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A. Neeharika Ist year PG

PHYSIOLOGY OF PAIN

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› Introduction› Definition› Classification & types of Pain› Theories of Pain› Pain Pathways

– Neural Anatomy– Neural Pathways

› Pain Assessment tools› Applied Anatomy

Contents

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› Pain is an unpleasant sensory action

› From perception of pain as ‘stimulus-response relationship’ , the concept of pain has evolved to be a consequence of complex interactions between sensory, emotional, and behavioral factors

› Derived from Greek – ‘Poin’ and Latin – ‘Poena’

Introduction

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› The concept of pain as a multidimensional experience has been described

in ancient texts, dating as far back as the Syriac Empire (circa 200 BC).

› In The Book of Medicines (Budge 2002), it is suggested that pain is the

product of bile and phlegm mingled with cold and heat.

› These simple combinations occur in the brain, and according to Syriac

medicine, pain is a product of the brain (a concept that has passed the test

of time and that we still hold true today).

History of Pain

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› The International Association for the Study of Pain defines pain as

“an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” .

Definition

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Though unpleasant, pain serves important adaptive purposes.

› Identifies and localizes noxious stimuli› Protective – withdrawal reflex response- limits injury› Experience of pain – avoids potentially harmful injuries› Immobility due to pain – assists in healing

Cambridge University Press978-0-521-87491-5 - Acute Pain Management

Purpose of pain

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› Based on duration:AcuteConvalescentChronic

Acute pain follows traumatic tissue injuries, is generally limited in

duration, and is associated with temporal reductions in intensity.

Chronic pain may be defined as discomfort persisting 3–6 months

beyond the expected period of healing.

Classification of Pain

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› Based on Pathophysiologic mechanisms: Physiologic Nociceptive / Inflammatory Neuropathic

Physiologic pain defines rapidly perceived nontraumatic discomfort of

very short duration.

Nociceptive pain is defined as noxious perception resulting from cellular

damage following surgical, traumatic, or disease-related injuries.

Inflammation and inflammatory mediators play a major role.

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› Somatic nociceptive pain is well localized and generally follows

a dermatomal pattern. It is usually described as sharp, crushing,

or tearing in character.

› Visceral nociceptive pain defines discomfort associated with

peritoneal irritation as well as dilation of smooth muscle

surrounding viscus or tubular passages. Non dermatomal.

Nociceptive pain

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Neuropathic pain is defined by the International Association for the

Study of Pain as “pain initiated or caused by a pathologic lesion

or dysfunction” in peripheral nerves and CNS.

Disease states associated with classic neuropathic sysmptoms

include infection (eg, herpes zoster), metabolic derangements

(eg, diabetic neuropathy), toxicity (eg, chemotherapy), and

Wallerian degeneration secondary to trauma or nerve

compression.

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› Neuropathic pain is usually constant and described as burning, electrical, lancinating, and shooting.

Cambridge University Press; 978-0-521-87491-5 - Acute Pain Management

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› By convention, symptoms related to peripheral lesions are termed

neuropathic, whereas symptoms related to spinal cord injuries are

termed myelopathic

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› Based on clinical context: Postsurgical Malignancy related Neuropathic Degenerative

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› Psychogenic pain was defined by the IASP as “Pain specifically attributable to the thought process, emotional state, or personality of the patient in the absence of an organic or delusional cause or tension mechanism.”

› Such a system hardly seems an improvement, as one is still forced to choose between psychological and physical factors.

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› Nociceptor: A high-threshold sensory receptor of the peripheral

somatosensory nervous system that is capable of transducing and

encoding noxious stimuli.

› Hyperalgesia: Increased pain from a stimulus that normally provokes

pain

› Allodynia: Pain due to a stimulus that does not normally provoke pain

Terms related to pain

International Association for the Study of Pain: http://www.iasppain.org/Content/NavigationMenu/GeneralResourceLinks/PainDefinitions/default.htm

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› Sensitization: Increased responsiveness of nociceptive neurons to their

normal input, and/or recruitment of a response to normally

subthreshold inputs.

› Hyperpathia: Increased or exaggerated pain intensity with minor

stimulation

› Dysesthesia: Unpleasant sensation at rest or movement

› Paresthesia: Unpleasant often shock-like or electrical sensation

precipitated by touch or pressure (CRPS-II causalgia)International Association for the Study of Pain: http://www.iasppain.org/Content/NavigationMenu/GeneralResourceLinks/PainDefinitions/default.htm

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› The propagation of pain is initiated with the activation of physiological

receptors, called nociceptors

› Present on skin, mucosa, membranes, deep fascias, connective tissues of

visceral organs, ligaments and articular capsules, periosteum, muscles,

tendons, and arterial vessels

Peripheral receptors

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› The receptors correspond to free nerve endings and represent the

more distal part of a first-order afferent neuron consisting of

small-diameter fibers, with little or unmyelinated, of the A-Delta

or C type, respectively.

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Nociception is by › Transduction: It is defined as responses of peripheral nociceptors

to traumatic or potentially damaging chemical, thermal, or mechanical stimulation.

› Conduction: It refers to the propagation of action potentials from peripheral nociceptive endings viamyelinated andunmyelinated nerve fibers.

› Transmission: Transmission refers to the transfer of noxious impulses from primary nociceptors to cells in the spinal cord dorsal horn.

Nociception

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PAIN PATHWAYS

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› The portions of the nervous system responsible for the sensation

and perception of pain may be divided into three areas:

1. Afferent pathways

2. CNS

3. Efferent pathways

Neuroanatomy of Pain

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The afferent portion is composed of:

a) Nociceptors (pain receptors)

b) Afferent nerve fibres

c) Spinal cord network

Afferent pathways terminate in the dorsal horn of the spinal cord

(1st afferent neuron).

2nd afferent neuron creates spinal part of afferent system.

Afferent Pathway

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The portion of CNS involved in the interpretation of the pain signals are the

› Limbic system

› Reticular formation

› Thalamus

› Hypothalamus

› Cortex

CNSCortex

Thalamus

Cerebellum

PeriqueductalGray matter

Limbic structures

hypothalamus

Reticular formation

Spinal tract nucleus of V

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The efferent pathways, composed of the fibers connecting the

reticular formation, midbrain, and substantia gelatinosa, are

responsible for modulating pain sensation

Efferent pathways

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› The propagation of pain is initiated with the activation of

physiological receptors called nociceptors.

› The receptors correspond to the free nerve endings i.e., A-Delta,

A-Beta or C type fibers.

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› Many types of pain arise as a result of infection or damage to tissue.

› Both events initiate an inflammatory response that is intimately linked

with pain.

› The passage of nociceptive impulses generated in the peripheral

nerve fibers depends on

› The release of various neurotransmitters. These neurotransmitters act

either peripherally of centrally.

Pain mediators:

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Various Pain mediators include:

› Plasma kinins: e.g. bradykinin › Serotonin› Histamine › Prostaglandins › Leukotrienes › Cytokines › Neuropeptides

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› These pain mediators/ pain producing substances acts as

stimulators for nociceptors / the free nerve endings.

› These pain mediators are released by activation of Phospholipase

A2, which is activated by trauma or infection.

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Nociceptive Pain Pathway

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› The four most influential theories of pain perception include the

› Specificity (or Labeled Line) Theory› Intensity Theory› Pattern Theory› Gate Control Theory

Theories of Pain

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› One of the earliest ideas, termed the Specificity theory, was proposed

by Descartes.

› The theory suggested that specific pain fibers carry specific coding that

discriminates between different forms of noxious and nonnoxious

sensation.

› The fundamental tenet of the Specificity Theory is that each modality

has a specific receptor and associated sensory fiber (primary afferent)

that is sensitive to one specific stimulus (Dubner et al. 1978)

Specifity Theory

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Moayedi M, Davis KD. Theories of pain: from specificity to gate control.J Neurophysiol 109: 5–12, 2013.

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› The intensity theory, proposed by Sydenham, suggested that the

intensity of the peripheral stimulus determines which sensation is

perceived.

› The theory defines pain, not as a unique sensory experience but

rather, as an emotion that occurs when a stimulus is stronger than

usual

Intensity Theory Of Pain

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› Goldscheider suggested a neurophysiological model to describe

this summation effect: repeated subthreshold stimulation or

suprathreshold hyperintensive stimulation could cause pain.

› But the theory lost support with Sherrington’s evolutionary

framework regarding existence of sensory receptors that are

specialized to respond to noxious stimuli, for which he coined

the term “nociceptor”.

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› J. P. Nafe postulated this “quantitative theory of feeling” in 1929.

› This theory ignored findings of specialized nerve endings and many of the

observations supporting the specificity and/or intensive theories of pain.

› The theory stated that any somaesthetic sensation occurred by a specific

and particular pattern of neural firing and that the spatial and temporal

profile of firing of the peripheral nerves encoded the stimulus type and

intensity

PATTERN THEORY OF PAIN

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› In 1965, Ronald Melzack and Charles Patrick (Pat) Wall (Melzack and

Wall 1965) proposed a theory that would revolutionize pain research

› Melzack and Wall accepted that there are nociceptors (pain fibers) and

touch fibers and proposed that that these fibers synapse in two different

regions within the dorsal horn of the spinal cord: cells in the substantia

gelatinosa and the “transmission” cells.

GATE CONTROL THEORY

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› The model proposed that signals produced in primaryafferents from stimulation of the skin were transmitted to threeregions within the spinal cord: 1) the substantia gelatinosa,2) the dorsal column, and 3) a group of cells that they called ‘transmission cells’.

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› They proposed that the gate in the spinal cord is the substantia

gelatinosa in the dorsal horn, which modulates the transmission

of sensory information from the primary afferent neurons to

transmission cells in the spinal cord.

› This gating mechanism is controlled by the activity in the large

and small fibers.

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› Large-fiber activity inhibits (or closes) the gate, whereas small-fiber

activity facilitates (or opens) the gate.

› When nociceptive information reaches a threshold that exceeds the

inhibition elicited, it “opens the gate” and activates pathways that

lead to the experience of pain and its related behaviors.

› Therefore, the Gate Control Theory of Pain provided a neural basis

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Applied Anatomy

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Anesthesia Dolorosa: It is defined as perception of pain in an area that

is anesthetic. A complication of post-treatment of TN.

Deafferentation pain: Deafferentation pain is due to partial or complete

interruption of peripheral or central afferent neural activity.

› Examples are postherpetic neuralgia, central pain (pain after CNS

injury), and phantom limb pain (pain felt in the region of an amputated

body part).

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› Congenital analgesia - nociceptive stimuli are not processed

and/or integrated at a level of brain.

› Patient does not feel any pain

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› Referred pain is pain that is present in an area removed or

distant from its point of origin. The area of referred pain is

supplied by the nerves from the same spinal segment as the

actual site of pain.

› Phantom limb pain - is pain that an individual feels in amputated

limb

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Hemiagnosia :

› It is a loss of ability to identify the source of pain on one side

(the affected side) of the body. Application of painful stimuli to

the affected side thus produces anxiety, moaning, agitation and

distress but no attempt to withdrawal from or push aside the

offending stimulus.

› Hemiagnosia is associated with stroke that produces paralysis

and hypersensitivity to painful stimuli in the affected side

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Pain Assessment tools

Visual Analogue Scale:

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Verbal Scale:

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McGill Pain Questionnare:

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› Sambulingam: Concise Textbook of Physiology› Monheims: Textbook of Local Anaesthesia› Moayedi M, Davis KD. Theories of pain: from specificity to gate

control. J Neurophysiol 109: 5–12, 2013.› Cambridge University Press; 978-0-521-87491-5 - Acute Pain

Management› International Association for the Study of Pain:

http://www.iasppain. org/Content/NavigationMenu/GeneralResourceLinks/PainDefinitions/default.htm

References

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