ABSTRACT
Endogenous opioids and opioid pain-relieving drugs ■ Opioid receptors ■ Actions of morphine ■ Clinical uses of morphine and other opioids ■ Adverse effects of morphine and diamorphine ■ Methadone ■ Morphine antagonists ■ Tolerance
and dependence ■ Morphine (and heroin) withdrawal symptoms
Learning objectives Be able to give an account of the gate theory of pain. ■ Be able to use the theory to hypothesise the sites of action of pain-■ relieving opioids. Have a defi nition for the term ‘opioid’. ■ Be ready to give examples of endogenous and synthetic opioids. ■ Know what is meant by antagonists and partial agonists with examples. ■ Be able to list important actions and clinical uses of morphine. ■ Know what is meant by tolerance and dependence with regard to opioid ■ abuse and how addicts are helped to get off the drugs. Be aware of the properties and uses of methadone. ■
Perception of pain Is perceived by the brain. ■ Pain is not felt if nervous pathways carrying pain to the brain are ■ damaged (see Figure 33.1). The ‘gate theory’ of pain proposes: ■ *
that high intensity stimuli from the body (damaging stimuli) open ❚ a ‘gate’ in the posterior horn of the spinal cord and ascend to the thalamus where the pain is ‘felt’, and from there to the cerebral cortex where discrimination and interpretation occur that the ‘gate’ is also controlled by descending impulses from the brain, ❚ which decides whether the gate should open or close, which explains how we can be distracted from pain that low intensity ascending impulses unrelated to pain can close the ❚ gate and by doing so block pain impulses, which explains how heat therapy at a painful area of the body may ease pain.
