Physiology Of Pain Sensation

Question 1

Definition of pain

Answer 1

Unpleasant sensory and emotional experience associated with actual or potential tissue damage

Question 2

Pain receptors

Answer 2

Nociceptor

- Detect noxious stimuli that may indicate damage taking place

Question 3

Types of pain

Answer 3

Nociceptive
Neuropathic
Central (thalamus)

Question 4

Nociceptive pathway

Answer 4

Spinothalamic
Spinorericular (more alert)
Spinomesencephalic: regulation of pain transmission

Question 5

Primary Hyperalgesia

Answer 5

Damaged cells release mediators that cause increased sensitivity to pain:

• K+ - when cells are damaged, K+ leaks out causing hyperkalemia -> depolarization of nociceptive endings
• H+ - Increases sensitivity if a are becomes more acidic
• Bradykinin - receptors in sensory terminals produce Calcium signals
• Prostaglandins - Stimulate receptors in nerve terminal

Question 6

Secondary Hyperalgesia

Answer 6

Near to damaged cells

• Substance P - vasodilation
• Bradykinin - Vasodilation
• Histamine - released by activated mast cells -> vasodilation -> hypersensitivity of nerve endings

Question 7

Descending Analgesic Pathway function

Answer 7

Inhibits the spinothalamic tract neurons

Question 8

Part of descending analgesic pathway

Answer 8

• Raphe nucleus: serotonin production of the medulla

* Periaqueductal gray matter of midbrain

Question 9

Analgesia

Answer 9

Occurs via an endogenous opiate system

1. Afferent nociceptive axons release excitatory transmitters
   * Glutamate
   * Substance P
   - Opiates inhibit neural activity of these nociceptive pathways by inhibiting both pre- and postsynaptic afferent neuron terminals

Question 10

Opiates on presynaptic neuron

Answer 10

• Pain is induces when depolarized, axon terminal open VD-Ca channels
• Substance P or glutamate is released = pain
• Opiate receptor on the axon terminal -> Gi -> inhibits Calcium channel

Question 11

Opiates on postsynaptic neuron

Answer 11

Gi -> K - channel open -> hyperpolarization

Question 12

Spinothalamic tract pathway

Answer 12

Receptor: nociceptor
1. order neuron: Dorsal root ganglion (pseudounipolar)
2. order neuron: Posterior horn
- Lateral spinothalamic tract for pain and temperature
(Anterior is for crude touch)
3. order neuron: VPL in thalamus
4. order neuron: Primary somatosensory cortex (Broadman 3,1,2)