Pain sensation physiology

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.


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


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


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.


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.



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


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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