In this Chapter:
8.1. Opioid (narcotic) analgesics and their antagonists
8.2. Non-opioids of central action with analgesic activity
8.3. Analgesics with mixed mechanism of action (opioid + non-opioid)
Both organic and psychogenic disorders can cause acute and chronic pain. Pain is felt after injuries to the skin, mucous membranes, ligaments, muscles, joints and visceral organs. Pain may be also caused by a dysfunction of the nervous system itself. These is so-called neuropathic pain, associated with direct injury of peripheral nerves or cerebral tissue due to ischemia, infection, tumor growth, etc.
The pathologic processes, causing pain, are very widespread and can last for months and years, and therefore it is difficult to overestimate the significance of analgesic drugs1. Elimination or relief of pain by means of analgesics leads to the improvement in physical and psychological condition of the patient and, therefore, favourably affects their professional and social life.
Painful sensations are perceived by special receptors, which are called «nociceptors2». They are located on the endings of afferent fiber arborisation in the skin, muscles, joint capsules, periosteum, visceral organs, etc. Damage causing (nociceptive) stimuli can be mechanical, thermal and chemical. The cause of pain is often a pathological process (for example, inflammation). There are endogenous substances that affect nociceptors and are able to cause painful sensations (bradykinin, histamine, serotonin, potassium ions, etc.). Prostaglandins (for example, E2) increase nociceptor sensitivity to chemical and thermal stimuli.
Impulses, caused by painful stimuli, spread along the Cand Aδ-fibres and enter the posterior horn of the spinal cord