Understanding Basic Principles of Nociception and Pain Flashcards
What is Pain
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
What is Nociception
The neuronal process of encoding and process noxious stimuli
Noxious stimuli is the insult causing the tissue damage
Five Steps of Nociception
- Transduction
- Transmission
- Modulation
- Projection
- Perception
Transduction
- 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
Types Of Nociceptors
Mechano-sensitive
Mechano-thermal
Chemical
Polymodal
Silent
Mechano-Sensitive Nociceptor
Respond to intense pressure, stretch or strain (A-delta fiber)
Mechano-Thermal Nociceptor
Respond to extreme temperature: < 5 C / 41 F ; >45 C / 113 F)
(A-delta fiber)
Chemical Nociceptor
Respond to chemicals released from tissue
damage (i.e: bradykinin, prostaglandin, substance P,
histamine) or from external chemicals (i.e.: topical
capsaicin) (C-fiber)
Polymodal Nociceptor
Sensitive to combinations of mechanical,
thermal, and chemical (C-fiber)
Silent Nociceptor
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
Transmission
- 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
Two primary afferent fibers : Transmission
- (A-delta) Fibers
- myelinated: associated with thermal and
mechanosensitive nociceptors - Fast pain, easy to localize location
- myelinated: associated with thermal and
- C fibers
- unmyelinated: associated with chemical,
polymodal, silent receptors - Slow pain, difficult to localize specific location
- unmyelinated: associated with chemical,
Clinical Note” Referred Pain
- 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
Clinical Note: Wind Up Pain
- 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
Projection
- The processing of
sending information via
ascending pathways to
the higher brain centers - AKA – ”projecting it up
to the brain”
HOW NOCICEPTIVE SIGNALS TRAVELS UP THE CORD
- 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
Perception
- 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!
Modulation
- 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
MODULATION: ENDOGENOUS ANALGESIA SYSTEM
- 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
Gate-Control Theory
- 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
Pain Pathway Summary
- 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
Clinical Applications of Pain
Dysesthesia
Parasthesia
Allodynia
Hyperesthesia
Dysesthesia
Unpleasant abnormal sensations – itching, burning,
tingling
Parasthesia
Spontaneous or evoked abnormal sensation
Allodynia
Pain response to non-noxious stimulus of normal skin
Hyperesthesia
Increased sensitivity (exaggerated response) to a normally noxious stimulus
Pain Assessment in Animals
- Assessing pain in animals is focused on evaluating behavior and response to stimuli
- Glasgow Pain Scale
Glasgow Pain Scale
- Behavioral assessment
- Can be subjective between observers
Pain on Neurological Exam
- 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
- I personally do NOT ventroflex the neck on every case
- Firm palpation along back
- Two fingers – lightly at first!!
