Pain anatomy and physiology
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Pain – Anatomy and Physiology. Definition. An unpleasant sensory and emotional experience... ...caused by actual or potential tissue injury, ...or described in terms of such injury. International Association for the Study of Pain. Sources of Pain.

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An unpleasant sensory and emotional experience...

...caused by actual or potential tissue injury,

...or described in terms of such injury.

International Association for the Study of Pain

Sources of pain
Sources of Pain

Nociceptive: pain that is usually transmitted after normal processing of noxious stimuli

Neuropathic: results from injury or abnormal functioning of peripheral nerves or CNS

Psychogenic: unknown physical cause

Classification of pain nociception
Classification of Pain Nociception

  • Proportionate to the stimulation of the nociceptor

  • When acute

    • Physiologic pain

    • Serves a protective function

    • Normal pain

  • Pathologic when chronic

Describe the different types of pain sensation
Describe the different types of pain sensation

  • bright,

  • sharp,

  • stabbing types of pain

  • dull,

  • throbbing,

  • aching types.

Where does pain come from
Where Does Pain Come From?

  • Cutaneous Pain – sharp, bright, burning; can have a fast or slow onset

  • Deep Somatic Pain – stems from tendons, muscles, joints, periosteum, & b. vessels

  • Visceral Pain – originates from internal organs; diffused @ 1st & later may be localized (i.e. appendicitis)

  • Psychogenic Pain – individual feels pain but cause is emotional rather than physical

Neuropathic pain
Neuropathic Pain

  • Sustained by aberrant processes in PNS or CNS

  • Disproportionate to the stimulation of nociceptor

  • Serves no protective function

  • Pathologic pain

Pain anatomy and physiology




Injury, Inflammation

Heat, Cold




Electrical Activity to spinal cord and onto the Brain

BRAIN = Electrical activity becomes the experience of


Sensory receptors
Sensory Receptors

  • Mechanoreceptors – touch, light or deep pressure

    • Meissner’s corpuscles (light touch), Pacinian corpuscles (deep pressure), Merkel’s corpuscles (deep pressure)

  • Thermoreceptors - heat, cold

    • Krause’s end bulbs ( temp & touch), Ruffini corpuscles (in the skin) – touch, tension, heat; (in joint capsules & ligaments – change of position)

  • Proprioceptors – change in length or tension

    • Muscle Spindles

  • Nociceptors – painful stimuli

    • mechanosensitive

    • chemosensitive

Nerve endings
Nerve Endings

  • “A nerve ending is the termination of a nerve fiber in a peripheral structure.”

  • Nerve endings may be sensory (receptor) or motor (effector).

  • Nerve endings may be:

    • Respond to phasic activity - produce an impulse when the stimulus is  or ,)

    • Superficial – Merkel’s corpuscles/disks, Meissner’s corpuscles

    • Deep – Pacinian corpuscles,

Nerve endings1

Merkel’s corpuscles/disks -

Sensitive to touch & vibration

Slow adapting

Superficial location

Most sensitive

Meissner’s corpuscles –

Sensitive to light touch & vibrations

Rapid adapting

Superficial location

Pacinian corpuscles -

Sensitive to deep pressure & vibrations

Rapid adapting

Deep subcutaneous tissue location

Krause’s end bulbs –


Ruffini corpuscles/endings


Sensitive to touch & tension

Slow adapting

Free nerve endings -


Detects pain, touch, temperature, mechanical stimuli

Nerve Endings

The skin receptors transducers
The skin receptors (transducers)

a) Merkel

b) Meissner

c) Ruffini

d) Pacinian


  • Sensitive to repeated or prolonged stimulation

  • Mechanosensitive – excited by stress & tissue damage

  • Chemosensitive – excited by the release of chemical mediators

    • Bradykinin, Histamine, Prostaglandins, Arachadonic Acid

  • Primary Hyperalgesia – due to injury

  • Secondary Hyperalgesia – due to spreading of chemical mediators


Transduction Conduction Transmission




“Ouch” Pain

primary sensory neuron central neuron

Pain anatomy and physiology

Nociception – Transduction

Nociceptor Activators











generator potential

action potentials

COX-2 Insensitive

Pain nerve fibers fast pain and slow pain
Pain nerve fibers – fast pain and slow pain

  • From the pain receptors, the pain stimulus is transmitted through peripheral nerves to the spinal cord and from there to the brain. This happens through two different types of nerves fibers:

  • A-delta "fast pain” and

  • C-fibers “slow pain” nerve fibers.

What is fast pain and slow pain
What is “fast pain” and “slow pain”?

  • A pain stimulus, e.g. if you cut yourself, consists of two sensations.

  • first “fast pain” sensation-is experienced as sharp, bright and localized pain followed by

  • “slow pain”, more a dull, burning and diffused.

  • So,pain occurs after a short time, and

  • lasts a few days or weeks,

  • Chronic pain-if inappropriately processed by the body, it can last several months

Fast pain
Fast pain

  • Relatively thick size nerve fibers allow the pain stimulus to be transferred very fast (at a speed of five to 30 meter/second), hence the name

  • Due to activity of A delta fibers

  • This is all to make the body withdraw immediately from the painful and harmful stimulus, in order to avoid further damage.

Slow pain
Slow Pain

  • starts immediately after the fast pain

  • is transmitted by very thin nerve fibers, called C-nerve fibers (their diameter is between 0.2 to 1 thousandth of a millimeter).

  • pain impulse can only be transmitted slowly to the brain, at a speed of less than 2 meters per second.

  • Body response -immobilization (guarding, spasm or rigidity), so that healing can take place.

Types of nerves
Types of Nerves

  • Afferent (Ascending) – transmit impulses from the periphery to the brain

    • First Order neuron

    • Second Order neuron

    • Third Order neuron

  • Efferent (Descending) – transmit impulses from the brain to the periphery

First order neurons
First Order Neurons

  • Stimulated by sensory receptors

  • End in the dorsal horn of the spinal cord

  • Types

    • A-alpha – non-pain impulses

    • A-beta – non-pain impulses

      • Large, myelinated

      • Low threshold mechanoreceptor; respond to light touch & low-intensity mechanical info

    • A-delta – pain impulses due to mechanical pressure

      • Large diameter, thinly myelinated

      • Short duration, sharp, fast, bright, localized sensation (prickling, stinging, burning)

    • C – pain impulses due to chemicals or mechanical

      • Small diameter, unmyelinated

      • Delayed onset, diffuse nagging sensation (aching, throbbing)

Second order neurons
Second Order Neurons

  • Receive impulses from the FON in the dorsal horn

    • Lamina II, Substantia Gelatinosa (SG) - determines the input sent to Transmission cells from peripheral nerve

    • Travel along the spinothalmic tract

    • Pass through Reticular Formation

  • Types

    • Wide range specific

      • Receive impulses from A-beta, A-delta, & C

    • Nociceptive specific

      • Receive impulses from A-delta & C

  • Ends in thalamus

Third order neurons
Third Order Neurons

  • Begins in thalamus

  • Ends in specific brain centers (cerebral cortex)

    • Perceive location, quality, intensity

    • Allows to feel pain, integrate past experiences & emotions and determine reaction to stimulus


  • Chemical substances that allow nerve impulses to move from one neuron to another

  • Found in synapses

    • Substance P - thought to be responsible for the transmission of pain-producing impulses

    • Acetylcholine – responsible for transmitting motor nerve impulses

    • Enkephalins – reduces pain perception by bonding to pain receptor sites

    • Norepinephrine – causes vasoconstriction

    • 2 types of chemical neurotransmitters that mediate pain

      • Endorphins - morphine-like neurohormone; thought to  pain threshold by binding to receptor sites

      • Serotonin - substance that causes local vasodilation &  permeability of capillaries

      • Both are generated by noxious stimuli, which activate the inhibition of pain transmission

  • Can be either excitatory or inhibitory

Pain control theories
Pain Control Theories

  • Gate Control Theory

  • Central Biasing Theory

  • Endogenous Opiates Theory

Endogenous opiates theory
Endogenous Opiates Theory

  • Least understood of all the theories

  • Stimulation of A-delta & C fibers causes release of B-endorphins

  • Mechanism of action – similar to enkephalins to block ascending nerve impulses

  • Examples: TENS (low freq. & long pulse duration)

Descending neurons
Descending Neurons

  • Descending Pain Modulation (Descending Pain Control Mechanism)

  • Transmit impulses from the brain (corticospinal tract in the cortex) to the spinal cord (lamina)

    • release enkephalins

    • release serotonin

Natural opioids endorphins
Natural Opioids-Endorphins

  • released from their storage areas in the brain when a pain impulse reaches the brain,

  • bind to receptors in the pain pathway to block transmission and perception of pain.

Central biasing theory
Central Biasing Theory

  • Descending neurons are manipulated by: stimulation of A-delta & C neurons, cognitive processes, anxiety, depression, previous experiences, expectations

  • Mediates release of enkephalins and serotonin

Gate control theory
Gate Control Theory

  • Gate - located in the dorsal horn of the spinal cord

  • Smaller, slower n. carry pain impulses

  • Larger, faster n. fibers carry other sensations

  • Impulses from faster fibers arriving @ gate 1st inhibit pain impulses (acupuncture/pressure, cold, heat, chem. skin irritation).



Gate (T cells/ SG)

Heat, Cold, Mechanical

Gate control theory melzack and wall 1965
Gate Control TheoryMelzack and Wall 1965.

  • Physiological and psychological interactions

  • Suggested spinal gates in the dorsal horn at each segment of the spinal cord

  • Competition at each gate for heat, touch or pain to be transmitted at each point

Pain inhibitory and pain facilitatory mechanisms within the dorsal horn
Pain-Inhibitory and Pain-Facilitatory Mechanisms Within the Dorsal Horn





_ _


Neuronal circuitry

within the dorsal horn.

Primary afferent neuron

axons synapse onto

spinothalamic neurons

and onto inhibitory and

excitatory neurons.



+ +


Mechanisms of pathophysiologic pain central sensitization processes
Mechanisms of Pathophysiologic Pain: Dorsal HornCentral Sensitization Processes

  • Repeated impulse activity in C nociceptive neurons produces sensitization of neurons over time

  • Causes hyperalgesia, allodynia, and spontaneous pain

Origin of pain
Origin of Pain Dorsal Horn

Physical cause — cause of pain can be identified

Psychogenic — cause of pain cannot be identified

Referred — pain is perceived in an area distant from its point of origin

What is referred pain
What is Referred Pain? Dorsal Horn

  • Occurs away from pain site

  • Examples: McBurney’s point

Phantom limb
Phantom limb Dorsal Horn

Phantom limb1
Phantom Limb Dorsal Horn

Pain threshold
Pain threshold Dorsal Horn

Is the lowest intensity of stimulus that causes the subject to recognize pain

Common responses to pain
Common Responses to Pain Dorsal Horn

Physiologic: ↑BP, ↑HR,↑RR, pupil dilation, muscle tension and tension rigidity, pallor, ↑adrenaline level, ↑blood glucose

Behavioral: grimacing, moaning, crying, restlessness

Common responses to pain1
Common Responses to Pain Dorsal Horn

Affective: exaggerated weeping, withdrawal, anxiety, depression, fear, anger, anorexia, fatigue, hopelessness, powerlessness.

The 4 components of pain
The 4 components of pain Dorsal Horn

  • Sensory / Physical

    • Action in pain nerves (actually just sensory nerves)

    • NB: Activity in pain nerves ≠ pain

  • Beliefs

    • Knowledge, expectations, fears, and attributions

  • Behavioural

    • The effect of pain on behaviour, physical and emotional

    • coping strategies

  • Emotions

    • The effect of the other three on mood and mood on the other three

Cortical pain processing
Cortical pain processing Dorsal Horn

  • Sensory aspects of pain seem to be processed in the Somatosensory cortex.

  • Emotional distress associated with pain seems to be processed in the Anterior Cingulate Cortex (ACC).

Factors affecting pain experience
Factors Affecting Pain Experience Dorsal Horn


Ethnic variables

Family, gender, and age variables

Religious beliefs

Environment and support people

Anxiety and other stressors

Past pain experience

Manipulating pain experience factors
Manipulating Pain Experience Factors Dorsal Horn

Remove or alter cause of pain

Alter factors affecting pain tolerance

Initiate nonpharmacologic relief measures

Nonpharmacologic pain relief measures
Nonpharmacologic Pain Relief Measures Dorsal Horn








Therapeutic touch

Relaxation Dorsal Horn

Acupuncture Dorsal Horn

Hypnosis Dorsal Horn

Therapeutic touch
Therapeutic touch Dorsal Horn

Pharmacologic pain relief measures
Pharmacologic Pain Relief Measures Dorsal Horn

Analgesic administration

Nonopiod analgesics e.g. NSAIDs

Opioids or narcotic analgesics

Adjuvant drugs e.g. anticonvulsants, antidepressants, ..

Additional methods for administering analgesics
Additional Methods for Administering Analgesics Dorsal Horn

Patient controlled analgesia

Epidural analgesia

Local anesthesia

Local anesthesia
Local anesthesia Dorsal Horn

Pain anatomy and physiology

Spinal Dorsal Horn


Epidural catheter

Pain anatomy and physiology

TENS Dorsal Horn

Preemptive analgesia

If the body doesn’t sense the pain during the procedure, the pain will be easier to deal with post-operatively

A patient in surgical anesthesia is not aware of pain, but the body is still responding sensitizes the nervous system


Hot peppers

Excites then fatigues nerve transmissionlocal analgesia

Also get endorphin release

St johnswort

Arthritic pain


Stimulates neural inhibitory pathways analgesia