Nociception

Be able to describe the path of the Anterolateral System (for both the right and left sides of the body), including the fiber types, the locations of the decussation and locations of synapses.
Contrast the fiber types for pain: their function, fiber type, and how they are stimulated.
Contrast the anterior spinothalamic with the lateral spinothalamic. Include which fiber types they carry and their overall function

1. First order neuron

   • Ending in skin

   • Cell body in Dorsal root ganglion

   • Saltatory A delta or contiguous

   • Enter at spinal segment, through dorsal root, bifurcates in the dorsal horn

     • one becomes withdrawal reflex

     • one synapses with second order neuron in the dorsal horn on ipsilateral side

2. Second order neuron

   • Cell body starts at ipsilateral dorsal horn

   • Decussates through the anterior commissure

   • ascend to the thalamus

     • Lateral + anterior tract

   • Synapses in ventral posterior lateral nucleus of thalamus

3. third order neuron

   • Cell body in the Ventral posterior lateral nucleus

   • axon goes through posterior limb of the internal capsule

   • Terminates in primary somatosensory cortex

4. Difference between the anterior and lateral spinothalamic tracts

   • Anterior = older

     • C fibers: slow pain

     • ligand gated

   • Lateral = newer

     • some C fibers

     • A delta fibers

       • fast pain

       • mechanically gated

Define the effects of the spinomesencephalic and the spinoreticular tracts

• Spinoreticular: responsible for alertness

• Spinomesencephalic: stimulates descending endogenous opioid tracts

Given a lesion at any point along either of these tracts, predict what the patient will still be able to experience, and what they will no longer experience. Be able to accomplish the opposite: Given a case be able to identify the location of a lesion responsible for a given clinical presentation.

Describe the descending tracts for pain. Briefly, the origin, the function, the neurotransmitters involved, and how opioids affect the membrane potential of the neurons

• Periaqueductal grey matter to spinal chord.

  • presynaptic inhibition on first order nociceptor and postsynaptic inhibition on second order nociceptor (IPSP)

  • opens K+ channels

Describe the phenomenon of referred pain

• When visceral pain is perceived in a different location

  • first order cutaneous nociceptor converges with first order visceral nociceptors

  • must be sensitized first

Contrast hyperalgesia with allodynia

Due to inflammation

• Allodynia: Excessive pain caused by a non painful stimulus

• Hyperalgesia: excessive pain caused by a painful stimulus (over-reaction)

Inflammation causes upregulation of Na+ channels = depolarization

Describe diabetic neuralgia, trigeminal neuralgia and post herpetic neuralgia, and any treatment for these disorders

1. Diabetic Neuropathy

   • Nerve ischemia

2. Trigeminal neuralgia

   • Chronic, episodic, progressive, unilateral pain

   • due to vascular disease or MS

3. Post-herpatic neuralgia

   • Varicella Zoster latent in Dorsal root ganglion

   • migration of virus to the skin

   • rash on body

   • Herpes Zoster Ophthalmicus:

     • rash associated with nasociliary branch of V1

     • tip of nose - Hutchinson sign

     • risk of corneal involvement

   • Nerve pain after shingles

     • damage to first order nociceptor

   • Tx: Zosterex (capsaicin)

     • apply after lesions crust

Describe analgesics and their mechanisms of action

Treatment to decrease pain

• Anti-inflammatory drugs

• Anesthetics

Describe the effects of opiates on pain and how they can cause dependence. Include the effect of opiates on the pupil

Decrease pain and inhibits sympathetic nervous system

• Natural

  • Morphine + codeine

• Semisynthetic

  • Hydrocodone and oxycodone

• Synthetic

  • Methadone

  • Fentanyl

• Effects:

  • tolerance

  • dopaminergic reward system stimulation

Describe Narcan and its mechanism of action

Treatment for overdose

• higher affinity for opioid receptors than drug.