The Somatosensory System: Pain

Question 1

What is pain?

Answer 1

Pain is a protective sense that protects you from damaging your tissues

Question 2

How is pain defined?

Answer 2

Pain is a response to a sensory input that has the potential to damage your tissue.

Question 3

What is involved in pain?

Answer 3

A combination of sensory (discriminative) and affective (emotional) components.

Question 4

What is an important note for future doctors regarding pain?

Answer 4

Pain is always subjective; never dismiss a patient’s report of pain.

Question 5

What is nociception?

Answer 5

The sensory component of pain alone.

Question 6

How is pain perceived?

Answer 6

A sensory stimulus that has the potential to cause damage activates free nerve endings (nociceptors) found all over the body.

Question 7

What types of stimuli can activate nociceptors?

Answer 7

Blunt force injury, cutting injury, high heat, extreme cold, inflammation, low pH.

Question 8

What are the two types of nociceptors?

Answer 8

Mechanical nociceptors and polymodal nociceptors.

Question 9

What do mechanical nociceptors respond to?

Answer 9

Strong shearing force in skin, causing sharp well-localized pain.

Question 10

What do polymodal nociceptors respond to?

Answer 10

Many stimuli including sharp blows, damaging heat, and chemicals released by damaged tissue, resulting in poorly localized dull burning pain.

Question 11

What are the common features of primary sensory neurons?

Answer 11

All primary afferent fibers are excitatory, their cell body is in the dorsal root ganglion, and they have a single axon that splits.

Question 12

What is the axon type of mechanical nociceptors?

Answer 12

Mechanical nociceptors are found on A-δ (delta) axon fibers.

Question 13

What is the axon type of polymodal nociceptors?

Answer 13

Polymodal nociceptors are found at the end of C axon fibers.

Question 14

How do A-δ and C fibers differ?

Answer 14

A-δ fibers are myelinated and have faster conduction.

Question 15

What is the result of different conduction rates in A-δ and C fibers?

Answer 15

They result in feeling two different types of pain from the same stimulus.

Question 16

What is first pain?

Answer 16

A sharp pain experienced quickly due to A-δ fibers.

Question 17

What is second pain?

Answer 17

A dull, burning pain experienced later due to C fibers.

Question 18

Where does a primary sensory neuron have its cell body?

Answer 18

In the dorsal root ganglion (outside CNS).

Question 19

Where does a primary sensory neuron synapse in the spinal cord?

Answer 19

With a second-order neuron in the substantia gelatinosa (lamina 2 of dorsal horn).

Question 20

Where do noxious fibers synapse in the dorsal horn?

Answer 20

C fibers synapse in lamina 2 and A-δ fibers synapse in lamina 1.

Question 21

Where do primary afferent neurons providing cutaneous innervation terminate?

Answer 21

At lamina I-II.

Question 22

Where do primary afferent neurons providing visceral innervation terminate?

Answer 22

In lamina I, V, X.

Question 23

What is the consequence of the difference between visceral and cutaneous innervation?

Answer 23

It is easier to localize pain from cutaneous sources than from visceral sources.

Question 24

What neurons make up the majority of the dorsal horn?

Answer 24

Local interneurons.

Question 25

What is the function of interneurons in the dorsal horn?

Answer 25

They modulate the activity of projection neurons.

Question 26

What must happen for a pain signal to reach the brain?

Answer 26

The pain signal must overcome inhibition by interneurons.

Question 27

How does a noxious stimulus overcome inhibitory drive of interneurons?

Answer 27

Nociceptor fibers synapse with interneurons to inhibit other inhibitory inputs.

Question 28

How can rubbing a painful area help relieve pain?

Answer 28

It activates non-noxious primary afferent fibers, increasing inhibition and closing the spinal gate.

Question 29

What is the clinical significance of gate control theory?

Answer 29

Increased non-noxious afferent input can provide analgesic effects.

Question 30

Which ascending tract carries pain information to the brain?

Answer 30

The spinothalamic tract (lateral part).

Question 31

Where does pain information end up in the brain?

Answer 31

In the cortex via tertiary neurons and in subcortical areas.

Question 32

When is pain actually perceived?

Answer 32

Pain is perceived once it reaches the brain.

Question 33

When is pain actually perceived?

Answer 33

You only perceive pain when subcortical areas of the brain are activated (pain is perceived at subcortical level, localised in cortical level).

Question 34

Where is the input localised in the brain?

Answer 34

You localise the input at the cortical level at the somatosensory cortex. This occurs to inputs that come from cutaneous afferents.

Question 35

How are the emotions associated with pain added?

Answer 35

You add the emotion associated with the pain through activation of the limbic system – a subcortical region of the brain.

Question 36

How can we see which parts of the brain are activated?

Answer 36

With Functional MRI you can see different parts of the brain that are activated. ## Footnote E.g. we know the cortex is involved in localisation and limbic area is big on emotion.

Question 37

How does the brain stem communicate with the spinal cord?

Answer 37

Brain stem nuclei (rich in opioids) send axons down to the dorsal horn of spinal cord via descending pathways.

Question 38

What are some neurotransmitters released in the dorsal horn of the spinal cord?

Answer 38

In the dorsal horn of the spinal cord we can get release of endogenous opioid: 5-HT (serotonin), noradrenaline, enkephalin from neurones that have cell bodies in the brain stem.

Question 39

What is this descending system involved in preventing pain called?

Answer 39

This is called the Intrinsic Analgesia System. ## Footnote E.g. battlefield injury when soldier stands on land mine and leg is blown off they are capable of getting up, picking up amputated leg and hopping off to hospital without feeling pain.

Question 40

Describe the rate of afferent input in normal physiological pain.

Answer 40

In normal physiological pain (non-facilitated) the sensation of pain is directly proportional to afferent input firing rate.

Question 41

How is chronic pain different from non-facilitated pain?

Answer 41

In Persistent/chronic pain states we see an increased sensitivity to pain.

Question 42

What are the two mechanisms underlying the increase in sensitivity to pain in chronic pain?

Answer 42

Process of peripheral and central sensitisation.

Question 43

What is this change in sensitivity known as?

Answer 43

This is known as neuroplasticity.

Question 44

Define hyperalgesia and allodynia.

Answer 44

HYPERALGESIA: enhanced painful response to a normally painful stimulus. ALLODYNIA: painful response to a normally non-painful stimulus.

Question 45

What are the mechanisms underlying heightened pain?

Answer 45

Process of peripheral and central sensitisation.

Question 46

What is the response to antidromic AP conduction?

Answer 46

In response to antidromic AP conduction you get the release of substances at the peripheral nerve ending called substance P & CGRP.

Question 47

What is the effect of the inflammatory soup on the nerves?

Answer 47

What they do is lower the threshold at which the nerve ending will fire the AP.

Question 48

What are the consequences of this decreased threshold?

Answer 48

Once they have been sensitised however, their thresholds are lowered and they will start firing AP that are at a much lower stimulus threshold.

Question 49

Which nociceptors show sensitisation in response to inflammatory soup?

Answer 49

It is the polymodal nociceptors (C fibres) that show the sensitisation in response to inflammatory soup.

Question 50

What is this sensitisation to inflammatory soup called?

Answer 50

This whole process is called neurogenic inflammation.

Question 51

What type of hyperalgesia does peripheral sensitisation result in?

Answer 51

Primary hyperalgesia: Due to damaged tissue we have a sensitised nerve ending which has got a lower threshold at which it fires AP.

Question 52

How does peripheral sensitisation result in allodynia?

Answer 52

In peripheral sensitisation we have more AP being fired and so more noxious information is going into the SC and you will be feeling more pain as a result.

Question 53

What causes secondary hyperalgesia?

Answer 53

Secondary hyperalgesia is thought to be due to changes happening in CNS.

Question 54

Describe the mechanism of central sensitisation.

Answer 54

Peripheral sensitisation promotes central sensitisation.

Question 55

What neurotransmitters can be found in pre-synaptic nerve?

Answer 55

In the primary afferent nerve ending you can see common things: Glutamate (excitatory NT), Substance P.

Question 56

What receptors are found on second order neurones membrane?

Answer 56

The second order neurone has a number of receptors: AMPA, NMDA, NK-1.

Question 57

What is the excitatory neurotransmitter normally released from primary afferent nerve?

Answer 57

Glutamate is released and it acts on AMPA and NMDA receptors.

Question 58

Describe the signalling between primary and secondary neurone in acute pain.

Answer 58

Normal signalling at a primary afferent synapse in the dorsal horn with a second order neurone – This is called signalling of acute pain (normal).