GATE CONTROL THEORY

GATE CONTROL THEORY

Psychologist Ronald Melzack and the anatomist Patrick Wall proposed the gate control theory for pain in 1965 to explain the pain suppression. According to them, the pain stimuli transmitted by afferent pain fibers are blocked by gate mechanism located at the posterior gray horn of spinal cord. If the gate is opened, pain is felt. If the gate is closed, pain is suppressed.

Mechanism of Gate Control at Spinal Level

1. When pain stimulus is applied on any part of body, besides pain receptors, the receptors of other

sensations such as touch are also stimulated 2. When all these impulses reach the spinal cord through posterior nerve root, the fibers of touch sensation (posterior column fibers) send collaterals to the neurons of pain pathway, i.e. cells of marginal nucleus and substantia gelatinosa

3. Impulses of touch sensation passing through these collaterals inhibit the release of glutamate and

substance P from the pain fibers

4. This closes the gate and the pain transmission is Blocked

Role of Brain in Gate Control Mechanism

According to Melzack and Wall, brain also plays some important role in the gate control system of the spinal cord as follows:

1. If the gates in spinal cord are not closed, pain signals reach thalamus through lateral spinothalamic

tract

2. These signals are processed in thalamus and sent to sensory cortex

3. Perception of pain occurs in cortical level in context of the person’s emotional status and previous

experiences

4. 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.

5. To minimize the severity and extent of pain, brain sends message back to spinal cord to close the gate

by releasing pain relievers such as opiate peptides

6. Now the pain stimulus is blocked and the person feels less pain.

Significance of Gate Control

Thus, gating of pain at spinal level is similar to presynaptic inhibition. It forms the basis for relief of pain

through rubbing, massage techniques, application of ice packs, acupuncture and electrical analgesia. All

these techniques relieve pain by stimulating the release of endogenous pain relievers (opioid peptides),

which close the gate and block the pain signals.

APPLIED PHYSIOLOGY

1. Analgesia

Analgesia means loss of pain sensation.

2. Hyperalgesia

Hyperalgesia is defined as the increased sensitivity to

pain sensation.

3. Paralgesia

Abnormal pain sensation is called paralgesia

FROM VISCERA 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 nerve

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

Substances released during ischemic reactions such as bradykinin and proteolytic enzymes stimulate the pain receptors of viscera.

2. Chemical Stimuli

Chemical substances like acidic gastric juice, leak from ruptured ulcers into peritoneal cavity and produce pain.

3. Spasm and Overdistention 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. Overdistention of hollow organs also causes pain.

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