Module D-04 Flashcards
What are the 2 major classes of pain?
1) Nociceptive pain
2) Neuropathic pain
What causes nociceptive pain?
Physiological, activation of nociceptors by stimuli that may cause tissue damage
What causes neuropathic pain?
Pathological, caused by aberrant processing in the CNS or PNS
2 types of Nociceptive pain
1)First Pain is recognized as a sharp, pricking sensation
2)Second Pain is recognized as a burning sensation that
has a slower onset
What type of Nociceptors are activated in First pain?
thermal or mechanical nociceptors
What type of fibers conduct the signal for First pain?
myelinated A-delta fibers (conduction velocities
between 4 and 46 m/sec)
What type of Nociceptors are activated in Second pain?
Polymodal Nociceptors
What type of fibers conduct the signal for Second pain?
unmyelinated C fibers (conduction velocities between 0.4 and 2 m/sec)
Where are no nociceptors found?
in the brain
Why do we still get headache if there are no brain nociceptors?
Because of the nociceptors in the meninges, periosteum and blood vessels
Through which fibers are Visceral or deep pain sensations conducted?
unmyelinated C fibers ( that is why visceral pain is felt as aching, burning pain)
Why is pain from Viscera felt as referred pain?
- Afferent fibers of visceral nociceptors enter the spinal cord through the dorsal roots, using the same route of entry as the nociceptors innervating the body’s surface
- neurons in the grey matter of the spinal cord receive convergent inputs from visceral and somatic afferent neurons and send ascending projections into the anterolateral system (ALS)
Describe the 3 neuron pathway for Pain
1) First order neurons (somatosensory receptor neuron): afferent fibers in peripheral nerve
2) Second order neurons: fibers crossing the midline and terminating in thalamus
3) Third order neurons: fibers running in posterior limb of internal capsule and ending in S1 (primary somatosensory cortex)
Where do the primary afferent fibers of spinal pain pathway synapse in the spinal cord?
dorsal horn of the grey matter of the spinal cord, in a region
called substantia gelatinosa
Where does cross over occur in the anterolateral system
immediately in the spinal cord
Where do the pain 2nd order neurons synapse
VPL of thalamus
Where do pain 3rd order fibers synapse
through the internal capsule=> corona radiata=> primary somatosensory cortex
Describe the trigeminal pathway for pain
1) Primary afferent fibers enter the brainstem at the pons and the fibers descend (!) ipsilaterally in the spinal trigeminal tract, to synapse in the lower third of the spinal nucleus of CN V, located in the lower medulla.
2) second order neurons immediately cross the body’s midline in the lower medulla and then ascend in the ventral trigeminothalamic tract until they reach the ventral posterior medial nucleus of the thalamus.
3) Axons of the third order neurons ascend through the posterior limb, close to the genu of the internal capsule, run through the corona radiata, and synapse in the primary somatosensory cortex (S1)
Ascending and descending fibers can be seen on both sides of the brain stem, the ascending fiber is the one ________ to the input and the descending fiber is _________
contralateral; ipsilateral
How are chemosensitive nociceptors activated ?
by chemical substances released due to tissue damage:
• Potassium from damaged cells
• Bradykinin from blood
• Histamine from mast cells
What is Hyperalgesia?
- an enhanced sensitivity and responsivity to stimulation of the area around the damaged tissue.
- Stimuli that are already painful under normal circumstances cause a more intense pain sensation
Why does hyperalgesia occur?
due to sensitization of nociceptors by chemicals released from damaged cells
What chemical substances released cause sensitization of nociceptors?
- Prostaglandins from damaged cells
- Leukotrienes from damaged cells
- Substance P from primary afferents
What is allodynia?
When a previously painless stimulus becomes painful
An effect of sensitization of nociceptors
2 neuronal mechanisms for pain control
1) Afferent Regulation of pain - gate control circuitry
2) Descending regulation of pain
Describe the gate control circuitry
large myelinated fibers carrying touch sensations activate
inhibitory interneurons within the dorsal horn of the spinal
cord, which then reduce the flow of nociceptive information through the “gate” between nociceptors (first order neurons of the pain pathway) and their second order neurons by hyperpolarizing it
What neurotransmitter do the interneurons release?
Enkephalin (opioid peptide)
Where are the opioid receptors found in the gate control circuitry?
- on the cell bodies of the primary afferent neurons,
- on their presynaptic ending,
- also on the postsynaptic membrane of the second order neurons (projection neurons)
Where are nuclei of descending pain located?
In the brainstem:
-midbrain = periaqueductal grey, (serotonergic)
-Medulla = nucleus raphe magnus (serotonergic)
-Pons = locus ceruleus (noradrenergic)
spinomesencephalic fibers) and the reticular
formation of pons and medulla (spinoreticular fibers)
Describe the descending pain regulation.
- nuclei where the descending pain control fibers originate, receive input from ascending ALS fibers carrying nociceptive information
- descending monoaminergic fibers run in the lateral and anterior funiculi of the spinal cord and terminate on the
opioidergic interneurons in the dorsal horn, where they
control the transmission of pain
What is the effect of Aspirin?
inhibit the enzyme cyclooxygenase thereby inhibiting synthesis of prostaglandins, agents which sensitize
sensory afferent fibers.
Example of an opioid
morphine
What type of pain is phantom limb pain?
Neuropathic (deafferentation) pain
