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. Fibers
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. 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.
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