Physiology of pain

Question 1

What is a nociceptor?

Answer 1

a sensory receptor that sends signals that cause the perception of pain in response to a potentially damaging stimulus

Question 2

What is pain?

Answer 2

unpleasant sensory + emotional experience associated with, or resembling that associated with, actual or potential tissue damage
- encourages rest + recovery

Question 3

What are the 3 types of nociceptors?

Answer 3

Thermal - activated by noxious heat or cold
Mechanical - respond to excess pressure, mechanical deformation + incisions to skin
Chemical - respond to spices (eg capsaicin), other chemicals, endogenous ligands + fatty acid amines (arise from damage to internal tissue)

Question 4

What are the 2 types of axon of nociceptors?

Answer 4

A-delta fibre axons = myelinated - fast action potential - initial extremely sharp pain (prick sensation)
C fibre axons = unmyelinated - slower action potential - prolonged + less intense pain (dull + poorly defined)

Question 5

What are the types of nociceptive pain?

Answer 5

Visceral = diffuse, difficult to locate pain - often referred to distant structure - sensitive to stretch, ischaemia + inflammation - described as sickening + dull

Deep somatic = initiated by stimulation of nociceptors in ligaments, tendons, bones, blood vessels, fasciae and muscles - dull, aching + poorly localised eg broken bones

Superficial = initiated by the activation of nociceptors in the skin or other superficial tissue - sharp, well-defined, clearly located eg minor wounds

Question 6

What is referred pain?

Answer 6

Pain felt in a location away from the site of the painful stimulus

Question 7

What is transduction?

Answer 7

generation of an action potential
(tissue damage releases chemical mediators which activate primary afferent nociceptors, resulting in an action potential being produced)

Question 8

Describe transmission

Answer 8

action potential carried from nociceptor to spinal cord (superficial dorsal horn)
(action potential - dendrites of nerve terminal in spinal cord dorsal horn - electrical signal triggers calcium ion entry + neurotransmitter release into dorsal horn synapse)

Question 9

Describe the sensation and transmission of pain

Answer 9

initial pain sensed by nociceptors (free nerve endings in skin, muscle + other tissues)
pain transmitted by primary sensory neurons to dorsal horn of spinal cord (A delta = fast + acute, C = slow + dull)
in dorsal horn, primary sensory neuron will synapse with 2nd order neuron of spinothalamic tract
2nd order neuron decussates + passes up to the thalamus
In the thalamus, 2nd order neurons synapse with 3rd order neurons leading to sensory cortex + limbic system