Pain Pathophysiology

Question 1

What is pain (with reference)?

Answer 1

An unpleasant sensory and emotional response to real, potential, or perceived tissue damage.

Bogduk, (2009)

Question 2

Explain the ascending pathway of pain from transduction to modulation.

Transduction, Conduction, Transmission, Perception, Modulation

Answer 2

1. Pain receptors (nociceptors) transduce a noxious stimulus strong enough to cross the pain theshold (mechanical, thermal, or chemical) into an electrical signal (action potential) .
2. This signal is conducted through the primary afferent pain fibers (A-delta/C) to the dorsal horn of the spinal cord where they synapes with inter-neurons and second order neurons.
3. The signal is transmitted to these decussating second-order neurons via the release of pro-nociceptive transmitters (such as substance p) and relayed through the spinothelamic and spinoraticular tracts.
4. The signal is sent to the relevent area in the somatosensory cortex where the sensation of pain is produced/perceived.
5. This processes can be modulated at different levels and pain perception is a result of the balance between facilitatory and inhibitory interactions.

Question 3

What are the different types of pain receptors and how do they differ in function?

Answer 3

A-delta Fibres
* Characterisitcs: Fast-conducting, small-diameter, thinly mylinated nerve fibres
* Function: Generally responsible for sharp, intense, and well localised sensations of pain - responsible for the “pain withdrawal reflex”

C Fibers
* Characteristics: Slow-conducting, smaller diameter, unmylinated nerve fibres:
* Function: Generally Responsible for diffuse aching sensations of pain.

Question 4

Explain Neuropathic Pain

Answer 4

Neuropathic pain is pain that is originating from damage or disease of the somatosensory system and can result from damage to anywhere along the neuraxis and can result in incorrect signals been sent to other pain centres:
* Central neuropathic pain is defined as ‘pain caused by a lesion or disease of the central (brain and spinal cord) somatosensory nervous system’.
* Peripheral neuropathic pain is defined as ‘pain caused by a lesion or disease of the peripheral somatosensory nervous system’.
* Neuropathic pain is subject to the same ascending pathway as nociceptive pain but in the abcesce on the intial nociceptive input and transduction.

Question 5

Define somatic refered pain, explain the associated mechanism, and how it presents (with reference).

Answer 5

Somatic reffered pain is pain that is perceived in regions innervated by nerves other than those that innervate the site of noxious stimulation and ocurs in the distinct abcense of any nerve root stimulation.

Mechanism
The leading proposed mechanism is convergence of nociceptive afferents on second-order neurons in the spinal cord that that share the same segmental innervation as the source.

Presentation
Dull, aching, gnawing, and difficult to localise pain; with an inconsistant and non-dermotomal pattern.

Bogduk (2009)

Question 6

Define radicular pain, explain the associated mechanism, and how it presents.

Answer 6

Pain evoked by ectopic discharges emanating from a dorsal root or its ganglion.

Mechanism
Compression and/or inflamation leads to ectopic discharges eminating from the doral root or its ganglion

Presentstion
An intense band-like localisation of lanciating, shocking, electic like pain.

Bogduk (2009)

Question 7

Define radiculopathy, explain the associated mechanism, and how it presents.

Answer 7

A neurological state in which conduction is blocked along a spinal nerve or its roots.

Mechanism
Compression and/or inflamation that leads to demyelination and axonal damage, causes reduced/impeded implulse conduction alond the nerve.

Presentation
Loss of function in reflexes, power, and/or sensation. (May or may not present with radicular pain).

(Bogduk, 2009)

Question 8

What is Allodynia

Answer 8

A nociceptive responce to a non-noxious stimulus

Question 9

What is hyperalgesia?

Answer 9

Increased sensitivity to noxious stimuli

Question 10

Define & Explain Peripheral Sensitisation

Answer 10

Increased responsiveness and reduced threshold of nociceptive neurons in the periphery to the stimulation of their receptive fields. Peripheral sensitisation is limited to the site of tissue injury and requires ongoing activity of the neural pathways to be sustained.

Mechanisms
* Chemicals, in the case of inflammation, can linger in the tissues and more easily attach to detectors causing them to stay open for longer. Signals are therefore sent more frequently.
* Mechanical detectors are now activated more easily during movement.
 The brain becomes more interested in the painful area and allows more detectors to be formed.
* New nerve endings grow into the surrounding tissues. This is why pain can be felt in areas where there is no damage.
* Over a longer period of time nerves which do not carry ’danger signals’ start doing so and sensations of touch are now experienced as pain.

Question 11

Define & Explain Central Sensitisation

Answer 11

An abnormal state of responsiveness or increased gain of the central nociceptive somatosensory system.

Mechanisms
Enhancement in the functional status of neurons and circuits in nociceptive pathways throughout the neuraxis caused by
* increases in membrane excitability
* Incrased synaptic efficacy
* or/and reduced inhibition