Referred pain

The pain sensation produced in some parts of the body is felt in other structures away from the place of pain development. This is called referred pain. The deep pain and some visceral pain are referred to other areas. But, the superficial pain is not referred.

EXAMPLES OF REFERRED PAIN

1. Cardiac pain is felt at the inner part of left arm and left shoulder

2. Pain in ovary is referred to umbilicus

3. Pain from testis is felt in abdomen

4. Pain in diaphragm is referred in right shoulder

5. Pain in gallbladder is referred to epigastric region

6. Renal pain is referred to loin.

MECHANISM

Dermatomal rule: Pain is referred to a structure, which is developed from the same dermatome from which the pain producing structure is developed.

A dermatome includes all the 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.

NEUROTRANSMITTER INVOLVED IN PAIN SENSATION

Substance P is the neurotransmitter involved in pain sensation. It is secreted by the ending of pain nerve fibers in dorsal gray horn.

CONTROL OF PAIN SENSATION

The central nervous system has its own control system, which inhibits the impulses of pain sensation. This is also called analgesia system. This control system is present in both brain and spinal cord.

PAIN CONTROL SYSTEM IN BRAIN

This actually blocks the pain impulses before entering the brain. This system is present in:

1) Gray matter surrounding aqueduct of Sylvius and

2) Raphe magnus nucleus in pons.

The neurotransmitters involved in inhibition of pain by control system in brain are serotonin and opiate receptor substances namely endorphin, encephalin and dimorphic.

PAIN CONTROL SYSTEM IN SPINAL CORD

This is in posterior gray horn. The posterior gray horn is considered as the gateway for the pain impulses to reach the brain via spinothalamic tracts.

Gate Control Theory

The psychologist Ronald Melzack and the anatomist Patrick Wall Gate proposed the gate control theory for pain in 1965. They suggested that the gating system in the central nervous system opens and closes to allow the pain signals to the brain or to block the signals.

According to them, the nerve fibers with smaller diameter carry pain stimuli through the gate mechanism present in spinal cord. But, the nerve fibers with larger diameter, which carry other stimuli such as touch, pass through the same gate. The larger nerves inhibit the transmission of pain signals by smaller nerves through the gate.

When pain sensation is produced in any part of body, along with pain fibers, the other afferents particularly the touch fibers reaching the posterior column of spinal cord are also activated. The dorsal column fibers send collaterals to the cells of substantia gelatinosa in the posterior gray horn. Thus, some of the impulses ascending via dorsal column fibers pass through the collaterals and reach substantia gelatinosa. Here, the touch impulses inhibit release of substance P by the pain fibers ending on substantia gelatinosa. So the pain sensation is suppressed. Thus, the gating of pain in posterior gray horn level is similar to presynaptic inhibition.

This forms the basis for relief of pain through rubbing, massage techniques and application of ice packs. The endogenous opioid peptides (pain relievers) released in response to pain stimulus also close the gate and block the pain signals. Acupuncture and electrical analgesia relieve pain by stimulating the release of endogenous pain relievers.

Melzack and Wall also suggested the role of brain in gate control. The sequence of events is as follows. The peripheral nerve fibers transmit the pain signals through open gates in spinal cord to thalamus. The signals are processed in thalamus and sent to sensory cortex. The perception of pain occurs in cortical level in context of the person's emotional status and previous experiences. The person responds to the pain based on the integration of all these information in the brain. Thus, the brain determines the severity and extent of pain. To minimize the severity and extent of pain, brain sends message back to spinal cord to close the gate. Now the pain stimulus is blocked and the person feels less pain. Brain sends the message to block the pain signals through opiate peptides and other natural pain relievers.

If the brain sends message to open the gate wider, then the pain stimulus intensifies resulting in severe and prolonged pain.