Neurophysiology of Pain

DR. KARISHMA R. PANDEYASSISTANT PROFESSOR

DEPARTMENT OF BASIC AND CLINICAL PHYSIOLOGY

Topics 1) What is pain?2) Types3) Receptors4) Stimulated by5) Pathways 6) Visceral pain7) Referred pain8) Projected pain/ phantom pain9) Modulation

What is pain?Pain is an unpleasant experience associated with acute tissue damage.

Experienced: 1) following injury,2) organic diseases such as advanced cancer, 3) its origin within the CNS itself (not associated with tissue damage)Accompanied by4) emotional reaction (fear or anxiety)5) autonomic responses such as sweating and an increase BP and HR.

Sherrington “Pain is the physical adjunct of an imperative protective reflex." Painful stimuli generally initiate potent withdrawal and avoidance responses.

Functions of Pain

Protects body against further injury Guides functional recovery May enhance disability

ClassificationFast Pain Slow Pain

2-5 micro met dia 0.4-1.2 miro met dia12-30 m/s 0.5-2 m/s

A fibre C fibreMonomodal Polymodal

Activated by either high intensity mechanical stimuli or thermal stimuli(<50C)

Activated by high intensity mechanical, chemical and thermal (both hot and cold) stimuli

Glutamate Substance P or CGRPFollow neospinothalamic tract Follow paleospinothalamic tract

Nociceptors

• Responsive to noxious/ painful stimuli

• Free nerve endings

• Non-adapting nature

• Stimuli1) Mechanical2) Thermal: <5 or >45

0C3) Chemical

Receptors

Vanilloid receptor-1 (VR1): Vanillins, capsaicin, protons and to potentially harmful temp >43 °

VRL-1: temp > 50 °

Organization of Spinal Cord

Anterolateral Pathway: Spinothalamic Pathway

Sensations of pain, thermal and crude touch

Neospinothalamic Pathway: Fast Pain (More Localized)

Paleospinothalamic Pathway: Slow Pain (Less Localized)Diffuse projection to cerebral cortex and limbic system

Visceral pain

Deep pain Pain in deeper region of the body especially due to injuries to bones, tendons, and

joints dull aching and poor localization (probably d/t relative deficiency of Aδ nerve

fibers) Usu accompanied by prolonged contraction of skeletal muscles--- induce

ischaemia and aggravate pain

Pain in visceral organs. chronic-aching-suffering type of pain, st burning and intense sharp pain

Diffuse and poor localization Causes of pain are distention of organ, chemical irritation or ischemia Muscle spasm and rigidity (guarding) Pain impulses are carried by C fibers. pain often radiates or is referred to other areas

Referred Pain Often a person feels pain

in a part of the body that is fairly remote from the tissue causing the pain is called referred pain

Mechanism of Referred Pain

Projected Pain

Phantom Pain

Modulation of Pain

Gate Control Theory: 1960, Ronald Melzack and Patrick Wall

Endogenous Pain Relief System

Revisited Gate Control Theory

Interneurons activated by fibres descending from PAG and associated areas