Under certain conditions, animals may reduce the severity of their own pain, however, it is unclear how these mechanisms are recruited and when they are used.
Lambs appear unable to use such mechanisms to cope with the intense acute pain produced by castration, which dominates the lambs’ physiology and behaviour for at least two hours.
On the other hand, the neural activity responsible for some chronic pains may be accommodated so well by such ‘coping’ mechanisms that physiological and behavioural changes can only be detected when these ‘coping’ mechanisms are suppressed or temporarily overpowered, sometimes referred to as ‘breakthrough pain’.
Pain may be modulated using one or more of the known mechanisms below or by as yet unknown mechanisms:
Various parts of the brain can act together to produce self-analgesia by sending signals to the brainstem and spinal cord where they reduce transmission of nociceptive signals [see animations] and in this way can prevent or alleviate pain.
Stress induced analgesia has been demonstrated but its significance for particular pains in particular species is unclear. Particular examples may be the use of a ‘twitch’ in horses and acupuncture analgesia.
Signals from the skin and other peripheral tissues (e.g. from massage) and conducted by ‘thick’ nerve fibres to the brain and spinal cord can reduce the transmission of nociceptive signals from damaged tissue. This is used in transcutaneous electrical nerve stimulation (TENS) for pain relief in humans but was ineffective for controlling pain associated with removal of antler velvet in deer (Woodbury et al 2001).
Some self-analgesia depends on release of natural opioids to reduce transmission and this can be revealed by blocking their effects e.g. with naloxone.
Opioid drugs such as Morphine activate the membrane receptors for these natural opioids and can provide effective analgesia.