Answers to Quiz 120 · Physiology of Pain, Itch and Temperature
Slow pain reaches the spinal cord along:
Answer: C fibers that end in lamina II
Slow (second) pain travels on fine unmyelinated C fibers, which terminate in lamina II of the dorsal horn. Aδ fibers carry fast pain and end in laminae I and V. Revise pain: concepts, receptors and fibers
Besides capsaicin, the VR-1 receptor at C fiber endings is activated by:
Answer: A rise in temperature above 43°C
VR-1 answers capsaicin, heat above 43°C and a change in pH. VRL-1 needs heat above 50°C and ignores capsaicin, while cold and menthol act on CMR1. Revise pain: concepts, receptors and fibers
In the lateral spinothalamic tract, the fibers coming from sacral segments lie:
Answer: Most laterally, at the outer edge of the tract
In the spinothalamic tract the lower body is outside: sacral fibers are outermost and cervical fibers innermost. The dorsal column runs the other way round, with sacral fibers most medial. Revise pain pathways and perception
The second order neurons of the paleospinothalamic pathway end mainly in which thalamic nuclei?
Answer: The midline and intralaminar nuclei
Paleospinothalamic fibers end in the medial nuclear group, that is the midline and intralaminar or non-specific nuclei, which is why slow pain is poorly localized. The ventrobasal and posterior complexes (VPL) are the neospinothalamic target. Revise pain pathways and perception
Which substance, released by the primary afferent fiber itself from its C fiber endings in skin, sensitizes nociceptors and heightens pain perception?
Answer: Substance P
Substance P is the polypeptide the afferent releases back at its own ending, and it also drives the other features of inflammation. Histamine and serotonin come from the damaged tissue, and glutamate is the fast transmitter at the dorsal horn synapse. Revise pain pathways and perception
A man with an acute myocardial infarction feels pain running down the inner border of his left arm. The best explanation is:
Answer: Referred pain, because heart and inner arm arise from the same embryonic segment
Visceral pain is referred to somatic structures of the same embryonic segment, so cardiac pain is felt on the ulnar side of the left arm. The arm itself is not ischemic and no impulse spreads along a plexus. Revise visceral and referred pain
The convergence theory of referred pain holds that:
Answer: Visceral and somatic afferents synapse on the same second order neuron
Second order neurons are fewer than the afferents arriving, so visceral and somatic fibers share them and the cortex cannot tell which end the signal came from. Raising the gain on a somatic neuron by a collateral is the facilitation theory. Revise visceral and referred pain
Which of these is NOT a feature of visceral pain?
Answer: Its afferents run in somatic sensory nerves
Visceral pain afferents travel with autonomic nerves, in the vagus, the sympathetic supply and the pelvic roots, which is exactly why sweating and blood pressure changes come with it. Distension of a hollow organ is its usual stimulus. Revise visceral and referred pain
Which statement about the endogenous opioid system is NOT correct?
Answer: Opioids inhibit the postsynaptic neuron by lowering its K⁺ conductance
Opioids raise postsynaptic K⁺ conductance, which hyperpolarizes the cell, and presynaptically they stop substance P release. The other three statements are all correct. Revise endogenous pain control and gate control theory
The dorsal horn interneurons of the descending analgesia system release enkephalin, which relieves pain by:
Answer: Preventing the release of substance P from the primary afferent ending
Enkephalin acts presynaptically and stops substance P release, so the pain message never crosses the dorsal horn synapse. The pathway above it runs from cortex and hypothalamus to the periaqueductal gray, then to the nucleus raphe magnus and down as serotonergic raphespinal fibers. Revise endogenous pain control and gate control theory
The gate control theory of pain proposes that:
Answer: Collaterals of large tactile afferents presynaptically inhibit Aδ and C fibers in the dorsal horn
Activity in the large myelinated touch fibers acts as a spinal gate on the output of the pain receptors. The raphespinal fibers are a descending system, not the gate, and sympathetic sprouting into the ganglion causes causalgia rather than relieving pain. Revise endogenous pain control and gate control theory
Pain produced by a stimulus that should not hurt at all, such as light touch, is called:
Answer: Allodynia
Allodynia is an excessive response to a mild stimulus. Hyperalgesia is a lowered threshold to stimuli that are already painful, and hyperpathia is an exaggerated reaction once pain has been perceived. Revise specific pain syndromes
A young man has continuous burning pain in the hand after a nerve injury in the forearm. It persists although the nerve has recovered fully, and it settles after sympathectomy. This is:
Answer: Causalgia
Causalgia is burning pain after traumatic nerve injury that outlives the damage, because noradrenergic sympathetic fibers grow into the dorsal root ganglia and every sympathetic discharge then hurts. Tic douloureux is brief stabbing facial pain, and thalamic syndrome needs no peripheral lesion at all. Revise specific pain syndromes
The receptors for itch are:
Answer: Endings of unmyelinated C fibers, separate from the pain C fibers
Itch is a modality of its own, carried by C fiber endings that are not the C fibers of pain, and histamine is the chief chemical that excites them. Itch and tickle then ascend in the lateral spinothalamic tract. Revise itch and temperature sensation
Cold receptors in the skin respond over a temperature range of about:
Answer: 10 to 40°C
Cold receptors work between 10 and 40°C and warmth receptors between 30 and 50°C, so the two overlap around body temperature. Cold afferents are Aδ and C fibers, heat afferents are C fibers, and both run in the lateral spinothalamic tract. Revise itch and temperature sensation
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