What Is Morphine? - Definition, Uses, Withdrawal & Addiction

Instructor: Mary Lou Perin
This lesson describes the transmission of pain in the nervous system and how it can be interrupted to provide relief from pain. You'll learn about morphine, its origin, how it relieves pain, and the problems associated with its use.

Origin of Morphine

Just as humans live in families, so do medicines. Morphine is in the family of opioids. It is named after Morpheus, the Greek god of sleep, because of its tendency to cause drowsiness. Morphine is an alkaloid that occurs naturally in the opium poppy. For almost 200 years, the plant extraction has been used in treating pain. Because the effectiveness and dosing of other opioids is measured against it, morphine is considered the gold standard of analgesics. It is the mainstay of modern pain medicine.

Transmission of Pain

All medicines in the opioid family act alike. Once introduced into the body, they are broken down or metabolized in the liver and then used to block the perception of pain. Morphine may be taken orally, intravenously (injected directly into a vein), subcutaneously (just under the skin), or epidurally (into the space around the spinal cord). It can be administered rectally for patients unable to swallow, and it can even be inhaled through a mask. Because morphine can be administered in so many different ways, its versatility increases its value as a therapeutic agent.

How does morphine relieve pain? The transmission of pain is like a three-stage relay race. The body uses runners or chemicals to pass a pain message from the periphery of the body, where injury occurs, to the brain. The nervous system has two highways, ascending and descending. On the ascending highway (afferent nerve pathway), fast nerve messengers called small A nerve fibers transmit acute pain signals. Larger C fibers transmit slower signals and play a role in chronic pain. In stage one of the relay, runners carry a chemical message from the original site of pain, i.e., the finger you burned, to the back side of the spinal cord, or the dorsal horn. Once there, chemicals carry the message across a gap and pass it to a second set of chemical messengers. This set of messengers carry the nociceptive pain signal up to the midbrain, a central receiving station. There the nociceptive impulse passes to the last set of messengers who deposit the message in the cortex or frontal section of the brain. It is only when the message reaches the cortex of the brain that you actually perceive pain.

Pain transmission does not end there. The body is not passive in this process. It has a natural defense system activated when pain occurs. The brain produces its own chemicals, which flood the descending nerve pathway, or efferent system, interrupting or inhibiting the incoming pain signals. Success in controlling pain occurs when the body's own internal responses combined with external measures, e.g., medication, heat or cold, and immobilization, reduce or drown out the incoming nociceptive messages to the brain.

How Morphine Works

Over the last fifty years, as research on the physiology of pain has revealed the secrets of pain transmission, drug research has focused on improving medicines whose therapeutic action is to block the pain message. Interrupting pain signals is the basic mechanism of all analgesics, whether they are opioids like morphine or non-opioids like ibuprofen and acetaminophen. Once an opioid, such as morphine, has entered the body, it migrates to certain uptake sites, called receptors. Specific morphine receptors exist in different locations in the nervous system, such as the spinal cord, the cortex of the brain, and the limbic system. They are also in the bowel. The location of these sites is important because opioids not only produce pain relief but also trigger side effects related to the site location, e.g., constipation in the bowel.

Morphine locks into receptor sites with a mechanism similar to a lock and key. The correct medicine and correct dose will provide relief as long as the opioid remains as a key in the lock turning off the pain signal. Once the medicine has been completely used up, or metabolized, the site becomes vacant, the pain chemicals start flowing again, and the sensation of pain returns. Morphine administered by injection usually takes about five minutes to begin providing relief, with maximum effect at about twenty minutes. Although it acts fast, the action is of short duration and pain returns within one to two hours. Given orally, morphine may take up to one hour to bring relief but the effect may last as long as three to four hours. Long acting products provide sustained relief to reduce chronic pain and are prescribed only once or twice daily.

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