Analgesics

Analgesics is a kind of drug which could decrease the pain. It could be be divided into two categories according to their intensity of action. One is weak analgesics which has mild analgesic effect, and it is antipyretic and has anti-inflammatory efficacy. So It is rarely used in clinical anesthesia. Another one is a strong analgesic, which has a strong analgesic effect. And it has no anti-pyretic and anti-inflammatory effect, so it is an important adjuvant in anesthesia.
Analgesics mainly work on the central nervous system, so it known as narcotic analgesics and also refers to morphine and morphine drugs, including natural opioids, semi-synthetic and synthetic morphine-like drugs. Apart from it’s role as analgesics, these drugs are easily addictive, so they should be strictly controlled. It could effect the selective inhibition of pain sensation to weaken the feelings of pain and response and it also  increase the threshold of feeling, but it does not affect other senses (tactile, optical and auditory). There is no consensus view on the acting site of Analgesic. Generally it is believed that it mainly effect the hypothalamus, the limbic system, and also has the role of inhibiting the spinal reflex, withstand the central and peripheral spasm. Domestic studies have found that is the paraventricular nuclei(PVN) of thalamic fasciculus maybe is the acting site of analgesic.
Since 1973, it has been confirmed that there exist morphine receptors in the brain. And human body can also produce morphine-like substances and produce analgesic effect by combining with this receptor. This kind of endogenous substance is called called endorphin. As the distribution of the receptor is roughly parallel to the distribution of endorphins, when we measured the concentration of endorphins in various parts of the brain, It is found that the receptors are mostly distributed in the limbic system and striatum, and the distribution of white matter and cerebellum is minimum. Long-term application of morphine-like drugs can produce feedback mechanism that morphine-like drugs occupy the receptor and inhibit the endogenous morphine-like composition. Once stop using morphine drugs, it could result in the lack of endorphin and cause withdrawal syndrome such as mydriasis, sweating, tachycardia, temperature rising, nausea, vomiting, diarrhea, dehydration, insomnia and muscle tremors occur Signs. But it will not cause death. Barbiturates, alcohol, and other hypnotics could not reduce withdrawal syndrome. Different narcotic analgesics has cross-resistant and addiction.