26 Pain and touch

Question 1

Why do people with congenital neuralgia tend to die early (around 22 years old)?

Answer 1

Cardiac failure caused by the strain of multiple blood infections. If you don’t know you’re hurt, you don’t change behaviour to allow infection to heal.

Question 2

How did Hardy et al. (1952) show relationship between pain and tissue damage?

Answer 2

The experiment showed that level of pain was not necessarily related to level of tissue damage, but to potential for tissue damage if noxious stimulus is allowed to proceed.

Heated probe to different temperatures - 45 C and 49 C.

At 45 C probe would cause damage in several hours.

At 49 C probe would cause damage in several minutes.

Skin of individuals was heated to these temperatures for only a few seconds. 49 C resulted in more pain than 45 C although neither caused tissue damage.

Pain can thus be correlated with potential for tissue damage, as well as actual tissue damage.

Question 3

What are the sensory-informative functions of pain?

Answer 3

• Tells you where injury occurred

- How serious the injury is

Question 4

What are the behavioural-motivational functions of pain?

Answer 4

• Immediate response is aggression and fear

- Acute response is recuperative behaviours

Question 5

What are the cognitive functions of pain?

Answer 5

• Our cognitive powers are pulled towards appraising source and meaning of pain
• Inform behavioural responses
• Learn from experience, avoid similar situations

Question 6

Why do chili peppers seem hot?

Answer 6

Because they contain capsaicin, which stimulates heat receptors

Question 7

What are the primary afferent nerves involved in touch?

Answer 7

The Aβ myelinated fibre controls fine touch (reaching in bag and naming object by feeling with fingers).

The Aδ myelinated fibre causes a wide dynamic range signals – from touch (OK) to heavy touch (painful).

The NON-myelinated C-fibres carry protopathic pain.

Question 8

What accounts for the delay between immediate pain and slow pain?

Answer 8

Difference in conduction speeds between myelinated Aδ-fibres and non-myelinated C-fibres

Question 9

What is the difference in organisation of white/grey matter between spine and brain?

Answer 9

Brain has grey matter on the outside, white matter on the inside.

Spine has white (axons) on outside, grey on inside (where nerves come in from the body).

Question 10

What are the four levels of the spinal cord from top to bottom

Answer 10

Cervical
Thoracic
Lumbar
Sacral

Question 11

Where do touch fibres (Aβ, Aδ, C) connect to the spinal column?

Answer 11

At the dorsal horn, where they synapse with ascending neurons which go up to the brain.

Question 12

What is the spino-thalamic tract?

Answer 12

The pathway from the spinal cord to the thalamus to the somatosensory cortex

Question 13

What is the formal definition of pain?

Answer 13

An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage

Question 14

What areas of the brain does the thalamus project to when pain is experienced?

Answer 14

A variety of areas, in particular…

The medulla, responsible for alertness - which is why pain wakes you up. Rugby players slap each other before games.

Parabrachial nucleus and amygdala - involved in stress response - affective areas

Hypothalamus, important for autonomic, sympathetic responses

Insular cortex - feelings of disgust

Question 15

What is hyperalgesia?

Answer 15

Hyperalgesia is an exaggeratedresponse to noxious stimuli, caused by sensitization of nociceptors.

It can occur after an injury. Hit thumb with hammer, for days after that thumb is more sensitive to stimulation. Repeated stimulation sensitizes nociceptors - wind-up of neural activity.

Question 16

How does LTP influence chronic pain?

Answer 16

When constant information of pain coming through, neurons transmitting this information wire together through LTP - activity dependent plasticity –and make it more likely this signal will be transmitted.

Question 17

What is secondary hyperalgesia?

Answer 17

NMDA-­receptor mediated strengthening of synapses to pain transmission neurons

When neurons of CNS become more sensitive to pain information through LTP.

Question 18

What will blocking NMDA receptors at time of injury do later?

Answer 18

Block neuroplasticity effects related to pain, such as phenotypic changes to neurons

Question 19

What longer-term neuroplasticity effects can occur if pain persists?

Answer 19

Changes in phenotype of neurons - start to express NTs and receptors that wouldn’t normally be expressed.

Silent neurons start making new pain receptors and responding to pain.

Aβ-fibres (touch neurons) start releasing NTs in the spinal cord that are normally only released by C-fibres (pain neurons).

Question 20

What changes in dorsal horn occur with chronic pain?

Answer 20

Touch fibres in dorsal horn start to wire themselves to the pain fibres and vice versa. Touch becomes associated with pain, and pain with touch - system loses integrity.

Question 21

How can changes in neural wiring in spinal cord cause referred pain?

Answer 21

Signals get confused in spinal cord. Signals from kidney start to talk to neurons usually associated with skin.

Question 22

What is Allodynia?

Answer 22

Touch becomes painful - shirt on back hurts.

Question 23

What is Paraesthesia?

Answer 23

Pins and needles - often severe and constant

Question 24

What morphological changes/rewiring are associated with secondary hyperalgesia?

Answer 24

– Aβ (touch) fibres start to connect to ascending pain transmission neurons (touch = pain)

– Spreading of pain fibres to more ascending pain transmission neurons (wider painful area)

– Referred pain

Question 25

How do glial cells enable secondary hyperalgesia?

Answer 25

They stop regulating glutamate at the synapses, and so allow spreading of activation from one synapse to another.

Question 26

What affective disruptions occur in secondary hyperalgesia?

Answer 26

Activation increases in anterior cingulate cortex, responsible for affective aspect of pain, and people become hypersensitive to emotion of pain.

Activation increases in parabrachial nucleus and amygdala - people become hypervigilant and stressed.

Question 27

What causes phantom limb syndrome?

Answer 27

Activation of adjacent areas in somatosensory cortex. People who pucker lips feel phantom sensation in fingers - adjacent areas. Similarly, genitals and toes.

Question 28

How did Ramachandran use a mirror to treat phantom limb pain?

Answer 28

Subjects with a missing arm would position mirror so it appeared they had two arms. Visual percept of two healthy arms appeared to override pain response.

Question 29

What happens when blind people read braille?

Answer 29

Stimulates visual cortex (if person lost sight at young age)

Question 30

Describe the endogenous opioid system

Answer 30

Three opioidreceptors: mu, delta, kappa

Three important opioid peptides: endorphin, enkephalin,dynorphin

Question 31

How can the endogenous opioid system reduce pain?

Answer 31

Interneurons release endogenous opiods that can regulate the transmission of pain at level of spinal cord (induced by neurons from above, or locally)

Also see this at thalamus, medulla and cortices.

Question 32

What is Gate Theory of Pain?

Answer 32

Touch afferents (Aδ fibres) activate enkephalin releasing interneurons in spinal cord, which inhibit both pain (C fibres) and touch. This is why when you rub your thumb after hitting it with a hammer the pain goes away.

Question 33

What is the debilitating effect of chronic pain?

Answer 33

Main reason is that people catastrophise, which causes a fear and avoidance loop. Pain, muscles become tighter, more pain.