Nervous System

How do anesthetics and pain medicines work on the nervous system?
Answered by Craig C. Freudenrich and Discovery Channel
  • Craig C. Freudenrich

    Craig C. Freudenrich

  • Discovery Channel

    Discovery Channel

  1. Imagine the world before anesthetics and pain medicine -- surgeries performed or bullets removed while patients were conscious and relieved only by the effects of alcohol, opium or hashish. Nitrous oxide came along in the late 18th century, and it was nearly 50 years until patients felt the relief of ether. Pain relief has come a long way since then. Understanding how today's anesthetics and pain medicines work is based on how the body's nervous system transmits and processes sensory information. Specialized nerve cells called sensory receptors are located throughout the body in the skin and organs. Receptors that sense pain are called nociceptors; they respond to touch, heat and pressure. Nociceptors also respond to chemicals released from injured or damaged tissues, such as prostaglandins, substance P, potassium, histamines and bradykinin.

    Once stimulated, nociceptors send action potentials, or nerve impulses, to the spinal cord. Some impulses participate in reflex pathways at the spinal cord level. In these pathways, the nociceptor nerve impulse directly stimulates a muscle pair to move a limb. For example, when you touch a hot pot, you immediately withdraw your hand and arm. These impulses also travel through the spinal cord to the brain, where the information gets processed. Scientists are not sure how, but some go to the motor cortex, somatosensory cortex and hypothalamus. Pain signals also can travel from damaged nerves through the spinal cord to the brain. The brain can send pathways down the spinal cord to suppress the incoming signals from damaged nerves. This is how you adapt to painful stimuli. For example, if you bang your finger with a hammer, the pain is intense at first, but eventually subsides.

    Pain medications and anesthetics work by influencing various parts of the pain pathways. For example, some medicines work at the site of injury by blocking the nociceptor's ability to sense painful stimuli or to transmit pain nerve impulses. These include nonopioid analgesics (acetaminophen, ibuprofen, naproxen and aspirin), local anesthetics (lidocaine, procaine, novocaine and tetracaine) and capsaicin. Some medicines act on the nerves in the spinal cord pathways to suppress pain transmission to the brain. Opioid analgesics, such as morphine, meperidine, oxycodone and fentanyl and spinal anesthetics (higher doses of local anesthetics applied directly to regions of the spinal cord) work in this manner. Finally, other medicines work directly on the brain to suppress pain information and induce sleep. These include general anesthetics, such as inhaled isoflurane/desflurane gas with nitrous oxide, or injected thiopental sodium. Anesthesiologists closely monitor a patient's breathing heart rate and respiration when the patient is under general anesthesia.

    Pain Qa2

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  2. With an advanced understanding of how neurons pass along signals to the brain, scientists have developed methods that suppress the action potentials in neurons. Though some of these efforts -- such as early forms of anesthesia -- were discovered by accident, the development of modern medicines depended on our knowledge of the nervous system. Anesthesia and pain medicines can limit the response of neurons by hindering the movement of action potentials along axons and preventing a signal from reaching the brain. And because pain is interpreted in the brain, blocking the signal beforehand is an effective way to block pain itself to some extent.

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