Mechanisms for Pain and Injury in Musculoskeletal Disorders - Fundamentals and Assessment Tools for Occupational Ergonomics - page 388

articular
cartilage

spinal cord

articular
joint

bone

intervertebral
disc

nerve root

ligaments

dorsal root
ganglion

nerve

FIGURE 16.2 Schematic illustrating varied tissue components capable of sensing pain, particularly following
mechanical or inflammatory injuries. While this schematic is based on the spinal column and highlights tissues
relevant to its anatomy, it highlights all such tissues in the body, including neural tissue, hard and soft tissue,
ligaments and bone. Muscle tissue is not shown here, but is innervated and is the tissue component particularly
relevant to pain sensation.

16.1.2 Tissue Injury, Central Sensitization, and Pain
Injury to any of a variety of tissue components, including muscle, disc, ligament, and neural tissue can
induce many cascades and physiological signals leading to pain perception. Most broadly, neuroplasticity
and subsequent CNS sensitization include altered function of chemical, electrophysiological and
pharmacological systems. 3,13,15,55,58,64 The interplay of these systems is intricate and involves compli-
cated cross-system effects between injury and changes in both the peripheral nervous system and CNS
(Figure 16.3). The integration of multiple physiologic systems that occurs as a result of a painful
injury contributes to the overall challenges in preventing such syndromes since a given mechanical
injury may initiate a host of different responses, which can be established and maintained remote
from the actual site of injury. Nonetheless, there is a generalized series of responses, which occurs follow-
ing a painful injury in the periphery.
Nociceptive afferents are specific for sensing different noxious stimuli (i.e., thermal, mechanical, and
chemical). Some nociceptors are polymodal and sense all types of stimuli. Sensory nerve fibers range in
diameter from

0.05 to 20 mm and can be thickly, thinly, or not myelinated. Conduction velocities of
nerve fibers range from 0.5 to 120 m

,

sec, depending on their axon diameter and the presence of myeli-
nation. The fibers evoke sharp and pricking pain sensation, sometimes also aching pain. The largest,
myelinated sensory axons, Aa, are generally classified as mechanoreceptiors. Ab fibers are primarily
proprioceptive (sensing mechanical movement in joints and muscles) and have diameters of

/

10 mm
in size and slower conduction velocities than Aa fibers. The smallest myelinated fibers are Ad fibers;
they primarily mediate pin-prick, itching, and other mechanical sensations of pain. Unmyelinated, C
fibers, mediate thermal sensation, in addition to mechanical pain. Stimulation of C fibers at high
enough magnitude induce a burning sensation. C and some A fibers are primary high-threshold
nociceptors. They have similar functions with A fibers having greater response frequencies and more
communications to the spinal cord.
Pain can result from direct injury or from inflammation, which induces alterations in the local per-
ipheral milieu. For a given insult, local nerve fibers become activated. The sensation of pain is initiated