centres, reduces the efficient handling of the multiple messages. There become too many incoming messages for the brain to process and manage effectively. Added to this is a reduction in the brain’s ability to ‘turn down’ its response to those incoming messages from the spinal cord. The descending pathways are often inhibitory and normally help to control the intensity of pain experienced. In persistent pain, the loss of descending inhibition can lead to a loss of control over pain intensity.
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