INTRODUCTION
Pain is
defined as an unpleasant and emotional experience associated with or without
actual tissue damage. Pain sensation is described in many ways like sharp,
pricking, electrical, dull ache, shooting, cutting, stabbing, etc. Often it
induces crying and fainting.
Pain is
produced by real or potential injury to the body. Often it is expressed in
terms of injury. For example, pain produced by fire is expressed as burning
sensation; pain produced by severe sustained contraction of skeletal muscles is
expressed as cramps.
Pain
may be acute or chronic. Acute pain is a sharp pain of short duration with
easily identified cause. Often it is localized in a small area before spreading
to neighboring areas. Usually it is treated by medications. Chronic pain is the
intermittent or constant pain with different intensities. It lasts for longer
periods. It is somewhat difficult to treat chronic pain and it needs
professional expert care.
BENEFITS
OF PAIN SENSATION
Pain is
an important sensory symptom. Though it is an unpleasant sensation, it has
protective or survival
benefits
such as:
1. Pain
gives warning signal about the existence of a problem or threat. It also
creates awareness of injury.
2. Pain
prevents further damage by causing reflex withdrawal of the body from the
source of injury
3. Pain
forces the person to rest or to minimize the activities thus enabling rapid
healing of injured part
4. Pain
urges the person to take required treatment to prevent major damage.
COMPONENTS
OF PAIN SENSATION
Pain
sensation has two components:
1. Fast
pain
2. Slow
pain.
Fast
pain is the first sensation whenever a pain stimulus is applied. It is
experienced as a bright, sharp
and
localized pain sensation. Fast pain is followed by the slow pain, which is
experienced as a dull, diffused and unpleasant pain.
Receptors
for both the components of pain are same, i.e. the free nerve endings. But,
afferent nerve
fibers
are different. Fast pain sensation is carried by Aδ fibers and slow pain
sensation is carried by C type of nerve fibers.
PATHWAYS
OF PAIN SENSATION
Pain
sensation from various parts of body is carried to brain by different pathways
which are:
1.
Pathway from skin and deeper structures
2.
Pathway from face
3.
Pathway from viscera
4.
Pathway from pelvic region.
1. FROM
SKIN AND DEEPER STRUCTURES
Receptors:
Receptors
of pain sensation are the free nerve endings, which are distributed throughout
the body.
First
Order Neurons:
First
order neurons are the cells in posterior nerve root-ganglia, which receive the
impulses of pain sensation from pain receptors through their dendrites. These
impulses are transmitted to spinal cord through the axons of these neurons.
Fast
pain fibers: Fast pain sensation is carried by Aδ type afferent fibers which
synapse with neurons of marginal nucleus in the posterior gray horn.
Slow
pain fibers: Slow pain sensation is carried by C type afferent fibers, which
synapse with neurons of substantia gelatinosa of Rolando in the posterior gray
horn.
Second
Order Neurons:
Neurons
of marginal nucleus and substantia gelatinosa of Rolando form the second order
neurons. Fibersfrom these neurons ascend in the form of the lateral
spinothalamic tract.
Fast
pain fibers: Fibers of fast pain arise from neurons of marginal nucleus.
Immediately after taking origin, the fibers cross the midline via anterior gray
commissure, reach the lateral white column of the opposite side and ascend.
These fibers form the neospinothalamic fibers in lateral spinothalamic tract.
These nerve fibers terminate in ventral posterolateral nucleus of thalamus.
Some of the fibers terminate in ascending reticular activating system of
brainstem.
Slow
pain fibers: Fibers of slow pain, which arise from neurons of substantia
gelatinosa, cross the midline and run along the fibers of fast pain as
paleospinothalamic fibers in lateral spinothalamic tract. One fifth of these
fibers terminate in ventral posterolateral nucleus of thalamus. Remaining
fibers terminate in any of the following areas:
i.
Nuclei of reticular formation in brainstem
ii.
Tectum of midbrain
iii.
Gray matter surrounding aqueduct of Sylvius.
Third
Order Neurons:
Third
order neurons of pain pathway are the neurons in:
i.
Thalamic nucleus
ii.
Reticular formation
iii.
Tectum
iv.
Gray matter around aqueduct of Sylvius.
Axons
from these neurons reach the sensory area of cerebral cortex. Some fibers from
reticular formation reach hypothalamus.
Center
for Pain Sensation: Center for pain sensation is in postcentral gyrus of
parietal cortex. Fibers reaching hypothalamus are concerned with arousal
mechanism due to pain stimulus.
2. FROM
FACE
Pain
sensation from face is carried by trigeminal nerve
3. FROM
VISCERA
Pain
sensation from thoracic and abdominal viscera is transmitted by sympathetic
(thoracolumbar) nerves. Pain from esophagus, trachea and pharynx is carried by
vagus and glossopharyngeal nerves.
4. FROM
PELVIC REGION
Pain
sensation from deeper structures of pelvic region is conveyed by sacral
parasympathetic nerves.
VISCERAL
PAIN
Pain
from viscera is unpleasant. It is poorly localized.
CAUSES
OF VISCERAL PAIN
1.
Ischemia
Substances
released during ischemic reactions such as bradykinin and proteolytic enzymes
stimulate the pain receptors of viscera.
2.
Chemical Stimuli
Chemical
substances like acidic gastric juice, leak from ruptured ulcers into peritoneal
cavity and produce pain.
3.
Spasm and Over-distention of Hollow Organs
Spastic
contraction of smooth muscles in gastrointestinal tract and other hollow organs
of viscera cause pain by stimulating the free nerve endings. Over-distention of
hollow organs also causes pain.
REFERRED
PAIN
DEFINITION
Referred
pain is the pain that is perceived at a site adjacent to or away from the site
of origin. Deep pain
and
some visceral pain are referred to other areas. But, superficial pain is not
referred.
EXAMPLES
OF REFERRED PAIN
1.
Cardiac pain is felt at inner part of left arm and left shoulder
2. Pain
in ovary is referred to umbilicus
3. Pain
from testis is felt in abdomen
4. Pain
in diaphragm is referred to shoulder
5. Pain
in gallbladder is referred to epigastric region
6.
Renal pain is referred to loin.
MECHANISM
OF REFERRED PAIN
Dermatomal
Rule: According to dermatomal rule, pain is referred to a structure, which is
developed from the same dermatome from which the pain producing structure is
developed.
Sites
of referred pain
A
dermatome includes all the structures or parts of the body, which are
innervated by afferent nerve fibers of one dorsal root. For example, the heart
and inner aspect of left arm originate from the same dermatome. So, the pain in
heart is referred to left arm.
NEUROTRANSMITTERS
INVOLVED
IN PAIN
SENSATION
Glutamate
and substance P are the neurotransmitters secreted by pain nerve endings. Aδ
afferent fibers
which
transmit impulses of fast pain secrete glutamate. The C type fibers, which
transmit impulses of slow pain secrete substance P.
ANALGESIA
SYSTEM
Analgesia
system means the pain control system. Body has its own analgesia system in
brain, which
provides
a short term relief from pain. It is also called endogenous analgesic system.
Analgesia system has got its own pathway through which it blocks the synaptic
transmission of pain sensation in spinal cord and thus attenuates the
experience of pain. In fact analgesic drugs such as opioids act through this
system and provide a controlled pain relief.
ANALGESIC
PATHWAY
Analgesic
pathway that interferes with pain transmission is often considered as
descending pain pathway, the ascending pain pathway being the afferent fibers
that transmit pain sensation to the brain
Role of
Analgesic Pathway in Inhibiting Pain Transmission
1.
Fibers of analgesic pathway arise from frontal lobe of cerebral cortex and
hypothalamus
2.
These fibers terminate in the gray matter surrounding the third ventricle and
aqueduct of Sylvius (periaqueductal gray matter)
3.
Fibers from here descend down to brainstem and terminate on:
i.
Nucleus raphe magnus, situated in reticular formation of lower pons and upper
medulla
ii.
Nucleus reticularis, paragigantocellularis situated in medulla
4.
Fibers from these reticular nuclei descend through lateral white column of
spinal cord and reach the
synapses
of the neurons in afferent pain pathway situated in anterior gray horn Synapses
of the afferent pain pathway are between:
i. Aδ
type afferent fibers and neurons of marginal nucleus
ii. C
type afferent fibers and neurons of substantia gelatinosa of Rolando.
5. At
synaptic level, analgesic fibers release neurotransmitters and inhibit the pain
transmission
before
being relayed to brain.
Neurotransmitters
of Analgesic Pathway:
Neurotransmitters
released by the fibers of analgesic pathway are serotonin and opiate receptor
substances namely enkephalin, dynorphin and endorphin.
GATE
CONTROL THEORY
Psychologist
Ronald Melzack and the anatomist Patrick Wall proposed the gate control theory
for pain in 1965 to explain the pain suppression.
According
to them, the pain stimuli transmitted by afferent pain fibers are blocked by
gate mechanism
Pain
pathway and analgesic pathway located at the posterior gray horn of spinal
cord. If the gate is opened, pain is felt. If the gate is closed, pain is
suppressed.
Mechanism
of Gate Control at Spinal Level
1. When
pain stimulus is applied on any part of body, besides pain receptors, the
receptors of other sensations such as touch are also stimulated.
2. When
all these impulses reach the spinal cord through posterior nerve root, the
fibers of touch sensation (posterior column fibers) send collaterals to the
neurons of pain pathway, i.e. cells of marginal nucleus and substantia
gelatinosa
3.
Impulses of touch sensation passing through these collaterals inhibit the
release of glutamate and substance P from the pain fibers
4. This
closes the gate and the pain transmission is blocked.
Role of
Brain in Gate Control Mechanism:
According
to Melzack and Wall, brain also plays some important role in the gate control
system of the spinal cord as follows:
1. If
the gates in spinal cord are not closed, pain signals reach thalamus through
lateral spinothalamic tract
2.
These signals are processed in thalamus and sent to sensory cortex
Gate
control system
3.
Perception of pain occurs in cortical level in context of the person’s
emotional status and previous
experiences
4. The
person responds to the pain based on the integration of all these information
in the brain.
Thus,
the brain determines the severity and extent of pain.
5. To
minimize the severity and extent of pain, brain sends message back to spinal
cord to close the gate
by
releasing pain relievers such as opiate peptides
6. Now
the pain stimulus is blocked and the person feels less pain.
Significance
of Gate Control
Thus,
gating of pain at spinal level is similar to presynaptic
inhibition.
It forms the basis for relief of pain
through
rubbing, massage techniques, application of
ice
packs, acupuncture and electrical analgesia. All
these
techniques relieve pain by stimulating the release
of
endogenous pain relievers (opioid peptides),
which
close the gate and block the pain signals.
THOSE ABOVE ARE COLLECTED FROM SOME BOOKS AND WEBSITES..
(ESSENTIALS OF MEDICAL PHYSIOLOGY-K Sembulingam, Prema
Sembulingam)
THANK YOU,
SRIKUMARAN
PHYSIOTHERAPY CLINIC & FITNESS CENTER
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