2/8: Thermal Sensation and Nociception I Flashcards
Where are thermal receptors (thermoreceptors) found on?
Free nerve endings and in the skin
Also- hypothalamus, spinal cord, and deep tissues
What are the sensitivities of temperature sensation categorized by?
Cool and warm
How many cool vs warm receptors?
3-10 times as many cool receptors at any skin surface
What are nociceptors activated by?
Extreme cold or heat
Which do you perceive first- a thermal or mechanical sensation?
Mechanical because use beta fibers which are myelinated and have larger diameter
What are warm signals transmitted by?
Mainly C
and Alpha delta fibers
What are cool signals transmitted by?
Mainly Alpha delta
and C fibers
Mechanism of thermal sensation are most likely achieved by sensing?
A change in metabolic rate
What increases the rate of intracellular chemical reacitons?
Temperature
Transient Receptor Potential (TRP) channels are identified and each are sensitive to what?
A different temperature range
Some chemicals (capsaicin, menthol) elicit thermal sensations, too
What is temperature perception based on?
Activation of a combination of the receptors
What are examples of cold pain?
Tickling
Pricking
Aching
Burning
numbing
What are examples of hot pain?
Sharp
Pricking
Stinging
Burning
Throbbing
If skin reaches freezing, __________ fibers are no longer stimulated?
Cold/pain
What kind of fibers are activated at higher temperatures?
Heat/pain fibers
Sometimes cold fibers are activated too, paradoxical cold (when cold fibers are activated at warm temps)
What do thermoreceptors adapt to?
They mostly (but never completely) adapt to constant temperature; BUT
they quickly change their activity in response to changes in temperature
What are thermoreceptors very sensitive to?
Changes in temperature
If temperature reaches one of the pain thresholds, what happens to the sensation?
Becomes more persistent throughout the stimulus
What are warm and cool receptors best able to detect?
A change at the mid-range of their temperature sensitivity (that is where they are most sensitive)
What happens if nociceptors are simultaneously activated?
The system is even better to discern small changes in temp
As temperature increases (or decreases) within a
range of a thermoreceptor’s sensitivity, more and
more receptors are _________
Activated, based on their varying thresholds
Thus, perception is increased
There is also an increase in the rate that thermal
receptors fire (but not nociceptors)
The greater the area of skin affected by a thermal stimulus, the greater the number of ________, __________, and __________ activated, and thus the _____________ the perceived sensation
Receptors; receptive fields; first order neurons; greater
- There is a much greater ability for detection of a
temperature stimulus if a large region is activated
Receptors in the oral cavity are primarily sensitive to?
Temperature, but certain receptors are also sensitive to chemicals
What is the receptor for vanilloid receptor?
TRPV1 receptor
What is vanilloid receptor activated by?
Capsaicin, high temperatures, and protons (hot peppers are hot depending on how much capsaicin)
What does vanilloid receptor decrease?
The threshold of channel activation so that heat is perceived at 33*C
What are the receptors for cold-menthol receptor type I?
CMR1 (cold menthol receptor)/TRPM8
What are cold-menthol receptor type I associated with?
Menthol and related compounds
What does cold-menthol receptor type I decrease?
The threshold of the channels so that warmer compounds are perceived as cold
What kind of receptive fields do thermoreceptors in the orofacial region have?
Small
What kind of receptive fields do nociceptive thermal receptors have?
Large receptive fields
Does the mouth have more cool or warm receptors?
More cool than warm
BUT, whole mouth studies indicate that subjects are
better able to accurately detect increments of
warming rather than cooling
What is the face more sensitive to?
The face is 2-4 times as sensitive to thermal change
(warming) as the inner mucosa
What are thermoreceptors in tongue most sensitive to?
Changes in temperature
What does activation of nociceptors result in the perception of?
PAIN, an unpleasant & emotional experience
associated with actual or potential tissue damage
All nociception produces ______, though not all ______ results from nociception
Pain; pain
Unlike most sensory modalities, _______ can be evoked by any stimuli (mechanical, thermal or chemical) if it is strong enough
Pain
Why do we sense pain?
So the body can detect, localize, and limit tissue damage
What is acute pain?
Physiologic pain; <6 months duration
What are the two types of acute pain?
- Somatic
- Visceral
Where is somatic pain located?
Skin, subcutaneous tissues or mucus membranes (superficial) or muscles, tendons, joints or bones (deep)
What is superifical somatic pain?
Localized, sharp, pricking and burning
What is deep somatic pain?
Dull, aching, diffuse, and can be referred
What is visceral pain due to?
A disease process or abnormal function involving an internal organ (visceral) or its covering (parietal)
What is true visceral pain?
Dull, diffuse, poorly localized and associated with nausea and auronomic symptoms
What is parietal visceral pain?
Sharp, stabbing and better localized than true pain
Both somatic and visceral pain can be…
Referred
Referred pain most frequently occurs with pain of _________ origin
Visceral (& sometimes parietal)
Where do the two nociceptic afferent neurons from different regions of the body converge?
Same second order neuron
- the brain doesn’t know which is the true source of input and may make a mistake in interpretation
What is chronic pain?
Occurs beyond the usual course of an acute disease or after a reasonable time to healing to occur
What are three types of chronic pain?
- Nociceptive
- Neuropathic
- Mixed pain
What is nociceptive pain due to?
Activation of nociceptors
What is neuropathic pain due to?
Neuronal injury
Describe neuropathic pain
Paroxysmal, sharp and stabbing
What is neuropathic pain associated with?
Hyperalgesia (increased sensitivty to pain)
“Neuropathic pain includes pain associated with diabetic neuropathy,
causalgia, phantom limbs, postherpetic neuralgia, stroke, spinal cord injury,
and multiple sclerosis. Cancer pain and chronic low back pain may have
prominent neuropathic components.”
Autonomic responses are only a part of _______ pain
Acute
Moderate to severe pain can affect function of every organ AND adversely influence postoperative _______&_________
Morbidity & mortality
- poor pain control will worsen patient outcomes
Where are nociceptors found?
On free nerve endings
What 3 things are nociceptors sensitive to?
- Mechanical stimuli
- Thermal stimuli
- Chemical stimuli
What are chemicals known to excite nociceptors?
Substance P, ATP,
calcitonin gene related peptide (CGRP), glutamate, aspartate, Bradykinin, potassium, Histamine, serotonin
What chemicals known to sensitize nociceptors?
Substance P, Prostaglandins, Histamine, Bradykinin
What chemicals are known to inhibit nociceptors?
Enkephalins, β
Endorphin, Cannabinoids
What chemical sensitizes and excites nociceptors?
Substance P
What is summation of nociception based on?
Spatial
- based on # of receptors activated
Minimal adaptation anda. nociceptors activity _______ if the painful stimuli continues (hyperalgesia) due to nociceptor ___________
INCREASES; sensitization
What is allodynia?
Painful sensation to a innocuous stimulus
What can stimulation of nociceptors also lead to?
antidromal (reverse) activation of nociceptive nerve terminals and release of substance P and calcitonin gene related peptide
What does release of substance P and calcitonin gene related peptide cause?
mast cell degranulation, vasodilation and edema,
and further sensitization and activation of nociceptors (neurogenic
inflammation)
What is the triple response?
- Red flush around site of
injury (flare) - Local tissue edema
- Sensitization to noxious
stimuli
- Tissue injury leads to local release of endogenous inflammatory mediators (ex.
Histamine, Prostaglandins, Bradykinin, etc.) that excite or sensitize nociceptors causing hyperalgesia
What fibers are small and myelinated?
Alpha delta fibers
What neurotransmitters are included in alpha delta fibers?
Glutamate
What pain is associated with alpha delta fibers?
Sharp, localized pain (FAST PAIN)
What stimuli influences alpha dental fibers?
Thermal and mechanical stimuli
What fibers are unmyelinated?
C fibers
What neurotransmitters are included in C fibers?
Substance P
What pain is associated with C fibers?
Dull, diffuse pain (SLOW PAIN)
What stimuli are associated with C fibers?
Thermal, mechanical and chemical stimuli
Where does the second order neuron cross over?
In the anterior commissure to the opposite side of the spinal cord to ascend contralaterally
What is the major pain pathway in the CNS?
The lateral spinothalamic tract
What are the two parts of the lateral spinothalamic tract?
Neospinothalmic
Paleospinothalamic
What fibers is the neospinothalmic pathway made of?
A delta fibers
What does the neospinothalmic pathway provide?
Location, intensity and duration information
What fibers make up the paleospinothalamic tract?
Mostly C fibers
What do paleospinothalamic fibers synapse with?
Brainstem structures (reticular formation, Periaqueductal Gray Region, Limbic System, Hypothalamus) and also with diffuse areas of the cortex for poorly localized sense of pain
What are the alternate pathways for the pain pathway in the CNS?
i. Spinoreticular pathway.
Mediates arousal &
autonomic responses
ii. Spinomesencephalic
pathway. Activates anti-
nociceptive, descending
pathways
iii. Spinohypothalamic pathway
In the lateral spinothalamic pathway, where is the cell body in the first order neuron?
ell body in dorsal root ganglion (or somatic afferent ganglion of cranial nerves)
In the lateral spinothalamic pathway, where is the cell body in the second order neuron?
cell body is in dorsal nuclei & axons decussate via the anterior commissure and
terminate in thalamus. Can be solely nociceptive OR
Wide-Dynamic Range (WDR) neurons
In the lateral spinothalamic pathway, where is the cell body in the third order neuron?
Cell body in thalamus, axons
project to the sensory cortex. Sensory Homunculus
What are the two ways that second-order neurons are in the dorsal horn?
- Nociceptive-specific
Neurons - Wide Dynamic Range (WDR) Neurons
Nociceptive-specific neurons receive only _______ stimuli
Noxious
WDR neurons receive ____________ input from ____,____,and ____ fibers
Non-noxious afferent; alpha beta; alpha delta; and C fibers
What neurons are the most prevalent in the dorsal horn?
WDR (wide dynamic range) neuron (most abundant in lamina V)
Where do first and second order neurons synapse?
In the dorsal horn of the spinal cord
During repeated stimulation, describe WDR diring rate?
WDR neurons characteristically
increase their firing rate
exponentially (wind-up), even with the same stimulus
intensity.
Do WDR neurons or nociceptive specific neurons have a larger receptive field?
WDR
Where are nociceptive specific neurons found?
In lamina I and have discrete, somatic receptive fields
What do nociceptive specific neurons respond to?
they are normally silent
and respond only to high-
threshold noxious stimulation, poorly encoding stimulus intensity
