Pain : Peripheral Mechanisms & Central Pathways

Question 1

Explain the process of sensation

Answer 1

1- Stimulus activates receptor in skin
2- action potential is elected in the dorsal route ganglion nerves
3- AP enters the spinal cord and synapses onto ascending nerve in Dorsal route horn
4- nerves travel across spinal cord, cross over and ascend into brain usually via thalamus
5- signal travels to sensory cortex and is processed

Question 2

Where is the sensory cortex located

Answer 2

Outer area of the cerebrum and closely aligned to motor cortex

Question 3

Explain what are somatic senses ( give examples )

Answer 3

Senses that aren’t dependent on a specialized organ. For ex: thermoception, magnetoception, proprioception, nociception, equilibrioception.

Question 4

Explain what are interoceptive senses

Answer 4

Senses perceived from internal organs.

Question 5

What are the two components of pain

Answer 5

1- nociception: unconscious detection of harmful stimuli using sensory receptors
2- emotional and subjective response to to the nociception

Question 6

What is the Nociceptor

Answer 6

The receptor type associated with pain. Has a simple free nerve ending.

Question 7

Where are nociceptors located

Answer 7

Located all throughout the skin, arterial walls, joint surfaces. Sparsely distributed throughout deeper body tissue.

Question 8

Where are the nociceptors in the Brian

Answer 8

in the walls of the blood vessels and the meninges

Question 9

What fibres are associated with nociceptors

Answer 9

1- A delta: myelinated nerve fibres that have rapid conduction velocity
2- C : unmyelinated fibres with slow conduction

Question 10

Activity in the A delta fibres results in which sensations

Answer 10

Sharp, immediate pain that is highly localized.

Question 11

Activity in the C fibres results in which sensations

Answer 11

Dull pain that is hard to locate, lingers and occurs after slow delay. ( SLOW pain )

Question 12

What can evoke A delta fibre and C fibre pain

Answer 12

A : mechanical or thermal stimuli

C: mechanical, thermal or chemical stimuli

Question 13

Where is Fast pain not felt

Answer 13

In most deep tissue

Question 14

What are the chemical mediators of pain

Answer 14

1- Potassium and Histamine bradykinin are released from damaged cells to activate nociceptors

Question 15

What is histamine bradykinin responsible for

Answer 15

characteristic changes occurring during inflammation

Question 16

Explain the process of chemical mediator release as nociceptor activators ( 5 different chemicals )

Answer 16

1- IgE coated antigens attach to the surface of mast cells and basophils
2- K+ and histamine bradykinin are released from the mast cells and basophils
3- Tissue damage stimulates prostaglandins production = increases sensitivity. of nociceptors without activating them
4- Excess H+ during high intensity exercise can elicit pain
5- 5-HT ( serotonin ) is released from activated platelets when they aggregate at a site of blood vessel damage. Activate nociceptors

Question 17

What Can activate nociceptors other than chemicals

Answer 17

1- extreme temperature

2- strong pressure

Question 18

What channels are involved with pain responses and how

Answer 18

Transient Receptor Potential ( ion )channels
1- TRPV1 : sensitive to high temp, H+ and mechanical stimuli and post burn allodynia
2- TRPM8 : methanol and cold receptor
3- TRPAA1: irritant and cold receptor

Question 19

What is Allodynia

Answer 19

A type of pain where a person is extremely sensitive to touch. pain in response to something not normally painful. Ex: post burn allodynia

Question 20

What is Gracile fasiculus and how does it work

Answer 20

A tracts that carries visceral pain signals ( kidney stones, stomach ache ).
1- Tract carries A delta fibres up spinal cord without crossing over and into the gracile nucleus in the medulla oblongata.
2- Crosses over before traveling to thalamus and cortex

Question 21

How do the facial pain signals reach the brain

Answer 21

1- Travel primarily via the trigeminal nerve , through the pons and to the medulla.
2- second order neurones, usually C fibres bring the signal through the thalamus and to the cortex ,

Question 22

Explain what happens when a nociceptor in the torso or limbs is activated

Answer 22

1- first order pain fibres carry the signal from affected tissue to dorsal horn of spinal cord
2- single synapses onto second order neurons that crossover and ascend into spinothalamic or spin-reticular tracts
3- If spinothalamic tract: nerve synapses in thalamus and then to somato-sensory cortex
4- If in spin-reticular tract: nerve synapse in reticular formation in the Brian stem and then to somato-sensory cortex

Question 23

Branches of the spin-reticular third order nerves innervate which structures

Answer 23

The hypothalamus and the limbic system

Question 24

What is released by C fibres to pass on signal

Answer 24

Glutamate and substance P

Question 25

What is released by A delta fibres to pass on signal

Answer 25

Glutamate

Question 26

List some autonomic responses to pain

Answer 26

1- Pupil dilation 
2- urination 
3- sweating 
4- naseau 
5- pallor 
6- bradycardia 
7- hypotension 
8- syncope

Question 27

What is referred pain

Answer 27

When pain from organ is referred to superficial areas of the skin

Question 28

Why might visceral pain be referred to superficial pain

Answer 28

Visceral pain fibres synapse in the spinal cord at the same point as pain fibres from the skin. As there are more receptors in the skin ht pain is interpreted to come from the skin rather than visceral organ since it is more likely to come form skin

Question 29

What is Congenital insensitivity to pain

Answer 29

Rare genetic condition in which there is an abnormally low expression of nociceptors.

Question 30

What is physiological/acute pain

Answer 30

Nociceptive or inflammatory pain that has a sudden onset in response to an event, doesn’t last long and lessens during the healing process.

Question 31

What is Pathological/chronic pain

Answer 31

neuropathic pain that continues long after recovery ( 3 months ) , difficult to tie to a specific event.

Question 32

Which pain is unresponsive to analgesics

Answer 32

Pathological/chronic pain ( neuropathic )

Question 33

Difference between Nociceptive pain and inflammatory pain

Answer 33

N: due to abnormal somatic structures. Stimulus dependent pain by high intensity noxious stimuli
I : due to tissue injury with inflammation. stimulus dependent by high or low intensity stimuli ( noxious or not )

Question 34

How does neuropathic pain occur and what happens

Answer 34

As a result of lesion or disease in the PNS or CNS.
1- Causes sensitization of Peripheral and central neurons involved in nociception.
2- Increase of nociceptive signals from C and A fibres causes secondary changes at spinal cord as frequent release of glutamate form 1st order fibres overwhelms Dorsal horn neurons and they increase expression of associated receptor ( NMDA)

Question 35

What is Hyperalgesia

Answer 35

increases intensity of pain sensation for a given nociceptive stimulus

Question 36

What happens to the A and C fibres with neuropathic pain

Answer 36

They can show spontaneous activity or be activated by much weaker stimuli

Question 37

What is linked to Peripheral fibres responding to much weaker stimuli

Answer 37

Increased expression of Na+ channels in peripheral nerves , channels become sensitive sites

Question 38

Explain the NMDA receptor

Answer 38

Ionotropic receptor permeable to sodium and calcium. When there is an increased expression of Na and CA the threshold potential of the DHN is easier to be reaches and weaker stimulus can result in a response ( hyperalgesia ).
Expression of NMDA increases due to increase of glutamate release from the increase of A and C fibre nociceptive signals

Question 39

What is PLP

Answer 39

Phantom limb pain , a type of neuropathic pain experienced by amputees

Question 40

Explain what happens in phantom limb pain

Answer 40

there is rearrangement of remaining cortical circuits so that neurons from other areas terminate where the amputed part would, but brain still thinks signals come form the amputated area. So when other areas that now terminate in the cortex instead of the amputated part are touched the brain believes it comes from the amputated limb.

Question 41

What can influence pain perception in a person

Answer 41

Psychological factors