Definition and Description

Pain sensation is described in many ways like sharp, prickling, electric, 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 muscle is expressed as cramps.

Benefits of Pain Sensation

Pain is an important sensory symptom. Though it is an unpleasant sensation, it has protective or survival benefits.

Protective or Survival Benefits of Pain

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 source of injury.
3. Pain forces the person to rest or to minimize activities thus enabling the rapid healing of injured part.
4. Pain urges the person to take required treatment to prevent major damage.

Pain is classified by four methods.

I. Classification Depending Upon Duration

1. Acute Pain

Often it is localized in a small area before spreading to neighboring areas.

Acute pain is usually caused by injury to tissues like muscle, bone or visceral organs.

Injury stimulates nociceptors (pain receptors). This type of pain disappears when the injury heals.

Usually, it is treated by medications.

2. Chronic Pain

Chronic pain is the intermittent or continuous pain with different intensities.

It lasts for longer periods. It is not easily described as acute pain.

Chronic pain is usually associated with a long-term illness such as osteoarthritis.

It is somewhat resistant to medical treatment.

II. Pathophysiological Classification

1. Nociceptive Pain

Nociceptive pain is produced due to activation of nociceptors during tissue injury.

Nociceptive pain is divided into two types:

1. Somatic pain.
2. Visceral pain.

i. Somatic Pain

Somatic pain is caused by stimulation of pain receptors situated in skin, mucus membrane and deeper tissues such as bones, muscles, ligaments, tendons, joints and connective tissue.

ii. Visceral Pain

Visceral pain is caused by stimulation of pain receptors situated in visceral organs.

It is unpleasant and 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 such as acidic gastric juice leaks from ruptured sources into peritoneal cavity and produce pain.
3. Spasm 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.
4. Overdistension of hollow organs: Overdistention of hollow organs also causes pain.

2. Neuropathic Pain

Neuropathic pain occurs due to dysfunction or damage, compression or disease of nervous structures.

Neuropathic pain is of two types:

1. Peripheral neuropathic pain which is caused by disease or lesion of peripheral nerve or dorsal root or dorsal root ganglion.
2. Central neuropathic pain which occurs due to disease or lesion of structures in brain or spinal cord.

III. Anatomical Classification

Often, pain is classified on the basis of location in the body or affected tissue.

Examples are headache, neck pain, back pain, shoulder pain, stomach pain, muscle pain, neurological pain and myofascial pain.

IV. Classification Depending Upon Components

Pain sensation has two components and accordingly it is subdivided into fast pain and slow pain.

1. Fast Pain

Fast pain is the first sensation whenever a pain stimulus is applied.

It is experienced as a bright, sharp and localized pain sensation.

2. Slow Pain

Fast pain is followed by slow pain, which is experienced as a dull, diffused and unpleasant pain.

Receptors for both 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 fibers.

Definition

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 in testis is felt in abdomen.
4. Pain in diaphragm is referred to right 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 same dermatome from which the pain producing structure developed.

A dermatome includes structures or parts of the body, which are innervated by afferent nerve fibers of one dorsal root.

For example, heart and inner aspect of left arm originate from the same dermatome. So, the pain in heart is referred to left arm.

Pain sensation from various parts of body is carried to brain by four pathways:

1. Pathway from skin and the deeper structures.
2. Pathway from face.
3. Pathway from viscera.
4. Pathway from pelvic region.

1. Pathway of Pain Sensation from Skin and Deeper Structures

Receptors

Receptors of pain sensation are free nerve endings which are distributed throughout the body.

First Order Neurons

First order neurons are the neurons in posterior nerve root ganglia which receive the impulses of pain sensation from pain receptors through their dendrites.

Impulses of pain are transmitted to spinal cord through axons of neurons in posterior nerve root ganglia.

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.

Their fibers ascend in the form of lateral spinothalamic tract.

Third Order Neurons

Third order neurons are ventral posterolateral nucleus of thalamus and reticular formation.

Their axons 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. Pathway of Pain Sensation from Face

Pain sensation from face is carried by trigeminal nerve.

3. Pathway of Pain Sensation from Viscera

Pain sensation from thoracic and abdominal viscera is transmitted by thoracolumbar (sympathetic) nerves.

Pain from esophagus, trachea and pharynx is carried by vagus nerve and glossopharyngeal nerve.

4. Pathway of Pain Sensation from Pelvic Region

Pain sensation from deeper structures of pelvic region is conveyed by sacral parasympathetic nerves.

Glutamate is secreted by Aδ afferent fibers, which transmit impulses of fast pain.

Substance P is secreted by C type fibers, which transmit impulses of slow pain.

Definition

Components of Analgesia System

1. Gray matter surrounding the III ventricle.
2. Gray matter surrounding aqueduct of Silvius.
3. Reticular formation of brainstem.

Analgesia system has got its own pathway through which it blocks the synaptic transmission of pain sensation in spinal cord and suppresses pain.

In fact, analgesic drugs such as opioids act through this system to produce a controlled pain relief.

Analgesic Pathway

Analgesic pathway is considered as a descending pain pathway, ascending pain pathway being the afferent fibers that transmit pain sensation to brain.

Analgesic pathway commences from brainstem.

Nerve Fibers Forming Analgesic Pathway

1. Nerve fibers of analgesic pathway arise from two sources namely, periventricular nucleus and periaqueductal gray matter (PAG) situated in midbrain.
2. Nerve fibers from above areas terminate in two areas called nucleus raphe magnus and nucleus locus coeruleus situated in reticular formation of midbrain.
3. Fibers from above nuclei descend through dorsal horn of spinal cord and synapse with interneurons.
4. Interneurons form synapses with neurons of marginal nucleus and substantia gelatinosa of Rolando. Neurons of these nuclei form second order neurons of afferent (ascending) pathway for pain.
5. Enkephalin released by interneuron acts on presynaptic nerve terminals as well as on postsynaptic neurons.
6. Ultimately transmission of pain sensation to brain is inhibited.