CHAPTER 9, STRESS-INDUCED ANALGESIA

I. Why have pain, rather than a neon light on our forehead? (Motivates us as well as informs us)

II. The basics of pain perception

A. Cellular messengers spill out and trigger pain receptors.

B. Some receptors: only pain (slow, unmyelinated) ; others: pain and touch (faster, myelinated).

C. Information sent to dorsal horn of spinal cord: Substance P is the transmitter.

1. Reflex is initiated here. Faster than if info had to go to cortex first.

2. Some neurons ascend on opposite side of spinal cord to brain.

D. Information is sent to thalamus and several areas of cortex.

1. Somatosensory cortex in parietal lobe: location, type of pain

2. Cingulate cortex: emotional content

E. Hypothalamus and other areas à autonomic nervous system and also CRF

III. Modulation of pain perception (see diagram in Sapolski, p. 164).

A. Melzak & Wall

B. The fast neuron excites the Y interneuron, which in turn inhibits the X neuron that carries the pain information in the spinal cord. The slow pain neuron actually inhibits the Y interneuron.

C. Note error on p. 165: Sapolski says that a diabetic who loses the fast pain fibers loses the ability to shut down the Y interneuron. However, you don’t want to shut down the Y interneuron—you want to excite it!

D. Patients with views of trees requested less pain medication after gall bladder surgery. (Also, sense of control)

E. Descending axons from the brain to the spinal cord, influence pain sensitivity of neuron X.

III. Stress-induced analgesia

A. Henry Beecher (WWII): 80% of civilians and 33% of soldiers requested morphine for similar injuries.

B. Exercise: after 30 min: pain melts away and you feel euphoria.

C. Animals: hot plate test: swim or social threat stress increases time to withdraw foot.

D. Endorphins, enkephalins, dynorphins (opioids vs opiates)

1. Acupuncture (humans and animals)

2. Naloxone

3. Roger Guillemin: beta-endorphin from pituitary

4. Relatively brief effect: depletion of transmitters