The Physiology of Pain

Pain Receptors

bare sensory nerve endingsinnervate all organs and tissues (except the brain)

respond to all types of noxious stimuli

Damaged cells release Bradykinin (the most potent pain producing chemical/enzyme known) which binds to pain receptors

**** this does not always lead to the perception of pain ****

Neural Pathways in Pain

impulses travel to the central nervous system through two different fibers

1.Neospinothalamic (lateral) Tract

A- delta fibers transmit impulses quickly and end in the motor and sensory areas of the cortex.

Believed to mediate localized, sharp, pricking, brief pain

mediates the sensory-discriminative dimension

via the cerebral cortex mediates the cognitive-evaluative dimension

2. Paleospinothalamic (medial) Tract

C – fibers transmit impulses more slowly and end in the lower regions of the forebrain.

Believed to mediate dull, burning, aching, prolonged pain.

mediates emotional-motivational dimension

Reticular Activating Formation involved in aversive drive & similar pain related behaviorintegrates pain experience with pain behavior

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Pain is a physical stressor

Oxygen from the air we breathe combines with digested food to provide energy. This process generates dangerous byproducts such as free radicals – electronically unstable atoms or molecules capable of stripping electrons from any other molecules they meet in an effort to achieve stability. In their wake they create even more unstable molecules.

Retrieved from http://www.drweil.com/drw/u/QAA400537/Stumped-by-Oxidative-Stress.html

Putative Mechanisms for Relationship between Stress & Pain

Acute Pain

chronic pain

Stress releases CRF

response = hypersecretion of ACTH and excessive CORT release

Result = the negative feedback system (cort-hippo) fails

Unimpeded, stress increases … pain increases

Severe tissue damage elevates interleukins (an immune system component) in the blood.

These stimulate the pituitary which leads to corticotropin and vasopressin release

i.e. initiates the stress response

ANXIETY

a common symptom of stress

when anxiety increases pain increases

when pain/anxiety decreases, so does the other

suggests a common mechanism

80% of PTSD (an anxiety disorder) have chronic pain

Endocannabinoids and anxiolytics decrease pain and anxiety

but you do not have to be anxious to derive the benefits

PERSONALITY

pain felt more in catastrophizers & hot-reactors

pain felt more in people with external locus of control

chronic pain felt more in people with low self-esteem

i.e. pain felt more in types with poor coping skills for stress

FEELINGS OF CONTROL

people who feel they have adequate control feel less pain

makes sense, controllable stressors are less stressful

if stress causes pain, then controllable pain is less painful

HYPNOSIS

has been used to decrease pain

has been used to decrease stress

is the decrease in stress responsible for the decrease in pain?

do they share a common mechanism?

LAUGHTER

laughter decreases stress and pain - both through endorphin release

PAIN AFTER ALL HEALING

“Usually” occurs after a traumatic injury

is the chronic pain the result of chronic stress?

STRESS INDUCED ANALGESIA Acute stress can decrease pain perception!could this account for some cases of episodic analgesia? MECHANISM FOR STRESS INDUCED ANALGESIA

Endogenous opioid and non-opioid mechanisms are implicated, as are the amygdala and the PAG CRF triggers the release of opioid peptides (endorphins) from immune cells in the area of injury Endocannabinoid receptors in the PAG: if blocked, eliminate SIA

Contradictory evidence? chronic stress suppresses the immune systemtherefore, should suppress release of endorphins, pain

Memory mechanisms for Stress & Pain

Stress can be induced via memoriesand negative self-talk

A memory-like mechanism has beenSuggested for pain (e.g. phantomlimb pain)

Memories may trigger one whichTriggers the other, or both are Triggered concurrently