Physiology of pain

Pain • unpleasant sensory & emotional

feeling, connected with true or potential damage of tissue or organ, which is described in the terms of such a damage.

International expert commiteeJ. “Pain” 6, 248-252, 1979

Pain • Doesn’t give any info about the external

environment;• Adequate stimulus – any suprathreshold

stimulus damaging the tissue or causing the danger of damage:

MechanicalThermal (burn or frostbite)Chemical (metabolism disorders)

pain• Danger signal that occurs at the damage

or the threat of damage of:SkinPeritoneumMeninxPericardithis.

Nociception • Nociception - sensor modality in

animals which causes pain feeling in man.

• Nociceptors - pain receptors

Types of painPain

Somatic Visceral

superficial deep

Early or primaryepicritical

Delayed or secondaryprotopatic

Superficial pain localization - skin

Early or primary – strictly localized,

Dissapears with the dissapearence of the stimulus

(pinch, hit, pin)

Latent period – sec

Superficial pain localization - skin

Delayed or secondary- Not localized, dyes out

slowly.Dull, diffuse

Latent period – 0,5-1,0 sec

diffuse

Deep painLocalization - connective tissue, muscles, bones, joints, teethTypes – muscle cramps, headache, toothache. Characteristics - dull, non-localized,irradiating Latent period 1-3 minAcute, sub-active, chronic

Visceral pain

Diffuse pain Pain with irradiation

Visceral pain• Localization – internal organs• Types - kidney, liver, intestinal cramps,

gastric ulcer pains, appendicitis, cardiac pain • Characteristics - dull, non-localized,

irradiating to other organs & tissue. May be acute but diffuse.

• Reasons – quick & excessive stretching of hollow organs, cramps, spastic contractions, ischemia

Pain duration• Acute pain – localized in the damaged area, its

intensity depends on the stimulus intensity, has signaling function, quickly dissapears.

• Chronic pain – lasts up till half a year, has stable & recurrent forms. No connections between pain intensity & level of organic damage.

• May become a separate syndrome

Other types of pain• Psycogenic – no peripheral organic

reason – neurosis.• Itching – caused by the increased

concentration of hystamine in skin

COMPONENTS OF PAIN

1. SENSORY DISCRIMINATIVE2. AFFECTIVE (EMOTIONAL)3. VEGETATIVE4. LOCOMOTOR5. COGNITIVE (intensity evaluation)

SENSORY DISCRIMINATIVE• Is enabled by thalamus & cortex.• When the hand is deepened into the water with

t0>450С skin receptors are excited, they send info to the cortex about the localization of hot stimulus, its intensity, the starting point & the end point of its action.

• Sensation is formed• This component prevails in superficial pain

Affective or emotional• Is enabled by limbic system• Negative emotions are formed• Is the prevailing component in chronic pain

vegetative• Is enabled by ANS• BP elevation, HR increase, pupil dilation,

changed rhythm of respiration • Sympatho-adrenal system is activated,

vasopressin (АDH) is produced.• Is the strongest in visceral pain

locomotor

• Is enabled by motor zones of cortex• Is displayed in flexor reflexes (defence

reflex), abdominal muscles tension, pscycomotor behavioral reactions

• Accompanies all types of pains

cognitive• Present pain is evaluated in comparison to

previous pains.• This evaluation depends on many factors:Social statusBringing up in the familyEthnic originCircumstances at which the pain occured

Pain neurophysiology

Pain theories

1. Theory of specificity – M.Frey – end of XIX century

2. Theory of intensity – Goldshteiner - end of XIX century

Theory of specificity

• Pain is an independent feeling with specialized nervous apparatus of receptors, conducting pathways & centres

• Prof – the correlation between skin pain dots and the dots of pressure & temperature is 9:1

Pressure & pain dots on the skin

Pressure dot

Pain dots

Intensity theory

• Pain may be caused by suprathreshold stimuli of different modality.

• Not proved to be true.

NociceptorsFree nervous endings of 2 types:• Non-myelinized fibres of C type – the velocity

of impulse conduction is up to 1 m/sec – are present everywhere (skin, joints, internal organs)

• Myelinized fibres of Аδ type – the velocity of impulse conduction is up to 20

m/sec– only in skin

Nociceptors Аδ • High threshold receptors;• May be sencibilized ;• Have small receptive fields.• 3 types:Mechano-Тhermo-Mechano-thermo-

Nociceptors of С type fibres

• High threshold, may cause sencibilization, have big receptive fields (17mm2).

• Polymodal

Nociceptors blocking

• Local anaestetics in low concentration block С type fibres

• Pressure – blocks just Аδ fibres. This activates С fibres

Algogenic substanses• Substances from damaged cells – potassium,,

АТP. • From plasma – bradikinins, Н+

• From must cells – hystamine• From platelets – serotonin• From nervous afferent fibres – substance Р• SNS mediators – adrenalin,noradrenalin

Nociceptive system

Receptive fields

Spinal cordЖелатинозная субстанция

skin

Internal org.

Аδ

Аδ

С

С

С

n. Vagus 70%

VPLThalamus

SI-SII

Thalamusn.medialis

5 –Associative cortex.(temporal & frontal)RF

3

45

12

1 –Tr. Neospino-thalamicus2 – tr. Paleospinothalamicus3 – Hypothalamus4 – Limbic cortex

Asparaginic acid

Sub. Р

Conducting pathways• Tr. Neospino-thalamicus – in the anterior

funiculus, has somatotypical organization. Enables primary pain conduction.

• Tr. paleospinothalamicus – non-specific system (RF) – has many synapses on one level in the spinal cord, makes diffuse connections in cortex. Enables emotional component of pain – secondary pain

First danger (bacterial infection, inflamation, mechanical influence)

Cortex- feeling of pain

Afferent fibres

Spinal cord

Supraspinal centers

Impulse conduction

Transduction & transformation

Nociceptors

Algogenic substanses formation

Refered pains

Pain irradiation

Pain progection in the cortex (due to lateral spino-thalamicus tract)

Antinociceptive systemGigantic-cell nucleus RF

Spinal cord neurons secreting endoopiates

Inhibition of afferent nociceptive neurons & neurons of posterior horn ІІ & Y plates

serotonin

Antinociceptive systemCentral grey matter

Ruph nucleiNoradrenalin, alpha-2 adrenoreceptors

serotonin

Spinal cord neurons releasing endoopiates

OPIOID ANALGESICS• Relieve pain as a symptom• Perception of pain & reaction to it are both

altered• Opioid receptor activation reduces intracellular

c-AMP formation,opens K-channels or suppresses voltage-gated Ca- channels, hyperpolarization of a neuron, decreased neurotransmitter release by CNS & myenteric neurons

MECHANISM OF OPIOID ACTION