How is Pain Produced?

Central Mechanisms

Central mechanisms are those changes that occur in the brain and spinal cord.

Thin nerve fibres from nociceptors reach the grey matter of the spinal cord mainly via the dorsal roots of spinal nerves. These nerve fibres converge on nociceptive neurons.

Nociceptive relay neurons in the spinal cord contribute to local spinal reflex responses such as withdrawal from the stimulus and to complex defensive responses involving the brain and to the experience of pain.

Pathways from the spinal cord such as the spino-thalamic tract relay nociceptive signals to the brain.

Referred pain

Nociceptive signals from internal organs such as the heart are relayed by the same spinal neurons that relay nociceptive signals from parts of the skin, muscle and other tissues. The brain does not distinguish the sources of these signals and refers the source of signals and the location of the pain from the heart to the inside of the forelimb.

Gate-control hypothesis 

This helps to explain how non-noxious mechanical stimulation such as stroking or rubbing the skin can provide pain relief. Signals in thick nerve fibres produced, for example, by rubbing can inhibit relay of signals by nociceptive relay neurons (hypoalgesia). It also helps to explain how the brain can exert control on the relay of nociceptive signals by the spinal cord through pathways from the brainstem to the spinal cord. 

Secondary hyperalgesia

This involves increased relay of nociceptive signals due to upset of the balance of activity reaching the relay neurons, either a reduction in the signals from non-nociceptors or an increase in signals from nociceptors. It contributes to the development and maintenance of chronic pain.

Sensory-discrimination of pain

The precise localisation and quality of a pain Is achieved through specific relays in the brain (thalamus) and after processing in somato-sensory parts of the cerebral cortex.

Motivational-affective qualities of pain

These are registered in the ‘limbic’ parts of the brain and bring about responses through associated parts of the brain such as the hypothalamus.

Autonomic responses

These, which include ‘fight or flight’ responses are generated through the hypothalamus and control centres in the brainstem.

Cognition

Complex adverse experiences of pain are generated both directly and by association at the highest levels of brain function in the cerebral cortex.

• Animation showing signals produced in CNS during rubber ring castration of a sheep
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• Animation showing a cross-section of the spinal cord
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