Opioid drugs are derivatives, or synthetic duplicates, of morphine or codeine extracted from the opium poppy plant Papaver somniferum. Doctors prescribe opioid drugs for therapeutic purposes because of the way this class of drugs works to ease pain and calm anxiety. Recreational users favor opioids because these drugs create a pleasant, euphoric feeling. Patients and recreational users rely on the primary physiological and neurological effects of opioids but both may suffer from unwanted side effects associated with opioid use.

Neurological Effects of Opioid Use

Opioids work by attaching to specific proteins, known as opioid receptors, in the brain, spinal cord, digestive tract, and elsewhere in the body. In the central nervous system, opioid receptors are located mainly in the brainstem and spinal cord. When opioids bind to these receptors, they change the way the brain perceives pain. Opioids cause other effects in the brain, including drowsiness and confusion. The drug also affects areas of the brain associated with judgment and decision-making.

Opioids also act directly on the part of the brain controlling respiration in a way that suppresses breathing. This condition, known as respiratory depression, sometimes occurs after large doses of opioids and can result in death.

This class of drugs also acts on the brain’s reward system. Human brains are wired to repeat life-sustaining activities, such as reproduction and eating food, through reward pathways in the brain that make these activities seem enjoyable. Opioids change these pathways by over- stimulating reward circuits in the brain.

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Repeated use of opioids causes changes in the structure and function of the brain. Over-stimulation of the reward circuits causes the brain to try toning down the activity by reducing the number of receptors in the brain. Fewer receptors decreases the euphoric effects of opioids so the consumer must take a stronger dose of the drug to achieve the same effects, meaning the individual has developed a high tolerance to opioids.

Physiological Effects of Opioid Use

Opioids also cause physiological effects in various locations in the body, especially in those areas that contain large numbers of opioid receptors. One type of muscle tissue, known as smooth muscle, is rich in opioid receptors. Smooth muscle allows hollow organs, such as the heart, blood vessels, and the intestines and other digestive organs, to contract. Contraction of smooth muscles allows the heart to pump blood, blood vessels to control blood pressure, and the gastrointestinal tract to move food and stool.

Opioids stiffen smooth muscle in a way that makes body tissues too rigid to function correctly. Smooth muscle rigidity causes side effects. Opioids cause constipation because smooth muscle stiffening prevents the intestines from pushing stool along the digestive tract, for example, while smooth muscle rigidity in blood vessels causes blood pressure problems and stiff heart muscles can result in cardiovascular problems. Opioids can also affect the smooth muscles in the eyes to cause pinpoint pupils, especially in cases of overdose. There can also be effects on the smooth muscles in the skin.

Implications of Opioid Abuse and Withdrawal on Physiological and Neurological Systems

The human body alters its own neurological and chemical makeup to help it survive long periods of opioid use. With continued use, the body alters itself to better deal with a sluggish digestive tract and poor circulation. In time, the body becomes dependent on a certain level of opioids to function correctly.

As opioid levels plummet, a drug dependent body reacts in those areas rich in opioid receptors to cause specific opioid withdrawal symptoms. Vomiting and diarrhea indicate the struggle for chemical balance in the gastrointestinal tract, smooth muscle action in the blood vessels cause blood pressure to rise, and smooth muscle response in the heart causes rapid heartbeat during withdrawal. When an opioid-dependent person quits “cold turkey“ the smooth muscles in the skin cause it to become pale, cold, clammy with goose bumps – much like the skin of a plucked bird.

In addition to increased tolerance, long-term opioid use can lead to neurological changes that prolong substance abuse. The changes in the reward pathways make the individual feel less rewarded by things that used to bring him pleasure, for example, so that he eventually feels a sense of satisfaction only through drug abuse. The neurological changes cause significant changes in his thought processes so that all decisions lead back to doing more drugs, and behavioral changes that cause separation from those friends and family that might otherwise intervene in his drug use.

Opioids have many therapeutic effects, especially for those suffering from pain. However, this class of drugs also causes other physiological and neurological effects that may cause ill health.

Sources

Opioid Analgesics and the Gastrointestinal Tract

Expression of Functional Delta Opioid Receptors in Vascular Smooth Muscle

Opioid Analgesic Drugs

Neurological Consequences of Repetitive Opioid Use

How Drugs Affect Neurotransmitters