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Abstract
Pain is a universal indicator to relinquish an association
with a threatening agent, calling one into action to seek
restoration and relief. Pain manifests itself in several
forms and need not correlate to identifiable stimuli. The
gate theory of pain spawned progress in new discoveries of
its underlying pathophysiology and the critical effect of
endorphins in the body’s natural efforts to fight pain.
Discussed are the processes of how an individual senses
pain. Additionally, types of pain examined include
nociceptive, neuropathic, and idiopathic as well as chronic
and acute. Finally, the psychological state of an
individual and its effect on the pain process is considered.
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Pain is a
primitive component of humanity, integrating itself at all
stages of life regardless of cognitive development. Whether
one is highly educated, or severely impaired, similar pain
stimuli can produce identical responses, both in reflex and
deterrence. Yet despite its fundamental core, science
still has much to research before coming to a full
understanding of the pain process. Throughout human
history, theories on the process of pain and its management
have been numerous and varied. The ancient Greeks debated
whether pain was felt in the head or the heart, and some
even believed that the gods shot arrows at individuals, which resulted
in painful disease (Scarborough,
1969). Descartes, in 1644, compared the process of pain to someone
standing at the bottom of a tower and pulling on a rope,
striking a bell at the top (Sarason & Sarason, 2005, p
221). While different civilizations have had their own
prominent theories on the causes and treatments of pain,
science nonetheless continues to strive forward to reach a
unified answer to managing and understanding one of the
greatest horrors that has plagued humanity, namely, the
suffering of pain. Currently, modern medicine identifies
three primary types of pain: nociceptive, idiopathic, and
neuropathic. While there are clear delineations in each
type, there has yet to be a full understanding of them.
Even today, for example, Scadding (2004) notes that there
does not exist a uniformly accepted definition of
neuropathic pain, illustrating the limitations in modern
understanding of its underlying pathophysiology.
The Gate
Theory of Pain
Originally
proposed in 1965, Wall and Melzack described a synapse in
the spinal chord that modulates or “gates” the flow of pain
to the brain. They postulated the idea of a group of second
order neurons that basically “spool” incoming pain signals,
upon which, once a certain minimum threshold of excitement
is exceeded, the theoretical “gate” opens, sending the
signals to higher order processing centers in the brain.
This theory birthed out of their observations of patients
that were impervious to pain during the insertion of a
needle while undergoing acupuncture. They speculated
similarly, that since the gate at the second order neuronal
site of the spinal chord could allow pain to flow by
“opening,” conversely certain triggers could cause the brain
to turn off pain by sending impulses down the spinal chord,
and closing the pain gate. This theory led to an important
discovery about pain, namely that incoming pain signals are
subject modulation, increasing and decreasing, greatly
changing the final perception of its degree by the patient (Choderrer
& Choiniere, 2000).
Prior to the
introduction of the gate theory of pain, neuroscientists had
already known the importance that chemicals had on
conducting nerve signals. The gate theory was instrumental
in stimulating a rush of new research in pain
pathophysiology, or pain pathways. In addition, it gave
rise to research in neurotransmitters and their abilities to
excite or inhibit the ability of a cell to generate an
electrical impulse. The eventual discovery of endorphins,
the brain’s natural pain suppressing compound, was a major
one that gave credence to the gate theory (NIH, 1997).
Endorphins are endogenous opioid compounds and are produced
by the brain, working as natural painkillers. Cumulative
research from the American Medical Association states that
the work of these compounds play a crucial role in pain
modulation by limiting the amount of impulses transmitted to
the brain through the act of binding to special subsets of
receptors, which limit the amount of pain signal that is
allowed to reach the brain. Nerve cells are equipped with
opiate receptors, and when activated, the postsynaptic cell
loses its ability to freely transmit signals across the
synaptic gap, thus inhibiting pain. What this means, in
essence, is that the response of endorphins indeed function
as a kind of ‘gate’ similar to the one originally theorized
by Wall and Melzack, in that they can ‘close’ the pain
pathway.
Neurotransmitter Modulation of Pain
McCaffrey, Frock, & Garguilo (2003) describe
neurotransmitters as endogenous chemical messengers that
affect the perception of pain, responding to compounds such
as serotonin, norepiniephrine, endorphins and substance P.
Substance P is a neurotransmitter that functions directly in
the transmission of pain impulses especially involved in the
modulation of inflammatory and immune responses. Along with
substance P, the aforementioned compounds all act similarly
as ‘neuromodulators’ that regulate pain. Serotonin and
norepiniephrine contribute to modulation by being
responsible for both inhibitory and excitatory functions
within the pons and medulla areas of the brain (Pert,
1997). In other words, depending upon their levels of
release, they have the ability to both increase and decrease
the level of pain felt by the individual.
As
previously mentioned, endorphins are a powerful ally in a
person’s natural ability to fight pain. Chemically,
endorphins are a protein, and compose a class of several
times. These proteins are able to bind to the opiate
receptors on nerve cells, blocking the release of substance
P and the action of those cells, which ultimately lessens
the severity of pain felt by the individual. As a result,
the higher amount of endorphins released in the body, the
less pain an individual will suffer. Along with pain,
pleasurable experiences such as laughing, touching, or being
outside on a nice day also releases endorphins. Drugs such
as morphine and codeine are effective painkillers because
they bind to the same opiate receptors as endorphin, acting
in a similar fashion.
Sensing Pain
The
sensation of pain results from the firing of specialized
first order neurons called nociceptors that transmit signals
carried into the central nervous to nociceptive cells,
forming connections to second order spinal neurons,
eventually leading to the brain (McCleskey, 2003). Once
reached by the brain, third order neurons originating in the
thalamus transmit the signals into the cerebral cortex, thus
the sensation of pain. Nociceptors, when functioning
normally, have a much higher threshold than other sensory
receptors, which can trigger at the slightest touch of a
feather. This, of course, is a good thing, as it would be
hard to imagine something as gentle as a soft breeze being
physically painful (types of neuropathic pain, however, can
produce exactly this). When a noxious stimulus, defined as
any stimulus that produces pain, is received by the
peripheral pain receptors exceeding this threshold, an
action potential is created that initiates the traversing of
signals across the pain pathway, which ultimately results in
the sensation of pain.
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