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Conductive ways

First order neurons are the cells in posterior nerve root ganglia. These neurons receive impulses of pain sensation from pain receptors through their dendrites and their axons reach the spinal cord. The fibers of fast pain sensation are carried by A5 afferent fibers. After reaching spinal cord, fibers synapse with marginal cells in posterior gray horn.

Fibers transmitting impulses of slow pain belong to С type and these fibers synapse with substantia gelatinosa in the posterior gray horn.

Second Order Neurons

Marginal cells and cells of substantia gelatinosa form the second order neurons. Fibers from these cells ascend in the form of the lateral spinothalamic tract.

Fibers of marginal cells for fast pain are long. Immediately after taking origin, the fibers cross midline via anterior gray commissure, reach anterolateral white column and ascend. These fibers form neo-spinothalamic tract—a part of lateral spinothalamic tract. These nerve fibers terminate in ventral posterolateral nucleus of thalamus. Some of fibers terminate in ascendng reticular system of brainstem.

Fibers of slow pain which arise from substantia jelatinosa cross the midline and run along with fibers of fast pain as paleospinothalamic fibers in lateral spinothalamic tract. One fifth of these fibers terminate in posterolateral nucleus of thalamus. The remaining terminate in nuclei of reticular formation in brainstem in tectum of midbrain or in the gray matter surrounding deduct of Sylvius.

Third Order Neurons

The third order neurons of pain pathway are the neurons of thalamic nucleus, reticular formation, tectum and gray matter around aqueduct of Sylvius. Axons from these neurons reach the sensory area of cerebral cortex. Some ones from reticular formation reach hypothalamus.

Centeгs for Pain Sensation

Cortical center for pain sensation is in the postcentral gyrus of parietal cortex. Fibers reaching hypothalamus are concerned with arousal mechanism due to pain stimulus.

I PATHWAY OF PAIN SENSATION FROM VISCERA AND FACE

The 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. Pain sensation from lace is carried by trigeminal nerve.

II PATHWAY OF PAIN SENSATION 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: The substances released during ischemic reactions like bradykinin and proteolytic enzymes stimulate the pain receptors of viscera.

2. Chemical stimuli: The chemical substances like acidic gastric juice leaks from ruptured ulcers into peritoneal cavity and produce pain.

3. Spasm of hollow organs: Spastic contraction of muscles in gastrointestinal tract and other hollow organs of viscera cause pain by stimulating the free nerve endings.

4. Overdistention of hollow organs also causes pain.

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