Understanding and Managing Pain.pdf

Understanding and Managing P CHAPT ER 7 Understanding and Managing Pain Lecture Outline I. Pain and the Nervous System All sensory stimulation, including pain, starts with activation of sensory neurons and proceeds with the relay of neural impulses toward the brain. A. Somatosensory System The somatosensory system conveys sensory information from the body through the spinal cord to the brain. 1. Afferent Neurons Afferent (sensory) neurons convey sensory information from sense organs to the spinal cord and then to the brain. Efferent (motor) neurons result in the movement of muscles. Interneurons connect afferent and efferent neurons. Primary afferents are those neurons that have receptors in the sense organs and that originate the neuron’s message. The vast number of neurons and their interconnections makes neural transmission complex. 2. Involvement in Pain Nociceptors are neurons capable of sensing pain stimuli. Three different types of neurons are involved with transmitting pain impulses. The large A-beta fibers and smaller A-delta fibers are covered with myelin, which speeds neural transmission. The smaller and more common C fibers require high levels of stimulation to fire. These different fibers with their different thresholds and transmission speeds may relate to different types of pain sensation. B. The Spinal Cord Primary afferents from the skin enter the spinal cord where they synapse with neurons in the dorsal horns of the spinal cord. The dorsal horns contain several laminae (layers). Laminae 1 and laminae 2 form the substantia gelatinosa, a structure that receives sensory input from the A and C fibers. Complex interactions of sensory input occur in the laminae of the dorsal horns, and these interactions may affect the perception of sensory input before it gets to the brain. C. The Brain The thalamus receives sensory input from the different neural tracts in the spinal cord. The skin is mapped in the somatosensory cortex in the parietal lobe of the cerebral cortex, and the proportion of cortex devoted to an area of skin is proportional to that skin’s sensitivity to stimulation (see Figure 7.2). Sensory information from internal organs are not mapped as precisely as the skin, leading people to have the ability to identify stimulation from the skin but less distinct sensory perceptions of their internal organs. This is also the reasoning behind referred pain, when pain is experienced in a part of the body other than the site where the pain stimulus originates. D. Neurotransmitters and Pain The neurotransmitters that form the basis for neural transmission also play a role in pain perception. The discovery of the endogenous opiates—enkephalin, endorphin, and dynorphin—led to the discovery of neural receptors specialized for lOMoAR cPSD| Chapter 7 these neurotransmitters and the conclusion that opiate drugs produce analgesia because of the brain’s own chemistry. The neurotransmitters glutamate and substance P and the chemicals bradykinin and prostaglandins may exacerbate pain stimulation. Proinflammatory cytokines produced by the immune system are also involved in pain, possibly creating chronic pain by sensitizing neurons in the spinal cord. E. Modulation of Pain When the periaqueductal gray, a structure in the midbrain, is stimulated, pain relief occurs. The neurons in the periaqueductal gray synapse with neurons in the nucleus raphé magnus, a structure in the medulla (see Figure 7.3). These neurons descend to the spinal cord and may constitute a descending control system for pain perception. Multiple Choice Questions 1. The system allows us to interpret certain sensory information as pain. a. somatosensory b. endocrine c. skeletal d. muscular ANS: a REF: Pain and the Nervous System 2. Primary afferents convey sensory impulses to the a. spinal cord. b. peripheral nervous system. c. brain. d. motor neurons. ANS: a REF: Pain and the Nervous System 3. Myelinated afferent neurons are called a. A fibers. b. beta afferents. c. C fibers. d. delta afferents. ANS: a REF: Pain and the Nervous System 4. More than half of all sensory afferents are a. A-beta fibers. b. A-delta fibers. c. C fibers. d. myelinated. ANS: c REF: Pain and the Nervous System