Regulation of Pain Impulses

Awareness of pain occurs when pain impulses reach the level of the thalamus—that is, even before they reach the cerebral cortex. However, the cerebral cortex must judge the intensity of pain and locate its source. The cerebral cortex is also responsible for emotional and motor responses to pain.

C>0 Reconnect to chapter 10, Postsynaptic Potentials, page 380 Still other parts of the brain, including areas of gray matter in the midbrain, pons, and medulla oblongata, regulate the flow of pain impulses from the spinal cord. Impulses from special neurons in these areas descend in the lateral funiculus (see chapter 11, p. 422) to various levels within the spinal cord. The impulses stimulate the ends of certain nerve fibers to release biochemicals that can block pain signals by inhibiting presynaptic nerve fibers in the posterior horn of the spinal cord.

Among the inhibiting substances released in the posterior horn are neuropeptides called enkephalins and the monoamine serotonin (see chapter 10, p. 381). Enkephalins can suppress both acute and chronic pain impulses; thus, they can relieve strong pain sensations, much as morphine and other opiate drugs do. In fact, enkephalins were discovered because they bind to the same receptor sites on neuron membranes as does morphine. Serotonin stimulates other neurons to release enkephalins.

Cannabinoids are substances in the plant Cannibus sativa, the source of marijuana, that may relieve pain. Anecdotal evidence for such an effect dates to 315 a.d. Neurons in areas of the brain, brain stem, and peripheral nervous system have receptors for cannabinoids. In some states, marijuana is legal for use to treat pain associated with such conditions as AIDS or cancer.

Another group of neuropeptides with pain-suppressing, morphine-like actions are the endorphins. They are found in the pituitary gland and in regions of the nervous system, such as the hypothalamus, that transmit pain impulses. Enkephalins and endorphins are released in response to extreme pain impulses, providing

Shier-Butler-Lewis: I III. Integration and I 12. Somatic and Special I I © The McGraw-Hill

Human Anatomy and Coordination Senses Companies, 2001

Physiology, Ninth Edition

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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