Pain Peripheral Mechanisms and Central Pathways

Question 1

Describe how a pain signal travels to the brain to be processed.

Answer 1

1. Stimulus activates receptor
2. AP elicited in Dorsal Route Ganglion nerves
3. AP enters spinal cord and synapses onto ascending nerve in dorsal route horn
4. Nerve crosses over in spinal cord and ascends to brain (usually via thalamus)
5. Signal travels to sensory cortex where it is processed and acted upon

Question 2

What are pain receptors called?

Answer 2

Nociceptors

Question 3

What kind of sense is pain and what does this mean?

Answer 3

Somatic sense - not dependent on a specific organ

Question 4

Where are nociceptors found?

Answer 4

Widespread throughout body

- not so many in brain

Question 5

What kind of stimuli can cause slow, dull pain?

Answer 5

chemical, mechanical, thermal

Question 6

What kind of stimuli can cause fast, sharp pain?

Answer 6

Mechanical, thermal

Question 7

Which chemicals can activate nociceptors?

Answer 7

Histamine, bradykinin, 5-HT (serotonin), prostaglandin, K+, H+ (pH)

Question 8

What is the cellular response when nociceptors are activated?

Answer 8

Ca2+ influx
Sets up AP
Na+ channel-dependent conduction in nerve to spinal cord + brain

Question 9

What are the different types of nociceptors?

Answer 9

Myelinated neurons - A Delta fibres
Unmyelinated neurons - C fibres
Gracile fasciculus - A delta (synapse and cross over in medulla_
Facial nerves (trigeminal) - C fibres (immediately cross over, dental pain)

Question 10

Compare myelinated neurons and unmyelinated neurons in terms of pain.

Answer 10

Myelinated - rapid conduction velocity, sharp, immediate, highly localised, superficial tissue
Unmyelinated - slow, dull, delay (+linger), hard to locate, superficial and deep tissue

Question 11

Describe the spino-reticular tract.

Answer 11

• Ascending pathway in white matter of spinal cord
• Responsible for transmission of pain, temperature and crude touch to somatosensory region of thalamus
• Increases our level of alertness
• First, second and third order neurons
• First - dorsal route ganglion
• Second - send afferents to thalamus
• Third - send afferents to sensory cortex
• Glutamate release in synapses

Question 12

Why would it be advantageous to inform the hypothalamus of injury when a person is in a potentially harmful situation?

Answer 12

Activate fight or flight response to get out of a harmful situation

Question 13

How does the hypothalamus contribute to nociception?

Answer 13

• Part of limbic system (part of brain involved in behavioural and emotional responses)
• Influences outlook on and response to pain (e.g. crying)
• Amplify pain signal, behaviours for survival

Question 14

What are some of the autonomic responses to pain?

Answer 14

Sympathetic - fear, pupillary dilation, pallor (blood diverted away from skin), sweating
Parasympathetic - bradycardia, syncope, hypotension, urination, nausea (less useful)

Question 15

What is referred pain?

Answer 15

When pain from a visceral organ is felt in a superficial area of the body

Question 16

What is the purpose of pain?

Answer 16

It is necessary and protective

- when is it absent, can lead to ulcers forming after surgery, amputations etc.

Question 17

Compare acute vs chronic pain.

Answer 17

Acute - physiological, nociceptive and inflammatory, sudden onset in response to discrete event, recedes during healing

Chronic - pathological, neuropathic pain, persists long after recovery (>3 months), often difficult to tie to specific event, often unresponsive to analgesics

Question 18

Describe the characteristics of neuropathic pain.

Answer 18

• Occurs as a result of lesion or disease in P/CNS
  E.g. diabetic neuropathy, MS, fibromyalgia, spinal cord injury
• Can present as burning or tickling or dull aches
• May be associated with allodynia or hyperalgesia
• Due to sensitization of peripheral and central neurones

Question 19

What is allodynia?

Answer 19

Sensation of pain in response to something not normally painful in nature e.g. light touch near a wound

Question 20

What is hyperalgesia?

Answer 20

Increased intensity of pain sensation for a nociceptive stimulus (small stimulus, large response)

Question 21

Describe peripheral sensitization of neurones that contribute to neuropathic pain.

Answer 21

• Increased sensitivity of C and A delta fibres due to prostaglandin release
• Increased voltage-dependent Na+ channel expression - increased likelihood that threshold potential is reached

Question 22

Describe central sensitization that contributes to neuropathic pain.

Answer 22

• increased glutamate release due to peripheral sensitization
• increased expression of glutamate receptors
  (both cause hyperalgesia)
• sprouting of A beta (mechanoreceptor) fibres to make new connections with ascending neurons
  (cause allodynia)

Question 23

What is phantom limb pain?

Answer 23

• Type of neuropathic pain due to reorganisation of cortex and sensitization
• Area in sensory cortex no longer receiving signals
• Nerves migrate into adjacent areas of brain
• Nerves respond to signals coming from other parts of body

Question 24

How is the psychological aspect of pain treated?

Answer 24

• Chronic pain and depression form a vicious cycle

- Talk therapy, peer support, inpatient or outpatient pain programs, stress-reduction skills