Aches and Pains Lab briefing 1 Flashcards
Define pain
Unpleasant sensory and emotional experience associated with actual or potential tissue damage
Describe the 2 types of nociceptive fibres conveying nociceptive pain
Afferents: A delta and C fibres
They have free nerve endings, located in skin and viscera of organs
what is nociceptive pain characterised by?
high threshold (need a highly painful stimulus to feel pain) and limited duration
what sensation do meissner’s corpuscles detect?
light touch
what sensation do meissner’s corpuscles detect?
light touch
what sensation do merkel’s discs detect?
mechanical deflection
what sensation do hair follicle afferents detect?
gentle brushing
what sensation do Pacinian corpuscles detect?
Gross pressure changes
Vibration
what sensation do Ruffini endings detect?
stretching of skin
define referred pain
referred pain is pain felt in a part of the body other than its actual source
explain the mechanism behind referred pain and give an example
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
what is chronic pain also called?
maladaptive pain
how is chronic pain induced?
it is either evoked by a stimulus that doesn’t usually induce pain or occurs spontaneously in the absence of a stimulus
what are the afferent fibres involved in chronic pain?
A beta, A delta and C fibres
what is allodynia?
pain which is elicited by stimuli that normally only evokes innocuous sensation (Paine evoked by stimulus that usually doesn’t cause pain)
what are the 3 aspects of a triple response to pain? describe them
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
what chemicals are released by the body in response to tissue damage and what is released by plasma and platelets?
K+ and Prostaglandins
Plasma: bradykinin
Platelets: serotonin
what is the axon reflex and describe its role in response to tissue damage
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
what is the function of Substance P?
plasma extravasation (leakage of fluid)
Oedema
Release of bradykinin
what is the function of CGRP?
dilatation of blood vessels
what is the first component to appear after mild tissue damage and what is this followed by?
white reaction
Followed by triple response
what test can be done to prove that components of the triple response depend of blood vessels?
using a sphygmomanometer to cut off the blood supply to the arm and scratching oneself will only produce a white reaction
how does local anaesthetic work to prevent the flare response from occurring?
it prevents the propagation of action potentials along the terminal branches of nociceptive afferents
what is ischaemic pain and what is it caused by?
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.
describe the mechanism of ischaemic pain
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
describe the 2 theories used to explain how a TENS machine modulates the perception of painful stimuli
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
how does the cold affect blood pressure and heart rate?
Cold causes blood vessels to constrict which reduces blood flow and oxygen to heart. This increases the BP and HR
Explain the Gate control theory
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.
