Neuroanatomy of Pain

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Neuroanatomy of Pain
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Introduction

• What is pain?
• An unpleasant sensory and emotional experience
  associated with actual or potential tissue
  damage, or described in such terms as tissue
  damage

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Introduction

• Pain is subjective
• Does pain have a purpose?
• Types of Pain
• Acute Pain
• Chronic Pain
• Referred Pain
• Radiating

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Neurophysiology of Pain

• Afferent Peripheral Receptor Types
• Special Receptors
• Visceral Receptors
• Deep Receptors
• Superficial Receptors

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Afferent Peripheral Receptors

• Special
• Transmit taste, sight, smell
• Visceral Receptors
• Transmit hunger, distention, nausea
• Deep
• Kinesthesia, joint position sense, deep pressure
  and pain
• Muscle -- GTOs, muscle spindles
• Articular Structures fast and slow adapting
  mechanoreceptor and nociceptors

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Afferent Peripheral Receptors

• Superficial
• Warm/cold, touch, pressure, itch and pain from
  skin level
• 3 categories
• Mechanorecptors
• Thermoreceptors
• Nociceptors

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Mechanorecepetors

• Mechanical stimuli
• Touch, pressure, stroking

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Mechanorecpetors

• Mechanical stimuli
• Touch, pressure, stroking
• Fast-adapting Receptors
• Pacinian Corpuscles
• Meissner Corpuscles
• Respond to pressure and touch very quickly

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Mechanorecpetors

• Mechanical stimuli
• Touch, pressure, stroking
• Fast-adapting Receptors
• Slow-adapting Receptors
• Merkel Cell Ending
• Ruffinni Endings
• Stimulated by sustained stimuli

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Thermoreceptors

• Respond to temp and temp changes
• Slow-adapting structures
• Warm vs cold receptor firing

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Nociceptors

• Transmit pain impulses
• Described as free nerve ending
• Stimulated by
• Mechanical
• Chemical
• Thermal

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What is the initial response to pain stimulus?
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Release of Chemical Mediators
Cell trauma

• Pain always begin with chemical response

Release dopamine and NE from Cell membrane
Activates phospholipase
Converted to prostaglandin or bradykinin
Release Arachidonic Acid
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First Order Neurons

• Transmit sensory information from receptors to
  dorsal horn
• Types of Primary Receptors in Pain Management
• A-delta
• A-beta
• C fibers

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All Types of First-Order Neurons
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Closer Analysis of those Receptors Involved in
Pain Management

• A-delta
• Large, lightly myelinated
• Conduction velocities
• Respond to?
• Protopathic pain (AKA Primary Hyperalgesia)

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Closer Analysis of those Receptors Involved in
Pain Management

• A-delta
• A-beta
• Large, myelinated
• Conduction velocities
• Respond to?

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Closer Analysis of those Receptors Involved in
Pain Management

• A-delta
• A-beta
• C-fibers
• Unmyelinated
• Conduction velocity
• Respond to
• Epicritic pain (AKA Secondary Hyperalgesia)

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Dorsal Horn of the Spinal Cord
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Spinal Cord Level

• SC divided into several Laminae
• A-delta and C terminate in Laminae I and II
• A-delta has input in V also
• Laminae I
• Laminae II
• Laminae III and IV
• Laminae V

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Second Order Neurons

• Synapse with Laminae I and V cross midline
• Synapse with Spinothalmic Tract
• Immediate warning of presence, location and
  intensity of pain

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Second Order Neurons

• Spinothalamic Tract
• Receives info from A-delta in Laminae I and V
• Classification of ST Tract Cells
• Low-threshold
• Wide dynamic range
• High threshold
• Thermosensitive

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Spinoreticulothalamic Tract (FYI)

• Input from Laminae VII and VIII
• Mostly from C-fibers
• A bilateral tract
• From reticular formation to thalamus
• Behavioral and emotional aspects of pain

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Third and Fourth Order Neurons

• Second order synapse to higher brain center
• 3rd reticular formation to thalamus
• 4th between and within higher brain centers

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Higher Brain Centers

• Reticular Formation
• Thalamus
• Cortex

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Structures involved with Descending Control of
Pain

• Periaqueductal Grey Matter
• Nucleus Raphe Magnus
• Corticospinal Tract