Pain and Temperature Sensations Flashcards
Core Body Temperature
37⁰ C
Body temperature has a
circadian fluctuation of
- 5-
0. 7°C (1-1.5° F)
Temperature is lowest at
—, highest in the —
6:00am
evening
Temperature regulation is less
precise in
young children;
normally have a temperature
that is 0.5°C or so above adults.
Thermal Receptors (thermoreceptors) are on
free nerve endings
& commonly found in the skin. Also in hypothalamus, spinal cord,
& deep tissues.
Categorized by their different sensitivities: (2)
cool
warm
Cool and warm receptors innervate
discrete regions of skin
There are – times as many cool receptors at any skin surface
3-10
— are activated by extreme cold or heat
Nociceptors
Which do you perceive first- a thermal or mechanical sensation?
mechanical
After thermoreceptors are activated,
Warm signals are transmitted by:
Cool signals are transmitted by:
C and Aδ fibers
Aδ and C fibers
The mechanism of thermal sensation is most likely achieved by
sensing a change in
metabolic rate
— increases the rate of intracellular chemical
reactions
Temperature
Several Transient Receptor Potential (TRP) channels
have been identified, each sensitive to a different
temperature range. Some chemicals (Capsaicin, Menthol)
elicit thermal sensations, too.
Temperature Perception
Based on the Activation of a
Combination of the Receptors
COLD PAIN: (5)
Tickling, Pricking, Aching, Burning, Numbing
HOT PAIN: (5)
Sharp, Pricking, Stinging, Burning, Throbbing
If skin reaches —, cold/pain fibers are no longer stimulated.
freezing
At ~ –°C, heat/pain fibers are activated.
Sometimes cold fibers are activated, too—
45
paradoxical cold
Average skin
temperature =
34-35ºC;
equal activation of cool
and warm receptors.
Adaptation of Thermoreceptors • They mostly (but never completely) adapt to --- ---; BUT they quickly change their activity in response to ---
constant temperature
changes in temperature.
They are very sensitive to changes
in temperature.
Adaptation of Thermoreceptors
• If the temperature reaches one of the
pain thresholds, the sensation
becomes
more persistent throughout
the stimulus
Sensitivity of Thermoreceptors
• Warm and Cool receptors are best able to detect a change at the
— of their temperature sensitivity (that is where they are
most sensitive)
mid-range
Sensitivity of Thermoreceptors
• If nociceptors are simultaneously activated, the system is even
better able to
discern small changes in temperature
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 — receptors fire (but not nociceptors)
thermal
The greater the area of skin affected by a thermal stimulus, the greater the
number of receptors, receptive fields, and first order neurons activated and thus the greater the perceived sensation.
There is a much greater ability for detection of a — stimulus if a large region is activated.
temperature
Receptors are primarily sensitive to —, but certain
receptors are also sensitive to —
temperature
chemicals
Vanilloid Receptor Subtype (3)
- TRPV1 Receptor
- Activated by capsaicin, temperature >43°C, and protons
- Decreases the threshold of channel activation so that
heat is perceived at 33°C.
Cold-Menthol Receptor Type I (CMR1/TRPM8) (2)
- Menthol and related compounds
- Decreases the threshold of the channels so that warmer
compounds are perceived as cold.
Characteristics of Thermoreceptors in Orofacial Region
receptive field size:
Small receptive fields (nociceptive thermal receptors
have large receptive fields)
Characteristics of Thermoreceptors in Orofacial Region
warm vs cool
More cool than warm receptors
BUT, whole mouth studies indicate that subjects are
better able to accurately detect increments of
warming rather than cooling
Characteristics of Thermoreceptors in Orofacial Region
the face vs inner mucosa
The face is 2-4 times as sensitive to thermal change
(warming) as the inner mucosa
Characteristics of Thermoreceptors in Orofacial Region
Thermoreceptors in — are the most sensitive to
changes in temperature.
tongue
Due to activation of nociceptors. This usually results in the
perception of
PAIN, an unpleasant & emotional experience
associated with actual or potential tissue damage.
nociception and pain relationship
All nociception produces pain, though not all pain
results from nociception.
Unlike most sensory modalities, pain can be evoked
by
any stimuli (mechanical, thermal or chemical) if it is strong enough.
We sense pain so that the body can (3)
detect, localize, and limit tissue damage.
PAIN, whatever its cause, is never —. Pain is the
most common reason patients (2)
benign
seek healthcare AND the most common symptom of disease.
types of acute pain (2)
somatic
visceral
acute pain
Physiologic Pain; <6 months duration
Somatic:
From skin, subcutanous tissues or mucus
membranes (Superficial) or muscles, tendons, joints or
bones (Deep).
– Superficial Somatic: localized, sharp, pricking and burning
– Deep Somatic: dull, aching, diffuse and can be referred
Visceral:
Due to a disease process or abnormal function
involving an internal organ (Visceral) or its covering
(Parietal).
– True Visceral: dull, diffuse, poorly localized and associated with
nausea and autonomic symptoms.
– Parietal Visceral: sharp, stabbing and better localized than true
pain.
Both types of visceral pain can be –
referred
Referred pain most frequently occurs with pain of
— Origin
visceral (& sometimes parietal)
Two nociceptive afferent neurons—from different regions of the
body—converge on the same second order neuron. The brain
doesn’t know which is the true source of input and may
make a
mistake in interpretation.
chronic pain
Pathologic Pain; occurs beyond the usual course of an acute
disease or after a reasonable time for healing to occur
3 types of chronic pain
nociceptive
neuropathic
mixed pain
Nociceptive:
Due to activation of nociceptors
Neuropathic:
Due to neuronal injury
• Pain is paroxysmal, sharp and stabbing
• Pain is associated with Hyperalgesia (increased
sensitivity 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.”
Mixed Pain
combo of nociceptive and neuropathic
autonomic responses are only a part of — pain
acute
Moderate to severe pain can
affect function of every
organ AND adversely
influence postoperative (2)
morbidity & mortality.
Poor pain control will
worsen patient outcomes!
Nociceptors are usually found on free nerve endings and
are sensitive to: (3)
- Mechanical Stimuli
- Thermal Stimuli
- Chemical Stimuli
Chemicals known to excite nociceptors: (9)
Substance P, ATP, calcitonin gene related peptide (CGRP), glutamate, aspartate, Bradykinin, potassium, Histamine, serotonin
Chemicals known to sensitize nociceptors: (4)
Substance P,
Prostaglandins,
Histamine,
Bradykinin
Chemicals known to inhibit nociceptors: (3)
Enkephalins,
β Endorphin,
Cannabinoids
Summation of
nociception is —
spatial
(based on # of
receptors activated).
Minimal adaptation and a nociceptors activity actually --- if the painful stimuli continues (hyperalgesia) due to nociceptor sensitization
INCREASES
Allodyina
painful
sensation to a
innocuous stimulus
can occur too
“Stimulation of nociceptors also leads to antidromal (reverse) activation of nociceptive nerve terminals and release of (2). Release of these peptides causes (5)
substance P and calcitonin gene related peptide
mast cell degranulation, vasodilation and edema, and further sensitization and activation of nociceptors (neurogenic inflammation).”
Triple Response (3)
1. Red flush around site of injury (flare) 2. Local tissue edema 3. Sensitization to noxious stimuli
Tissue injury leads to local release of — that excite or sensitize nociceptors causing —
endogenous inflammatory mediators (ex.
Histamine, Prostaglandins, Bradykinin, etc.)
Hyperalgesia
Aδ Fibers:
Small, Myelinated, 12-30 m/sec (fast)
– Neurotransmitter includes Glutamate
– Sharp, localized pain (FAST PAIN)
– Thermal and Mechanical Stimuli
C Fibers:
Unmyelinated, 0.5-2 m/sec (slow) – Neurotransmitter includes Substance P – Dull, diffuse pain (SLOW PAIN) – Thermal, Mechanical and Chemical Stimuli
The second order neuron crosses
over in the — — to
the opposite side of the spinal cord
to ascend contralaterally.
anterior commissure
divisions of the Spinothalamic Tract (2)
Neospinothalmic
Paleospinothalamic
Neospinothalmic:
Mostly A delta fibers. Provides location, intensity, & duration information.
Paleospinothalamic:
Mostly C fibers. Some of these 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.
There are multiple alternate pathways: (3)
i. Spinoreticular pathway. Mediates
ii. Spinomesencephalic pathway. Activates
iii. Spinohypothalamic pathway. Activates
arousal & autonomic responses
anti-nociceptive, descending pathways
the hypothalamus
The Spinothalamic Pathway includes the
Neospinothalamic and Paleospinothalamic
Tracts
Spinothalamic Pathway
First Order Neuron:
cell body in dorsal root
ganglion (or somatic afferent ganglion of cranial
nerves).
Spinothalamic Pathway
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.
Spinothalamic Pathway
Third Order Neuron:
cell body in thalamus,
axons project to the sensory cortex. Sensory
Homunculus
First- and Second-Order Neurons
Synapse in the
Dorsal Horn of the Spinal
Cord
Spinal cord gray matter was divided
by Bror Rexed in the 1950s into ten
—. I-VI are in the dorsal horn.
laminae
Second-order neurons in the
dorsal horn are either:
- Nociceptive-specific
Neurons - Wide Dynamic Range (WDR)
Neurons
Nociceptive-specific neurons
receive
only noxious stimuli
WDR neurons also receive
non-
noxious afferent input from Aβ, Aδ,
and C fibers
--- neurons are the most prevalent cell type in the dorsal horn (most abundant in Lamina V).
WDR
Wide Dynamic Range Neuron
During repeated stimulation, WDR neurons characteristically increase their firing rate exponentially (wind-up), even with the same stimulus intensity. They have larger receptive fields than nociceptive-specific neurons.
Nociceptive Specific Neuron
Found in lamina I and have discrete, somatic receptive fields; they are normally silent and respond only to high- threshold noxious stimulation, poorly encoding stimulus intensity.
Mechanisms for CENTRAL Modulation of Pain (3)
Facilitation of Pain
1. Wide-Dynamic Range (WDR) Neurons found in the dorsal horn
of the spinal cord. They respond to all somatosensory
modalities. Normally only sensitive to non-noxious stimuli
UNLESS the stimulus is highly noxious.
Allodynia = Perception of non-noxious stimuli as pain
2. Receptor Field Expansion of dorsal horn neurons so adjacent
neurons become responsive to stimuli (whether noxious or not)
3. Some second order neurons increase their frequency of
activation following prolonged discharge—”wind-up.” May
continue, even after the nociceptive stimulus is removed.
Mechanisms for CENTRAL Modulation of Pain
Inhibition of Pain (2)
gate control theory of pain
pain inhibits pain
Gate Control Theory of Pain:
Activation of Ab fibers from the same
region inhibits the Spinothalamic pathway
and reduces pain perception
The A-β fibers activate an inhibitory
interneuron that causes post-synaptic
inhibition of the Second-order neuron for
the Pain Pathway. This inhibition leads to
a weaker pain signal being sent to the
thalamus and somatosensory cortex.
This is the basis for how massage therapy
and TENS units work as a treatment for
chronic pain conditions and is why rubbing
a painful region makes it feel better.
Pain Inhibits Pain
Stimulate pain in other regions of the body to inhibit pain (via GABA) at second order, WDR, neurons in spinal cord (Diffuse Noxious Inhibitory Control (DNIC) or Conditioned Pain Modulation (CPM))
Conditioned Pain Modulation
CPM
a second noxious stimulus leads to the activation of PAG, NRM and RVM in the brainstem, which results in diffuse analgesic effect over the rest of the body.
“Pain Inhibits Pain”
Interneurons in the dorsal horn of the spinal
cord are activated by 5-HT/NE and release
endogenous opiates (Enkephalin).
Enkephalin in the dorsal horn inhibits the
first-order and second-order neurons
(nociceptive specific and/or WDR neurons).
In —, there is evidence of
low concentrations of serotonin and
NE in the CSF, which suggests this
pathway might have reduced activity.
Fibromyalgia
Analgesia is the
selective suppression of pain without
effects on consciousness or other sensations.
We use many mechanisms to achieve pain relief: (5)
- Pharmacological agents: COX Inhibitors (aspirin,
ibuprofen), Opioids, Antidepressants (TCAs),
Anticonvulsants (GABApentin), Topical Lidocaine
& Capsaicin - Massage (Gate Control Theory of Pain)
- Acupuncture (seems to be linked to activation of
the endogenous opioid pathways) - Transcutaneous Electrical Stimulation (TENS)
- Nerve Blocks
Pain information from the orofacial region is
conveyed into the CNS via the (2)
Trigeminospinothalamic Tract and the
Trigeminoreticular Tract
Pain can be referred to the orofacial region
particularly teeth) in response to: (5
- Maxillary Sinusitis
- Angina
- Migraine
- Nasal Mucosa
- Ear Pain
It is often difficult to – tooth pain.
localize
Dentin and enamel has limited space to swell in the
— state, so results in
inflammatory
exaggerated pain even with mild inflammation
Painfully noxious hot/cold stimuli do not cause pain in an
uninflamed healthy tooth due to thermal insulating of the
enamel. But, with missing enamel and exposed dentin, a
slight thermal stimuli will cause
sudden and stinging pain.
– It is difficult to distinguish between between noxious hot
or cold stimuli.
Weak air puffs to exposed dentin result in
intense pain
while in skin this would result in the sensation of light touch.
Stimulation of tooth pulp by any kind of stimulus results in a
painful sensation
While afferent fibers innervating teeth comprise only 0.1% of
trigeminal neurons, each of these fibers has extensive branching,
such that each afferent nerve innervates
multiple teeth.
Large receptive fields =
Poor Localization
The pulp is highly innervated with sensory afferents, mostly for pain, but the dentin has limited innervation but is sensitive due to
dentinal tubules.
Nerve fibers penetrate the dentinal tubules, but only for a short distance into the inner dentin.
Odontoblasts have long processes that are located in the — — and their cell body is on the surface of the — —
dentinal tubule
dental pulp
Studies have shown that sensitive teeth have many more (8X) and wider (2X) --- --- compared to non- sensitive teeth. --- --- thought to be more important than number since flow is proportional to the fourth power of the radius.
dentinal tubules
Tubule
radius
Tissue is highly vascular, contains many neurons, but is a
— tissue because it is surrounded by
low-compliance
hard tissue
Any change in volume within the tooth pulp translates to large
changes in pressure (3)
a. Inflammation due to mechanical or bacterial trauma
b. Increases in pulpal temperature secondary to laser radiation
c. Can lead to hypoxia and eventually tissue necrosis
Dentin is a mineralized, avascular tissue (3)
a. Suprasensitive to extreme temperatures, particularly at the neck
of the tooth (gingival recession, wear of cementum and enamel)
b. Sensitive to hyperosmotic solutions
c. NOT sensitive to KCl, bradykinin, and histamine
Dentinal tubules contain odontoblasts and perhaps they also
contain
nerve terminals
The mechanism of — — in dentin is not currently
understood.
pain transduction
Dentinal and pulp nociceptor information travels via Aδ and C fibers along with Aβ. The nerve fibers in tooth pulp are ~90%
C-fibers
Dentinal Pain:
sharp pain
Pulp Pain:
dull pain
Dentin and Pulp are separated
by
tight junctions between
epithelial cells—selectively
permeable barrier
Neural Theory
Free nerve endings in dentinal tubules are activated
Hydrodynamic Theory
Stimulus displaces fluid in dentinal tubules which activates mechanoreceptors in nerve endings of dentin or pulp.
Odontoblast
Transducer Theory
Odontoblasts are excited and transduce the signal to nearby nerve cells
TRPV1
Polymodal receptor activated by painful chemicals (Capsaicin) and noxious temperatures (above 42°C)
TRPM8
Low Temperature (threshold 25°C)
TRPA1
Low Temperature (threshold 17°C) Mechanosensation?
The periodontal ligament has (2)
nociceptors AND Ruffini Endings (a mechanoreceptor/proprioceptor).
Periodontal nociceptive information from free
nerve endings travels via
Aδ/C fibers
The --- --- regulate the forces applied by the teeth in occlusion, mastication and biting. These receptors are slowly adapting, show directional sensitivity and their response varies with the force applied to the tooth.
periodontal mechanoreceptors (Ruffini endings)
Subjects with dentures (who lack corresponding
PDL receptors), show impaired – —-
perception in tasks as biting force discrimination
intraoral sensory
the apical portion of the
periodontal ligament is the
most heavily —
innervated
