348 ◆ CHAPTER 6
A multidimensional problem, pain is seemingly simple but, at the same time, enor-
mously complex. The severity of pain, from a simple paper cut to chronic debilitating low
back pain, depends upon a balance between the organic cause of the discomfort and the
psychological status of the person.
To set the stage for this discussion, it is helpful to further categorize pain into acute,
chronic, and cancer forms. Acute pain is caused by immediate tissue damage resulting from
an injury, such as from a bone fracture or kidney stone. This pain is self-limited and serves
a useful, protective role. When a person stubs a toe, there is significant discomfort; the foot
is then rested, and the pain eventually recedes. If one’s hand gets too close to a fire, it hurts,
and the person quickly pulls it back, with pain serving a protective function. On the other
hand, chronic pain (of noncancerous origin) is long lasting, usually more than three months.
Extending past the time of normal healing, chronic pain serves no biological purpose, has
no protective function, and is often associated with suffering, depression, and disability.
This form of pain is not just acute discomfort that lasts longer, but is a disease in of its
own. The American Academy of Pain Medicine has proposed the term maldynia (Greek
for “bad pain”) to describe this maladaptive discomfort because it can occur in the absence
of ongoing noxious stimulation and does not promote healing and repair.9 Cancer pain has
components of both acute pain, in that the cancer may be causing ongoing tissue destruc-
tion, and chronic pain, as it is of long duration with associated suffering and disability.
No single medical or surgical specialty claims pain as its own because this problem
crosses many specialty lines. Those physicians who typically have a practice in pain medi-
cine include anesthesiologists, physiatrists, and neurologists.
Neurobiology of Pain
The pain pathway transmitting painful information directly from an injury or diseased area
to the brain is a three-neuron circuit, consisting of first-, second-, and third-order neurons.
Figure 6-1. Pain receptors in the skin and tissues of the body, termed nociceptors, are free
nerve endings of sensory nerves. Stimulation of the nociceptors generates an impulse
carried by the sensory fibers in the peripheral nerves, termed the first-order neurons, toward
the central nervous system (see chapter 5). The cell bodies for these neurons are found in
the dorsal root ganglia along the spinal cord and the trigeminal ganglion in the head. More
specifically, only specific afferent (sensory) fibers that are thinly myelinated (A-delta) or
unmyelinated (C fibers) transmit pain information. Figure 6-2.
In addition to these pain fibers, peripheral nerves contain larger diameter myelinated
fibers (A-beta) that carry the sensations of touch, pressure and position (proprioception)
senses. In the gate control theory of pain proposed in the 1960s by Melzack and Wall,
these larger diameter fibers are suggested to close neural “gates” within the spinal cord
by stimulating inhibitory neurons that block the incoming pain-producing A-delta and
C-fiber input. Figure 6-3. The gate control theory explains why rubbing an injured
area seems to help reduce the pain and is the basis of the use of TENS (transcutaneous
electrical nerve stimulator) units.
Transmitting pain information from the head, the trigeminal nerve (cranial nerve V) is the
first-order neuron that sends pain signals from a pain-generating condition, such as from a
toothache or broken nose, toward the brain stem. Figure 6-4. In the rest of the body, however,
the spinal nerves transmit this information toward the spinal cord. Figure 6-5. Within the
spinal cord, the dorsal nerve root makes synaptic connection in the dorsal horn with the
second-order neuron in this pathway. Figure 6-1. The second-order neuron for sensation from
the head (trigeminal system) is located within the trigeminal nucleus in the brain stem.
Second-order neurons from the dorsal horn form the spinothalamic tracts, which cross
over to the opposite side and run up through the brain stem to the thalamus within the
brain. Figure 6-1. Because of this spinothalamic tract crossover, injury on one side of the
brain or spinal cord results in sensory symptoms/loss on the opposite side of the body.
Within the thalamus, the second-order neurons synapse on the third-order neurons,
which, in turn, project up to the higher centers of the brain, especially the cerebral cortex,