Lecture 6- Pain pathways

Question 1

What are the steps involved in nervous sensation?

Answer 1

1. Stimulus activates temperature, pain, touch or pressure receptor
2. AP elicited in dorsal route ganglion nerve
3. AP enters spinal cord and synapses onto ascending nerve in DR horn
4. Nerves travel across spinal cord, crossing over and ascend to brain (usually via thalamus)
5. Signal > sensory cortex, where it is processed and acted upon

Question 2

How is pain sensed?

Answer 2

Nociceptors- free nerve endings founnd widespread throughout body e.g skin, arteries, bone, not abundant in brain
Respond to damaging or potentially damaging stimuli- inflammation

Question 3

Nociceptor stimuli

Answer 3

Chemical- Slow, dull

Mechanical and thermal- Slow/dull or fast/ sharp pain

Question 4

Features of myelinated neurone (A delta) conduction

Answer 4

Rapid velocity- 6 to 30 m/s
Sharp
Seems immediate
Highly localised
Superficial tissue

Question 5

Features of unmyelinated neurones (C)

Answer 5

Slow conduction velocity ( 0.5 to 2 m/s)
Dull
Delay (lingers)
Hard to locate
Superificial and deep tissue

Question 6

Less important pathways to be aware of

Answer 6

Gracile fasciculus- A delta fibres (stomach ache, kidney stones)- visceral pain signals, no crossing over
Facial nerves (trigeminal)- C fibres, through the pons to the medulla (2nd order neurones)

Question 7

Pathway of initial nociception

Answer 7

K+ released from damaged cells/ bradykinin and histamine (inflamm) from mast cells and basophils activate nociceptors when IgE antigens attach.
- can occur at initial appearance of foreign antigen (pain can preempt tissue damage, warning)
Stimulates production of prostaglandins, increase sensitivitity of nocicieptors with actually activating.

Question 8

How is serotonin involved in nociception?

Answer 8

Activates nociceptors- consider it is released from activated platelets when they aggregate at site of BV damage.
Can be activated due to chemical, temperature, pressure etc.

Question 9

Pathway of nociceptor activated in torso/limbs

Answer 9

C or Ad (1st order) carry signals to DH of spinal cord, synapse onto 2nd order neurones (Ad or dorsal root horn)
crossover from one side of the spine and ascend in either ST or SR tract
Final destination is somatosensory cortex on other side of the brain

Question 10

Difference between spinothalmic and spinoreticular tracts

Answer 10

Spinothalmic tract- ascending nerves synapse in thalamus
Spinoreticular tract- synapse in reticular formation in brain stem
Branches of SR 3rd order nerves innvervate HT and limbic system

Question 11

What is the difference in what is released by C fibres and Ad fibres?

Answer 11

C fibres- Gluatamate and substance P

Ad- only glutamate

Question 12

Autonomic response to pain

Answer 12

Dilated pupils, pallor, urination, sweating and nausea

CV- bradycardia, hypotension and syncope

Question 13

Neuropathic pain

Answer 13

Chronic pain, results as lesion/disease of P/CNS (e.g diabetic neuropathy, MS, fibromyalgia or spinal cord injury)
May have burning/ tingling quality, and may be associated with allodynia (pain with something not usually painful) or hyperalgesia ( inc intensity to pain)

Question 14

Cause of neuropathic pain

Answer 14

Sensititsation of peripheral/central neurones involved in nociception. Peripheral C/Ad fibres normally only active when stimulated- after insult can show spontaneous activity/respond to weaker stimuli
this is linked to increased expression of Na+ channels in peripheral nerves