This document provides an overview of the anatomy and physiology of pain. It begins by outlining the learning objectives which are to gain insight into the anatomy and physiology of pain, recognize terminology, identify the body's physiological response to pain, recognize the effects of pain on different body systems, and apply knowledge to assess and treat pain. It then discusses the types of pain including acute, chronic, nociceptive, neuropathic, and nociplastic pain. It describes the pain pathway from peripheral nerves to the central nervous system. It outlines the processes of transduction, transmission, modulation, and perception of pain. It discusses the functional effects of pain on different body systems and the psychological effects of pain. It concludes by emphasizing the complexity

1. TheAnatomyandPhysiologyofPain
CO’s CURE
Hospital Medicine Pilot

2. Learning Objectives
• Gain insight into of the anatomy and physiology of pain
• Recognize terminology
• Identify the body’s physiological response to pain
• Recognize the effects of pain on the different body
systems
• Apply knowledge to assess and treat pain

3. Pain
THE TEACHING:
• Medically - pain is a symptom of
an underlying condition
THE MISSION:
• Find the source of pain
• Holistically treat the pain
THE GOAL:
• Return to a realistic, productive
life
“An unpleasant sensory and emotional experience associated with actual or
potential tissue damage.”

4. The Purpose of Pain
1. Prevents serious injury
• A light touch to something hot forces a quick reaction before serious injury occurs
2. Teaches avoidance
• A painful activity teaches what not to repeat or when to seek help
3. Prevents permanent damage
• Joint pain limits activity
• Surgery requires down time for healing

5. The Pain Roadmap
Spinal Cord
Brain
Ascending
Pathways
Descending
Pathways
Peripheral
Nerves
• Pain sensation involves a series
of complex interactions between
peripheral nerves and the central
nervous system
• Pain is a dynamic, bidirectional
process
• Multiple areas of the nervous
system help process pain signals
• Normal and pathologic processes
underlie pain mechanisms

6. The Origin of Pain
Pain Origin
Somatic or Cutaneous Pain • Arises from nociceptive receptors in the skin and mucous
membranes
• Superficial pain
• Feels like sharp, burning, pricking and is constant
• Fast or slow onset
Deep Somatic Pain • Stems from tendons, muscles, joints, periosteum and
blood vessels
Visceral Pain • Originates from internal organs: pelvis, abdomen, chest
and intestines
• Activates nociceptors of the viscera (internal organs in
main cavities of the body)
• Poorly localized and is an achy and dull sensation
• Visceral structures are highly sensitive to stretching,
ischemia and inflammation but insensitive to other
stimuli that normally provoke pain
Psychogenic Pain • Individuals “feel” pain but cause is emotional rather than
physical

7. The Process of Pain Physiology
• Pain sensation is modulated by two types of neurotransmitters or
neurochemicals:
oNeurochemicals that excite pain or try to initiate pain
oNeurochemicals that inhibit or try to stop the pain
• Pain sensation is composed of four basic processes:
oTransduction
oTransmission
oModulation
oPerception

8. The Process of Pain Physiology
Process Action
Transduction • Processes by which tissue-damaging stimuli activate nerve endings
• Three types of stimuli can activate pain receptors in peripheral
tissues: mechanical, heat, chemical
• Pain stimuli is converted to energy
• Electrical energy is known as “transduction”
• Stimulus sends an impulse across the peripheral nerve fibers known
as the nociceptor never fibers
Transmission • Nociceptive message is transmitted to the central nervous system
(CNS)
• A delta fibers send sharp, localized and distinct sensations
• C fibers relay impulses that are poorly localized, burning and
persistent
• This is the route by which the CNS is informed of impending or
actual tissue damage
Modulation • Natural inhibition or modulation of pain by the body
• Inhibitory neurotransmitters like endogenous opioids (enkephalins,
dynorphin and endorphins) that work to hinder pain transmission
Perception • Person is aware of the pain
• Somatosensory cortex identifies the location and intensity of the
pain
• Person unfolds a complex reaction – physiological and behavioral

9. The Transmission of Pain: Afferent Axons
Nerve Fibers Involved in
Transmission of Pain
A – Alpha Fibers
muscle sense
Diameter: 13-20 um
Speed: 80-120 m/s
C - Fibers
pain/temperature/itch
 Small
 Unmyelinated
 Very slow conducting
 Respond to all types of
noxious stimuli
 Transmit prolonged,
dull pain
 Require high intensity
stimuli to trigger
response
 Diameter: . – . um
 Speed: . – . m/s
A - Beta Fibers
touch
A - Delta Fibers
pain/ temperature
 Small
 Lightly myelinated
 Slow conducting
 Respond to heat,
pressure, cooling and
chemicals
 Sharp sensation of pain
 Diameter: 1-5 um
 Speed: 5-35 m/s
 Large
 Myelinated
 Fast conducting
 Low stimulation
threshold
 Respond to light touch
 Diameter: 6-12 um
 Speed: 35-75 m/s
PRIMARY AFFERENT AXONS

10. The Transmission of Pain: Timing
• The thickness of a nerve fiber correlates directly to the speed with which information travels
• The thicker the nerve fiber, the faster information travels
• A-alpha, A-beta and A-delta nerve fibers are insulated with a protective covering called the myelin
sheath, which helps with nerve conductivity
• C nerve fibers are unmyelinated
• A-delta and C fibers are the main fibers responsible for the transmission of pain, however new studies
suggest A-beta fibers may have an important role to play in the diagnosis and treatment of pain
Fast Pain Slow Pain
• Transmitted by the A-delta nerve fibers at a
velocity of 6-30 m/second
• Felt about 0.1 seconds after a pain stimulus is
applied
• Pin prick, cutting or burning of skin
• Caused by mechanical or thermal stimuli
• Fast, sharp pain is not felt in most deeper tissues
• Neurotransmitter released – glutamate
• Transmission route: Neo-spinothalamic tract
• Transmitted by the C-nerve fibers at a velocity
of 0.5-2 m/second
• Begins after 1 second or more and may range
from seconds to minutes
• Slow, burning, aching, throbbing, nauseous
pain and chronic pain
• Associated with tissue destruction
• Caused mainly by chemical stimuli
Neurotransmitter released – Substance P
• Transmission route: Paleo-spinothalamic tract

11. Transmission of Pain: Pull it Together
Peripheral Sensory Nerve
Spinal Cord
Thalamus
Cortex
Pain!
PAG Periaqueductal Gray Matter
C
A-delta
Afferent neurons
Modulation
Transduction
Perception
• Serotonin
• Endorphins
• Enkephalins
• Dynorphin
Neurochemical
Pain Inhibitors
Descending Inhibition
• Epinephrine
• Cortisol
• ACTH
Neurochemical
Pain Initiators
• Glutamate – Central
• Substance P – Central
• Bradykinin - Peripheral
• Prostaglandins -
Peripheral
Transmission

12. Breakout of Pain
CO’s CURE Module Five

13. Acute Pain
• Occurs in response to injury or illness
• Responds well to interventions
• Resolves well as healing proceeds
• Short-lived
• Less than three months
• Often accompanied by sympathetic
nervous system arousal
• Adaptive pain response
• Helpful pain response, which produces a
behavior that promotes healing
Chronic Pain
• Multiple underlying mechanisms
• Requires multi-modal and interdisciplinary
treatment approaches
• Pain that persists beyond the “normal”
time expected to heal
• Longer than three months
• Changes in both peripheral and central
nervous system processing
• If acute pain is not managed well will
move into the chronic phase
Acute Pain vs. Chronic Pain

14. Acute Pain vs. Chronic Pain
Nociceptive Pain
ACUTE
Neuropathic Pain
CHRONIC
Nociplastic Pain
TYPICAL
CHRONIC

15. Acute Pain: Nociceptive
What is nociceptive pain?
• Normal response to an injury of tissues
• Most common type of pain
What are nociceptors?
• Nerves that detect or find noxious stimuli
What is nociception?
• Process whereby signals are sent to the brain by nociceptor receptors
What is the cause of nociceptive pain?
• Injury to body tissue to the skin, muscle and bones
Examples:
• Postoperative pain, bruises, burns, fractures, overused joints
• Patients will present with dull, heavy, aching pain that spreads over a wide area

16. Chronic Pain: Neuropathic
What is neuropathic pain?
• Chronic pain lasts more than six-months
What is the cause of neuropathic pain?
• Pain caused by a primary lesion or disease in the somatosensory nervous system causing varying
degrees of pain sensations
What is some the cause of neuropathic pain?
• Nerve damage due to some type of a viral infection or a disease involving the central or peripheral
nervous system (neuralgias)
Examples:
• Arthritis, migraines, shingles, multiple sclerosis, shingles
• Patients will present with a variety of symptoms from numbness to burning to stinging, pins and
needles and pricking sensations

17. Chronic Pain: Nociplastic
What is nociplastic pain?
• Nonnociceptive and nonneuropathic pain
• Inflammatory response
How does nociplastic pain work?
• Activation and sensitization of nociceptive pain pathway by a variety of mediators released at a site
of tissue
What causes nociplastic pain?
• Abnormal processing in the central nervous system, however the reason for this abnormality is
generally unknown
Examples:
• Fibromyalgia, irritable bowel syndrome, chronic fatigue syndrome
• Patients will present with a variety of symptoms seen in both acute and chronic pain patients

18. Mixed Pain
What is mixed pain?
• Mixed pain share common clinical characteristics of all three types of pain
• nociceptive, neuropathic and nociplastic
Potential Examples:
• Sciatica, cancer pain, lumbar spinal stenosis

19. Other Types of Pain
Type of Pain Elements of the Pain
Breakthrough Pain • Pain is intermittent, transitory and an increase in pain occurs at a greater intensity
• Usually lasts from minutes to hours and can interfere with functioning and quality of
life (e.g., neuropathic pain and lower back pain)
Complex Regional Pain
Syndrome (CRPS)
• Pain condition that most often affects one limb (arm, leg, hand or foot) usually after
an injury
• CRPS is believed to be caused by damage to, or malfunction of, the peripheral and
central nervous system
Phantom Limb Pain • Pain in the absence of a limb
Referred Pain • Pain sensation produced in some part of the body is felt in other structures away
from the point of origin
• Deep pain and some visceral pain are referred to other areas
• Superficial pain is not referred
• Most common areas of referred pain include: heart, esophagus, kidneys, stomach,
colon, appendix, gallbladder, stomach, ureters (e.g., pain from a myocardial
infarction has referred pain to the left arm, neck and chest)
Chronic Pain Syndrome
(CPS)
• Different than chronic pain
• Combination of the original pain and the secondary
complications that are making the pain worse

20. Chronic Pain Syndrome: CPS
PAIN
inactivity
work
issues
altered
social
life
family
life
issues
weak
muscles
stress
anxiety
anger
fear
sadness
depression
fatigue
“A chronic pain syndrome is the
combination of chronic pain
and the secondary
complications that are making
the original pain worse”
Institute for Chronic Pain

21. Interaction of Complex Pain
Nociceptive Pain
ACUTE
Neuropathic Pain
CHRONIC
Nociplastic Pain
TYPICALLY
CHRONIC

22. Functional Effects of Pain
Body System Anticipated Change
Brain • Anxiety and fear
• Depression
• Poor concentration
• Inhibition or promotion of pain
Cardiovascular • Increased heart rate and blood pressure
• Increased need for oxygen
• Water retention
• Potential fluid overload
Endocrine • Increased blood glucose
• Increased cortisol production
Gastrointestinal • Reduced gastric emptying and intestinal motility
• Nausea and vomiting
• Constipation

23. Functional Effects of Pain
Body System Anticipated Change
Immune • Increased susceptibility to infection
• Increased or decreased sensitivity to pain
• Activation of hypothalamic-pituitary-adrenal axis (HPA)
• HPA is the central stress response system in the brain
Musculoskeletal • Tense muscles local to injury
• Shaking or shivering
• Pilo-erection or goose bumps
Nervous • Changes in pain processing
Respiratory • Increased respiratory rate
• Shallow breathing
• Increased risk for infection
Urinary • Urge to urinate/incontinence

24. Psychological Effects of Pain
Anticipated Change
Physical • Sleep disturbances
• Chronic fatigue
• Inability to keep up with daily activities
• Adverse Rx effects
Psychological • Rapid escalation or changes in mood
• Crying, anger, anxiety, irritability
• Low emotional distress tolerance
• Irrational thinking or behavior
• Fear
• Helplessness
Social • Work-related challenges
• Relationship challenges
• Intimacy challenges
• Social isolation
• Loss of role/identity
Spiritual • Hopelessness
• Questioning faith
• Guilt
• Self-pity

25. Sustained
currents
Peripheral
Nociceptive
Fibers
Transient
Activation
ACUTE
PAIN
Surgery
or
injury
causes
inflammation
Impact of Pain: Acute to Chronic Path
Sustained
Activation
Peripheral
Nociceptive
Fibers
Sensitization
CHRONIC
PAIN
CNS
Neuroplasticity
Hyperactivity
Structural
Remodeling

26. • Infants and children
• Elderly
• Racial and ethnic groups
• Women
• People with current or past history of substance abuse/addiction
• Cognitively impaired, non-verbal people
Groups at High Risk for Pain Management

27. Take Home Points
• The body’s reaction to pains to is complex and multifaceted
• Pain is transmitted from the site of injury to the brain by electrical signals
• Physiological changes triggered by pain are helpful in the beginning but become
harmful if they continue
• Understanding the anatomy and physiology of pain helps health professionals to
find better ways to treat pain
• Key interventions to prevent and treat pain begin with holistic pain assessments
• Check the Society of Hospital Medicine Pain Pathways for treatment guidelines.

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