Neurophysiology of Nociception: Sensory Transduction Flashcards
The processing of nociceptive information is accomplished via?
electrochemical reactions that occur along specific nociceptive pathways
Can the perception of pain occur without nociception?
YES
What are the 4 distinct neurophysiological events of nociception?
Transduction
Transmission
Modulation
Perception
What is transduction?
converting noxious stimuli into electrochemical impulses
What is transmission?
transmitting electrochemical impulses along afferent fibers to various nervous system regions
What is modulation?
altering the perception of noxious stimuli by peripheral or central mechanisms
What is perception?
the conscious experience of pain created by the interpretation of nociceptive information by higher centers of the CNS
What is sensory transduction?
the process in which information from various forms of noxious stimuli (chemical, thermal and/or mechanical) is transformed into electrochemical energy by sensory nerve ending (nociceptors)
Nociceptors are free nerve endings of what?
specialized afferent fibers
Nociceptors respond to?
intense, noxious stimuli
The primary cell bodies of neurons carrying noxious info from the body are located where?
in dorsal root ganglia (DRG)
Primary cell bodies of neurons carry noxious info from the face are located where?
trigeminal ganglia
All nociceptive neurons utilize what as primary neurotransmitters?
glutamate and substance P
What are the properties of peripheral nociceptor fibers?
myelinated A delta and unmyelinated C fibers
Myelinated nociceptors (20%)
- A delta fiber group
- Large myelinated axons
- conduct at ~20 m/s
- Bimodal: respond to noxious mechanical & thermal stimuli
- Small receptive fields (clusters of sensitive spots)
- Project to lamina I of spinal cord grey
Unmyelinated nociceptors (80%)
- C fiber group
- Small axons in remak bundles
- Conduct at ~2 m/s
- Polymodal: respond to mechanical, thermal and chemical stimuli
- Large receptive fields
- Project to lamina II and III of spinal cord grey
Myelinated bimodal nociceptors (A delta):
What is the adequate noxious stimuli for activation?
Mechanical: > 10 grams of pressure
Thermal: Heat (>45 C); Cold (11° C)
Unmyelinated polymodal nociceptors (C fiber):
What is the adequate noxious stimuli for activation?
Mechanical: >10 grams of pressure
Thermal: Heat (>45 C); Cold (11° C)
Chemical: numerous substances
What is the perception of first and second pain?
pain perception has been described:
- A first sharp pain and a more delayed diffuse and longer-lasting second pain
What fiber will be activated when intensity of a stimulus initially reaches the threshold of nociception?
Fast-conducting A-delta fibers
- producing a sharp painful sensation (first pain)
If a noxious stimulus persists which fibers are then activated? and what is there response?
Slowly adapting C-fibers will be activated
- producing a less well-localized pain of a dull quality
How fast do A-delta and C-fibers deactivate?
- A-delta fibers rapidly deactivate
- C-fibers deactivate slowly –> extending sensation of dull pain
Somatosensory quality (modality) processing form?
Receptor type (transduction) - i.e. temp, pressure, pain
Somatosensory intensity (magnitude) processing form?
Impulse (frequency)
- i.e. light or heavy pressure
Somatosensory spatial (location) processing form?
Receptive field
- i.e. superficial or deep, on leg or arm
After an injury to peripheral tissue, numerous chemical substances are released, what is the functional purpose of this?
Inflammatory response to protect the injured area (by inducing hyperalgesia and allodynia) and also to promote healing and guard against infection
What else may also result with peripheral inflammation?
inflammatory and neuropathic pain
What two mechanisms can inflammation enhance the perception of pain?
- Modifying the degree of nociceptor activation/excitation
2. Sensitization of nociceptors (peripheral sensitization)
What does modifying the degree of nociceptor activation/excitation result in?
an increased transmission of nociceptive information and enhanced perception of pain
What does sensitization of nociceptors (peripheral sensitization) result in?
Increased responsiveness (due to decreased threshold of excitation) to either thermal, mechanical, or chemical stimuli that may be translated to nociceptive information
What is peripheral sensitization?
increased sensitivity of a peripheral nociceptor to nociceptive stimuli (lowered threshold) which results in subsequent hyperalgesia and allodynia
What cellular mechanisms underly the sensitized state of peripheral nociceptors?
- Increased expression of Na ion channels
- R/c up regulation along the afferent fibers
When can peripheral sensitization occur?
- Exposure to inflammatory mediators
- Repeated application of noxious stimulus
K+ ion & histamine (origin and action)
O- escape from damaged cells
A- Activate polymodal nociceptors
Prostaglandins (origin and action)
O- Synthesized by enzymes released from substrates created by tissue damage
A- Sensitize nociceptors
Bradykinin, 5HT & ATP (origin and action)
O- Arrive following plasma effusion or lymphocyte migration
A- Activate and sensitize nociceptors
Substance P, calcitonin gene-related peptide (CGRP) (origin and action)
O- Secreted by nociceptor activity
A- Contribute to inflammatory response by initiating release of other substances
