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.
Pathways of pain sensation
Pain sensation from various parts of body is carried to brain by different pathways which are:
- Pathway from skin and deeper structures.
- Pathway from face.
- Pathway from viscera.
- Pathway from pelvic region..
Pathway 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 Ao 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 from these neurons ascend in the form of the lateral spinothalamic tract.
Fast pain fibers :
Fiber 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 arise from neurons of substantia gelatinosa, cross the midline and run along with fibers of fast pain as Paleospinothalamic fibers in lateral spinothalamic tract. One fifth of these fibers of fast pain as paleospinothalamic fibres in lateral spinothalamic tract. One fifth these fibers terminate in Central posterolateral nucleus of thalamus. Remaining fibers terminate in any of the following areas:
- Nuclei of reticular formation in brainstem.
- Tectum of mid brain
- Gray matter surrounding aqueduct of sylvius.
Third order neurons
Third order neurons of pain pathway are the neurons in :
- Thalamic nucleus.
- Reticular formation.
- Tectum.
- 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.
Pathway from face :
Pain sensation from face is carried by trigeminal nerve
Pathway from viscera:
Pain sensation from thoracic and abdominal viscera is transmitted by (thoracolumbar) nerves. Pain from esophagus, trachea and pharynx is carried by vagus and glossopharyngeal nerves.
Pathway from pelvic region:
Pain sensation from deeper structures of pelvic region is conveyed by sacral parasympathetic nerves.
Dr. Mahalakshmi Raghunath
Referrence :Medical physiology
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