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 |
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 |
| Nociceptive: 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 |
| Nociceptive: visceral |
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. |
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.
Neurotransmitters:
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-
A-BETA (β) FIBERS–
- Large
- Myelinated
- Fast conducting
- Low stimulation threshold
- Respond to light touch
A-DELTA (δ) FIBERS-
- Small
- lightly myelinated
- slow conducting
- respond to heat, pressure, cooling, and chemicals
- sharp sensation of pain
C FIBERS-
- 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:
Resource-
- Table 1, Table 2 from physical rehabilitation book by Susan B. O’Sullivan, Thomas J. Schmitz, George D. Fulk.
- YouTube. https://www.youtube.com/watch?v=uOaiaYDoUnA&t=1s
This is a good article on pain.
Concise and comprehensive.