Aches and Pains Lab briefing 1

Question 1

Define pain

Answer 1

Unpleasant sensory and emotional experience associated with actual or potential tissue damage

Question 2

Describe the 2 types of nociceptive fibres conveying nociceptive pain

Answer 2

Afferents: A delta and C fibres

They have free nerve endings, located in skin and viscera of organs

Question 3

what is nociceptive pain characterised by?

Answer 3

high threshold (need a highly painful stimulus to feel pain) and limited duration

Question 4

what sensation do meissner’s corpuscles detect?

Answer 4

light touch

Question 5

what sensation do meissner’s corpuscles detect?

Answer 5

light touch

Question 6

what sensation do merkel’s discs detect?

Answer 6

mechanical deflection

Question 7

what sensation do hair follicle afferents detect?

Answer 7

gentle brushing

Question 8

what sensation do Pacinian corpuscles detect?

Answer 8

Gross pressure changes

Vibration

Question 9

what sensation do Ruffini endings detect?

Answer 9

stretching of skin

Question 10

define referred pain

Answer 10

referred pain is pain felt in a part of the body other than its actual source

Question 11

explain the mechanism behind referred pain and give an example

Answer 11

Viscera afferent fibre has a collateral side branch that synapses with and activates a secondary sensory SKIN neuron. The brain thinks the signal has come from the skin instead of visceral organs = referred pain.

E.g. heart pain is detected as referred pain in the chest and left shoulder/arm

Question 12

what is chronic pain also called?

Answer 12

maladaptive pain

Question 13

how is chronic pain induced?

Answer 13

it is either evoked by a stimulus that doesn’t usually induce pain or occurs spontaneously in the absence of a stimulus

Question 14

what are the afferent fibres involved in chronic pain?

Answer 14

A beta, A delta and C fibres

Question 15

what is allodynia?

Answer 15

pain which is elicited by stimuli that normally only evokes innocuous sensation (Paine evoked by stimulus that usually doesn’t cause pain)

Question 16

what are the 3 aspects of a triple response to pain? describe them

Answer 16

Red reaction - a red line immediately at site of mild injury due to vasodilation, mediated by histamine
Wheal (swelling) - slight oedema at site, mediated by histamine
Flare - surrounding, ill-defined erythema (redness of skin) due to axon reflex

Question 17

what chemicals are released by the body in response to tissue damage and what is released by plasma and platelets?

Answer 17

K+ and Prostaglandins
Plasma: bradykinin
Platelets: serotonin

Question 18

what is the axon reflex and describe its role in response to tissue damage

Answer 18

Axon reflex aka flare response: response stimulated by peripheral nerves of the body that travels away from the nerve cell body and branches to stimulate target organs.

When the axon potential propagates along the nerve cell axon to the cell body, on its way the AP travels down other branches of the same neuron and when it gets to nociceptors on these branches, they release substance P and CGRP.

This causes mast cells to release histamine and causes vasodilation

Question 19

what is the function of Substance P?

Answer 19

plasma extravasation (leakage of fluid)
Oedema
Release of bradykinin

Question 20

what is the function of CGRP?

Answer 20

dilatation of blood vessels

Question 21

what is the first component to appear after mild tissue damage and what is this followed by?

Answer 21

white reaction

Followed by triple response

Question 22

what test can be done to prove that components of the triple response depend of blood vessels?

Answer 22

using a sphygmomanometer to cut off the blood supply to the arm and scratching oneself will only produce a white reaction

Question 23

how does local anaesthetic work to prevent the flare response from occurring?

Answer 23

it prevents the propagation of action potentials along the terminal branches of nociceptive afferents

Question 24

what is ischaemic pain and what is it caused by?

Answer 24

pain associated with lack of adequate blood supply to active tissues.
Caused by compromised blood supply due to atheroma laid down in the intimate of arteries.

Question 25

describe the mechanism of ischaemic pain

Answer 25

lack of blood supply to muscles means oxygen cannot be used to contract and there is a buildup of metabolites e.g. lactic acid which causes pain

Question 26

describe the 2 theories used to explain how a TENS machine modulates the perception of painful stimuli

Answer 26

1) It stimulates the large diameter axons of other sensory neurons (nociceptors have small diameter axons) which creates a “jamming” effect in the dorsal horn of spinal cord. This reduces the perception of pain because of interference with transmission in pain pathways
2) stimulation of the receptors releases endorphins in the brainstem, which activates descending pathways and suppresses transmission in pain pathways

Question 27

how does the cold affect blood pressure and heart rate?

Answer 27

Cold causes blood vessels to constrict which reduces blood flow and oxygen to heart. This increases the BP and HR

Question 28

Explain the Gate control theory

Answer 28

APs propagating along unmyelinated axons e.g. C fibres excite projection neurons and inhibit inhibits interneurons that usually inhibit the projection neurons. Activity in C fibres therefore successfully excites these projection neurons and is perceived as pain.

However, APs arriving simultaneously along large diameter axons e.g. A beta/delta fibres excite the inhibitory interneurons and inhibits the C fibres which reduces the perception of pain.