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Understanding the anatomy and physiology of pain transmission
systems is important for the pain management specialist. Injuries
to these areas may cause the common pain syndromes for which patients
seek help (e.g., diabetic neuropathy, postherpetic neuralgia). Interventions
at distinct anatomic sites may provide the relief the patient seeks
(nerve blocks, implantable devices). And ongoing research may reveal
new modalities or pharmaceutical agents to provide relief, such
as the cyclooxygenase-2 (COX-2) inhibitors. This chapter is designed
as an overview for the clinician and not as a comprehensive review
of the anatomy and physiology of the nervous system, about which
textbooks have been written.
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This chapter reviews the transmission of a nociceptive or pain
impulse from the site of stimulus in the periphery to the central
nervous system. The basic anatomic pathways of nociceptive transmission
and of descending nociceptive modulations are described. Some of
the basics of physiology also are discussed, with the assumption
that the reader is knowledgeable about the fundamentals of neuronal
transduction, such as action potential propagation and the relationship of
the cell body to the dendrites and axon. Although some mention of
pathophysiology and disease states is made, for further discussion,
please see the specific chapters in this book.
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This chapter focuses on the ability of the nervous system to
transmit and modulate nociceptive stimuli. The studies reviewed
here likely apply more to acute pain than to chronic pain. Although there
is intense interest in developing an animal model of chronic pain,
most of the experimental paradigms used are more closely analogous
to the injury of acute pain than chronic pain. The latter is both
a physical and emotional entity and may result from plasticity in
the nervous system.1 The complexities of chronic
pain syndromes, including such theories as chronic pain as a variant of
depressive disorders,2 are discussed elsewhere.
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Knowledge of nociceptive transmission has advanced considerably
since Descartes outlined his concept of the nerve as tubes with
delicate threads to convey a painful impulse.3 It
is now widely believed that stimulation of a primary afferent neuron
in the peripheral nervous system results in activation of neurons
in the dorsal horn of the spinal cord and then in transmission rostrally
to the brain.
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Sensory neurons, called primary afferents, have a cell body in
the dorsal root ganglia (DRG) of the spinal cord or in the ganglia
of the cranial nerves. Cranial nerves V, VII, IX, and X receive inputs
from primary afferents and, thus, sensory information from the head,
face, and throat. Cells in the DRG are a heterogenous population
of size and function; a reflection of the heterogeneity of the sensory
inputs processed by the central nervous system.4
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This heterogeneity is reflective of the multiple functions of
the peripheral sensory nervous systems, as well as the supportive
function of the glial system. The classic nomenclature of peripheral
neurons relies on the size of the axon ...