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Chapter 7

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Ryerson University
PSY 606
Trevor Hart

Chapter 7 Understanding Pain congenital insensitivity - a very rare genetic disorder that prevents one from experiencing any pain at all. While it may sound good, the consequences are very serious. somatosensory system - The part of the nervous system that carries sensory information from various parts of the body (e.g., from the skin and muscles) to the brain. afferent neurons - are also called sensory neurons. They relay information from the sense organs (e.g., touch) toward the brain. efferent neurons - are also called motor neurons. They carry neural impulses from the brain to the muscles. primary afferents - Are specialized sensory neurons in the skin and other places. They respond to physical energy (e.g., touch) and convert it to neural signals that travel to the spinal cord and brain. action potential - is the technical term for when a neuron "fires." In response to sufficient amounts of neurotransmitters, or physical stimulation (for the primary afferents), a signal travels down the neuron's axon and causes release of neurotransmitters. nocioception - is the process of perceiving pain. nociceptors - Sensory receptors in the skin and organs that are capable of responding to various types of stimulation that may cause tissue damage (heat, cold, cutting, or burning). myelin - A fatty substance that acts as insulation for the axons of some neurons. Neural impulses are transmitted much faster by neurons with myelinated axons. fiber size - larger fibers (of larger neurons) transmit messages faster than smaller fibers. A fiber is basically the same thing as an "axon." A beta fibers - are both large and myelinated. They transmit information "100 times faster" than C fibers. They rapidly transmit sensory information. A-delta fibers - are small sensory fibers that are involved in the experience of “fast” pain that is "sharp" or "pricking." C fibers - are smaller and unmyelinated. Their slower transmission results in sensations that are dull, aching, or lingering. About 60% of all sensory afferents are "C fibers." nerve - A group or bundle of fibers (axons) that travel together (like a large electrical cable made up of hundreds of wires). Some nerves (afferent and efferent) are referred to as "Tracts." the cranial Nerves - Sensory information from the head and neck area travel to the brain via "12 cranial nerves." Information from all other body areas travels to the brain via the spinal cord. spinal cord - carries afferent (sensory) information to the brain and efferent (motor) information from the brain to the muscles (see Figure). dorsal horns - The back part of the spinal cord (away from the stomach) that receives sensory input (via the sensory tracts). There, the sensory fibers connect to small "secondary afferents" which, in turn, connect to efferent (motor) fibers that leave the ventral (front) side of the spinal cord. The messages also travel up the spinal cord via other neurons to the brain. transmission cells - "secondary afferents" and "interneurons" are all different names for the same thing, cells that connect neurons to other neurons. laminae - Are different layers of the dorsal horns which are composed of cell bodies. substantia gelatinosa - Is made up of layers 1 and 2 of the dorsal horns and is important in modulating sensory input. thalamus - Structure in the forebrain that acts as a relay center for incoming sensory information and outgoing motor information (from and to the spinal cord). somatosensory cortex - The part of the brain that receives and processes sensory input from the body. primary somatosensory cortex - receives information from the thalamus that allows the entire surface of the skin to be mapped onto the somatosensory cortex. The more sensitive areas of the skin (face and lips) take up a much larger part of the cortex relative to less sensitive parts like the back (see Figure). emotional and physical pain - May be all the same to the brain. Eisenberg et. al. (2003) found that experimentally induced emotional pain (social rejection) had the same activating effect on certain brain areas as physical pain. endorphins - Along with the enkephalins and dynorphin are naturally occurring neurochemicals whose pain lowering effects resemble those of the opiates. glutamate and substance p - Are neurotransmitters that sensitize (or excite) the neurons that relay pain messages. periaqueductal gray - An area of the midbrain that, when stimulated, decreases pain. It sends signals, via the medula, to the substantia gelatinosa in the spine's dorsal horn. Endorphin release blocks pain transmission (see Figure) medulla - The structure of the hindbrain just above the spinal cord. Sensory and motor messages travel through it. how pain is viewed - Prior to about 1900, pain was thought to be directly proportional to the extent of tissue damage. C. A. Strong then proposed that the experience of pain reflected BOTH sensory information AND the person's interpretation of it. the IASP definition
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