Understanding Basic Principles of Nociception and Pain

Question 1

What is Pain

Answer 1

The unpleasant sensory and emotional experience associated with tissue damage

Pain is a variable experience and requires cortical involvement to be able to experience it

Question 2

What is Nociception

Answer 2

The neuronal process of encoding and process noxious stimuli

Noxious stimuli is the insult causing the tissue damage

Question 3

Five Steps of Nociception

Answer 3

• Transduction
• Transmission
• Modulation
• Projection
• Perception

Question 4

Transduction

Answer 4

• The process of turning a noxious stimulus to an electrical signal
• Nociceptors are free nerve endings with no capsule
• Variable types of nociceptors that respond to specific stimuli
• RECEPTORS ARE CALLED NOCICEPTORS
• Nociceptors are found throughout most of the body
• NOT FOUND IN THE BRAIN (but are found in the meninges)
• Activation requires an adequate stimuli to cause a receptor potential
• Nociceptors are NON-ADAPTING
• They will not adapt to a persistent stimulus

Question 5

Types Of Nociceptors

Answer 5

Mechano-sensitive
Mechano-thermal
Chemical
Polymodal
Silent

Question 6

Mechano-Sensitive Nociceptor

Answer 6

Respond to intense pressure, stretch or strain (A-delta fiber)

Question 7

Mechano-Thermal Nociceptor

Answer 7

Respond to extreme temperature: < 5 C / 41 F ; >45 C / 113 F)
(A-delta fiber)

Question 8

Chemical Nociceptor

Answer 8

Respond to chemicals released from tissue
damage (i.e: bradykinin, prostaglandin, substance P,
histamine) or from external chemicals (i.e.: topical
capsaicin) (C-fiber)

Question 9

Polymodal Nociceptor

Answer 9

Sensitive to combinations of mechanical,
thermal, and chemical (C-fiber)

Question 10

Silent Nociceptor

Answer 10

Silent nociceptors must be first activated or “awakened” by tissue inflammation
before responding to a mechanical, thermal, or chemical stimulus. Most visceral
nociceptors are silent nociceptors

Question 11

Transmission

Answer 11

• Process of sending signal from receptor to CNS
• Transmission is carried via TWO different afferent fibers ( A-delta and C fibers)
• Travel via nerve to dorsal root to synapse in the dorsal horn
• Sensory fibers synapse on different lamina in the dorsal horn gray matter
• Do not need to know which lamina

Question 12

Two primary afferent fibers : Transmission

Answer 12

• (A-delta) Fibers
  
  • myelinated: associated with thermal and
    mechanosensitive nociceptors
  • Fast pain, easy to localize location
• C fibers
  
  • unmyelinated: associated with chemical,
    polymodal, silent receptors
  • Slow pain, difficult to localize specific location

Question 13

Clinical Note” Referred Pain

Answer 13

• Signals from nociceptors in the viscera can be felt as pain ELSEWHERE in the body
• A heart attack leads to referred pain in the chest and left arm
  • The convergence of visceral and somatic afferent fibers is suspected as the underlying cause

Question 14

Clinical Note: Wind Up Pain

Answer 14

• Also called central sensitization
• Chronic pain -> chronic enhancement of the central pain-processing circuits
• The synapse becomes MORE EFFICIENT at relaying noxious information;
  synaptic activity may persist after healing
  
  • Phantom limb pain
  • Chronic pain syndromes

Question 15

Projection

Answer 15

• The processing of
  sending information via
  ascending pathways to
  the higher brain centers
• AKA – ”projecting it up
  to the brain”

Question 16

HOW NOCICEPTIVE SIGNALS TRAVELS UP THE CORD

Answer 16

• Travels predominantly via the Spinothalamic
  Pathway
• The majority cross the midline of the spinal cord at
  the spinal cord segment they originate from and
  travel in the lateral funiculus to the thalamus.
• These pathways do travel up both sides of the cord
• Contralateral predominates

Question 17

Perception

Answer 17

• Involves many cortical areas whose
  activity is widely influenced critically
  by the context in which the noxious
  stimulus is presented
• Also altered by an individual’s prior
  experience
• Nociception is interpreted as pain by
  the CONTRALATERAL cortex.
• Relay nuclei in the thalamus receive the signal
  from the spinal cord
• Relay nuclei send projections to limbic structures,
  hypothalamus, and amygdala
• Allows for an emotional response with pain
• Other areas of brain stimulated in humans
• VERY COMPLICATED PROCESS!

Question 18

Modulation

Answer 18

• This neuronal process reduces the activity of
  pain transmission system – OCCURS AT THE
  LEVEL OF THE SPINAL CORD
• Involves DESCENDING signals from portions of
  the brain
• Two Primary Systems:
• Endogenous Opioids
• Gate Control Theory

Question 19

MODULATION: ENDOGENOUS ANALGESIA SYSTEM

Answer 19

• Collection of neurons that function to reduce
  the activity of nociceptive neurons
• These neurons are located in:
• Periaqueductal gray matter (PAG)
• Pons/Medulla
• Dorsal Horn (inhibitory interneurons)
• Activation of the endogenous analgesia system stimulates inhibitory
  interneurons in the dorsal horn gray matter via opioid receptors
• Decreases in synaptic activity between peripheral neuron (C-Fiber) and the spinal cord
  tract
• Leads to inhibition of local excitatory neurotransmitters

Question 20

Gate-Control Theory

Answer 20

• Mechanical stimulation of the A-Beta (Aβ) fiber reduces the activation of the
  relay in the dorsal horn by increasing inhibitory interneuron activity
• Examples:
• Transcutaneous Electrical Nerve Stimulation (TENS)
• Rubbing/ Massage of area close to injury
• Ice/Warm packing of site

Question 21

Pain Pathway Summary

Answer 21

• Pain is a PERSONAL EXPERIENCE
• Individuals will experience pain differently
• It is is the cortical awareness and response to
  nociception
• Nociception:
• A noxious stimuli damages tissue leading to transduction at the nociceptors
• Transmission of the signal occurs from the nociceptors to the spinal cord via
  specific fibers
• The signal is projected (sent) to the brain in a process known as Projection.
• That signal undergoes Modulation at the level of the spinal cord via
  descending pathways
• Finally, Perception occurs within the cortex and emotional reactions and
  sensations are experienced by organism

Question 22

Clinical Applications of Pain

Answer 22

Dysesthesia

Parasthesia

Allodynia

Hyperesthesia

Question 23

Dysesthesia

Answer 23

Unpleasant abnormal sensations – itching, burning,
tingling

Question 24

Parasthesia

Answer 24

Spontaneous or evoked abnormal sensation

Question 25

Allodynia

Answer 25

Pain response to non-noxious stimulus of normal skin

Question 26

Hyperesthesia

Answer 26

Increased sensitivity (exaggerated response) to a normally noxious stimulus

Question 27

Pain Assessment in Animals

Answer 27

* Assessing pain in animals is focused on evaluating behavior and response to stimuli * Glasgow Pain Scale

Question 28

Glasgow Pain Scale

Answer 28

* Behavioral assessment * Can be subjective between observers

Question 29

Pain on Neurological Exam

Answer 29

* Evaluate posture! * Firm palpation of neck * I personally do NOT ventroflex the neck on every case * Small dogs with atlantoaxial instability can become worse * May use food and see how the voluntarily follow food * Firm palpation along back * Two fingers – lightly at first!!