Chapter 22 Surgical Pain

Question 1

what is Plasticity?

Answer 1

– the ability of the pain sensing system to change in response to input.

Question 2

what is hyperalgesia?

Answer 2

• stimuli that would not normally have been felt as painful (both from the site of surgery and from other areas) may be felt as painful when the animal regains consciousness (allodynia)

Question 3

what is phantom limb pain?

Answer 3

when under certain circumstances, permanent changes to central connections may occur, leading to permanent perception of pain due to ongoing spontaneous production of nociceptive signals.

Question 4

Sensory nerve fibers recognize and transform (_____) environmental stimuli into electrical signals (_______) that are conveyed (________) to the dorsal horn of the spinal cord where they are processed (_______) and then relayed (______) to the brainstem and brain, where the singal is integrated, recognized, and identified (____) as pain.

Answer 4

Sensory nerve fibers recognize and transform (transduction) environmental stimuli into electrical signals (action potentials) that are conveyed (transmission) to the dorsal horn of the spinal cord where they are processed (modulation) and then relayed (projection) to the brainstem and brain, where the singal is integrated, recognized, and identified (perception) as pain.

Question 5

what is nociception?

Answer 5

– the creation and transfer of signals after activation of nerve endings by a noxious stimulus.

Question 6

what does pain imply?

Answer 6

– implies that noxious stimuli have been perceived at the thalamic or cortical level and interpreted as pain.

Question 7

what are the peripheral nociceptors?

Answer 7

a. Alpha delta afferents & C fiber afferents
i. Excited when stimulus intensities reach the noxious range, suggesting that they possess bioelectrophysical and moldecular properties that enable them to respond selectively to potentially injurious stimuli.
ii. Medium-diameter alpha delta afferents mediate acute, well-localized first or fast pain. These myelinated afferents differ considerably from the larger diameter and rapidly conducting alpha beta fibers that respond to innocuous mechanical stimulation (light touch)
iii. Small diameter unmyelinated C fibers the convey poorly localized second or slow pain.

Question 8

Most nociceptors are relatively nonselective ____ channels gated not by _____ but by the stimulus (temperature, chemicals, mechanical forces). Channels open and ___and ___ influx occurs depolarizing the membrane

Answer 8

Most nociceptors are relatively nonselective ion channels gated not by voltage but by the stimulus (temperature, chemicals, mechanical forces). Channels open and Na and Ca influx occurs depolarizing the membrane

Question 9

alpha delta -
Type ___
Type ___

Answer 9

i. Type I (high threshold mechanical nociceptors) respond to both mechanical and chemical stimuli but have relatively high heat thresholds (>50 degrees C).
ii. Type II have a much lower heat threshold, but a very high mechanical threshold

Question 10

C-fibers -

Answer 10

are polymodal, being both heat and mechanically sensitive (C-fiber nociceptors).

i. Silent nociceptors are heat responsive but mechanically insensitive and are not normally active.
ii. Also responsive to a variety of itch producing substances
iii. Approximately 10% of all C-fiber nerve endings respond to low intensity stimuli, such as stroking and may mediate pleasant touch.
iv. C-fibers release neuropeptides, substance P, and calcitonin gene-related peptide.

Question 11

Heat transduction

Answer 11

i. Human pain threshold to heat is around 43 degrees C
ii. Capsaicin and related vanilloid compounds produce burning pain by depolarizing specific subsets of C and alpha delta nociceptors.
iii. Transient receptor potential (TRP) ion channels (TRPV1) plays an important role in heat transduction

Question 12

cold transduction

Answer 12

i. TRPV1 and TRPM8 are expressed in different neuronal populations, consistent with the notion that hot and cold detection mechanisms are organized into anatomically and functionally distinct peripheral neurons. The receptor TRPA1 has also been suggested to detect cold, specifically within the noxious (<15 degrees C) range

Question 13

mechanical transduction

Answer 13

i. Somatosensory system detects diverse mechanical stimuli, ranging from a light brush of the skin to distention of viscera.
0. High threshold mechanoreceptors include C-fibers and slowly adapting alpha delta mechanoreceptor fibers, both of which terminate as free nerve endings in the skin.
1. Low threshold mechanoreceptors include alpha delta hair fibers that terminate on down hairs in the skin and detect light touch.
2. Alpha beta fibers that innervate Merkel cells, Pacinaian corpuscules, and hair follicles detect texture, vibration, and light pressure.
3. Very little is known about mechanosensation.

Question 14

chemical transduction

Answer 14

i. Detection of environmental irritants and endogenous factors such as inflammatory mediators, transient receptor potential channels have prominent roles, discovered from their response to plant-derived irritants, including capsaicin (TRPV1) methanol (TRPM8) and the pungent ingredients in mustard and garlic plants, isothiocyanates, and thiosulfinates (TRPA1)

Question 15

local modulation of transduction

Answer 15

i. Other molecules play a role in transduction by tuning or altering sensitivity of the transduction elements
0. Voltage gated sodium channels and voltage gated potassium channels.
ii. Primary afferents are different to other neurons in that proteins synthesized in the dorsal root ganglion or trigeminal ganglion can be distributed to both central (spinal cord) and peripheral terminals.
0. Neurogenic inflammation is the process whereby peripheral release of the neuropeptides, calcitonin gene-related peptide, and substance P induces vasodilation and extravasation of plasma proteins. At the same time, centrally released substances can alter the functioning of spinal cord tissues.

Question 16

transmission and projection

Answer 16

a. Type I and type II alpha delta fibers range in size from 1-5micrometers in diameter, are myelinated, and conduct impulses rapidly at a rate of 5-30 m/sec
b. The conduction rate is correlated with the initial sensation of pain – sharp, localized, and transient.
c. These afferents have small receptive fields and specific high-threshold ion channels that are activated by noxious thermal or mechanical input.
d. C-fibers are small, ranging in size from 0.25-1.5micrometer in diameter, are unmyelinated with conduction velocities of only 0.5-2m/sec, and have larger receptive fields compared with alpha delta fibers. These characteristics contribute to the poorly localized, burning, gnawing sensation of second pain, which persists after termination of the noxious stimulus.
e. If the depolarizing current initiated during transduction is of sufficient magnitude, voltage-gated ion channels are activated, further depolarizing the membrane and causing a burst of action potentials. Voltage gated sodium and potassium channels are critical to the generation of action potentials.
i. Several isoforms of sodium channels have been recognized for their specific role in nociception.
ii. Much of the analgesic effect of cold is considered to be due to inactivation of sodium channels.
iii. These voltage gated sodium channels are the targets of local anesthetics and are the focus of intense development of specific channel blockers.
0. The analgesic effects of serotonin and norepinephrine reuptake inhibitors may result in part from their ability to block sodium channels.

Question 17

Sensory nerve fibers enter the spinal cord primarily through the ____nerve root, but some fibers enter through the ____nerve root as well.

Answer 17

a. Sensory nerve fibers enter the spinal cord primarily through the dorsal nerve root, but some fibers enter through the ventral nerve root as well.

Question 18

thalamocortical system -

Answer 18

a. The sensory discriminiative asepcts of pain are produced in the lateral thalamocortical system, which consists of relay nuclei in the lateral thalamus and the primary and secondary somatosensory cortices.