Neuro-PhysiologyKamal Mohammad

Lecturer Of Physiology

Faculty Of Medicine Dept.Of Physiology

PAINPHYSIOLOGY

LECTURE NO (-)

What is Pain?

An unpleasant sensory & emotional experience associated with actual or potential tissue damage.

The International Association for the Study of Pain

Pain sensation

Pain is unpleasant sensation that is characterized by the

following :

1. It has protective function (initiate the withdrawal reflex)

2. It is widely distributed particularly in the skin

3. Its adequate stimulus is not specific i.e. pain can be produced

by mechanical, thermal or chemical stimuli that lead to tissue

damage

4. It is almost a non- adapting sensation

PAIN RECEPTORS

PAIN RECEPTORS (the nociceptors)

These are specific naked free nerve endings of which

there are 3 types called the mechanical, thermal &

chemical pain receptors

These are excited by noxious stimuli

Mechanisms of stimulation

of pain receptors

Thermal

Chemical

Mechanical

Mechanism of stimulation of

pain receptors :

Pain receptors are stimulated chemically by

certain substances released from the damaged

tissues e.g. bradykinin, histamine, serotonin,

acids, excess K+ ions, prostaglandins.

Types of Pain

According to site of origin:

1- Cutaneous pain.

2- Deep pain.

3-Visceral pain

Cutaneous pain

This type of pain is perceived as a result of

stimulation of pain receptors in the skin.

Afferents:

a) Thin myelinated A delta fibers

b) Non myelinated C fibers.

Quality of pain

Slow burningFast pricking

C fibersA delta fibersAfferent

Felt shortly after injury

Felt during the injury

Onset

LongerShortDuration

BurningPrickingNature

Poorly localizedWell localizedLocalization

thalamusSensory cortexCenter

Skin & subcutaneous

tissues.

Substance P

Skin.

Glutamate

Site of receptors

Chemical transmitter

Reaction to cutaneous pain

1-Somatic effects:-In the form of protective withdrawal reflexes

2-Autonomic effects:- Mild or moderate pain results in sympathetic effects in

the form of: Tachycardia & Rise of arterial blood pressure.

Sever pain results in parasympathetic effects produced as bradycardia & hypotension.

3-Emotional effects:- As crying, restlessness & anxiety.

Deep pain

Origin: pain receptors in the deep structures as

muscles , ligaments and joints.

Afferent: non myelinated C- fibers

Nature: dull aching.

Localization: poorly localized.

Associated changes

1- Somatic changes:

spasm of nearby skeletal muscles.

2- Autonomic reactions:

decrease heart rate & blood pressure.

3- Emotional reactions:

anxiety, crying & depression

Visceral Pain

Origin: from internal viscera.

Nature: dull aching.

Afferent: non myelinated C fibers pass mainly

with autonomic fibers.

Localization: poorly localized.

Associated changes

1- Somatic changes: spasm of overlying skeletal

muscles.

2- Autonomic reactions: decrease heart rate & blood

pressure.

3- Emotional reactions: anxiety, crying & depression

Ion Fluxes(H+/ K+)

Mechanism of pain

Tissue Injury

Mast Cell

SensitizedNociceptor

Aspartate, Neurotensin, Glutamate, Substance P

To brain

Histamine

Bradykinin

Leukotrienes

Dorsalhorn

Prostaglandins

Pathways

Spinothalamic tract

(spinothalamic pathway)

Receptor

1th order

neuron

2th order

neuron

thalamus

3th order

neuron

Sensory

cortex

Referred pain

It is pain felt away from site of it’s origin.

It is felt in skin area away from the diseased viscera.

1-Gall bladder pain: is referred to the tip of the right

shoulder.

2-Renal colic: is referred downward to the inguinal region ,scrotum as well as to the back.

3-Cardiac pain: referred to the base of the neck & left

sholder

4-Gastric pain: Usually referred to the surface between the xiphoid process and the umbilicus .

5-Pain from an inflamed appendix: It is referred to the

midline at an area around the umbilicus.

Gall bladder pain

Cardiac pain

Renal painAppendicitis

Referred pain

Examples

Mechanism of referred pain

1-Convergence projection theory: Afferent pain fibers from the skin area and the diseased

viscera converge on the same second order neuron and finally stimulate the same cortical neuron.

Cortex will feel pain as if it is coming from the skin because the sensory cortex is accustomed to receive pain from the skin.

Referred pain mechanism

Convergence projection theory

Spinal cord

spinothalamic Tract

spinothalamic

neuron

skin

viscus

Convergence

2-Facilitation theory

Visceral pain afferent fibers send collaterals to facilitate the second order neuron of pain pathway from the skin area of reference facilitating the pain sensation from this area.

Minor activity in the pain pathway from this area leads

to pain sensation.

Facilitation mechanism of referred pain

Spinal cord

skin

viscus

threshold

Facilitate theory

Any question