Topics and Speakers Amy B. MacDermott, PhD
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Synopsis
Pain was long considered a simply a disagreeable form of tactile stimulation. Yet, as Dr Amy B. MacDermott shows, pain is not a homogeneous experience. Multiple types exist, ranging from acute to inflammatory to neuropathic. In fact, the International Association for the Study of Pain defines pain broadly as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage." In this lecture, Dr MacDermott reviews the three most important elements in the pain pathway: the peripheral neurons, the dorsal horn of the spinal cord, and the projection neurons that extend into the brainstem and thalamus. She describes how pain is inhibited and regulated and unveils some of the mysteries surrounding chronic pain.
The peripheral neurons include large- and small-diameter "pain neurons" known as nociceptors, which are actually defined by their pain conduction velocity. They are activated by noxious factors such as changes in temperature and in the gradient of sodium ions. All of these nociceptors terminate in the dorsal horn of the spinal cord, where they meet with primary afferent fibers and projection neurons. Two events occur at the dorsal horn: there is both inhibition (through GABA, the amino acid) and excitation (through glutamate) at the pain synapses. If excitation exceeds inhibition, then the projection neurons will send the signal of pain up to the thalamus, where it sends off different collaterals into the brainstem, producing the emotional and physical aspects of pain. Interestingly enough, if glutamate is blocked, so is the pain signal, and therefore pain.
Dr MacDermott concludes by discussing what is known about the causes of chronic pain (i.e., neuropathic) diseases, such as fibromyalgia and allodynia. Chronic pain is marked by hyperexcitability at the synapses of peripheral neurons, which is caused by the release of substances such as prostaglandins, bradykinin, and histamine. In a chronic-pain patient, the dorsal horn of the spinal cord often reveals an accumulation of astrocytes, a type of glial cell. As Dr MacDermott argues, these and other findings point the way toward further research on targeting chronic pain at every level of the pain pathway, including the brain.
