There have always been two theories of the sensation
of pain, a quantitative or intensity theory and a stimulus-specific theory.
According to the former, pain results from excessive stimulation of every
kind: e.g., excessive heat or cold, excessive damage to the tissues. This
theory in its simplest form entails the belief that the same afferent nerve
fibers are activated by all of these various stimuli; pain is felt merely
when they are conducting far more impulses than usual. But knowledge
acquired in the 20th century has shown that the quantitative theory—at least
in its classic form—is wrong. Peripheral nerve fibers have been found to be
stimulus-specific, each one excited by certain forms of energy. The
stimulus-specific theory proposes that pain results from interactions
between various impulses arriving at the spinal cord and brain, that these
impulses are brought to the spinal cord in certain non-myelinated and small
myelinated fibers, and that the specific stimuli that excite these nerve
fibers are noxious, or harmful.
In the somatic tissues there are certain kinds of nerve fiber that do not
give rise to pain, no matter how many there are or how frequently they are
stimulated. Included in this category are mechanoreceptors that report only
deformation of the skin and the larger afferent nerve fibers from muscles
and tendons that form part of the organization of posture and movement. No
matter how they are excited, these receptors never give rise to pain. But
the smaller fibers from these tissues do cause pain when they are excited
mechanically or chemically.
Warmth and cold fibers are specific. Warmth fibers are excited by rising
temperature and quieted by falling temperature, and cold fibers respond
similarly with cold stimuli. Although pain arises from very hot and very
cold stimulation and with intense forms of mechanical stimulation, this
occurs only with the activation of afferent nerve fibers that specifically
report noxious events. When no noxious events are occurring, these nerve
fibers are silent.
In contrast, the quantitative theory seems to apply to the viscera, where
afferent nerve fibers used in reflex organization also report events giving
rise to pain. In the heart, for example, the same nerve fibers are excited
by mechanical stimulation as are excited by chemical substances formed in
the body that cause pain. Also, in the bladder, rectum, and colon, nerve
fibers activated by substances that cause pain are the same as those
activated by distension and contraction of the viscera. This means that the
same nerve fibers are reporting the state that underlies the desire to
urinate or defecate and the sensation underlying the pain felt when these
organs are strongly contracting in an attempt to evacuate their contents. In
the heart, rectum, and bladder, therefore, pain appears to be due to a
summation of impulses in sensory nerve fibers and is not mediated by a
special group of fibers reserved for reporting noxious events. It must be
pointed out, however, that not all researchers accept the argument.
Not all the tissues making up the body give rise to
pain; furthermore, each tissue must be stimulated in an appropriate way to
invoke its particular sensation of pain. Skin, being the outer covering of
the body, easily raises the warning of pain, but other tissues that do not
come in direct contact with the outer environment are just the opposite. The
brain, for example, can be pierced, cut, and burned in neurosurgery, while
the patient would require only local anesthesia of the pain-sensitive scalp.
The lung, liver, and spleen also do not give rise to pain, no matter how
they are stimulated. Pain arises from hollow viscera when the passage of
their contents is obstructed and the musculature must undergo strong
contraction and stretching. Pain cannot be induced by cutting or burning the
wall of the intestine, but pulling on the mesenteric tissue that fixes the
intestines to the posterior wall of the abdomen causes pain. The reason for
these differences is clear. Tissues are sensitive to the kinds of damage
they are likely to meet during life and not to those that they probably will
never meet.
Although the warning function of pain is obvious, it is not equivalent to
nociception, the perception of forces likely to
damage the tissues of the body. First, nociception can occur without pain
and pain without nociception; also, the sensation of pain is only a part of
the total act of nociception. There are reflex effects as well, such as a
local reflex withdrawal from a sudden noxious stimulation of the skin. There
are autonomic effects, such as a rise in blood pressure, quickening of the
heart rate and respiration, and other excitatory sympathetic nervous
effects. There may even be shrieking or howling, warning other animals that
something dangerous and painful is occurring.
Acute and chronic pain differ in many ways. Acute
pain occurs with sudden damage, such as stepping on a nail or biting the
tongue. Chronic pain is the pain of pathological conditions—the pain that
accompanies gout, arthritis, inflammation, or cancer. Each type of pain has
effects on the nervous system.
The effect of acute inflammation of the joints on nerves reporting the state
of the joint and on the central nervous system has been studied by inducing
arthritis in animals. In this condition, locally formed chemical substances
excite the small myelinated and nonmyelinated afferent fibers that report
noxious events. Most of these nerves, sensitized by the inflammatory
exudate, begin to fire impulses continuously. This flow of impulses to the
dorsal-horn neurons of the spinal cord increases their excitability so that
many of them also begin to fire continuously. Neurons that are normally
excited only by noxious stimulation now respond to light touch as well.
Meanwhile, motor neurons in related areas also fire spontaneously, and
stimuli that would not normally cause withdrawal reflexes now cause a
prolonged reflex response. There is no change in the motor neurons
themselves; the change is in the firing threshold of peripheral neurons
coming from the inflamed area and in the interneurons between the afferent
nerve fibers and the motor neurons. These interneurons are ultimately
connected to the brain, so that the state of increased sensitivity is passed
on to related cerebral neurons. Eventually, neurons in the cerebral
hemispheres continuously and spontaneously generate impulses. Other neurons
of the brain start responding to movements of the affected joints that
normally would not do so.
In some people with chronic painful conditions, the constant pain impulses
change the character of neurons of the thalamus and cortex. For example, one
patient who had had a toothache 10 days before he had an operation on the
thalamus for parkinsonism suddenly got the pain of toothache again when the
thalamus was stimulated electrically. Normally, no pain can be induced by
stimulating that part of the thalamus.
Most of the afferent nerves making up the dorsal
roots are nonmyelinated fibers. These fibers are activated by warmth within
a physiological range (and by higher temperatures likely to damage the
body), by chemical substances (including those made in the body), and by
strong mechanical stimulation such as pricking and crushing. The smaller
myelinated fibers report mechanical stimulation of the skin, noxious
stimulation, and cold.
As stated above, pain is not the inevitable result of the firing of
nonmyelinated fibers reporting noxious events. These fibers may fire at a
slow rate without causing pain; they may even continue to fire for an hour
or so without pain. Furthermore, the pain threshold does not correspond with
the onset of activity in the nonmyelinated fibers, for pain can increase
while the discharge of nerve impulses decreases.
The stimulus-specific organization of the peripheral
nerve fibers is not continued within the spinal cord, as the various
afferent nerve fibers do not transmit their impulses exclusively to neurons
of only one kind of sensibility. In the dorsal horns (the spinal region that
receives afferent impulses) a few neurons are purely nociceptive, but most
neurons reporting noxious events receive both noxious and mechanoreceptive
input. These latter are called convergent neurons. The size of the
peripheral field (the area of the body from which it receives stimuli) of a
dorsal-horn neuron continually varies, depending on the state of
excitability of the neuron. Furthermore, events in the peripheral field
affect future responses. For example, repeated input along a group of
afferent nerve fibers produces a gradually decreasing response in the
central nervous system. This is called habituation. Also, the region of
decreased response spreads from local neurons that received the input to
neighbouring neurons.
The state of excitability of a dorsal-horn neuron depends on many variables.
If it is very excitable, it will respond to impulses from many afferent
peripheral nerve fibers; if it is relatively inexcitable, it will be
affected only by those peripheral fibers that are habitually connected to it
and located near it. A neuron excited by many afferent fibers receives input
from a larger area than a neuron receiving only from the fibers most nearly
related to it. For this reason the area of skin or deep tissue connected to
neurons of the dorsal horn varies and changes. In experiments using damaged
skin, it has been found that a barrage of nerve impulses from the damaged
region increases the excitability of the dorsal-horn neurons. Once this
hyperexcitable state has been set up, it continues for a time without
further input from peripheral nerves. In this state of local excitability,
some dorsal-horn neurons receive an input from the area of damaged skin that
they would not receive were the skin in a normal state.
The convergent neurons mentioned above can have their activity inhibited by
tactile stimulation of a region near their peripheral fields or of a
homologous region on the opposite side of the body. Also, their
responsiveness to stimuli can be increased by damage to the skin in their
peripheral fields.
From these convergent neurons and from other neurons of the dorsal horns,
there arise tracts of long fibers that cross the midline and lead to the
thalamus and other nuclei of the brain. These constitute the spinothalamic
tracts. The other main pathway of pain impulses ends in the reticular
formation of the medulla oblongata and pons and is known as the
spinoreticular tract. It is believed that spinoreticular input to the brain
serves the autonomic responses and emotional components associated with
pain, whereas the spinothalamic serves conscious sensation, with its exact
temporal and spatial aspects. Neurons around the central spinal canal that
receive input from the bladder and colon and their overlying somatic tissues
may be connected to an ascending tract that stays within the gray matter in
the neighbourhood of the central canal.
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