Neurophysiology of Pain and Gate Control Theory

Think about yourself touching a hot surface or got a cut on your body, you will feel pain. But can you think how you feel this pain? How does this piece of information go to our brain? Let’s know about the process that is called neurophysiology of pain.

What is Neurophysiology of pain ?

Neurophysiology is a branch of physiology and neuroscience that is concerned with the study of the functioning of the nervous system.

According to this definition, the meaning of Neurophysiology of pain is the study of the functioning of the nervous system on pain sensation.

What is Pain?

The International Association for the Study of Pain (IASP) is defined as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.

Pain may be acute or chronic. Acute pain is a sharp pain of short duration, Chronic pain is the intermittent or constant pain with different intensities.

Advantage of Pain Sensation:

  • Gives a warning signal for a problem
  • Creates awareness of injury
  • Prevent further damage by causing reflex
  • Due to pain person minimize the activity so rapid healing of the injured part occurs
  • It urges the person to take the required treatment to prevent major damage

Components of Pain Sensation:

Pain sensation has two components- 1) Fast Pain, 2) Slow Pain.

Features Fast pain Slow pain
Nerves A-delta(myelinated C (unmyelinated)
Stimulus Pinprick, heat Tissue damage
Sensation Sharp, pricking, burning, dermatomal Slow, dull, sclerotomal
Diameter 1-4 μm 0.1-1 μm
Conduction velocity 5-30 m/sec 0.4-1.4 m/sec
Distribution Body surface All tissue except CNS
Reflex response Withdrawal Muscle spasm or tone
Biological value Avoidance of tissue damage Enforced rest
Effects of morphine Very little Suppresses pain
CNS target Thalamus, cortex Limbic, hypothalamic
Affective response No Yes
Autonomic signs No Yes
Localized receptor field Yes No
Dorsal horn connection laminae I and V laminae II and III 
Table 1: Features of Fast and Slow Nociceptive Pain.

Types of Pain:

Types of Pain Tissue Source Subjective Objective
Nociceptive: Cutaneous or superficial Skin and subcutaneous tissue(predominantly A- delta fibers) Well-localized, stabbing, burning, cutting Clear, consistent, proportional pain reproduced through movement or mechanical testing of target tissue
Deep somatic
Bone, muscles, blood vessels, connective tissue(predominantly C fibers) Often referred to other location, tearing, cramping, pressing, aching Vague, sometimes referred pain reproduced through movement or mechanical testing of deeper tissues, spasm, trigger points common
Organs and the linings of the body cavities (predominantly C fibers) Often referred to other location; poorly localized, diffuse, deep cramping or splitting, sharp, stabbing Vague pain reproduction on movement or mechanical testing of visceral tissues
Peripheral neurogenic Nerve fibers (axon or neural connective tissue Pain variously described as burning, shooting, sharp, aching or “electric-shock-like Pain or symptoms provocation with movement or mechanical tests that move, load, or compress neural tissue.
Central Spinal cord and central nervous system Disproportionate, non-mechanical, unpredictable pattern of pain provocation in response to multiple, nonspecific aggravating or easing factors Disproportionate, inconsistent, non-mechanical or non-anatomical pattern of pain provocation in response to movement or mechanical testing.
Table 2: Subjective and objective characteristics associated with different types of pain and tissue sources

Pain Receptors(Nociceptors):

Pain receptors, also called nociceptors, are a group of sensory neurons with specialized nerve endings widely distributed in the skin, deep tissues (including the muscles and joints), and most of the visceral organs. or The receptors that respond to noxious stimuli are termed as nociceptors.

Depending on the type of noxious stimuli, they can be classified as 1)Mechanical, 2)Thermal, 3)Chemical. Their structure is that of free nerve ending and they are therefore the termination of A-delta and C afferent fibers.


Pain initiators

  • Glutamate- central
  • Substance P- Central
  • Bradykinin- Peripheral
  • Prostaglandins- peripheral
  • Aspartate

Pain inhibitors

  • Serotonin
  • Endorphins
  • Enkephalins
  • Dynorphin
  • GABA
  • Glycine

Pain Pathway:

Pain pathway: A pathway for the sensation of pain. This is a long pathway, in which neurons make connections in both the brain and the spinal cord. it is an important part to understand about Neurophysiology of pain

The nerve impulses giving rise to conscious pain travel in four regions of the nervous system, in each of which they can be modulated.

The four regions include:

  • The peripheral nervous system ( nociceptors and the afferent pathway)
  • Spinal cord (dorsal horn, spinothalamic tract and the multisynaptic ascending system)
  • Brainstem and thalamus
  • Cerebral cortex

Nerve fibers involved in pain transmission: A fibers and C fibers.

A FIBERS are classified in two types-


  • Large
  • Myelinated
  • Fast conducting
  • Low stimulation threshold
  • Respond to light touch


  • Small
  • lightly myelinated
  • slow conducting
  • respond to heat, pressure, cooling, and chemicals
  • sharp sensation of pain


  • Small and unmyelinated
  • Very slow conducting
  • Respond to all type of noxious stimuli
  • Transmit prolonged dull pain
  • Require high-intensity stimuli to trigger a response

Pathway of Pain Sensation:

1st ORDER NEURON [ Injury ⇒ Receptor(free nerve ending) ⇒ Posterior nerve root ganglion ⇒ Posterior gray horn of spinal cord; here (a)fast pain fibers Aδ synapse with the neuron of a marginal nucleus and release glutamate neurotransmitter (b) slow pain fiber C fiber synapse with neurons of substantia gelatinosa and release substance P neurotransmitter]  ⇒ SYNAPSE BETWEEN 1st ORDER AND 2nd ORDER NEURON ⇒ second-order neuron cross to the opposite side(lateral spinothalamic tract) and then ascend  ⇒   These 2nd ORDER NEURONS MAKES SYNAPSE WITH THE 3rd ORDER NEURON at the ventero lateral nucleus of the thalamus ⇒  Axon from 3rd order neuron reach the sensory area of cortex some fibers from reticular formation reach hypothalamus⇒ Center for pain sensation is in the postcentral gyrus of parietal cortex.

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 the spinal cord. if the gate is opened, pain is felt. if the gate is closed, pain is suppressed.

Transcutaneous Electrical Nerve Stimulation (TENS) is a stimulating device which works on gate control theory for relieving pain

Pain Measurement Scale:

Pain Measurement Scale