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86 m a s s a g e & b o d y w o r k n o v e m b e r / d e c e m b e r 2 0 1 7 model of pain receptors sending pain signals from the periphery to the brain. So how do pain signals actually work? NERVES AND PAIN To fully grasp how pain sensations are produced, it is helpful to review some basic principles of neuroanatomy. Not all massage therapists are taught these details about the nervous system in their basic training, so this is a great opportunity to polish your understanding of these concepts. Nerve fibers are classified according to their diameter. There are four primary types of nerves that play a major role in our experience of pain. They are named with letters from our alphabet as well as the Greek alphabet. These four primary types of nerve fibers and their key characteristics are: When a nerve fiber is myelinated, that means it is covered by a myelin sheath (Image 2). The myelin sheath helps the nerve impulse travel along the length of the nerve at a much faster rate. The rate of signal transmission plays a crucial role in pain perception and also how that sensation can be magnified or diminished. Nerves that carry nociceptive signals are primarily the Aδ and C fibers, although there is some indication that nociceptive input can travel along the Aβ fibers in some cases. Often when you have an acute injury, you feel a sudden, sharp, and strong pain first that is followed by a more persistent, dull, aching pain afterward. The strong and sharp pain is mostly from Aδ fiber signals, which arrive at the brain before the slower, non-myelinated nociceptive signals from the C fibers. The C fibers are responsible for the latent dull, aching pain that comes on after the immediate pain from an injury. There is also an indication that C fiber nociceptive signals are mainly responsible for many of the chronic-pain complaints that persist for long periods of time. 1 In 1965, two researchers, Ronald Melzack and Patrick Wall, published a paper outlining a new theory of pain modulation that emphasized an expanded role for the central nervous system and de-emphasized the notion of pain receptors in the periphery and the idea they were sending "pain signals" to the brain. This theory has come to be known as the gate control theory of pain. While it has been modified from its original presentation, there is still strong evidence to support the idea that signal transmission and the experience of pain can be modified in the way they originally described it. Let's take a look at how that works. THE GATE THEORY OF PAIN Nociceptive signals are sent from specialized sensory receptors in the periphery of the body. Once those sensory receptors are activated, they send a message primarily along the Aδ and C fibers. But the body is also getting sensory information from other receptors simultaneously. Proprioceptive signals about the body's position in space and signals about joint position from mechanoreceptors are traveling on the much faster Aα and Aβ nerve fibers. They get to "processing stations" in the spinal cord and central nervous system faster than the nociceptive signals traveling on the Aδ and C fibers (Image 3). The gate theory suggests there is a neurological "gate" (not truly a mechanical gate, but a metaphorical one) in the spinal cord that closes down to limit the amount of information being sent to the brain for processing. When the proprioceptive signals arrive at the gate first, the gate shuts down to the slower-traveling nociceptive signals. With fewer nociceptive signals getting through, there is decreased pain sensation. The benefit of mechanoreceptors and proprioceptors outpacing the nociceptive signals means that the most important stimulus is perceived by the brain first, as in the example of the person with the sprained ankle sprinting to the sidewalk to avoid a bus. This mechanism also explains Fiber Type Name Myelinated Primary Responsibility Aα (alpha) Yes Proprioception: muscle spindle and golgi tendon organ Aβ (beta) Yes Mechanoreception Aδ (delta) Thinly myelinated Free nerve endings and nociceptors for touch and pressure, cold receptors C Non- myelinated Nociceptors and warmth receptors 2 Myelin sheath surrounding a nerve fiber. Image courtesy Wikipedia. Myelin sheath