Page 28 Guide to Pain Management in Low-Resource Settings

16 Nilesh B. Patel

but which in essence remains valid. Th e theory propos- system of internal pain modulation and the subjective
es that the transmission of information across the point variability of pain.
of contact (synapse) between the Aδ and C nerve fi bers
Descending inhibitory nerve system
(which bring noxious information from the periphery)
and the cells in the dorsal horn of the spinal cord can Nerve activity in descending nerves from certain brain-
be diminished or blocked. Hence, the perception of the stem areas (periaqueductal gray matter, rostral me-
painfulness of the stimulus either is diminished or is not dulla) can control the ascent of nociceptive informa-
felt at all. Th e development of transcutaneous electrical tion to the brain. Serotonin and norepinephrine are the
nerve stimulation (TENS) was the clinical consequence main transmitters of this pathway, which can therefore
of this phenomenon. be modulated pharmacologically. Selective serotonin
Th e transmission of the nerve impulse across reuptake inhibitors (SSRIs) and tricyclic antidepressants
the synapse can be described as follows: Th e activation (e.g., amitriptyline) may therefore have analgesic prop-
of the large myelinated nerve fi bers (Aβ fi bers) is associ- erties (Fig. 3).
ated with the low-threshold mechanoreceptors such as
touch, which stimulate an inhibitory nerve in the spinal
Cerebral Cortex
cord that inhibits the synaptic transmission. Th is is a
possible explanation of why rubbing an injured area re-
duces the pain sensation (Fig. 2). Thalamus

C fiber (nociceptive signals) Midbrain PAG
Brain Stem
Projection neuron
(nociceptive signal)
+ Raphe nucleus
Locus ceruleus
Inhibitory Spinal Cord
interneuron Spinothalamic tract
−
+
SPINAL CORD
Aα and Aβ (mechanoceptors)
Aδ & C
nociceptive
fibers
Fig. 2. Th e gate control theory of Pain (Melzack and Wall).
+ excitatory synapse; – inhibitory synapse
Fig. 3. Ascending (solid lines) and descending pain pathways. Th e
raphe nucleus and locus ceruleus provide serotoninergic (5-HT) and
Endogenous opioid system adrenergic modulation. PAG = periaqueductal gray matter, part of
the endogenous opioid system.
Besides the gating of transmission of noxious stimuli,
another system modulates pain perception. Since 4000 Referred pain
BCE, it has been known that opium and its derivatives Visceral organs do not have any Aδ nerve innervation,
such as morphine, codeine, and heroin are powerful but the C fi bers carrying the pain information from the
analgesics, and they remain the mainstay of pain relief visceral organs converge on the same area of the spinal
therapy today. In the 1960s and 1970s, receptors for the cord (substantia gelatinosa) where somatic nerve fi bers
opium derivatives were found, especially in the nerve from the periphery converge, and the brain localizes the
cells of the periaqueductal gray matter and the ventral pain sensation as if it were originating from that somatic
medulla, as well as in the spinal cord. Th is fi nding im- peripheral area instead of the visceral organ. Th us, pain
plied that chemicals must be produced by the nervous from internal organs is perceived at a location that is
system that are the natural ligands of these receptors. not the source of the pain; such pain is referred pain.
Th ree groups of endogenous compounds (enkephalins,
endorphins, and dynorphin) have been discovered that Spinal autonomic refl ex
bind to the opioid receptors and are referred to as the Often the pain information from the visceral organs
endogenous opioid system. Th e presence of this system activates nerves that cause contraction of the skeletal
and the descending pain modulation system (adrener- muscles and vasodilation of cutaneous blood vessels,
gic and serotoninergic) provides an explanation for the producing reddening of that area of the body surface.